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Faye 1991 XXI
Pergamon
PII: SOO32-0633(96)00049-9
Planet. Space Sci., Vol. 44, No. I, pp. 625-635, 1996 Copyright (0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0032p0633/96 $15.00+0.00
Photometry and direct imaging of comet P/Faye 1991 XXI Hans-Georg Grothues Astronomisches
Institut
Received 9 November
der Ruhr-Universitgt, 1995; revised 5 February
Abstract. Broad-band
D-44780 Bochum, 1996; accepted
B and V photometry over a period of 74days including the perihelion passage, and CCD direct-imaging (V and I) is presented for the coma of short-period comet Faye 1991 XXI. The V lightcurve, corrected for the projected diaphragm diameter varying with geocentric distance, and for the cometary dust phase function, reveals a brightness “outburst” at perihelion, corresponding to an increase of the water production rate by about 10-l 5% during 2.0days. This “outburst” cannot be explained by an increase of the gas sublimation rate due to a change of temperature with heliocentric distance. Rather, it must be attributed to the activation of a “dormant” active area on a very slowly rotating nucleus, or to a change in the physical structure of the emission region, like an “explosion” of volatile ices, or an exothermal phase transition in the nucleus material. From the total visual brightness of the comet a water production rate of roughly 2 x 10” s-l can be estimated for the time of perihelion at 1.6 AU. The “apparent” post-perihelion brightness decrease is extraordinarily steep with n = 13.2 (or larger), which is difficult to explain by a simple dependence of the sublimation from heliocentric distance. The correction of the lightcurve for the cometary dust phase function of the coma, i.e. reduction to zero phase angle, leads to a “more normal” brightness decrease for Jupiter family comets of n = 6-7. Radial surface-brightness profiles show a p-’ dependence for the inner coma indicating an almost isotropic dust emission of the near-nucleus region. A cut-off of the p-l profile in the sunward sector can be interpreted as a dust reflection envelope at about 5 x lo3 km in front of the nucleus. The dust expansion velocities near perihelion are of the order of 200m s-’ for the smallest grains visible in the coma. Moreover, apart from the sun-tail elongation, a slight deviation from the isotropy of the coma can be recognized perpendicular to the sun-comet radius vector, and may be interpreted in terms of solar radiation pressure acting on cometary dust grains, or anisotropic outgassing of the nucleus. Copyright 0 1996 Elsevier Science Ltd
F.R.G.
5 February
1996
1. Introduction
Comet 4P/Faye is one of the most often recovered members of the Jupiter family of short-period comets with 19 apparitions observed from its discovery in 1843 to 1991. Its perihelion distance of about 1.6AU and the orbital period varying between 7.32 and 7.44years led to very favourable observing conditions in 1910, 1932, and 1969 when the comet passed its perihelion nearly in opposition to the sun. Consequently, the minimum geocentric distances at these apparitions were between 0.67 and 0.75 AU, leading to rather bright total visual magnitudes, V,,,, of 10-12” over long periods of observation. Thus the development of the comet’s brightness is very well documented during these apparitions by a considerable number of magnitude estimations and photographic measurements made by professional and amateur astronomers (Kamt?l, 1991, 1992). Despite significant changes in the shape and the time of maximum brightness of the lightcurve, especially from 1932 to 1969 (KamCl, 1992), the total visual magnitudes corrected for the varying geocentric distance, A, i.e. V, = V - 51og A, remained remarkably constant at V, = 10”.65~0”.15 in 1910, 1932, and 1969, when averaging from 20 days before to 20 days after perihelion (in 1910 the observations started only 6.5 days after perihelion ; cf. also Meisel and Morris (1982)). Using the correlation between V, and the water production rates of comets, QHZO.established by Jorda et al. (1992), the latter can be estimated at - 1.5 x 10” s-’ at about the time of perihelion. The 1991192 apparition of P/Faye was as favourable as the ones mentioned above. The comet passed its perihelion at q = 1.593AU on November 16, 1991, 4h.39mUT (defining tp = fO.Od) with a phase angle of 16‘.6, after having reached a minimum geocentric distance of 0.62 AU on October 28. Broad-band B and V photometry of the cometary coma was performed in order to investigate the brightness and (dust) activity development during the most active part of the cometary orbit. The use of broad-band filters is justified by the fact that many of the low and medium active short-period comets show comae dominated by the dust
626
H.-G. Grothues: Photometry and direct imaging of comet P/Faye 1991 XXI
continuum of reflected sunlight (e.g. Fink and Hicks. 1996). In their spectra molecular and atomic emission lines are almost absent. A classification of comets by their production rate ratios, established by A’Hearn et al. (1996), places P/Faye in the taxonomic class of carbondepleted comets, i.e. the typical molecular emission lines of C, Cj, and CN are weak in the spectrum of this comet. Additionally, two CCD direct-images (V and I) were taken on November 27, 1991. These images allow the V photometry to be corrected for the effect of the varying projected diaphragm diameters due to the changing geocentric distance of the comet, and also some conclusions to be drawn on the gas and dust emission of the nucleus from the surface brightness distribution in the coma. Moreover, a comparison between the V and I frames confirms the assumption of a dust dominated coma for P/Faye. In the following sections the observations and reductions are described in some detail, separately for the photometry (Section 2.1) and the imaging (Section 2.2). Subsequently, Section 3 discusses the coma lightcurve, while in Section 4 the coma morphology and its physical causes are investigated. Finally, Section 5 presents the most important conclusions.
2. Observations and reductions 2.1. Photomew? The B and V photometries were carried out at the Bochum 61 cm (24”) Telescope at La Silla, Chile, during every photometric night from November 9, 1991 (tP = -7d) to January 22, 1992 (tP = + 67 d), excluding some nights around full moon, using a single channel photometer with an EM1 9789 B photomultiplier and a standard Johnson BV filter set (cf. Gochermann et al., 1993). All measurements were obtained through a circular diaphragm of 45”.4 in diameter which was always centred on the sharply peaked brightness maximum of the coma (cf. Fig. 3) by means of an illuminated eyepiece cross-hair. The resulting positioning error is estimated to be 2-3”. No guiding on the comet was performed since the integration times were always shorter than 60s in each filter, at a maximum motion of the comet of - 0”.5 min-’ near perihelion. This can be entirely neglected against the positioning uncertainties of the diaphragm. Each measurement (B and V) consisted of the sequence “sky-comet-sky-cometsky”, in order to estimate the photometric errors induced by the positioning of the diaphragm onto the diffuse coma. The sky measurements were obtained about S-10’ to the east or west of the nucleus to minimize contamination of the coma. Despite this distance from the nucleus, the sky measurements may be affected by a very moderate “zeropoint” offset due to the far extension of the coma, as indicated by the CCD images in Fig. 3. A larger offset of the sky measurements of about 3&60’ towards the sun would have probably yielded a slightly better sky correction. Additionally, an appropriate number of E-region UBVRI standard-stars from the list of Graham (1982)
were observed to allow a determination of atmospheric extinction and a transformation to BV standard-magnitudes. Each of the two comet measurements in the sequence. taken about 447min apart, were corrected by interpolated sky counts, and transformed to V magnitudes and B-V separately. A detailed description of these reductions can be found in Gochermann et al. (1993). Finally, the cometary magnitudes and colours belonging to the same measurement sequence were averaged, with half of the differences, AV and A(B -V), giving a clue to the photometric positioning uncertainties which are of the order of 0”.01-0”.04 for both, V and B-V. Table 1 presents the final results of the photometry. The photometric standard deviations given in this table, ov and gB_“, represent the final errors caused by the photometry itself (cf. Gochermann et al., 1993) and the positioning uncertainties mentioned above. The times of observation quoted in the table refer to the arithmetic time averages of each measurement sequence. In addition to the direct measurements, Table 1 also lists the magnitudes, V ‘,, reduced to a geocentric distance of A = l.OAU and a constant projected diaphragm diameter (km) at the distance of the nucleus. All magnitudes, V, = V- 5logA, were reduced to V’,, which represents the V magnitude within a projected diaphragm diameter 2p = 32,930 km, i.e. 4Y.4 at A = l.OAU. In order to determine the corrections to be applied, the cumulative brightnesses, I,, in the cometary coma were deduced from the CCD V-image (cf. Section 2.2) by integrating over circular diaphragms of consecutive growing radii, p, centred on the nucleus (peak brightness). The resulting relation, I,(p), was then fitted by a second-order polynomial to be able to interpolate to the actual diaphragm radii. The corrections applied to V, ranged from -0”.47 for the observations at the beginning of the campaign, to +O”. I5 at its end. They are based on the assumption that the shape of the cometary coma did not change substantially during the whole time interval covered by the photometry. A systematic error in the sky background subtraction of the CCD image (Section 2.2) by typically 1% will lead to shifts in the correction terms of only f 0”.003 for an underestimation (+) or overestimation ( -) of the sky. Larger, but still not serious errors in the correction terms are caused by a temporary steepening or flattening of the coma brightness distribution due to possible timedependent coma structures, like jets or an expanding halo after the “outburst” close to perihelion (Section 3). An alteration of the average coma surface-brightness slope (approximately - 1.O, Fig. 5) by 0.1 gives an estimate of the change of the correction to be applied to V, : at the beginning of the observations, the correction term would be changed from -0”.47 to -0”.52 in the case of steepening (slope - 1.1) and -0”.42 for a flattening (slope - 0.9). The corresponding values for the last measurement of the campaign, at tp = + 67 d, are + 0”. 17 and +O”. 13, respectively. Another important effect that influences the lightcurve is the phase function of the cometary dust in the coma. Especially for small scattering or phase angles, CI,i.e. for comets observed near opposition, this function displays a steep slope. but is not very sensitively depending on the
H.-G. Grothues:
Photometry
and direct imaging of comet P/Faye
Table 1. Photometric
tp (d)” -____ - 6.897 -6.167 - 6.075 - 5.963 -5.170 - 5.079 -4.988 -4.921 -4.172 -4.084 -3.975 -2.172 -2.097 - 1.962 -1.173 - 1.091 - 0.952 -0.175 - 0.092 0.026 0.050 0.062 0.880 1.051 1.864 1.972 7.855 7.949 8.064 8.858 8.945 12.855 12.938 13.064 13.872 13.939 14.856 14.935 15.870 15.940 16.947 17.861 17.940 18.878 18.925 19.882 19.923 20.873 20.914 21.872 21.911 36.853 37.891 38.907 39.899 40.849 40.900 41.849 41.896 45.868 45.899 46.866 46.912 47.866
observations
r (AU)b
A (AU)’
1.595 1.595 1.595 1.595 1.594 1.594 1.594 1.594 1.594 1.594 1.594 1.594 ,594 ,594 1,593 ,593 .593 ,593 .593 ,593 ,593 .593 ,593 ,593 ,594 ,594 1.596 1.596 1.596 1.596 1.596 1.599 1.599 1.599 1.600 1.600 1.601 1.601 1.602 1.602 1.603 1.604 1.604 1.605 1.605 1.607 1.607 1.608 1.608 1.609 1.609 1.638 1.641 1.643 1.646 1.648 1.648 1.651 1.651 1.662 1.662 1.665 1.665 1.668
0.630 0.631 0.63 1 0.632 0.634 0.634 0.634 0.634 0.636 0.637 0.637 0.642 0.642 0.643 0.645 0.645 0.646 0.648 0.649 0.649 0.649 0.649 0.652 0.653 0.656 0.656 0.681 0.681 0.682 0.685 0.686 0.706 0.706 0.707 0.711 0.711 0.716 0.717 0.722 0.723 0.729 0.734 0.734 0.740 0.740 0.746 0.747 0.753 0.753 0.759 0.759 0.871 0.879 0.888 0.896 0.905 0.905 0.913 0.914 0.950 0.950 0.959 0.960 0.969
and corrected
1991 XXI V magnitudes
627 (cf. Section 2.1)
cx(deg)d
V
0;
B-VE
%Vh
12.7 13.1 13.2 13.3 13.7 13.8 13.8 13.9 14.3 14.3 14.4 15.4 15.5 15.5 16.0 16.0 16.1 16.5 16.6 16.6 16.6 16.6 17.1 17.2 17.6 17.6 20.5 20.6 20.6 21.0 21.0 22.7 22.7 22.8 23.1 23.1 23.5 23.5 23.9 23.9 24.3 24.6 24.7 25.0 25.0 25.4 25.4 25.7 25.7 26.0 26.0 29.8 30.0 30.2 30.4 30.6 30.6 30.7 30.7 31.3 31.3 31.4 31.4 31.6
11.300 11.253 11.270
0.010 0.027 0.022 0.051 0.046 0.019 0.021 0.030 0.045 0.040 0.065 0.027 0.022 0.040 0.020 0.021 0.023 0.030 0.025 0.021 0.021 0.028 0.024 0.016 0.024 0.048 0.018 0.023 0.012 0.018 0.035 0.029 0.012 0.011 0.019 0.036 0.035 0.037 0.021 0.021 0.014 0.015 0.016 0.057 0.017 0.011 0.010 0.032 0.019 0.024 0.024 0.016 0.040 0.022 0.026 0.028 0.013 0.028 0.022 0.040 0.015 0.023 0.014 0.018
0.746 0.810 0.787 0.800 0.792 0.794 0.783 0.794 0.760 0.761 0.785 0.785 0.750 0.788 0.755 0.763 0.777 0.723 0.735 0.718 0.758 0.775 0.763 0.729 0.661 0.698 0.775 0.812 0.812 0.799 0.812 0.786 0.770 0.780 0.783 0.794 0.798 0.790 0.800 0.785 0.792 0.775 0.790 0.784 0.775 0.791 0.783 0.767 0.761 0.760 0.765 0.767 0.804 0.763 0.758 0.739 0.759 0.757 0.775 0.808 0.777 0.733 0.747 0.744
0.012 0.019 0.117 0.018 0.022 0.016 0.018 0.015 0.022 0.022 0.021 0.014 0.021 0.016 0.015 0.021 0.017 0.012 0.013 0.012 0.013 0.026 0.054 0.015 0.014 0.046 0.015 0.028 0.026 0.016 0.023 0.012 0.019 0.027 0.023 0.018 0.03 1 0.026 0.022 0.022 0.028 0.017 0.010 0.017 0.014 0.020 0.028 0.02s 0.013 0.027 0.027 0.023 0.025 0.013 0.023 0.016 0.026 0.030 0.031 0.066 0.022 0.019 0.019 0.02s
11.268 11.265 11.296 11.291
11.263 11.311 11.290 11.303 11.286 11.322 11.351 11.321
11.322 11.258
11.256 11.235 11.254 11.185 11.194 11.178 11.171 11.209 11.193 11.341 11.329 11.347 11.356 11.360 11.520 1.545 1.548 1 1.553 1.565 1.581 1.591 1.642 11.661 11.651 11.694 11.673
11.734 11.723 11.715 11.733 11.801 11.828 11.809 11.782 12.278 12.308 12.371 12.304 12.372 12.358 12.375 12.347 12.547 12.500 12.552 12.433 12.517
v ‘,’
V’ I” k
11.82 11.77 11.78 11.78 11.77 11.80 11.79 11.77 11.82 11.79 11.81 11.78 11.81 11.84 11.80 11.81 11.75 11.74 11.71 11.73 11.66 11.67 11.66 11.65 11.67 11.66 11.77 11.75 11.77 11.7s 11.78 11.91 11.93 11.93 11.94 11.95 11.95 11.96 12.00 12.02 12.00 12.04 12.02 12.07 12.06 12.04 12.06 12.12 12.14 12.12 12.09 12.43 12.45 12.51 12.43 12.49 12.49 12.48 12.45 12.61 12.56 12.60 12.48 12.56
11.54 11.48 II.49 11.48 11.46 11.49 11.48 11.46 11.50 11.47 11.4s 11.43 I .45 I .48 1.43 1.44 1.38 1.36 1.32 I .34 1.27 1.28 1.26 1.25 1.26 1 1.25 11.27 11.25 11.27 11.26 11.26 11.34 11.36 11.36 I 1.36 1 I .37 11.35 11.36 11.39 11.41 11.38 11.41 11.38 11.42 11.41 11.38 11.40 11.45 11.47 11.43 11.40 11.60 11.61 11.66 11.57 11.62 11.62 11.61 11.58 11.71 11.66 11.70 11.58 11.65 continued
628
H.-G. Grothues:
Photometry
and direct imaging of comet P/Faye
1991 XXI
Table 1. (Continued) tp (d)”
Y(AU)b
A (AU)’
a (deg)d
V
fl;,
B-VS
OB-bh
V’,’
47.918 48.865 48.912 50.865 66.871
1.668 1.671 1.671 1.677 1.734
0.969 0.978 0.979 0.997 1.166
31.6 31.7 31.7 31.9 32.8
12.527 12.520 12.502 12.495 12.994
0.012 0.018 0.035 0.019 0.013
0.779 0.759 0.779 0.749 0.766
0.020 0.011 0.013 0.022 0.030
12.57 12.55 12.53 12.50 12.82
“t,-time (UT) relative to perihelion at JD 2,448,576.694 (days). ‘r-heliocentric distance of comet (AU). ‘A-geocentric distance of comet (AU). ‘r-phase or scattering angle (deg). ‘V-observed visual magnitudes (4.5” diaphragm). ‘a,-standard deviation of V. “B-V-observed colour index (45” diaphragm). ‘kg_ v-standard deviation of B-V. IV’,-V magnitude corrected to A = 1.O AU and for variable, projected (Section 2.1). !V’ ,“-V’, magnitude corrected to z = 0’ (equation (1)).
material, the sizes, and the shapes of the dust particles (e.g. Divine, 1981 ; Grtin and Jessberger, 1990). Using the dust phase function presented by Divine (198 1), which has been deduced from photometry of comets, implies for the correction of V’, to zero phase angle, z = 0” (or of other “dust photometries” in the visible wavelength domain) Am0 = 2.51og(l -O.OlSc()
(1)
where CLis measured in deg. Equation (1) is valid only for the linear part of the phase function. i.e. for c(I 50”. The V’] magnitudes corrected in this way, V’],,, are also listed in Table I.
2.2. Direct imaging In addition to the photometry, and in order to correct for the variation in the projected radii of the diaphragm (cf. Section 2.1), two CCD images (V and I) were taken on November 27, 1991, 11 days after perihelion, with the Tektronics CCD-camera at the Dutch 90 cm Telescope at La Silla. With effectively 380 x 570 px used. and an image scale of 0”.38 pxx’ (corresponding to 192 km pxx’ at the distance of the comet) this provides a field of view (FOV) of 2’.4 x 3’.6. Table 2 gives a journal of observations. The seeing in each image frame was measured by fitting Gaussians to the unsaturated stars in the FOV. However, since no tracking on the comet was carried out during the Table 2. Journal of CCD observations (November 27, 1991) ; Y = 1.597 AU, A = 0.696AU, phase angle LY= 21-.9 Image V I
fstdr.ta 3hll” 3hl8”
tp (d)”
tcrp(s)’
AMd
Seeing’
+ 10.940 + 10.945
300 150
1.19 1.19
1.7kO.2 1.3kO.2
“t,,,,,-start of exposure (UT). brpdays after perihelion. ‘t,,,exposure time (s). ‘AM-airmass. ‘Seeing--estimated FWHM (arcsec).
diameter
v;,, 11.66 11.63 11.61 11.57 11.85
of the diaphragm
exposures, an additional smearing occurs in the direction of the cometary motion at a position angle of 106’ (Fig. 3). This smearing accumulates to about 2”.4 and I”.2 during the exposures for the V and the I image. respectively, resulting in an effective resolution of - 3” in that direction, almost perpendicular to the most important suntail axis at a position angle of 2 15 The CCD reduction was done in the standard way using averaged bias frames and sky flat-fields. Owing to the non-photometric weather conditions during the CCD observations no absolute photometric calibration of the images could be obtained. Subtraction of the (constant) sky background was performed by averaging the mean sky brightness in the south-western part of the images towards the direction to the sun, which is relatively undisturbed by contributions of the coma. The statistical uncertainties of the sky brightness were 0.6% for V and 0.9% for the I image. respectively. However, it cannot be ruled out that these parts of the images are contaminated by the comet’s surface brightness, and thus cause a systematic overestimation of the sky brightness (cf. the discussion in Section 2.1). The proper alignment of the image frames was ensured by comparing the field stars to the Digitized Sky Survey (POSS).
3. The lightcurve The V lightcurve of the coma, corrected for geocentric distance, varying diaphragm diameters, and the dust phase function (V’,,), is shown in Fig. lc (cf. also Table 1). For comparison, the “apparent” lightcurve which does not take into account the phase function effect is also presented (Fig. la). Starting at about 7days before perihelion, the lightcurve remains at an almost constant level until t, = - 1.1 d. At this time, it began to rise steeply by AV’,” = 0”.21 f0”.03, reaching its peak brightness at about f0.9d or later. Following this “outburst”, the lightcurve seems to have stayed nearly constant until about 10 days after perihelion. Unfortunately, a gap exists in the data owing to full moon between tp = +2 and
H.-G. Grothues:
Photometry
and direct imaging of comet P/Faye
1991 XXI
629
Comet PiFaye 1991XXI
(4
Comet P/Faye 1991XXI(Post-Perlhellon)
(b)
Projected diaphragm32930 km #
0
20
40
60
80
0200 P
0205
Days afterPerlhellon
20
40
0225
(d)
Projectea diaphragm32930 km
0
0220
CorretPiFaye 1991XX (Post-Perlhellor) 1'0
L
0215
lg(r/AU)
Come: P/Faye 1991XXI
1200 -:'O
0210
60
"outburst" 7
80
Days afterPerthellon
123' 0200
P
0205
0210
0215
6220
0225
lg(r/AU)
Fig. 1. (a) V lightcurve of comet 4P/Faye in 1991/92, reduced to a standard geocentric distance of A = 1.OAU, and corrected for the varying projected diameters of the 45” diaphragm (V ‘,), but not for the phase function effect (cf. Section 2.1). The perihelion passage on November 16, 1991 (JD 2.448,576.694) is indicated by “0”. (b) Post-perihelion V; -log r relation with two possible linear fits (single data point at t, = + 66.9 days omitted). While the steeper line (n = 16.0) fits all data points, the shallow one (n = 13.2) neglects the measurements possibly to be attributed to the “outburst” at perihelion. The perihelion is marked by “P”. (c) Same as (a), but considering the dust phase function effect. (d) Same as (b), but also taking into account the dust phase function effect. As in (b), the straight line with the steeper slope (n = 7.2) fits all data points, while the shallow one (tz = 6.2) neglects the “outburst”
+8 d. Thus the maximum brightness might have been reached as late as + 6 to + 8 d. At least from tp = + 12 d, the lightcurve maximum is followed by a steady decrease in brightness until the end of the observations at + 67 d. The increasing scatter of the data points with time is probably not due to intrinsic brightness variations, but is caused by systematic errors introduced by the fading coma surface-brightness approaching the detection limit of the telescope/detector combination. A period analysis (Stellingwerf, 1978) was applied to the data after having subtracted a third-order polynomial in order to suppress the long-term variations with heliocentric distance. No brightness variations (of amplitudes >0”.05 to Om.l) could be detected in the lightcurve which may indicate significant periodicities of 0.2-l 6 days related to the inner coma and the nucleus.
The B lightcurve varied almost parallel to V, resulting in constant colour of (B -V), = 0”.77 f 0”.03 during the whole period of observation (Table 1). This is significantly redder than the sun with (B-V), z 0”.65. Assuming that the emission from the coma is dominated by the dust continuum of scattered sunlight. i.e. molecular emission lines only make a minor contribution to the brightness (cf. Section 4, and Fink and Hicks (1996)), the reddening caused by the cometary dust can be estimated from (B-y),-(B-V), = 0”.14~0”.05 to be (13f5)%/ IOOOA. This is in good agreement with the results obtained by Gil-Hutton and Licandro (1994) and Jorda et al. (1995) from narrow-band photometries. The lightcurve of the total visual brightness of comet P/Faye for the 1991/92 apparition can be constructed from the brightness estimates published in the IAU Cir-
H.-G. Grothues: Photometry and direct imaging of comet P/Faye 1991 XXI
630 Comet “iFaye
1991 XXI 1
100
10 5 P E >
$
110
I
-100
-50
0
50
Days after Perlhellon
Fig. 2. Total visual lightcurve of comet P/Faye in 199 l/92. con-
structed from brightness estimates published in the IAU Circulars (1991/92). The data have been normalized according to the corrections given by Kamel(1992), i.e. they are not corrected for the dust phase function effect (cf. Section 2.1). Marginal evidence occurs for a lightcurve maximum at about -35 d, before the start of the photometry. The reason for the constant brightness from about 100 to 70days pre-perihelion remains unknown. but it is most probably caused by improper corrections applied (cf. Section 3)
culars (1991/92). Figure 2 presents this lightcurve, taking into account the corrections for geocentric distance, telescope apertures, sky brightness, and altitude (but no dust phase function effect), as applied to the earlier lightcurves ofthe comet since 1910, published by KamCl(l991, 1992). The total visual “perihelion brightness” (averaged over the time interval & 20 d) is IO”.49 & 0”. 19, corresponding to a water production rate of QHzo = (1.7 + 0.8) x 10z8s-’ (cf. Section 1). or, considering a correction of -0”.37 due to the dust phase function (equation (1)) QHIo = 2.0 x lo’* s-l. Again, this is in a remarkable agreement with the apparitions of 1910, 1932, and 1969. Scaling with an r-?.4 law (Fink, 1994 ; which is certainly not correct for P/Faye, as shown below), would lead to a production rate of - 5 x 10” ss’ at a standard distance of 1.OAU. It must, however, be noted that the water production rate of comet P/Faye determined from narrow-band photometries (GilHutton and Licandro, 1994 ; A’Hearn et al., 1996) is smaller by a factor of about 3, i.e. Q,?,z~ x lO”s~‘, when compared to the value given above. A comparison to other comets (e.g. Fink and Hicks, 1996; A’Hearn et al., 1996) ranges 4P/Faye among the medium active shortperiod comets, about a factor of 10 less active than comet Halley, but very similar to the (current) prime target of the ROSETTA mission, 46P/Wirtanen (e.g. Jorda and Rickman, 1995). Free sublimation at the comet’s nucleus surface would result in a maximum water sublimation rate (subsolar point) at 1.6 AU of about 2.5 x 102’ rnp2 s-’ (Rickman, 1992). From this, a minimum active area follows for the nucleus of P/Faye of about 7 km2, or, taking into account a nucleus radius of 2.7 km (Lamy and Toth, 1995), a fraction of more than 7% of its total surface (cf. also Gil-Hutton and Licandro, 1994).
A further comparison of the post-perihelion decline of the total visual magnitudes in 1991/92 to the decrease of the corrected photoelectric V magnitudes (V’,) reveals a constant difference between these two lightcurves of I”.4 f 0”.2. As the V,,, and V’, lightcurves are affected by the dust phase function effect in the same way, the (relative) changes of the water production of comet P/Faye can be consequently estimated from the variations in the V’,, lightcurve by means of the correlation given by Jorda et al. (1992). Correspondingly, the brightness increase close to perihelion translates to a growth of the water production rate by about l&15%, with almost no change in the heliocentric distance of the comet (Arc 10-4AU), its true anomaly (Av = 1>.2), and the phase angle (Acr = 1 ..l). Thus, it is improbable that the “outburst” was solely caused by an increase of the water sublimation rate owing to the increasing surface temperature with decreasing heliocentric distance. Rather, one could attribute the brightness increase to the activation of a hitherto non or less active (“dormant”) region on a nucleus rotating very slowly in an exited rotational state (precession). Such a very slow period of rotation (of the order of lOdays) may be indicated by the HST photometry of the nucleus presented by Lamy and Toth (1995). However, owing to their very few data points, this possibility remains somewhat questionable. Other explanations could be a non“explosion” (abrupt insolation) of a newly recurrent exposed pocket of volatile ices (e.g. CO or CO?), an exothermal phase transition for instance in the crystalline structure of the nucleus material, or a mechanical alteration of the physical structure of the active region (e.g. by a landslide). The continuous brightness decrease, which followed the brightness peak after perihelion until the end of the observations is corresponding to a decrease in the water production of roughly 30%. At the same time, the heliocentric distance of the comet had changed from 1.59 to 1.73AU. According to recent modelling (e.g. Rickman, 1992), this decline cannot entirely be explained by a sole adaptation of the water sublimation rate to the changing nucleus surface-temperature, which should result in a change of only 15-20%. However, due to the large uncertainties of the production rate determination and also the sublimation rates. this clue has to be regarded with some care. Traditionally, the (total visual) brightness increase or decrease of a cometary lightcurve with heliocentric distance is characterized by an absolute magnitude, H,, at Y = A = 1.OAU, and a slope, n. Table 3 lists separately for Table 3. (E&n) for comet 4P/Faye
(1991/92)
Lightcurve
HO 6.4kO.6 5.1kO.4 3.8kO.8 5.2iO.6 7.7kO.5 8.2kO.4
V,,,, pre-perihelion V,,,, post-perihelion V ‘,, post-perihelion V ;, post-perihelion” V ‘,,,, post-perihelion V’,,, post-perihelion” ““Outburst”
omitted
(cf. Section 3).
n 8.1k 1.1 10.8kO.7 16.0& 1.4 13.2_+ 1.1 7.2kO.9 6.2+_0.7
H.-G. Grothues:
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and direct imaging of comet P/Faye
63
1991 XXI
(a >
ItttI
20000
II
0 Posit,ion
20000 km
I
I
-20000 / km
-3
Fig. 3. (a) Logarithmized greyscale represention of the V image of comet 4P/Faye taken on November 27, 1991, about 1I days after perihelion. A fourfold jigsaw intensity transformation is used to illustrate the steep brightness gradient in the inner coma. (b) Contour plot of the I image of the same day. The contour levels in the frame are at 1. 2, 3, 5, 10, 20, 50, and 95% of the peak surface brightness (nucleus) at (0.0). The directions towards the sun (0) and the projected orbital velocity vectors of the cometary nucleus (Q) are marked by arrows. North is up, east to the left
I1III-l ,I,
-40000
632
H.-G. Grothues:
Photometry
and direct imaging of comet P,‘Faye 1991 XXI
Fig. 4. I image processed by a division of the original image (Fig. 3b) by its 360”-azimuthal average, and smoothed by a Gaussian (FWHM = 3px, - seeing). The greyscale representation indicates deficient surface brightness in the sunward hemisphere, while the tailward one is dominated by excess brightness. Note the asymmetry in the excess relative to the suncomet vector, which is obvious by the S-shaped “isophote” that intersects the nucleus (brightest pixel) at the centre (cross). The suncomet radius vector (0) is marked by an arrow. The radius of the circular image held corresponds to 25,500 km. North is up, east to the left
H.-G. Grothues: Photometry and direct imaging of comet P/Faye 1991 XXI the pre- and the post-perihelion observations the values of (&,n) determined from the total visual brightness of comet P/Faye as presented in Fig. 2, and compares them to (H,,n) as found from the photometry. Even if the first, very steep decline after perihelion is solely attributed to the “outburst” (Fig. Id), the slope, n = 6.2, turns out to be steeper than the canonical value of n = 4 (e.g Meisel and Morris, 1982). Nevertheless, the value fits rather well into the usually steeper slopes observed for the dynamically old Jupiter family comets. Evidently, the power law fits are not very satisfactory to describe the development of the lightcurve after perihelion. Thus, more sophisticated explanations than the usual distance law have to be assumed for comet P/Faye, e.g. illumination effects of the active region(s) in combination with the state of rotation of the nucleus (cf. the discussion of the “outburst”). It is quite interesting to note that in 1910 and 1932 the total visual magnitudes after perihelion decreased as n = 3.6 and 4.5, respectively, i.e. much more “ordinary” than in 1991/92 (KamCl, 1991, 1992), although the observation geometry was similar to 199 l/92 and no correction for the dust phase function effect has been applied to these data. In 1969, the brightness reached its peak only about 35days after perihelion to remain almost constant for more than 50days thereafter. The reason for this behaviour is not completely clear : the general activity pattern, i.e. the shift of the brightness maximum, can again be explained by the influence of the dust phase function; it can, however, not account for the long plateau of maximum brightness observed during that apparition. A possible, substantial change in the activity pattern, i.e. the number, strength, and spatial distribution of the active regions on the nucleus’ surface will most probably also result in a change of the gas and dust production rates, and hence the brightness of the coma, by a noticeable amount. However, the “perihelion brightness” has remained constant to less than 0”.2 since 1910 (cf. Section I), which in turn means the water production near perihelion to have varied by at most 20% from one apparition to the next only. The parameter, n, can be used as a guideline to estimate the r-dependence of the water production rate, again using the correlation given by Jorda et al. (1992) : QHZON r-0.6n. The approximate validity and the accuracy and limitation of this relation is demonstrated by a comparison of the pre-perihelion total visual lightcurve of comet Halley in 1985/86 (Green and Morris, 1987 ; n = 3.2 for r< 1.5 AU, of the water i.e. QH20- r -‘.9) to the direct determination production rates presented by Fink (1994), giving QH,o -‘4 (cf. also Green and Morris, 1987). Consequently, fz:comet P/Faye in 199 l/92, QH+, fell off as r-j 7*o.4 after perihelion. A much steeper dependence was found by Jorda et al. (1995) for the pre-perihelion dust production (Afp values) of comet P/Faye with r-6.7*o.5. The postperihelion decrease of Afp seems to have been similarly steep (cf. Fig. 3b of Jorda et al. (1995) and Gil-Hutton and Licandro (1994)). Thus the fast activity decline of comet 4P/Faye in 1991192 is confirmed by two independent measurements, and it must be regarded as real. As discussed above, the physical reasons are not yet completely understood.
633
4. Coma morphology Figure 3 presents a greyscale representation and a contour plot of the V and 1 CCD-images of comet P/Faye, respectively. A division of one image by the other reveals almost no differences in the shapes of the inner coma. This can be interpreted to support the assumption, which has been made above, that the coma is dominated by the continuous radiation of reflected sunlight scattered by cometary dust grains, rather than by molecular emission lines. Hence, the following discussion concentrates mainly on the V image, but the image processing was always performed on both images in order to confirm the mutual results. Primarily, the coma of comet P/Faye is characterized by the isophotes clearly elongated along the sun-comet radius vector, indicating the existence of a tail roughly opposite the sun. Besides the effect of radiation pressure forces acting on the dust grains, the eccentricity of these isophotes, e~O.5, may be interpreted in terms of fragmentation of dust particles inside the coma (Combi, 1994). The elongation at constant eccentricity can be traced down to a projected distance of p < 5 x 1O3km from the nucleus, indicating that the possible fragmentation process is already effectively acting on this small-scale length. However, in order to confirm the fragmentation theory, a real modelling had to be performed taking into account the viewing geometry from Earth. Furthermore, for both images a slight asymmetry can be recognized in the inner coma (p<2 x lo4 km or 40”) with respect to the radius vector, i.e. the coma shows a larger extent (- 20-30%) to the NW than to the SE. This asymmetry is by far too large to be explained by the lack of compensation of the comet’s apparent motion during observation (Section 2.2), acting in approximately the same direction as the effect under consideration. A possible explanation for this anisotropy may be the solar radiation pressure driving the ejected dust particles outside the cometary orbit, and leaving them back behind the nucleus, i.e. opposite to the comet’s velocity vector. Another explanation could involve anisotropic outgassing of the nucleus. In order to confirm the findings from the direct inspection of the images, and to search for possible additional coma structures like jets or fans, three independent image processing techniques were applied to the frames: (a) a division of the images by their 3601-azimuthal averages, (b) the azimuthal-renormalization algorithm introduced by A’Hearn et al. (1986), and (c) a division of the original by their Gaussian-smoothed counterparts images (FWHM = 4px for V and 3 px for I, roughly corresponding to the respective seeing). The brightest pixel in the coma was always taken to represent the nucleus (coordinate zeropoint) in these algorithms. The third technique has the advantage over the other two to avoid centring errors of the nucleus. For methods (a) and (b) these errors have been investigated by shifting the zeropoint by one pixel ( - error of centring) along the NS and EW directions and combinations. No substantial changes in the resulting images occurred with respect to the optimum centring. All three methods lead to qualitatively identical results.
H.-G. Grothues:
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1
i
l$j?!kT) Comet PiFaye 1991XX
Photometry
and direct imaging of comet P:Faye
1991 XXI
The radial profiles perpendicular to the sunxomet vector show also no significant deviations from linearity. The linearity of the log (&log p relation with the slopes near to - 1.O can most easily be explained in terms of an isotropic expansion of the dust coma at constant velocity. Moreover, the dust production rate must have been almost constant on a time scale necessary for the grains to leave the coma, i.e. for about 1 day or longer (steady-state condition). The constant slope of the tailward profile, if significant above - 1.O. could be another indication of continuous fragmentation of dust particles in the coma (cf. O’Dell et ((1. (I 988) for I P/Halley). The surface-brightness cut-off on the sunward side of the nucleus is most easily interpreted by a simple fountain model (e.g. Jewitt and Meech, 1987 ; Griin and Jessberger, 1990). In this model the dust grains ejected from the nucleus towards the sun are decelerated by the solar radiation pressure force at an apex distance or reflection point, X,, and then accelerated into the tail. The apex distance is connected to the grain velocity, f:prrand the ratio, /?, of the radiation pressure force affecting the particle to the local solar gravity, by the relation
(2)
MpW Fig. 5. Radial surface-brightness
profiles (p-projected radius from the brightness pixel) averaged over 90 ’ sectors centred on the direction towards the sun (a), and the prolonged radius
vector (b) in the V image. Error bars indicate f 0.6% of the sky background level (cf. Section 2.2). which is represented by a value of log([,,,) = 1.37. An “S” marks the radius (HWHM = 430 km) of the seeing disk. The fitted straight lines have the slopes - 1.08 kO.03 (a) and -0.95 f0.02 (b), respectively
where I’ is the heliocentric distance of the comet. G the gravitational constant, and M, one solar mass (Jewitt and Meech, 1987). Taking into account the observational geometry (phase angle c( = 21 .9) as modelled by Jewitt and Meech (their Fig. 5), XR can be estimated at - 5 x lo1 km. Hence. for p = 0.01-2.0, corresponding to spherical organic and silicate dust grains of roughly 0.1~ 1Opm in radius, which have been concluded for the dust tails of several comets (e.g. Burns et (I/.. 1979; Saito et ul.. 1981 ; Lamy and Perrin, 1988). the expansion velocities, l’gr = 15-220 m SC’, can be calculated from equation (2). This is comparable to the velocities found in other shortperiod comets (e.g. Jewitt and Meech, 1987).
5. Conclusions 4P/Faye is a medium active member of the Jupiter family of short-period comets with a water production rate of -2 x 10’ss-’ at its perihelion distance of I .6AU. This makes it comparable in its activity to the (current) prime target of the ROSETTA mission, comet 46P/Wirtanen. The steep decline of the (dust) activity in comet P/Faye with heliocentric distance, possibly as steep as -r-‘~.~‘, will, if preserved over a larger distance interval (which is still not proved), lead to almost no activity which can definitely be discovered from Earth beyond 3-5 AU (aphelion 5.96 AU). Near perihelion, irregular (?), shortterm activity variations on a time scale of few days may occur from time to time (at most a few “outbursts” per revolution), with the gas and dust production changing by amounts of the order of 10% or more. The expansion of dust from the innermost coma (p < lo3 km) seems to be nearly isotropic, with very weak deviations caused by an anisotropic distribution of the spatial activity pattern on the nucleus to be recognized outside this zone. The dust Comet
Besides the most distinct sun-tail morphology, the asymmetry of the coma relative to the sunxomet radius vector is clearly present (Fig. 4). Significant other features, however, were not uncovered by this techniques. It is not clear whether the asymmetry of the dust coma found by Jorda et al. (1995) for comet P/Faye in their R image taken on August 26, 1991. 82 days before perihelion, is related to the asymmetry detected in the images presented here. Radial surface-brightness profiles, log(l) vs. logp (pprojected distance from the nucleus), each averaged over a 90” sector centred on the sunward direction and the prolonged radius vector, respectively, are shown in Fig. 5. While the tailward profile can be well represented by a linear fit with a slope of -0.95+0.02 out to a radius of p = 2.4 x lo4 km (edge of FOV), the sunward profile (slope - I .08 + 0.03) displays an obvious cut-off from linearityatp=(llfl)~lO~km.
H.-G. Grothues: Photometry and direct imaging of comet P/Faye 1991 XXI expansion
200ms-’ coma.
velocities
near perihelion
are of the order
of
for the smallest particles to be visible in the
Acknolrled~ernents. I thank M. 0. Oestreicher and Th. Berghofer for carrying out considerable parts of the photometric observations, without which the lightcurve would not have been extended over such a long period. A. Collier Cameron and B. Foing kindly obtained the CCD images of the comet, and provided some technical information. Moreover, I am indebted to the referees, H. Bijhnhardt and L. Jorda, for their valuable comments and suggestions on the manuscript. The CCD reductions were carried out using the ESO-MIDAS image processing package.
References A’Hearn, M. F., Hoban, S., Birch, P. V., Bowers, C., Martin, R. and Klinglesmith, D. A., Cyanogen jets in comet Halley. Nature 324,649-65 I, 1986. A’Hearn, M. F., Millis, R. L., Schleicher, D. G., Osip, D. J. and Birch, P. V., The ensemble properties of comets : results from narrowband photometry of 85 comets, 1976-1992. Icarus
1996 (in press). Burns, J. A., Lamy, P. L. and Soter, S., Radiation forces on small particles in the solar system. Icarus 40, l-48, 1979. Combi, M. R., The fragmentation of dust in the innermost comae
of comets: possible evidence from ground-based images. Astron. J. 108, 304-312, 1994. Divine, N., A simple radiation model of cometary dust for P/Halley. ESA-SP 174,47-53, 1981. Fink, U., The trend of production rates with heliocentric distance for comet P/Halley. AstrophJ,s. J. 423, 461473. 1994. Fink, U. and Hicks, M. D., A survey of 39 comets using CCD spectroscopy. Astrophl;.~. J. 1996 (in press). Gil-Hutton, R. and Licandro, J., Photoelectric photometry of periodic comet Faye. Rec. Mesicarza 28, 3-6, 1994. Gochermann, J., Grothues, H.-G., Oestreicher, M. O., Berghiifer, Th. and Schmid-Kaler, Th., UBV photometry of galactic fore-
ground and LMC member stars. I. Galactic foreground stars. Astron. Astroph>x. Suppl. 99, 591-614, 1993.
635
Graham, J. A., UBVRI standard stars in the E-regions. Publ. Astron. Sot. Pac. 94, 244-265, 1982. Green, D. W. E. and Morris, C. S., The visual brightness behaviour of P/Halley during 198 l-l 987. Astrox Astrophys. 187, 560-568, 1987. Griin, E. and Jessberger, E. K., Physics and Chemistry qf Comets
(edited by W. F. Huebner). Chap. 4. pp. 113-176, 1990. Nos. 5325, 5378, 5348, 5356. 5362, 5382, 5407, and 5440. 1991/92. Jewitt, D. C. and Meech, K. J., Surface brightness profiles of 10 comets. Astrophq. J. 317, 992-I 00 1, 1987. Jorda, L. and Rickman, H., Comet PiWirtanen, summary of observational data. Planet. Space Sci. 43, 575-579, 1995. Jorda, L., Crovisier, J. and Green, D. W. E., The correlation between water production rates and visual magnitudes in comets, in Asteroids, Comets, Meteors 1991 (edited by A. W. Harris and E. Bowell), pp. 285-288, 1992. Jorda, L., Hainaut, 0. and Smette, A., Photometric study of comets P/Faye 1991 XXI and Zanotta-Brewington 1992 III. IAU Circulars, IAU Circulars.
PImet. Space Sci. 43, 737-745, 1995. Kam& L., The Conlet Light Cure Catalogue/.4tlas (CLICC,‘A). Part 11, Digital Data Bare (ftp.astro.uu.se). 1991. Kamkl, L., The comet light curve atlas (The comet light curve catalogue/atlas. 111.The atlas). Astron. Astroph>*s.Suppl. 92, 85-149, 1992. Lamy, P. L. and Perrin, J.-M., Opticat properties of organic
grains : implications for the inter-planetary and cometary dust. Icarus 76, 10&109, 1988. Lamy, P. L. and Toth, I., Direct detection of a cometary nucleus with the Hubble Space Telescope. Astrm. Asrroph~s. 293, L43-45, 1995. Meisel, D. D. and Morris, C. S., Cometary head photometry: past, present and future, in Cowets (edited by L. L. Wilkening), pp. 413432. 1982. O’Dell, C. R., Robinson, R. R., Krishna Swamy, K. S., McCarthy, P. J. and Spinrad, H., C, in comet Halley: evidence for
its being third generation and resolution of the vibrational population discrepancy. Astrophys. J. 334,476&488, 1988. Rickman, H., Physico-dynamical evolution of aging comets, in Interrelatiom bet,t,eerlPhJ,sicsarldDJnamicsfor Minor Bodies in the Solar System (edited by D. Benest and C. FroeschlC),
p. 197. 1992. Saito, K., Shuzo, I., Nishioka, K. and Ishii, T., Substances of
cometary grains estimated from evaporation and radiation pressure mechanisms. Icarus 47, 35 l-360, 198 1. Stellingwerf, R. F., Period determination using phase dispersion minimization. Astroph~~s.J. 224, 953-960, 1978.
PII: SOO32-0633(96)00049-9
Planet. Space Sci., Vol. 44, No. I, pp. 625-635, 1996 Copyright (0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0032p0633/96 $15.00+0.00
Photometry and direct imaging of comet P/Faye 1991 XXI Hans-Georg Grothues Astronomisches
Institut
Received 9 November
der Ruhr-Universitgt, 1995; revised 5 February
Abstract. Broad-band
D-44780 Bochum, 1996; accepted
B and V photometry over a period of 74days including the perihelion passage, and CCD direct-imaging (V and I) is presented for the coma of short-period comet Faye 1991 XXI. The V lightcurve, corrected for the projected diaphragm diameter varying with geocentric distance, and for the cometary dust phase function, reveals a brightness “outburst” at perihelion, corresponding to an increase of the water production rate by about 10-l 5% during 2.0days. This “outburst” cannot be explained by an increase of the gas sublimation rate due to a change of temperature with heliocentric distance. Rather, it must be attributed to the activation of a “dormant” active area on a very slowly rotating nucleus, or to a change in the physical structure of the emission region, like an “explosion” of volatile ices, or an exothermal phase transition in the nucleus material. From the total visual brightness of the comet a water production rate of roughly 2 x 10” s-l can be estimated for the time of perihelion at 1.6 AU. The “apparent” post-perihelion brightness decrease is extraordinarily steep with n = 13.2 (or larger), which is difficult to explain by a simple dependence of the sublimation from heliocentric distance. The correction of the lightcurve for the cometary dust phase function of the coma, i.e. reduction to zero phase angle, leads to a “more normal” brightness decrease for Jupiter family comets of n = 6-7. Radial surface-brightness profiles show a p-’ dependence for the inner coma indicating an almost isotropic dust emission of the near-nucleus region. A cut-off of the p-l profile in the sunward sector can be interpreted as a dust reflection envelope at about 5 x lo3 km in front of the nucleus. The dust expansion velocities near perihelion are of the order of 200m s-’ for the smallest grains visible in the coma. Moreover, apart from the sun-tail elongation, a slight deviation from the isotropy of the coma can be recognized perpendicular to the sun-comet radius vector, and may be interpreted in terms of solar radiation pressure acting on cometary dust grains, or anisotropic outgassing of the nucleus. Copyright 0 1996 Elsevier Science Ltd
F.R.G.
5 February
1996
1. Introduction
Comet 4P/Faye is one of the most often recovered members of the Jupiter family of short-period comets with 19 apparitions observed from its discovery in 1843 to 1991. Its perihelion distance of about 1.6AU and the orbital period varying between 7.32 and 7.44years led to very favourable observing conditions in 1910, 1932, and 1969 when the comet passed its perihelion nearly in opposition to the sun. Consequently, the minimum geocentric distances at these apparitions were between 0.67 and 0.75 AU, leading to rather bright total visual magnitudes, V,,,, of 10-12” over long periods of observation. Thus the development of the comet’s brightness is very well documented during these apparitions by a considerable number of magnitude estimations and photographic measurements made by professional and amateur astronomers (Kamt?l, 1991, 1992). Despite significant changes in the shape and the time of maximum brightness of the lightcurve, especially from 1932 to 1969 (KamCl, 1992), the total visual magnitudes corrected for the varying geocentric distance, A, i.e. V, = V - 51og A, remained remarkably constant at V, = 10”.65~0”.15 in 1910, 1932, and 1969, when averaging from 20 days before to 20 days after perihelion (in 1910 the observations started only 6.5 days after perihelion ; cf. also Meisel and Morris (1982)). Using the correlation between V, and the water production rates of comets, QHZO.established by Jorda et al. (1992), the latter can be estimated at - 1.5 x 10” s-’ at about the time of perihelion. The 1991192 apparition of P/Faye was as favourable as the ones mentioned above. The comet passed its perihelion at q = 1.593AU on November 16, 1991, 4h.39mUT (defining tp = fO.Od) with a phase angle of 16‘.6, after having reached a minimum geocentric distance of 0.62 AU on October 28. Broad-band B and V photometry of the cometary coma was performed in order to investigate the brightness and (dust) activity development during the most active part of the cometary orbit. The use of broad-band filters is justified by the fact that many of the low and medium active short-period comets show comae dominated by the dust
626
H.-G. Grothues: Photometry and direct imaging of comet P/Faye 1991 XXI
continuum of reflected sunlight (e.g. Fink and Hicks. 1996). In their spectra molecular and atomic emission lines are almost absent. A classification of comets by their production rate ratios, established by A’Hearn et al. (1996), places P/Faye in the taxonomic class of carbondepleted comets, i.e. the typical molecular emission lines of C, Cj, and CN are weak in the spectrum of this comet. Additionally, two CCD direct-images (V and I) were taken on November 27, 1991. These images allow the V photometry to be corrected for the effect of the varying projected diaphragm diameters due to the changing geocentric distance of the comet, and also some conclusions to be drawn on the gas and dust emission of the nucleus from the surface brightness distribution in the coma. Moreover, a comparison between the V and I frames confirms the assumption of a dust dominated coma for P/Faye. In the following sections the observations and reductions are described in some detail, separately for the photometry (Section 2.1) and the imaging (Section 2.2). Subsequently, Section 3 discusses the coma lightcurve, while in Section 4 the coma morphology and its physical causes are investigated. Finally, Section 5 presents the most important conclusions.
2. Observations and reductions 2.1. Photomew? The B and V photometries were carried out at the Bochum 61 cm (24”) Telescope at La Silla, Chile, during every photometric night from November 9, 1991 (tP = -7d) to January 22, 1992 (tP = + 67 d), excluding some nights around full moon, using a single channel photometer with an EM1 9789 B photomultiplier and a standard Johnson BV filter set (cf. Gochermann et al., 1993). All measurements were obtained through a circular diaphragm of 45”.4 in diameter which was always centred on the sharply peaked brightness maximum of the coma (cf. Fig. 3) by means of an illuminated eyepiece cross-hair. The resulting positioning error is estimated to be 2-3”. No guiding on the comet was performed since the integration times were always shorter than 60s in each filter, at a maximum motion of the comet of - 0”.5 min-’ near perihelion. This can be entirely neglected against the positioning uncertainties of the diaphragm. Each measurement (B and V) consisted of the sequence “sky-comet-sky-cometsky”, in order to estimate the photometric errors induced by the positioning of the diaphragm onto the diffuse coma. The sky measurements were obtained about S-10’ to the east or west of the nucleus to minimize contamination of the coma. Despite this distance from the nucleus, the sky measurements may be affected by a very moderate “zeropoint” offset due to the far extension of the coma, as indicated by the CCD images in Fig. 3. A larger offset of the sky measurements of about 3&60’ towards the sun would have probably yielded a slightly better sky correction. Additionally, an appropriate number of E-region UBVRI standard-stars from the list of Graham (1982)
were observed to allow a determination of atmospheric extinction and a transformation to BV standard-magnitudes. Each of the two comet measurements in the sequence. taken about 447min apart, were corrected by interpolated sky counts, and transformed to V magnitudes and B-V separately. A detailed description of these reductions can be found in Gochermann et al. (1993). Finally, the cometary magnitudes and colours belonging to the same measurement sequence were averaged, with half of the differences, AV and A(B -V), giving a clue to the photometric positioning uncertainties which are of the order of 0”.01-0”.04 for both, V and B-V. Table 1 presents the final results of the photometry. The photometric standard deviations given in this table, ov and gB_“, represent the final errors caused by the photometry itself (cf. Gochermann et al., 1993) and the positioning uncertainties mentioned above. The times of observation quoted in the table refer to the arithmetic time averages of each measurement sequence. In addition to the direct measurements, Table 1 also lists the magnitudes, V ‘,, reduced to a geocentric distance of A = l.OAU and a constant projected diaphragm diameter (km) at the distance of the nucleus. All magnitudes, V, = V- 5logA, were reduced to V’,, which represents the V magnitude within a projected diaphragm diameter 2p = 32,930 km, i.e. 4Y.4 at A = l.OAU. In order to determine the corrections to be applied, the cumulative brightnesses, I,, in the cometary coma were deduced from the CCD V-image (cf. Section 2.2) by integrating over circular diaphragms of consecutive growing radii, p, centred on the nucleus (peak brightness). The resulting relation, I,(p), was then fitted by a second-order polynomial to be able to interpolate to the actual diaphragm radii. The corrections applied to V, ranged from -0”.47 for the observations at the beginning of the campaign, to +O”. I5 at its end. They are based on the assumption that the shape of the cometary coma did not change substantially during the whole time interval covered by the photometry. A systematic error in the sky background subtraction of the CCD image (Section 2.2) by typically 1% will lead to shifts in the correction terms of only f 0”.003 for an underestimation (+) or overestimation ( -) of the sky. Larger, but still not serious errors in the correction terms are caused by a temporary steepening or flattening of the coma brightness distribution due to possible timedependent coma structures, like jets or an expanding halo after the “outburst” close to perihelion (Section 3). An alteration of the average coma surface-brightness slope (approximately - 1.O, Fig. 5) by 0.1 gives an estimate of the change of the correction to be applied to V, : at the beginning of the observations, the correction term would be changed from -0”.47 to -0”.52 in the case of steepening (slope - 1.1) and -0”.42 for a flattening (slope - 0.9). The corresponding values for the last measurement of the campaign, at tp = + 67 d, are + 0”. 17 and +O”. 13, respectively. Another important effect that influences the lightcurve is the phase function of the cometary dust in the coma. Especially for small scattering or phase angles, CI,i.e. for comets observed near opposition, this function displays a steep slope. but is not very sensitively depending on the
H.-G. Grothues:
Photometry
and direct imaging of comet P/Faye
Table 1. Photometric
tp (d)” -____ - 6.897 -6.167 - 6.075 - 5.963 -5.170 - 5.079 -4.988 -4.921 -4.172 -4.084 -3.975 -2.172 -2.097 - 1.962 -1.173 - 1.091 - 0.952 -0.175 - 0.092 0.026 0.050 0.062 0.880 1.051 1.864 1.972 7.855 7.949 8.064 8.858 8.945 12.855 12.938 13.064 13.872 13.939 14.856 14.935 15.870 15.940 16.947 17.861 17.940 18.878 18.925 19.882 19.923 20.873 20.914 21.872 21.911 36.853 37.891 38.907 39.899 40.849 40.900 41.849 41.896 45.868 45.899 46.866 46.912 47.866
observations
r (AU)b
A (AU)’
1.595 1.595 1.595 1.595 1.594 1.594 1.594 1.594 1.594 1.594 1.594 1.594 ,594 ,594 1,593 ,593 .593 ,593 .593 ,593 ,593 .593 ,593 ,593 ,594 ,594 1.596 1.596 1.596 1.596 1.596 1.599 1.599 1.599 1.600 1.600 1.601 1.601 1.602 1.602 1.603 1.604 1.604 1.605 1.605 1.607 1.607 1.608 1.608 1.609 1.609 1.638 1.641 1.643 1.646 1.648 1.648 1.651 1.651 1.662 1.662 1.665 1.665 1.668
0.630 0.631 0.63 1 0.632 0.634 0.634 0.634 0.634 0.636 0.637 0.637 0.642 0.642 0.643 0.645 0.645 0.646 0.648 0.649 0.649 0.649 0.649 0.652 0.653 0.656 0.656 0.681 0.681 0.682 0.685 0.686 0.706 0.706 0.707 0.711 0.711 0.716 0.717 0.722 0.723 0.729 0.734 0.734 0.740 0.740 0.746 0.747 0.753 0.753 0.759 0.759 0.871 0.879 0.888 0.896 0.905 0.905 0.913 0.914 0.950 0.950 0.959 0.960 0.969
and corrected
1991 XXI V magnitudes
627 (cf. Section 2.1)
cx(deg)d
V
0;
B-VE
%Vh
12.7 13.1 13.2 13.3 13.7 13.8 13.8 13.9 14.3 14.3 14.4 15.4 15.5 15.5 16.0 16.0 16.1 16.5 16.6 16.6 16.6 16.6 17.1 17.2 17.6 17.6 20.5 20.6 20.6 21.0 21.0 22.7 22.7 22.8 23.1 23.1 23.5 23.5 23.9 23.9 24.3 24.6 24.7 25.0 25.0 25.4 25.4 25.7 25.7 26.0 26.0 29.8 30.0 30.2 30.4 30.6 30.6 30.7 30.7 31.3 31.3 31.4 31.4 31.6
11.300 11.253 11.270
0.010 0.027 0.022 0.051 0.046 0.019 0.021 0.030 0.045 0.040 0.065 0.027 0.022 0.040 0.020 0.021 0.023 0.030 0.025 0.021 0.021 0.028 0.024 0.016 0.024 0.048 0.018 0.023 0.012 0.018 0.035 0.029 0.012 0.011 0.019 0.036 0.035 0.037 0.021 0.021 0.014 0.015 0.016 0.057 0.017 0.011 0.010 0.032 0.019 0.024 0.024 0.016 0.040 0.022 0.026 0.028 0.013 0.028 0.022 0.040 0.015 0.023 0.014 0.018
0.746 0.810 0.787 0.800 0.792 0.794 0.783 0.794 0.760 0.761 0.785 0.785 0.750 0.788 0.755 0.763 0.777 0.723 0.735 0.718 0.758 0.775 0.763 0.729 0.661 0.698 0.775 0.812 0.812 0.799 0.812 0.786 0.770 0.780 0.783 0.794 0.798 0.790 0.800 0.785 0.792 0.775 0.790 0.784 0.775 0.791 0.783 0.767 0.761 0.760 0.765 0.767 0.804 0.763 0.758 0.739 0.759 0.757 0.775 0.808 0.777 0.733 0.747 0.744
0.012 0.019 0.117 0.018 0.022 0.016 0.018 0.015 0.022 0.022 0.021 0.014 0.021 0.016 0.015 0.021 0.017 0.012 0.013 0.012 0.013 0.026 0.054 0.015 0.014 0.046 0.015 0.028 0.026 0.016 0.023 0.012 0.019 0.027 0.023 0.018 0.03 1 0.026 0.022 0.022 0.028 0.017 0.010 0.017 0.014 0.020 0.028 0.02s 0.013 0.027 0.027 0.023 0.025 0.013 0.023 0.016 0.026 0.030 0.031 0.066 0.022 0.019 0.019 0.02s
11.268 11.265 11.296 11.291
11.263 11.311 11.290 11.303 11.286 11.322 11.351 11.321
11.322 11.258
11.256 11.235 11.254 11.185 11.194 11.178 11.171 11.209 11.193 11.341 11.329 11.347 11.356 11.360 11.520 1.545 1.548 1 1.553 1.565 1.581 1.591 1.642 11.661 11.651 11.694 11.673
11.734 11.723 11.715 11.733 11.801 11.828 11.809 11.782 12.278 12.308 12.371 12.304 12.372 12.358 12.375 12.347 12.547 12.500 12.552 12.433 12.517
v ‘,’
V’ I” k
11.82 11.77 11.78 11.78 11.77 11.80 11.79 11.77 11.82 11.79 11.81 11.78 11.81 11.84 11.80 11.81 11.75 11.74 11.71 11.73 11.66 11.67 11.66 11.65 11.67 11.66 11.77 11.75 11.77 11.7s 11.78 11.91 11.93 11.93 11.94 11.95 11.95 11.96 12.00 12.02 12.00 12.04 12.02 12.07 12.06 12.04 12.06 12.12 12.14 12.12 12.09 12.43 12.45 12.51 12.43 12.49 12.49 12.48 12.45 12.61 12.56 12.60 12.48 12.56
11.54 11.48 II.49 11.48 11.46 11.49 11.48 11.46 11.50 11.47 11.4s 11.43 I .45 I .48 1.43 1.44 1.38 1.36 1.32 I .34 1.27 1.28 1.26 1.25 1.26 1 1.25 11.27 11.25 11.27 11.26 11.26 11.34 11.36 11.36 I 1.36 1 I .37 11.35 11.36 11.39 11.41 11.38 11.41 11.38 11.42 11.41 11.38 11.40 11.45 11.47 11.43 11.40 11.60 11.61 11.66 11.57 11.62 11.62 11.61 11.58 11.71 11.66 11.70 11.58 11.65 continued
628
H.-G. Grothues:
Photometry
and direct imaging of comet P/Faye
1991 XXI
Table 1. (Continued) tp (d)”
Y(AU)b
A (AU)’
a (deg)d
V
fl;,
B-VS
OB-bh
V’,’
47.918 48.865 48.912 50.865 66.871
1.668 1.671 1.671 1.677 1.734
0.969 0.978 0.979 0.997 1.166
31.6 31.7 31.7 31.9 32.8
12.527 12.520 12.502 12.495 12.994
0.012 0.018 0.035 0.019 0.013
0.779 0.759 0.779 0.749 0.766
0.020 0.011 0.013 0.022 0.030
12.57 12.55 12.53 12.50 12.82
“t,-time (UT) relative to perihelion at JD 2,448,576.694 (days). ‘r-heliocentric distance of comet (AU). ‘A-geocentric distance of comet (AU). ‘r-phase or scattering angle (deg). ‘V-observed visual magnitudes (4.5” diaphragm). ‘a,-standard deviation of V. “B-V-observed colour index (45” diaphragm). ‘kg_ v-standard deviation of B-V. IV’,-V magnitude corrected to A = 1.O AU and for variable, projected (Section 2.1). !V’ ,“-V’, magnitude corrected to z = 0’ (equation (1)).
material, the sizes, and the shapes of the dust particles (e.g. Divine, 1981 ; Grtin and Jessberger, 1990). Using the dust phase function presented by Divine (198 1), which has been deduced from photometry of comets, implies for the correction of V’, to zero phase angle, z = 0” (or of other “dust photometries” in the visible wavelength domain) Am0 = 2.51og(l -O.OlSc()
(1)
where CLis measured in deg. Equation (1) is valid only for the linear part of the phase function. i.e. for c(I 50”. The V’] magnitudes corrected in this way, V’],,, are also listed in Table I.
2.2. Direct imaging In addition to the photometry, and in order to correct for the variation in the projected radii of the diaphragm (cf. Section 2.1), two CCD images (V and I) were taken on November 27, 1991, 11 days after perihelion, with the Tektronics CCD-camera at the Dutch 90 cm Telescope at La Silla. With effectively 380 x 570 px used. and an image scale of 0”.38 pxx’ (corresponding to 192 km pxx’ at the distance of the comet) this provides a field of view (FOV) of 2’.4 x 3’.6. Table 2 gives a journal of observations. The seeing in each image frame was measured by fitting Gaussians to the unsaturated stars in the FOV. However, since no tracking on the comet was carried out during the Table 2. Journal of CCD observations (November 27, 1991) ; Y = 1.597 AU, A = 0.696AU, phase angle LY= 21-.9 Image V I
fstdr.ta 3hll” 3hl8”
tp (d)”
tcrp(s)’
AMd
Seeing’
+ 10.940 + 10.945
300 150
1.19 1.19
1.7kO.2 1.3kO.2
“t,,,,,-start of exposure (UT). brpdays after perihelion. ‘t,,,exposure time (s). ‘AM-airmass. ‘Seeing--estimated FWHM (arcsec).
diameter
v;,, 11.66 11.63 11.61 11.57 11.85
of the diaphragm
exposures, an additional smearing occurs in the direction of the cometary motion at a position angle of 106’ (Fig. 3). This smearing accumulates to about 2”.4 and I”.2 during the exposures for the V and the I image. respectively, resulting in an effective resolution of - 3” in that direction, almost perpendicular to the most important suntail axis at a position angle of 2 15 The CCD reduction was done in the standard way using averaged bias frames and sky flat-fields. Owing to the non-photometric weather conditions during the CCD observations no absolute photometric calibration of the images could be obtained. Subtraction of the (constant) sky background was performed by averaging the mean sky brightness in the south-western part of the images towards the direction to the sun, which is relatively undisturbed by contributions of the coma. The statistical uncertainties of the sky brightness were 0.6% for V and 0.9% for the I image. respectively. However, it cannot be ruled out that these parts of the images are contaminated by the comet’s surface brightness, and thus cause a systematic overestimation of the sky brightness (cf. the discussion in Section 2.1). The proper alignment of the image frames was ensured by comparing the field stars to the Digitized Sky Survey (POSS).
3. The lightcurve The V lightcurve of the coma, corrected for geocentric distance, varying diaphragm diameters, and the dust phase function (V’,,), is shown in Fig. lc (cf. also Table 1). For comparison, the “apparent” lightcurve which does not take into account the phase function effect is also presented (Fig. la). Starting at about 7days before perihelion, the lightcurve remains at an almost constant level until t, = - 1.1 d. At this time, it began to rise steeply by AV’,” = 0”.21 f0”.03, reaching its peak brightness at about f0.9d or later. Following this “outburst”, the lightcurve seems to have stayed nearly constant until about 10 days after perihelion. Unfortunately, a gap exists in the data owing to full moon between tp = +2 and
H.-G. Grothues:
Photometry
and direct imaging of comet P/Faye
1991 XXI
629
Comet PiFaye 1991XXI
(4
Comet P/Faye 1991XXI(Post-Perlhellon)
(b)
Projected diaphragm32930 km #
0
20
40
60
80
0200 P
0205
Days afterPerlhellon
20
40
0225
(d)
Projectea diaphragm32930 km
0
0220
CorretPiFaye 1991XX (Post-Perlhellor) 1'0
L
0215
lg(r/AU)
Come: P/Faye 1991XXI
1200 -:'O
0210
60
"outburst" 7
80
Days afterPerthellon
123' 0200
P
0205
0210
0215
6220
0225
lg(r/AU)
Fig. 1. (a) V lightcurve of comet 4P/Faye in 1991/92, reduced to a standard geocentric distance of A = 1.OAU, and corrected for the varying projected diameters of the 45” diaphragm (V ‘,), but not for the phase function effect (cf. Section 2.1). The perihelion passage on November 16, 1991 (JD 2.448,576.694) is indicated by “0”. (b) Post-perihelion V; -log r relation with two possible linear fits (single data point at t, = + 66.9 days omitted). While the steeper line (n = 16.0) fits all data points, the shallow one (n = 13.2) neglects the measurements possibly to be attributed to the “outburst” at perihelion. The perihelion is marked by “P”. (c) Same as (a), but considering the dust phase function effect. (d) Same as (b), but also taking into account the dust phase function effect. As in (b), the straight line with the steeper slope (n = 7.2) fits all data points, while the shallow one (tz = 6.2) neglects the “outburst”
+8 d. Thus the maximum brightness might have been reached as late as + 6 to + 8 d. At least from tp = + 12 d, the lightcurve maximum is followed by a steady decrease in brightness until the end of the observations at + 67 d. The increasing scatter of the data points with time is probably not due to intrinsic brightness variations, but is caused by systematic errors introduced by the fading coma surface-brightness approaching the detection limit of the telescope/detector combination. A period analysis (Stellingwerf, 1978) was applied to the data after having subtracted a third-order polynomial in order to suppress the long-term variations with heliocentric distance. No brightness variations (of amplitudes >0”.05 to Om.l) could be detected in the lightcurve which may indicate significant periodicities of 0.2-l 6 days related to the inner coma and the nucleus.
The B lightcurve varied almost parallel to V, resulting in constant colour of (B -V), = 0”.77 f 0”.03 during the whole period of observation (Table 1). This is significantly redder than the sun with (B-V), z 0”.65. Assuming that the emission from the coma is dominated by the dust continuum of scattered sunlight. i.e. molecular emission lines only make a minor contribution to the brightness (cf. Section 4, and Fink and Hicks (1996)), the reddening caused by the cometary dust can be estimated from (B-y),-(B-V), = 0”.14~0”.05 to be (13f5)%/ IOOOA. This is in good agreement with the results obtained by Gil-Hutton and Licandro (1994) and Jorda et al. (1995) from narrow-band photometries. The lightcurve of the total visual brightness of comet P/Faye for the 1991/92 apparition can be constructed from the brightness estimates published in the IAU Cir-
H.-G. Grothues: Photometry and direct imaging of comet P/Faye 1991 XXI
630 Comet “iFaye
1991 XXI 1
100
10 5 P E >
$
110
I
-100
-50
0
50
Days after Perlhellon
Fig. 2. Total visual lightcurve of comet P/Faye in 199 l/92. con-
structed from brightness estimates published in the IAU Circulars (1991/92). The data have been normalized according to the corrections given by Kamel(1992), i.e. they are not corrected for the dust phase function effect (cf. Section 2.1). Marginal evidence occurs for a lightcurve maximum at about -35 d, before the start of the photometry. The reason for the constant brightness from about 100 to 70days pre-perihelion remains unknown. but it is most probably caused by improper corrections applied (cf. Section 3)
culars (1991/92). Figure 2 presents this lightcurve, taking into account the corrections for geocentric distance, telescope apertures, sky brightness, and altitude (but no dust phase function effect), as applied to the earlier lightcurves ofthe comet since 1910, published by KamCl(l991, 1992). The total visual “perihelion brightness” (averaged over the time interval & 20 d) is IO”.49 & 0”. 19, corresponding to a water production rate of QHzo = (1.7 + 0.8) x 10z8s-’ (cf. Section 1). or, considering a correction of -0”.37 due to the dust phase function (equation (1)) QHIo = 2.0 x lo’* s-l. Again, this is in a remarkable agreement with the apparitions of 1910, 1932, and 1969. Scaling with an r-?.4 law (Fink, 1994 ; which is certainly not correct for P/Faye, as shown below), would lead to a production rate of - 5 x 10” ss’ at a standard distance of 1.OAU. It must, however, be noted that the water production rate of comet P/Faye determined from narrow-band photometries (GilHutton and Licandro, 1994 ; A’Hearn et al., 1996) is smaller by a factor of about 3, i.e. Q,?,z~ x lO”s~‘, when compared to the value given above. A comparison to other comets (e.g. Fink and Hicks, 1996; A’Hearn et al., 1996) ranges 4P/Faye among the medium active shortperiod comets, about a factor of 10 less active than comet Halley, but very similar to the (current) prime target of the ROSETTA mission, 46P/Wirtanen (e.g. Jorda and Rickman, 1995). Free sublimation at the comet’s nucleus surface would result in a maximum water sublimation rate (subsolar point) at 1.6 AU of about 2.5 x 102’ rnp2 s-’ (Rickman, 1992). From this, a minimum active area follows for the nucleus of P/Faye of about 7 km2, or, taking into account a nucleus radius of 2.7 km (Lamy and Toth, 1995), a fraction of more than 7% of its total surface (cf. also Gil-Hutton and Licandro, 1994).
A further comparison of the post-perihelion decline of the total visual magnitudes in 1991/92 to the decrease of the corrected photoelectric V magnitudes (V’,) reveals a constant difference between these two lightcurves of I”.4 f 0”.2. As the V,,, and V’, lightcurves are affected by the dust phase function effect in the same way, the (relative) changes of the water production of comet P/Faye can be consequently estimated from the variations in the V’,, lightcurve by means of the correlation given by Jorda et al. (1992). Correspondingly, the brightness increase close to perihelion translates to a growth of the water production rate by about l&15%, with almost no change in the heliocentric distance of the comet (Arc 10-4AU), its true anomaly (Av = 1>.2), and the phase angle (Acr = 1 ..l). Thus, it is improbable that the “outburst” was solely caused by an increase of the water sublimation rate owing to the increasing surface temperature with decreasing heliocentric distance. Rather, one could attribute the brightness increase to the activation of a hitherto non or less active (“dormant”) region on a nucleus rotating very slowly in an exited rotational state (precession). Such a very slow period of rotation (of the order of lOdays) may be indicated by the HST photometry of the nucleus presented by Lamy and Toth (1995). However, owing to their very few data points, this possibility remains somewhat questionable. Other explanations could be a non“explosion” (abrupt insolation) of a newly recurrent exposed pocket of volatile ices (e.g. CO or CO?), an exothermal phase transition for instance in the crystalline structure of the nucleus material, or a mechanical alteration of the physical structure of the active region (e.g. by a landslide). The continuous brightness decrease, which followed the brightness peak after perihelion until the end of the observations is corresponding to a decrease in the water production of roughly 30%. At the same time, the heliocentric distance of the comet had changed from 1.59 to 1.73AU. According to recent modelling (e.g. Rickman, 1992), this decline cannot entirely be explained by a sole adaptation of the water sublimation rate to the changing nucleus surface-temperature, which should result in a change of only 15-20%. However, due to the large uncertainties of the production rate determination and also the sublimation rates. this clue has to be regarded with some care. Traditionally, the (total visual) brightness increase or decrease of a cometary lightcurve with heliocentric distance is characterized by an absolute magnitude, H,, at Y = A = 1.OAU, and a slope, n. Table 3 lists separately for Table 3. (E&n) for comet 4P/Faye
(1991/92)
Lightcurve
HO 6.4kO.6 5.1kO.4 3.8kO.8 5.2iO.6 7.7kO.5 8.2kO.4
V,,,, pre-perihelion V,,,, post-perihelion V ‘,, post-perihelion V ;, post-perihelion” V ‘,,,, post-perihelion V’,,, post-perihelion” ““Outburst”
omitted
(cf. Section 3).
n 8.1k 1.1 10.8kO.7 16.0& 1.4 13.2_+ 1.1 7.2kO.9 6.2+_0.7
H.-G. Grothues:
Photometry
and direct imaging of comet P/Faye
63
1991 XXI
(a >
ItttI
20000
II
0 Posit,ion
20000 km
I
I
-20000 / km
-3
Fig. 3. (a) Logarithmized greyscale represention of the V image of comet 4P/Faye taken on November 27, 1991, about 1I days after perihelion. A fourfold jigsaw intensity transformation is used to illustrate the steep brightness gradient in the inner coma. (b) Contour plot of the I image of the same day. The contour levels in the frame are at 1. 2, 3, 5, 10, 20, 50, and 95% of the peak surface brightness (nucleus) at (0.0). The directions towards the sun (0) and the projected orbital velocity vectors of the cometary nucleus (Q) are marked by arrows. North is up, east to the left
I1III-l ,I,
-40000
632
H.-G. Grothues:
Photometry
and direct imaging of comet P,‘Faye 1991 XXI
Fig. 4. I image processed by a division of the original image (Fig. 3b) by its 360”-azimuthal average, and smoothed by a Gaussian (FWHM = 3px, - seeing). The greyscale representation indicates deficient surface brightness in the sunward hemisphere, while the tailward one is dominated by excess brightness. Note the asymmetry in the excess relative to the suncomet vector, which is obvious by the S-shaped “isophote” that intersects the nucleus (brightest pixel) at the centre (cross). The suncomet radius vector (0) is marked by an arrow. The radius of the circular image held corresponds to 25,500 km. North is up, east to the left
H.-G. Grothues: Photometry and direct imaging of comet P/Faye 1991 XXI the pre- and the post-perihelion observations the values of (&,n) determined from the total visual brightness of comet P/Faye as presented in Fig. 2, and compares them to (H,,n) as found from the photometry. Even if the first, very steep decline after perihelion is solely attributed to the “outburst” (Fig. Id), the slope, n = 6.2, turns out to be steeper than the canonical value of n = 4 (e.g Meisel and Morris, 1982). Nevertheless, the value fits rather well into the usually steeper slopes observed for the dynamically old Jupiter family comets. Evidently, the power law fits are not very satisfactory to describe the development of the lightcurve after perihelion. Thus, more sophisticated explanations than the usual distance law have to be assumed for comet P/Faye, e.g. illumination effects of the active region(s) in combination with the state of rotation of the nucleus (cf. the discussion of the “outburst”). It is quite interesting to note that in 1910 and 1932 the total visual magnitudes after perihelion decreased as n = 3.6 and 4.5, respectively, i.e. much more “ordinary” than in 1991/92 (KamCl, 1991, 1992), although the observation geometry was similar to 199 l/92 and no correction for the dust phase function effect has been applied to these data. In 1969, the brightness reached its peak only about 35days after perihelion to remain almost constant for more than 50days thereafter. The reason for this behaviour is not completely clear : the general activity pattern, i.e. the shift of the brightness maximum, can again be explained by the influence of the dust phase function; it can, however, not account for the long plateau of maximum brightness observed during that apparition. A possible, substantial change in the activity pattern, i.e. the number, strength, and spatial distribution of the active regions on the nucleus’ surface will most probably also result in a change of the gas and dust production rates, and hence the brightness of the coma, by a noticeable amount. However, the “perihelion brightness” has remained constant to less than 0”.2 since 1910 (cf. Section I), which in turn means the water production near perihelion to have varied by at most 20% from one apparition to the next only. The parameter, n, can be used as a guideline to estimate the r-dependence of the water production rate, again using the correlation given by Jorda et al. (1992) : QHZON r-0.6n. The approximate validity and the accuracy and limitation of this relation is demonstrated by a comparison of the pre-perihelion total visual lightcurve of comet Halley in 1985/86 (Green and Morris, 1987 ; n = 3.2 for r< 1.5 AU, of the water i.e. QH20- r -‘.9) to the direct determination production rates presented by Fink (1994), giving QH,o -‘4 (cf. also Green and Morris, 1987). Consequently, fz:comet P/Faye in 199 l/92, QH+, fell off as r-j 7*o.4 after perihelion. A much steeper dependence was found by Jorda et al. (1995) for the pre-perihelion dust production (Afp values) of comet P/Faye with r-6.7*o.5. The postperihelion decrease of Afp seems to have been similarly steep (cf. Fig. 3b of Jorda et al. (1995) and Gil-Hutton and Licandro (1994)). Thus the fast activity decline of comet 4P/Faye in 1991192 is confirmed by two independent measurements, and it must be regarded as real. As discussed above, the physical reasons are not yet completely understood.
633
4. Coma morphology Figure 3 presents a greyscale representation and a contour plot of the V and 1 CCD-images of comet P/Faye, respectively. A division of one image by the other reveals almost no differences in the shapes of the inner coma. This can be interpreted to support the assumption, which has been made above, that the coma is dominated by the continuous radiation of reflected sunlight scattered by cometary dust grains, rather than by molecular emission lines. Hence, the following discussion concentrates mainly on the V image, but the image processing was always performed on both images in order to confirm the mutual results. Primarily, the coma of comet P/Faye is characterized by the isophotes clearly elongated along the sun-comet radius vector, indicating the existence of a tail roughly opposite the sun. Besides the effect of radiation pressure forces acting on the dust grains, the eccentricity of these isophotes, e~O.5, may be interpreted in terms of fragmentation of dust particles inside the coma (Combi, 1994). The elongation at constant eccentricity can be traced down to a projected distance of p < 5 x 1O3km from the nucleus, indicating that the possible fragmentation process is already effectively acting on this small-scale length. However, in order to confirm the fragmentation theory, a real modelling had to be performed taking into account the viewing geometry from Earth. Furthermore, for both images a slight asymmetry can be recognized in the inner coma (p<2 x lo4 km or 40”) with respect to the radius vector, i.e. the coma shows a larger extent (- 20-30%) to the NW than to the SE. This asymmetry is by far too large to be explained by the lack of compensation of the comet’s apparent motion during observation (Section 2.2), acting in approximately the same direction as the effect under consideration. A possible explanation for this anisotropy may be the solar radiation pressure driving the ejected dust particles outside the cometary orbit, and leaving them back behind the nucleus, i.e. opposite to the comet’s velocity vector. Another explanation could involve anisotropic outgassing of the nucleus. In order to confirm the findings from the direct inspection of the images, and to search for possible additional coma structures like jets or fans, three independent image processing techniques were applied to the frames: (a) a division of the images by their 3601-azimuthal averages, (b) the azimuthal-renormalization algorithm introduced by A’Hearn et al. (1986), and (c) a division of the original by their Gaussian-smoothed counterparts images (FWHM = 4px for V and 3 px for I, roughly corresponding to the respective seeing). The brightest pixel in the coma was always taken to represent the nucleus (coordinate zeropoint) in these algorithms. The third technique has the advantage over the other two to avoid centring errors of the nucleus. For methods (a) and (b) these errors have been investigated by shifting the zeropoint by one pixel ( - error of centring) along the NS and EW directions and combinations. No substantial changes in the resulting images occurred with respect to the optimum centring. All three methods lead to qualitatively identical results.
H.-G. Grothues:
634
1
i
l$j?!kT) Comet PiFaye 1991XX
Photometry
and direct imaging of comet P:Faye
1991 XXI
The radial profiles perpendicular to the sunxomet vector show also no significant deviations from linearity. The linearity of the log (&log p relation with the slopes near to - 1.O can most easily be explained in terms of an isotropic expansion of the dust coma at constant velocity. Moreover, the dust production rate must have been almost constant on a time scale necessary for the grains to leave the coma, i.e. for about 1 day or longer (steady-state condition). The constant slope of the tailward profile, if significant above - 1.O. could be another indication of continuous fragmentation of dust particles in the coma (cf. O’Dell et ((1. (I 988) for I P/Halley). The surface-brightness cut-off on the sunward side of the nucleus is most easily interpreted by a simple fountain model (e.g. Jewitt and Meech, 1987 ; Griin and Jessberger, 1990). In this model the dust grains ejected from the nucleus towards the sun are decelerated by the solar radiation pressure force at an apex distance or reflection point, X,, and then accelerated into the tail. The apex distance is connected to the grain velocity, f:prrand the ratio, /?, of the radiation pressure force affecting the particle to the local solar gravity, by the relation
(2)
MpW Fig. 5. Radial surface-brightness
profiles (p-projected radius from the brightness pixel) averaged over 90 ’ sectors centred on the direction towards the sun (a), and the prolonged radius
vector (b) in the V image. Error bars indicate f 0.6% of the sky background level (cf. Section 2.2). which is represented by a value of log([,,,) = 1.37. An “S” marks the radius (HWHM = 430 km) of the seeing disk. The fitted straight lines have the slopes - 1.08 kO.03 (a) and -0.95 f0.02 (b), respectively
where I’ is the heliocentric distance of the comet. G the gravitational constant, and M, one solar mass (Jewitt and Meech, 1987). Taking into account the observational geometry (phase angle c( = 21 .9) as modelled by Jewitt and Meech (their Fig. 5), XR can be estimated at - 5 x lo1 km. Hence. for p = 0.01-2.0, corresponding to spherical organic and silicate dust grains of roughly 0.1~ 1Opm in radius, which have been concluded for the dust tails of several comets (e.g. Burns et (I/.. 1979; Saito et ul.. 1981 ; Lamy and Perrin, 1988). the expansion velocities, l’gr = 15-220 m SC’, can be calculated from equation (2). This is comparable to the velocities found in other shortperiod comets (e.g. Jewitt and Meech, 1987).
5. Conclusions 4P/Faye is a medium active member of the Jupiter family of short-period comets with a water production rate of -2 x 10’ss-’ at its perihelion distance of I .6AU. This makes it comparable in its activity to the (current) prime target of the ROSETTA mission, comet 46P/Wirtanen. The steep decline of the (dust) activity in comet P/Faye with heliocentric distance, possibly as steep as -r-‘~.~‘, will, if preserved over a larger distance interval (which is still not proved), lead to almost no activity which can definitely be discovered from Earth beyond 3-5 AU (aphelion 5.96 AU). Near perihelion, irregular (?), shortterm activity variations on a time scale of few days may occur from time to time (at most a few “outbursts” per revolution), with the gas and dust production changing by amounts of the order of 10% or more. The expansion of dust from the innermost coma (p < lo3 km) seems to be nearly isotropic, with very weak deviations caused by an anisotropic distribution of the spatial activity pattern on the nucleus to be recognized outside this zone. The dust Comet
Besides the most distinct sun-tail morphology, the asymmetry of the coma relative to the sunxomet radius vector is clearly present (Fig. 4). Significant other features, however, were not uncovered by this techniques. It is not clear whether the asymmetry of the dust coma found by Jorda et al. (1995) for comet P/Faye in their R image taken on August 26, 1991. 82 days before perihelion, is related to the asymmetry detected in the images presented here. Radial surface-brightness profiles, log(l) vs. logp (pprojected distance from the nucleus), each averaged over a 90” sector centred on the sunward direction and the prolonged radius vector, respectively, are shown in Fig. 5. While the tailward profile can be well represented by a linear fit with a slope of -0.95+0.02 out to a radius of p = 2.4 x lo4 km (edge of FOV), the sunward profile (slope - I .08 + 0.03) displays an obvious cut-off from linearityatp=(llfl)~lO~km.
H.-G. Grothues: Photometry and direct imaging of comet P/Faye 1991 XXI expansion
200ms-’ coma.
velocities
near perihelion
are of the order
of
for the smallest particles to be visible in the
Acknolrled~ernents. I thank M. 0. Oestreicher and Th. Berghofer for carrying out considerable parts of the photometric observations, without which the lightcurve would not have been extended over such a long period. A. Collier Cameron and B. Foing kindly obtained the CCD images of the comet, and provided some technical information. Moreover, I am indebted to the referees, H. Bijhnhardt and L. Jorda, for their valuable comments and suggestions on the manuscript. The CCD reductions were carried out using the ESO-MIDAS image processing package.
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