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Spectropolarimetry of comets Austin and Churyumov-Gerasimenko
ICARUS 58, 431--439 (1984)
Spectropolarimetry of Comets Austin and Churyumov-Gerasimenko ROY V. M Y E R S AND K E N N E T H H. NORDSIECK Washburn Observatory, University of Wisconsin, 475 North Charter Street, Madison, Wisconsin 53706 Received N o v e m b e r 10, 1983; revised February 2, 1984 Spectropolarimetric observations from 5000 to 8000 A, have been obtained for c o m e t s P/Austin (1982g) and P / C h u r y u m o v - G e r a s i m e n k o (1982f). The observations were spaced over phase angles of 50-125 ° for c o m e t Austin and 10-40 ° for comet C h u r y u m o v - G e r a s i m e n k o . The use of spectropolarimetry allowed an evaluation o f c o n t i n u u m polarization without molecular line contamination. Especially for c o m e t C h u r y u m o v - G e r a s i m e n k o , the curve of polarization versus phase angle resembles c u r v e s for asteroids, where the polarization is negative (electric vector m a x i m u m parallel to the scattering plane) for phase angles less than 20 ° and the most negative polarization is from - 1 to - 2 % . The negative polarization at backscattering angles m a y be due to multiple scattering in agglomerated grains, as a s s u m e d for asteroids, or to Mie scattering by small dielectric particles. If multiple scattering is important in c o m e t dust, polarization m e a s u r e m e n t s m a y imply a low albedo, less than 0.08. The polarization of c o m e t Austin remained steady during a large change in the dust production rate. Both c o m e t s increased c o n t i n u u m flux by a factor of 2 near perihelion. The c o n t i n u u m of c o m e t C h u r y u m o v - G e r a s i m e n k o had the shape of the solar s p e c t r u m with derivations less than 5%. The equivalent width of spectral features of C2, NH_,, and O varied as r 2.
1. I N T R O D U C T I O N
The history of comet polarimetry covers more than a hundred years. However, polarization measurements of backscattered light (small phase angles) from comets appear only in recent literature (Kiselev and Chernova, 1978, 1981). In these investigations and in one investigation of a comet tail (Weinberg and Beeson; 1976), the position angle of the electric vector maximum of backscattered light from comets was found to be parallel to the scattering plane instead of the unusual case of perpendicular for small-particle scattering. Two models have been used to explain the position angle: multiple scattering from adjoining dust grains (Boweil and Zellner, 1974; Gehrels, 1977) and Mie scattering from dielectric particles with sizes near the wavelength of light (Oishi et al., 1978; Michalsky, 1981). For both models, only the polarization at small phase angles (backscattering) is sensitive to changes in model parameters. Hence, the polarization of backscattering must be observed accurately. The precision of polarization measure-
ments of the comet continuum suffers from the presence of molecular emission lines. Since the strength of the molecular emission varies from comet to comet and varies with time for a given comet, broadband filter photopolarimetry involves uncertain corrections for contamination. All four previous measurements of the polarization at small phase angles (Kiselev and Chernova, 1978, 1981) were broadband measurements and only comet Ashbrook-Jackson was reported free of emission lines. Apart from the inherent problems of comet polarimetry, the measurements obtained for comets West, Ashbrook-Jackson, Chernykh, and Meier may have suffered from a systematic error. Assuming that the collection of comet dust particles has no preferential axis, the only special directions are toward the light source and toward the observer. Then, the position angle of the polarization must be either perpendicular or parallel to the scattering plane (Sun-Comet-Earth plane). The position angle of comet West, however, was advanced consistently 10° from the expected angle. The position angle deviations for the three dimmer comets
431 0019-1035/84 $3.00 Copyright © 1984by Academic Press, Inc. All rights of reproduction in any form reserved.
432
MYERS
AND NORDS1ECK
TABLE I COMET ORBITAL DATA Obs.
Date
A B C D E F G H 1
08/21/82 08/26/82 08/27/82 08/28/82 09/02/82 09/03/82 09/09/82 09/I 2/82 09/19/82
J K L M N
Phase (deg)
r IAUI
a lAD)
(1.65 0.65 0.65 0.65 (1.67 0.68 0.72 0.75 0.84
0.49 0.63 0.66 0.69 0.83 11.86 1.03 I.ll 1.2~
C o m e t Austin 123 104 I 01 98 84 81 68 62 52
Comet Churyumov-Gerasimenko 10/27/82 38 1.32 11/17/82 32 1.3 I 12/21/82 18 1.38 01/03/82 12 1,44 01/18/82 13 1.52
0.46 0.4 I
(1.43 (I.47 0.56
scatter wildly with averages again pointing to an a d v a n c e m e n t of position angle. Molecular line contamination and the possible instrumental error m a k e further investigation of backscattering polarization desirable. This p a p e r describes results of spectropolarimetry of the comets P/Austin (1982g) and P / C h u r y u m o v - G e r a s i m e n k o (1982f, hereafter CG). Unlike broadband polarimetry, s p e c t r o p o l a r i m e t r y allows determination of the polarization without molecular line contamination. In addition, the comet spectra are available for analysis. The observations are discussed in Section II. Spectral analysis is presented in Section III, polarization results in Section IV, and the relationship of the gas and dust production rates in Section V. 11. O B S E R V A T I O N S
O b s e r v a t i o n s were spaced in phase angle o v e r the range 125-50 ° for comet Austin and 40-10 ° for c o m e t CG. Table 1 contains the date of the o b s e r v a t i o n (UT) and position information: the Sun to comet dis-
tance, the Earth to comet distance, and the phase angle ( S u n - c o m e t - E a r t h angle). All observations were made with the intensified Reticon spectropolarimeter (Lupie and Nordsieck, 19831 at the 0.9-m telescope of the Pine Bluff Observatory. The instrument obtains the spectrum at a resolution of 8 A, together with polarimetric information at a resolution of 100 A. The dual-detector system allows simultaneous m e a s u r e m e n t of the comet and background from 5000 to 8000 A with a spatial separation of 168 arcsec. A rapidly varying background for comet Austin, which was at a small elongation from the Sun, makes the simultaneous background m e a s u r e m e n t especially important. The entrance slit size was 4.9 × 11.3 arcsec for comet Austin observations and the two slits were oriented e a s t - w e s t . For comet CG, the slit size was 6.5 ×21.1 arcsec with the slit orientation 45 ° from the scattering plane, Neither comet had an extended tail in the guiding c a m e r a to about five magnitudes dimmer than the c o m a , so the possibility of contamination due to a tail in the background slit could be ignored. The reduction procedure includes flat field correction and calibrations of wavelength, flux, and polarization efficiency. The flat field correction is a pixel by pixel divide by an observation of a tungsten lamp. Observations of standard stars (Breget, 19761 determine the instrument response and calibrate the flux level. H o w ever, on a given night the extinction may differ from the assumed average atmospheric extinction curve (Taylor, 19631, so the precision of the p h o t o m e t r y depends mainly on the precision of the extinction. For nights during which there were comet observations, the flux of self-calibrated standard stars typically displayed night-tonight variations less than 10% and that could be considered the uncertainty due to lack of absolute p h o t o m e t r y . The polarization efficiencies were evaluated by reducing an observation of an unpolarized star through a polaroid. Scans were taken such that the source alternated between slits and
COMET SPECTROPOLARIMETRY 10 b
32 a
433
i
i
i
t
84100
I 7200
80100
'7 9 E o g15
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~, 5 NH~
1
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[o0
CN
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,7
u.
5600
6400 Wavelength
7200 (,~)
I 5600
8000
Wavelength
(/~)
FIG. I. (a) Spectrum of comet Austin on 1982 August 28. (b) Spectrum of comet ChuryumovGerasimenko on 1982 November 17.
the background c a m e from an adjacent scan. To prevent an incorrect subtraction of background polarization when the background varied rapidly, the individual scans from c o m e t Austin were inspected to ensure that e x t r e m e variations from the previous scan did not exist. Figure 1 provides an example of each c o m e t ' s spectrum.
was near the horizon on a night with thick haze. I f the dust production were constant, the reflected flux would drop with heliocentric distance as r -2. As illustrated on the figure, a constant rate hypothesis is consis-
r (AU) o
d
d
.
.
d
.
.
d
~
~
,-:
~
,
111. CONTINUUM FLUX AND COLOR Relative continuum flux m e a s u r e m e n t s were obtained f r o m the spectra. Variations from the a s s u m e d extinction and a slit size smaller than the c o m e t image p r e v e n t e d absolute p h o t o m e t r y . Continuum data are discussed below in t e r m s of the flux at 7000 ,~, where emission lines are absent, and in terms of the deviation of the continuum from solar color. Figure 2 shows for both comets the continuum flux at 7000 A, reduced to a geocentric distance of I AU. The abscissa shown is the phase angle, to facilitate c o m p a r i s o n with the polarimetry. The error estimates reflect the noise in the s p e c t r u m and ignore the lack of absolute p h o t o m e t r y . C o m e t Austin shows a large dust production rate increase o v e r 3 days (points B - D ) near perihelion ( m a r k e d w i t h P). The dip (point E) after the abrupt rise is attributed to atmospheric fluctuations, since the comet
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120
I
100
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I 80 Phase
I I 60 40 (degrees)
l 20
i0
FIG. 2. Continuum flux at 7000 ,~ reduced to 1 AU from the Earth. The Austin point E is probably affected by haze. The phase angle of comet C h u r y u m o v Gerasimenko reached a minimum at 12.6 ° and increased after point M. With this exception, time increases from left to right. The scaled I U E point was taken before perihelion at r = 1.1 AU. The curves through the comet Austin data vary as r 2 and the curve through the comet C h u r y u m o v - G e r a s i m e n k o data varies as r 4
434
MYERS AND NORDSIECK
tent with the points D - I after a small correction for observing an extended object at different geocentric distances with a fixed aperture size. F o r the calculation of the correction, the flux was assumed to vary as the inverse of the distance from the comet nucleus. The reason for the lack of conformity of point H to the curve is unknown. Three more m e a s u r e m e n t s of the continuum flux of c o m e t Austin before perihelion are available from International Ultraviolet Explorer ( I U E ) observations (P. D. Feldman, 1982, personal communication: Feldman e t al. 1983). If the c o m e t continuum retains the shape of the solar spectrum (albedo independent of wavelength, see below), the flux at 2960 A can be scaled to obtain a flux at 7000 ,~. The 1UE observations at heliocentric distances 1.11, 0.88, and 0.8[ AU then would imply continuum fluxes at 7000 ° A of 9.5, 8.6, and 8.0 x 10 t~ erg cm 2 sec ~ A 1. After correction for a fixed aperture size, these fluxes alone vary as r 2 before perihelion, implying a constant dust production rate. The extrapolated brightness for this dust production rate, shown in Fig. 2, is a factor of 3 lower than the pre-perihelion point A. Thus, either the dust production rate increased significantly between the last 1UE observation on 1982 August 1 and the observation A on August 21, and/or the uv albedo is significantly smaller than the visual albedo. The color d e p e n d e n c e of the albedo of comets may be represented by the percentage deviation of the comet flux, C, from the solar continuum, S. at two wavelengths 1 and 2: c = (C,./$2 - C J S O / { C / S ) . Previous determinations of the continuum color of comets (Hanner, 1980) have ranged from neutral to a 30% red excess over the visible wavelengths. H o w e v e r , line contamination may have affected the broadband filter measurements. Estimates of the albedo color dependence of comets Austin and CG using parts of the spectrum with no significant contribution from lines are consistent with a neutral albedo, Molecular emission lines and atmo-
spheric absorption lines cluttered the spectra of comet Austin and prevented the appearance of a clean continuum. An upper limit of 30% to the albedo color d e p e n d e n c e of comet Austin was found from continuum segments (lines contribute less than 15% of flux) over the wavelength region from 6700 to 7900 A. The spectra of c o m e t CG were free of emission lines o v e r the wavelength region from 5300 to 8000 A. A linear leastsquares fit to comet spectra normalized to the solar spectrum produced a range of color d e p e n d e n c e from a 2.5% blue excess to a 5.9% red excess with a formal uncertainty of -+1%. H o w e v e r , the true uncertainty should reflect the repeatability of flux and extinction calibrations at < 5 % as verified by the lack of changes in the spectral shape greater than 5%. The aibedo color dependence for comet CG was therefore neutral to within 5%. IV. POLARIZATION Spectropolarimetry holds a great advantage o v e r broadband photopolarimetry in that the contamination by molecular emission can be eliminated. Dividing the c o m e t spectra by the solar spectrum identified a region of continuum of c o m e t Austin extending, with interruptions by emission lines, from 6300 to 8100 A and it verified that comet CG is free of emission lines from 5300 to 8000 A. The polarization was evaluated o v e r these ranges where line flux contributed less than 15% of the continuum flux. The polarization results are discussed below in terms of the color d e p e n d e n c e and the wavelength mean. Table Ii contains both the o b s e r v e d mean position angle ("free P A " ) and the position angle ('~fixed P A " ) corresponding to polarization perpendicular to the S u n - C o m e t Earth scattering plane (positive polarization). Within the uncertainties, the position angle was found to be perpendicular or parallel to the scattering plane. The degree of polarization is listed both as o b s e r v e d and as reduced on the assumption that the position angle is in fact either perpendicular
COMET
435
SPECTROPOLARIMETRY T A B L E II
COMET POLARIZATION DATA
Obs.
Date
Source time (sec)
Free PA Pol. (%)
Fixed PA PA (deg)
Pol. (%)
Color Ap/p
(%)
PA (deg)
Comet Austin A B C D E F G H I
08/21/82 08/26/82 08/27/82 08/28/82 09/02/82 09/03/82 09/09/82 09/12/82 09/19/82
4O0 3,200 3,200 5,200 6,000 4,800 4,000 3,200 4,000
J K L M N
10/27/82 11/17/82 12/21/82 01/03/83 01 / 18/83
7,200 16,800 20,000 24,000 22,400
22.9 ± 1.5 19.9 _+ 0.8 22.3_+ 0.5 20.5 ± 0.5 20.2 _+ 0.6 17.6 ± 0.8 14.5 ± 1.8 7.8 _+ 1.4
-+ 1.5 ± 0.7 ± 0.5 -+ 0.5 _+ 0.5 ± 0.7 _+ 1.8 ± 1.1
124 125 126 127 127 123 121 114
Comet C h u r y u m o v - G e r a s i m e n k o 4.2 _+ 0.7 180 ± 19 4.1 ± 0.5 3.3 ± 0.2 181 ± 8 3.4 ± 0.2 1.5 ± 0.2 51 ± 15 - 1 . 5 ± 0.2 1.9 ± 0.3 9 ± 18 - 1 . 9 ± 0.3 2.2±0.7 135_+37 -1.9_+0.7
177 174 142 104 59
(positive polarization) or parallel (negative) to the scattering plane. The difference in polarization values is not just a factor of cosine, since the polarization information is refit with one less free parameter (Lupie and Nordsieck, 1983) when the position angle is assumed. The dependence of the "fixed PA" polarization on phase angle is shown in Fig. 3. Comparison of the continuum flux in Fig. 2 and the polarization in Fig. 3 shows no correlation of polarization and flux variations. The polarization-sensitive parameter of the dust cannot have changed during the rapid and large increase of continuum flux for comet Austin. Table II gives the relative color dependence, Pz--P]/P, derived from a linear leastsquares fit of the polarization versus wavelength. For comet Austin, the two wavelengths (,kj, h:) and 6500 and 7800 A and for comet CG they are 5400 and 7500 ~,. No significant color dependence of the polarization was found for either comet. Averaging the best observations, for comet Austin, the polarization was neutral to
133 123 126 127 127 118 134 101
_+ 7 ± 5 ± 3 _+ 3 ± 3 + 5 _+ 14 ± 21
23.0 19.6 22.3 20.5 20.2 16.7 14.4 6.0
-13 32 11 -12 0 32 26 50
_+ ± ± -+ ± -+ ± ±
19 11 6 7 8 12 36 52
-32 + -5 ± 33 -+ -141 ± 115±
35 15 36 47 118
within about -+10% and for comet CG. to within about -+20%. The polarization versus phase angle r (AU) (D
(0
~-
(5
c;
o
6
I
I
I
I
t,
c~ I
~ I
~
I
I
20
t
f
v
CG
Austin
~0 ".C..
o ~°
I 120
i 100
I 80
I 60
I 40
I 20
0
Phase (degrees)
FIG. 3. Continuum polarization as a function of phase angle. The polarization is an average over the range 6300-8000 ,~ for comet Austin and 5300-8000 ,~ for comet C h u r y u m o v - G e r a s i m e n k o .
436
MYERS AND NORDSIECK
curve in Fig. 3 resembles curves obtained from asteroids. The p a r a m e t e r s that describe the asteroid curves at small phase angles are the most negative polarization (minimum polarization), the phase angle at c r o s s o v e r f r o m negative to positive polarization, and the slope of the curve near the c r o s s o v e r (polarimetric slope). The minim u m polarization occurs near 10° phase angle for asteroids and c o m e t CG. The observation of c o m e t CG at 12° phase angle establishes the minimum polarization at - 1 . 9 +_ 0.3%. A linear least-squares fit to the points of c o m e t CG between phase angles 15° and 40 ° yields a c r o s s o v e r at 22 -+ 1°. The slope of the line that determines the c r o s s o v e r phase angle is 0.31 _+ 0.3% per degree. The c o m e t Austin data appear to connect smoothly onto the c o m e t CG data, implying a similar c r o s s o v e r phase angle for comet Austin. The apparent continuity of the polarization data c o m e s from two comets with very different spectra: gaseous emission lines dominate the spectra of c o m e t Austin, but are absent from the spectra of c o m e t CG. The fundamental difference between these two comets, perhaps the n u m b e r of perihelion passages, does not seem to affect the polarization-sensitive parameter of the dust. The similarity between comet and asterold polarization curves suggests that correlations b e t w e e n the albedo and the minim u m polarization and polarimetric slope obtained for asteroids (Dollfus e t a l . , 1977: Dollfus and Zellner, 1979) might be applied to the c o m e t CG data. With this assumption, the minimum polarization of CG would imply an albedo of 0.05-0.08, and the polarimetric slope would imply an albedo less than 0.06. The albedo of comet dust near 90 ° phase angle, determined from comparison of infrared emission and visual scattering, scatters around 0.17 (Campins e t a l . , 1981; N e y , 1982) and the range goes down to 0.02. The albedo ranges obtained from the asteroid relations are on the low end of the range. The reasonable c o m e t albedo obtained from polarization data sug-
gests that the mechanism for producing the negative polarization in comet CG may be similar to that in asteroids, possibly multiple scattering (Gehrels, 1977). This would suggest that much of the c o m e t dust could be large c o m p a r e d to the wavelength of light and of complex shape, similar to the agglomerated particles seen in upper atmosphere collection experiments (Brownlee e t al., 1977). Of the comets with polarization measured at small phase angles (Kiselev and Chernova, 1978, 1981), comet CG mimics comet West in all three curve p a r a m e t e r s obtained above. The minimum polarization of comet Meier also may be - 2 % . However, the minimum polarizations that Kiselev and C h e r n o v a quote for comets A s h b r o o k - J a c k s o n and Chernykh are more negative and the polarimetric slopes steeper than observed tbr comet CG, even though the crossover also occurs near 22 ~' for these comets. Both parameters are far from the range observed in asteroids and the asteroid correlations fail to give any value for the albedo. The uncertainties in the polarization parameters for these two comets are large, but the inability to obtain an albedo may indicate that multiple scattering is not the mechanism for producing the negative polarization in these comets. Also, observations of a comet tail (Weinberg and Beeson, 1976) showed large negative polarization at moderate phase angles (<50°). The alternative, small-particle Mie models, can produce negative polarizations (Oishi e t a l . , 1978) much larger than those that have been produced in multiple scattering models. H o w e v e r , the Mie models suffer from a considerable sensitivity to size and composition, which is difficult to reconcile with polarization stability during dust outbursts. V. GAS AND DUST PRODUCTION A by-product of the spectropolarimetry of comet Austin is line equivalent width data which may be used to evaluate the gas/
COMET SPECTROPOLARIMETRY r (AU)
g
g
o
6
i
i
i
g
~
g
o
6
o
i
I
~0.5 3 ~r LB
0.0
120
I
I 100 Phase
I 80 (degrees)
60
I
FIG. 4. The variation of the equivalent width of molecular emission features for comet Austin normalized a perihelion. The points are means from nine spectral features of C2, NH2, and [0 l]. The error bars are the rms deviations of the individual features from the mean. The curve drawn through the data varies as r 2.2.
dust ratio. The n u m b e r of lines in the c o m e t Austin spectra m a k e s the continuum level impossible to m e a s u r e directly in the wavelength region 5000-6300 A. H o w e v e r , the continuum color of both c o m e t s Austin and CG (Section III) suggests that the assumption of a solar continuum is valid; a solar s p e c t r u m therefore was used that matches the c o m e t Austin continuum longward of 6300 A. Readily identifiable features of the molecules C2 (5170, 5630, 6110 ,&) and NH2 (5710, 5980, 6340, 6620 A), and atomic oxygen (6300, 6370 A) have been m e a s u r e d for equivalent width. M e a s u r e m e n t s of a single feature at several heliocentric distances can indicate variations of the gas production. Assuming that each feature sampled the gas production rate at the same instant, the equivalent widths of the sets of lines listed a b o v e were averaged to obtain a gas/dust ratio for each species. The equivalent widths determined for sets of C2 features and NH2 features were found to have essentially the same de-
437
pendence on heliocentric distance, r 2.3 for C2 and r 2.1 for NH2. Figure 4 shows the grand mean of normalized equivalent widths of all three species. The error bars show the dispersion about the mean of the equivalent widths for individual features. If the data for C2 only and NH2 only were plotted in Fig. 4, all points would be within the error bars of the means and the separation of the C2 and NH2 points on each night would be less than half of the error bar of the mean. Combining data f r o m these different molecules therefore appears to be a valid a s s e s s m e n t of the gas/dust ratio. A curve with an r - 2 2 d e p e n d e n c e is drawn through the data of Fig. 4. A formal uncertainty on the exponent probably would be an underestimate, so it is more appropriate to state the dependence as r -2 and not r t or r 3.
When the continuum flux is constant, the equivalent width variations reflect the gas production rate. Gas production is expected to vary as r -2 (Delsemme, 1982) and the second half of the c o m e t Austin observations verifies the r -2 dependence. For the first half of the observations, the equivalent widths are roughly constant even though the continuum flux m o r e than doubles. The gas production therefore must have increased in the same proportion as the dust production. The equivalent width provides an indicator of the n u m b e r of molecules c o m p a r e d to the n u m b e r of dust particles. The F i n s o n Probstein analysis of dust tail p h o t o g r a p h s provides a ratio of the masses of gas and dust. An application of both method would yield c o m p l e m e n t a r y information on the dust, but the equivalent width is much easier to obtain in that no dust tail is required. The mass gas/dust ratio for c o m e t West (Delsemme, 1982) varies by a factor of 5 around a dust outburst. Although the continuum flux increase of c o m e t Austin was one-fourth that of c o m e t West, the c o m e t Austin equivalent width gave no evidence for variation of the n u m b e r ratio of gas/ dust.
438
MYERS AND NORDSIECK Vl. CONCLUSIONS
1. T h e c o n t i n u u m flux o f both c o m e t s A u s t i n a n d C G i n c r e a s e d significantly n e a r p e r i h e l i o n : c o m e t A u s t i n i n c r e a s e d its flux by 60% o v e r a few days. T h e a l b e d o c o l o r d e p e n d e n c e was n e u t r a l to within 30% for c o m e t A u s t i n a n d to within 5% for c o m e t C G , e v e n d u r i n g the rapid c o n t i n u u m flux i n c r e a s e of c o m e t A u s t i n . 2. T h e e q u i v a l e n t width m a y b c a useful d i a g n o s t i c o f the n u m b e r gas/dust ratio. T h e e q u i v a l e n t w i d t h s of f e a t u r e s in spectra of c o m e t A u s t i n varied with h e l i o c e n t r i c d i s t a n c e as r ~-. T h e a d h e r e n c e to the c u r v e is r e m a r k a b l e d u r i n g large v a r i a t i o n s of c o n t i n u u m flux, i m p l y i n g that the n u m b e r ratio of g a s / d u s t r e m a i n e d u n c h a n g e d . 3. T h e p o l a r i z a t i o n of c o m e t s A u s t i n and CG was e i t h e r p e r p e n d i c u l a r or parallel to the s c a t t e r i n g p l a n e within u n c e r t a i n t i e s a n d the d e g r e e o f p o l a r i z a t i o n was w a v e length i n d e p e n d e n t to within +-10c~ for c o m e t A u s t i n . T h e p o l a r i z a t i o n was i n s e n sitive to the dust p r o d u c t i o n rate and to c o m e t d i f f e r e n c e s that are reflected by the gas c o n t e n t , since the p o l a r i z a t i o n v e r s u s p h a s e angle c u r v e a p p e a r s to be c o n t i n u o u s for the g a s s y c o m e t A u s t i n and the d u s t y c o m e t CG. 4. N e g a t i v e p o l a r i z a t i o n at small phase angles is c o n f i r m e d by the o b s e r v a t i o n s of c o m e t C G a n d is s u s p e c t e d to o c c u r in c o m e t A u s t i n by j u d g i n g the t r e n d of the p o l a r i z a t i o n p o i n t s . T h e p o l a r i z a t i o n as a f u n c t i o n o f p h a s e a n g l e for c o m e t CG imitates the c u r v e of c o m e t W e s t and resembles c u r v e s c h a r a c t e r i s t i c of a s t e r o i d s : the m i n i m u m p o l a r i z a t i o n is - 1 . 9 + 0.3%,, the c r o s s o v e r p h a s e angle is 22 _+ 1°, a n d the p o l a r i m e t r i c slope is 0.31 -+ 0.03% per degree. T h e ability to o b t a i n a r e a s o n a b l e c o m e t a l b e d o ( < 0 . 0 8 ) from c o r r e l a t i o n s of a s t e r o i d a l b e d o to p o l a r i z a t i o n p a r a m e t e r s l e n d s s u p p o r t to the s u g g e s t i o n that a multiple s c a t t e r i n g m e c h a n i s m used to e x p l a i n the b a c k s c a t t e r e d p o l a r i z a t i o n in a s t e r o i d s m a y be valid for c o m e t s . H o w e v e r , if the larger n e g a t i v e p o l a r i z a t i o n s a n d p o l a r i m e t -
ric slopes seen in earlier o b s e r v a t i o n s of o t h e r c o m e t s arc verified, Mie m o d e l s m a y be required to explain the p o l a r i z a t i o n . If the multiple s c a t t e r i n g e x p l a n a t i o n for the n e g a t i v e p o l a r i z a t i o n p r o v e s valid, the polarization of b a c k s c a t t e r i n g could p r o v e to be a sensitive way of o b s e r v i n g grain agg l o m e r a t i o n in the i n t e r p l a n e t a r y and interstellar m e d i u m . REFERENCES BOWEIA, E., AN[) B. ZEI I.NER(1974). Polarization of asteroids and satellites. In Planels, Slar.~ and Nebulae Studied with PhotopolarimetJ3' (T. Gehrels, Ed.L pp. 381-404. Univ. of Arizona Press, Tucson. BREedER, M. (1976). Catalog of spectrophotometric scans of stars. Astrophy,~. J. Suppl. 32, 7-87. BROWNI Ek. l). E.. R. S. RAJAN, AND D. A. T¢)MANDI
(1977). A chemical and textural comparison between carbonaceous chondrites and interplanetary dust. In ('ornery, Asteroid,~, Meteorile,~--lnlerrelation.~, Evolution and Ori~,,in,~ (A. H. Delsemme, Ed.), pp.
137-141. University of Toledo, Toledo. ('AMPINS, H., J. GRADIE, M. LEBOESKY. AND G . RIEU.k(1981). Infrared obserwltions of faint comets. In Modern Observational Techniques.[br Comets (J, (7. Brandl et al., Eds.), pp. 83-89. USGPO, Washington, I).C. DELSEMM~, A. H. (1982). Chemical composition of cometary nuclei. In Comers' (L. L. Wilkening, Ed.). pp. 85-130. Univ. of Arizona Press, Tucson. DOl IFUS, A., J. E. GEAKE, J. C. MANDEVIIL, AND B. Z~I.I NER (1977L The nature of asteroid surfaces, from optical polarimetry. In Comets. Asteroids, Meteorites-Interrelation,s, Evolution and Ori~eins (A. H. Delsemme, Ed.), pp. 243-251. University of Toledo, Toledo. DotJ~FUS, A., ',ND B. ZEt ENER(1979). Optical polarimetry of asleroids and laboratory samples. In A,steroids (T. Gehrels, Ed.t, pp. 170-183. Univ. of Arizona Press. Tucson. FELDMAN, P. D., M. F. A'HEARN, D. G. SCHLEI('HER, M. G. FEs'rou. M. K. WALLIS.W. M. BURION, D. W. HUGHES, H. U. KELLER, AND P. BENVENUn (1983). Evolution of the ultraviolet coma of comet Austin (1982g). Preprint. GEHRH S, T. (1977). The physical basis of the polarimetric method for deriving asteroid albedos. In Comets. A steroids. Meteorites--Interrelations. Evolution and Origins (A. H. Delsemme, Ed.t, pp.
253-256. University of Toledo, Toledo. HANNER, M. S. (1980L Physical characteristics of cometary dust from optical studies. In IA U Symposium No. 90 (1. Halliday and B. A. Mclntosh, Ed.), pp. 223-236. Reidel, Dordrecht. KfSE1EV. N. N., AND G. P. CHERNOVA(1978). Polar-
COMET SPECTROPOLAR1METRY ization of the radiation of comet West, 1975n. Soy. Astron. 22, 607-61 I. KISELEV, N. N., AND G. P. CHERNOVA (1981). Phase functions of polarization and brightness and the nature of cometary atmosphere particles. Icarus 48, 473-48 I. LUP~E, O. L., AND K. H. NORDSlECK (1983). Visible and infrared continuum spectropolarimetric observations of ten OB supergiant and O emission line stars. Wisconsin Astrophysics No. 183. Submitted for publication. MICHALSKY, J. J. (1981). Optical polarimetry of comet West 1976V1. Icarus 47, 388-396.
439
NEY, E. P. (1982). Optical and infrared observations of bright comets in the range 0.5/.Lm to 20 p~m, In Comets (L. L. Wilkening, Ed.), pp. 323-340. Univ. of Arizona Press, Tucson. OISHI, M., H. OKUDA, AND N. C. WICKRAMASINGHE (1978). Infrared observations of comet West (1975n). lI. A model of the cometary dust. Publ. Astron. Soc. Japan 30, 161-171. TAYLOR, D. (1963). Ph.D. thesis, Univesity of Wisconsin. WEINBERG, J. L., AND BEESON, D. E. (1976). Polarization reversal in the tail of comet Ikeya-Seki (1965 VIII). Astron. Astrophys. 48, 151-153.
Spectropolarimetry of Comets Austin and Churyumov-Gerasimenko ROY V. M Y E R S AND K E N N E T H H. NORDSIECK Washburn Observatory, University of Wisconsin, 475 North Charter Street, Madison, Wisconsin 53706 Received N o v e m b e r 10, 1983; revised February 2, 1984 Spectropolarimetric observations from 5000 to 8000 A, have been obtained for c o m e t s P/Austin (1982g) and P / C h u r y u m o v - G e r a s i m e n k o (1982f). The observations were spaced over phase angles of 50-125 ° for c o m e t Austin and 10-40 ° for comet C h u r y u m o v - G e r a s i m e n k o . The use of spectropolarimetry allowed an evaluation o f c o n t i n u u m polarization without molecular line contamination. Especially for c o m e t C h u r y u m o v - G e r a s i m e n k o , the curve of polarization versus phase angle resembles c u r v e s for asteroids, where the polarization is negative (electric vector m a x i m u m parallel to the scattering plane) for phase angles less than 20 ° and the most negative polarization is from - 1 to - 2 % . The negative polarization at backscattering angles m a y be due to multiple scattering in agglomerated grains, as a s s u m e d for asteroids, or to Mie scattering by small dielectric particles. If multiple scattering is important in c o m e t dust, polarization m e a s u r e m e n t s m a y imply a low albedo, less than 0.08. The polarization of c o m e t Austin remained steady during a large change in the dust production rate. Both c o m e t s increased c o n t i n u u m flux by a factor of 2 near perihelion. The c o n t i n u u m of c o m e t C h u r y u m o v - G e r a s i m e n k o had the shape of the solar s p e c t r u m with derivations less than 5%. The equivalent width of spectral features of C2, NH_,, and O varied as r 2.
1. I N T R O D U C T I O N
The history of comet polarimetry covers more than a hundred years. However, polarization measurements of backscattered light (small phase angles) from comets appear only in recent literature (Kiselev and Chernova, 1978, 1981). In these investigations and in one investigation of a comet tail (Weinberg and Beeson; 1976), the position angle of the electric vector maximum of backscattered light from comets was found to be parallel to the scattering plane instead of the unusual case of perpendicular for small-particle scattering. Two models have been used to explain the position angle: multiple scattering from adjoining dust grains (Boweil and Zellner, 1974; Gehrels, 1977) and Mie scattering from dielectric particles with sizes near the wavelength of light (Oishi et al., 1978; Michalsky, 1981). For both models, only the polarization at small phase angles (backscattering) is sensitive to changes in model parameters. Hence, the polarization of backscattering must be observed accurately. The precision of polarization measure-
ments of the comet continuum suffers from the presence of molecular emission lines. Since the strength of the molecular emission varies from comet to comet and varies with time for a given comet, broadband filter photopolarimetry involves uncertain corrections for contamination. All four previous measurements of the polarization at small phase angles (Kiselev and Chernova, 1978, 1981) were broadband measurements and only comet Ashbrook-Jackson was reported free of emission lines. Apart from the inherent problems of comet polarimetry, the measurements obtained for comets West, Ashbrook-Jackson, Chernykh, and Meier may have suffered from a systematic error. Assuming that the collection of comet dust particles has no preferential axis, the only special directions are toward the light source and toward the observer. Then, the position angle of the polarization must be either perpendicular or parallel to the scattering plane (Sun-Comet-Earth plane). The position angle of comet West, however, was advanced consistently 10° from the expected angle. The position angle deviations for the three dimmer comets
431 0019-1035/84 $3.00 Copyright © 1984by Academic Press, Inc. All rights of reproduction in any form reserved.
432
MYERS
AND NORDS1ECK
TABLE I COMET ORBITAL DATA Obs.
Date
A B C D E F G H 1
08/21/82 08/26/82 08/27/82 08/28/82 09/02/82 09/03/82 09/09/82 09/I 2/82 09/19/82
J K L M N
Phase (deg)
r IAUI
a lAD)
(1.65 0.65 0.65 0.65 (1.67 0.68 0.72 0.75 0.84
0.49 0.63 0.66 0.69 0.83 11.86 1.03 I.ll 1.2~
C o m e t Austin 123 104 I 01 98 84 81 68 62 52
Comet Churyumov-Gerasimenko 10/27/82 38 1.32 11/17/82 32 1.3 I 12/21/82 18 1.38 01/03/82 12 1,44 01/18/82 13 1.52
0.46 0.4 I
(1.43 (I.47 0.56
scatter wildly with averages again pointing to an a d v a n c e m e n t of position angle. Molecular line contamination and the possible instrumental error m a k e further investigation of backscattering polarization desirable. This p a p e r describes results of spectropolarimetry of the comets P/Austin (1982g) and P / C h u r y u m o v - G e r a s i m e n k o (1982f, hereafter CG). Unlike broadband polarimetry, s p e c t r o p o l a r i m e t r y allows determination of the polarization without molecular line contamination. In addition, the comet spectra are available for analysis. The observations are discussed in Section II. Spectral analysis is presented in Section III, polarization results in Section IV, and the relationship of the gas and dust production rates in Section V. 11. O B S E R V A T I O N S
O b s e r v a t i o n s were spaced in phase angle o v e r the range 125-50 ° for comet Austin and 40-10 ° for c o m e t CG. Table 1 contains the date of the o b s e r v a t i o n (UT) and position information: the Sun to comet dis-
tance, the Earth to comet distance, and the phase angle ( S u n - c o m e t - E a r t h angle). All observations were made with the intensified Reticon spectropolarimeter (Lupie and Nordsieck, 19831 at the 0.9-m telescope of the Pine Bluff Observatory. The instrument obtains the spectrum at a resolution of 8 A, together with polarimetric information at a resolution of 100 A. The dual-detector system allows simultaneous m e a s u r e m e n t of the comet and background from 5000 to 8000 A with a spatial separation of 168 arcsec. A rapidly varying background for comet Austin, which was at a small elongation from the Sun, makes the simultaneous background m e a s u r e m e n t especially important. The entrance slit size was 4.9 × 11.3 arcsec for comet Austin observations and the two slits were oriented e a s t - w e s t . For comet CG, the slit size was 6.5 ×21.1 arcsec with the slit orientation 45 ° from the scattering plane, Neither comet had an extended tail in the guiding c a m e r a to about five magnitudes dimmer than the c o m a , so the possibility of contamination due to a tail in the background slit could be ignored. The reduction procedure includes flat field correction and calibrations of wavelength, flux, and polarization efficiency. The flat field correction is a pixel by pixel divide by an observation of a tungsten lamp. Observations of standard stars (Breget, 19761 determine the instrument response and calibrate the flux level. H o w ever, on a given night the extinction may differ from the assumed average atmospheric extinction curve (Taylor, 19631, so the precision of the p h o t o m e t r y depends mainly on the precision of the extinction. For nights during which there were comet observations, the flux of self-calibrated standard stars typically displayed night-tonight variations less than 10% and that could be considered the uncertainty due to lack of absolute p h o t o m e t r y . The polarization efficiencies were evaluated by reducing an observation of an unpolarized star through a polaroid. Scans were taken such that the source alternated between slits and
COMET SPECTROPOLARIMETRY 10 b
32 a
433
i
i
i
t
84100
I 7200
80100
'7 9 E o g15
c2
~, 5 NH~
1
,l
[o0
CN
o
o
,7
u.
5600
6400 Wavelength
7200 (,~)
I 5600
8000
Wavelength
(/~)
FIG. I. (a) Spectrum of comet Austin on 1982 August 28. (b) Spectrum of comet ChuryumovGerasimenko on 1982 November 17.
the background c a m e from an adjacent scan. To prevent an incorrect subtraction of background polarization when the background varied rapidly, the individual scans from c o m e t Austin were inspected to ensure that e x t r e m e variations from the previous scan did not exist. Figure 1 provides an example of each c o m e t ' s spectrum.
was near the horizon on a night with thick haze. I f the dust production were constant, the reflected flux would drop with heliocentric distance as r -2. As illustrated on the figure, a constant rate hypothesis is consis-
r (AU) o
d
d
.
.
d
.
.
d
~
~
,-:
~
,
111. CONTINUUM FLUX AND COLOR Relative continuum flux m e a s u r e m e n t s were obtained f r o m the spectra. Variations from the a s s u m e d extinction and a slit size smaller than the c o m e t image p r e v e n t e d absolute p h o t o m e t r y . Continuum data are discussed below in t e r m s of the flux at 7000 ,~, where emission lines are absent, and in terms of the deviation of the continuum from solar color. Figure 2 shows for both comets the continuum flux at 7000 A, reduced to a geocentric distance of I AU. The abscissa shown is the phase angle, to facilitate c o m p a r i s o n with the polarimetry. The error estimates reflect the noise in the s p e c t r u m and ignore the lack of absolute p h o t o m e t r y . C o m e t Austin shows a large dust production rate increase o v e r 3 days (points B - D ) near perihelion ( m a r k e d w i t h P). The dip (point E) after the abrupt rise is attributed to atmospheric fluctuations, since the comet
o,,~
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o~
/
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CG
o
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120
I
100
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I 80 Phase
I I 60 40 (degrees)
l 20
i0
FIG. 2. Continuum flux at 7000 ,~ reduced to 1 AU from the Earth. The Austin point E is probably affected by haze. The phase angle of comet C h u r y u m o v Gerasimenko reached a minimum at 12.6 ° and increased after point M. With this exception, time increases from left to right. The scaled I U E point was taken before perihelion at r = 1.1 AU. The curves through the comet Austin data vary as r 2 and the curve through the comet C h u r y u m o v - G e r a s i m e n k o data varies as r 4
434
MYERS AND NORDSIECK
tent with the points D - I after a small correction for observing an extended object at different geocentric distances with a fixed aperture size. F o r the calculation of the correction, the flux was assumed to vary as the inverse of the distance from the comet nucleus. The reason for the lack of conformity of point H to the curve is unknown. Three more m e a s u r e m e n t s of the continuum flux of c o m e t Austin before perihelion are available from International Ultraviolet Explorer ( I U E ) observations (P. D. Feldman, 1982, personal communication: Feldman e t al. 1983). If the c o m e t continuum retains the shape of the solar spectrum (albedo independent of wavelength, see below), the flux at 2960 A can be scaled to obtain a flux at 7000 ,~. The 1UE observations at heliocentric distances 1.11, 0.88, and 0.8[ AU then would imply continuum fluxes at 7000 ° A of 9.5, 8.6, and 8.0 x 10 t~ erg cm 2 sec ~ A 1. After correction for a fixed aperture size, these fluxes alone vary as r 2 before perihelion, implying a constant dust production rate. The extrapolated brightness for this dust production rate, shown in Fig. 2, is a factor of 3 lower than the pre-perihelion point A. Thus, either the dust production rate increased significantly between the last 1UE observation on 1982 August 1 and the observation A on August 21, and/or the uv albedo is significantly smaller than the visual albedo. The color d e p e n d e n c e of the albedo of comets may be represented by the percentage deviation of the comet flux, C, from the solar continuum, S. at two wavelengths 1 and 2: c = (C,./$2 - C J S O / { C / S ) . Previous determinations of the continuum color of comets (Hanner, 1980) have ranged from neutral to a 30% red excess over the visible wavelengths. H o w e v e r , line contamination may have affected the broadband filter measurements. Estimates of the albedo color dependence of comets Austin and CG using parts of the spectrum with no significant contribution from lines are consistent with a neutral albedo, Molecular emission lines and atmo-
spheric absorption lines cluttered the spectra of comet Austin and prevented the appearance of a clean continuum. An upper limit of 30% to the albedo color d e p e n d e n c e of comet Austin was found from continuum segments (lines contribute less than 15% of flux) over the wavelength region from 6700 to 7900 A. The spectra of c o m e t CG were free of emission lines o v e r the wavelength region from 5300 to 8000 A. A linear leastsquares fit to comet spectra normalized to the solar spectrum produced a range of color d e p e n d e n c e from a 2.5% blue excess to a 5.9% red excess with a formal uncertainty of -+1%. H o w e v e r , the true uncertainty should reflect the repeatability of flux and extinction calibrations at < 5 % as verified by the lack of changes in the spectral shape greater than 5%. The aibedo color dependence for comet CG was therefore neutral to within 5%. IV. POLARIZATION Spectropolarimetry holds a great advantage o v e r broadband photopolarimetry in that the contamination by molecular emission can be eliminated. Dividing the c o m e t spectra by the solar spectrum identified a region of continuum of c o m e t Austin extending, with interruptions by emission lines, from 6300 to 8100 A and it verified that comet CG is free of emission lines from 5300 to 8000 A. The polarization was evaluated o v e r these ranges where line flux contributed less than 15% of the continuum flux. The polarization results are discussed below in terms of the color d e p e n d e n c e and the wavelength mean. Table Ii contains both the o b s e r v e d mean position angle ("free P A " ) and the position angle ('~fixed P A " ) corresponding to polarization perpendicular to the S u n - C o m e t Earth scattering plane (positive polarization). Within the uncertainties, the position angle was found to be perpendicular or parallel to the scattering plane. The degree of polarization is listed both as o b s e r v e d and as reduced on the assumption that the position angle is in fact either perpendicular
COMET
435
SPECTROPOLARIMETRY T A B L E II
COMET POLARIZATION DATA
Obs.
Date
Source time (sec)
Free PA Pol. (%)
Fixed PA PA (deg)
Pol. (%)
Color Ap/p
(%)
PA (deg)
Comet Austin A B C D E F G H I
08/21/82 08/26/82 08/27/82 08/28/82 09/02/82 09/03/82 09/09/82 09/12/82 09/19/82
4O0 3,200 3,200 5,200 6,000 4,800 4,000 3,200 4,000
J K L M N
10/27/82 11/17/82 12/21/82 01/03/83 01 / 18/83
7,200 16,800 20,000 24,000 22,400
22.9 ± 1.5 19.9 _+ 0.8 22.3_+ 0.5 20.5 ± 0.5 20.2 _+ 0.6 17.6 ± 0.8 14.5 ± 1.8 7.8 _+ 1.4
-+ 1.5 ± 0.7 ± 0.5 -+ 0.5 _+ 0.5 ± 0.7 _+ 1.8 ± 1.1
124 125 126 127 127 123 121 114
Comet C h u r y u m o v - G e r a s i m e n k o 4.2 _+ 0.7 180 ± 19 4.1 ± 0.5 3.3 ± 0.2 181 ± 8 3.4 ± 0.2 1.5 ± 0.2 51 ± 15 - 1 . 5 ± 0.2 1.9 ± 0.3 9 ± 18 - 1 . 9 ± 0.3 2.2±0.7 135_+37 -1.9_+0.7
177 174 142 104 59
(positive polarization) or parallel (negative) to the scattering plane. The difference in polarization values is not just a factor of cosine, since the polarization information is refit with one less free parameter (Lupie and Nordsieck, 1983) when the position angle is assumed. The dependence of the "fixed PA" polarization on phase angle is shown in Fig. 3. Comparison of the continuum flux in Fig. 2 and the polarization in Fig. 3 shows no correlation of polarization and flux variations. The polarization-sensitive parameter of the dust cannot have changed during the rapid and large increase of continuum flux for comet Austin. Table II gives the relative color dependence, Pz--P]/P, derived from a linear leastsquares fit of the polarization versus wavelength. For comet Austin, the two wavelengths (,kj, h:) and 6500 and 7800 A and for comet CG they are 5400 and 7500 ~,. No significant color dependence of the polarization was found for either comet. Averaging the best observations, for comet Austin, the polarization was neutral to
133 123 126 127 127 118 134 101
_+ 7 ± 5 ± 3 _+ 3 ± 3 + 5 _+ 14 ± 21
23.0 19.6 22.3 20.5 20.2 16.7 14.4 6.0
-13 32 11 -12 0 32 26 50
_+ ± ± -+ ± -+ ± ±
19 11 6 7 8 12 36 52
-32 + -5 ± 33 -+ -141 ± 115±
35 15 36 47 118
within about -+10% and for comet CG. to within about -+20%. The polarization versus phase angle r (AU) (D
(0
~-
(5
c;
o
6
I
I
I
I
t,
c~ I
~ I
~
I
I
20
t
f
v
CG
Austin
~0 ".C..
o ~°
I 120
i 100
I 80
I 60
I 40
I 20
0
Phase (degrees)
FIG. 3. Continuum polarization as a function of phase angle. The polarization is an average over the range 6300-8000 ,~ for comet Austin and 5300-8000 ,~ for comet C h u r y u m o v - G e r a s i m e n k o .
436
MYERS AND NORDSIECK
curve in Fig. 3 resembles curves obtained from asteroids. The p a r a m e t e r s that describe the asteroid curves at small phase angles are the most negative polarization (minimum polarization), the phase angle at c r o s s o v e r f r o m negative to positive polarization, and the slope of the curve near the c r o s s o v e r (polarimetric slope). The minim u m polarization occurs near 10° phase angle for asteroids and c o m e t CG. The observation of c o m e t CG at 12° phase angle establishes the minimum polarization at - 1 . 9 +_ 0.3%. A linear least-squares fit to the points of c o m e t CG between phase angles 15° and 40 ° yields a c r o s s o v e r at 22 -+ 1°. The slope of the line that determines the c r o s s o v e r phase angle is 0.31 _+ 0.3% per degree. The c o m e t Austin data appear to connect smoothly onto the c o m e t CG data, implying a similar c r o s s o v e r phase angle for comet Austin. The apparent continuity of the polarization data c o m e s from two comets with very different spectra: gaseous emission lines dominate the spectra of c o m e t Austin, but are absent from the spectra of c o m e t CG. The fundamental difference between these two comets, perhaps the n u m b e r of perihelion passages, does not seem to affect the polarization-sensitive parameter of the dust. The similarity between comet and asterold polarization curves suggests that correlations b e t w e e n the albedo and the minim u m polarization and polarimetric slope obtained for asteroids (Dollfus e t a l . , 1977: Dollfus and Zellner, 1979) might be applied to the c o m e t CG data. With this assumption, the minimum polarization of CG would imply an albedo of 0.05-0.08, and the polarimetric slope would imply an albedo less than 0.06. The albedo of comet dust near 90 ° phase angle, determined from comparison of infrared emission and visual scattering, scatters around 0.17 (Campins e t a l . , 1981; N e y , 1982) and the range goes down to 0.02. The albedo ranges obtained from the asteroid relations are on the low end of the range. The reasonable c o m e t albedo obtained from polarization data sug-
gests that the mechanism for producing the negative polarization in comet CG may be similar to that in asteroids, possibly multiple scattering (Gehrels, 1977). This would suggest that much of the c o m e t dust could be large c o m p a r e d to the wavelength of light and of complex shape, similar to the agglomerated particles seen in upper atmosphere collection experiments (Brownlee e t al., 1977). Of the comets with polarization measured at small phase angles (Kiselev and Chernova, 1978, 1981), comet CG mimics comet West in all three curve p a r a m e t e r s obtained above. The minimum polarization of comet Meier also may be - 2 % . However, the minimum polarizations that Kiselev and C h e r n o v a quote for comets A s h b r o o k - J a c k s o n and Chernykh are more negative and the polarimetric slopes steeper than observed tbr comet CG, even though the crossover also occurs near 22 ~' for these comets. Both parameters are far from the range observed in asteroids and the asteroid correlations fail to give any value for the albedo. The uncertainties in the polarization parameters for these two comets are large, but the inability to obtain an albedo may indicate that multiple scattering is not the mechanism for producing the negative polarization in these comets. Also, observations of a comet tail (Weinberg and Beeson, 1976) showed large negative polarization at moderate phase angles (<50°). The alternative, small-particle Mie models, can produce negative polarizations (Oishi e t a l . , 1978) much larger than those that have been produced in multiple scattering models. H o w e v e r , the Mie models suffer from a considerable sensitivity to size and composition, which is difficult to reconcile with polarization stability during dust outbursts. V. GAS AND DUST PRODUCTION A by-product of the spectropolarimetry of comet Austin is line equivalent width data which may be used to evaluate the gas/
COMET SPECTROPOLARIMETRY r (AU)
g
g
o
6
i
i
i
g
~
g
o
6
o
i
I
~0.5 3 ~r LB
0.0
120
I
I 100 Phase
I 80 (degrees)
60
I
FIG. 4. The variation of the equivalent width of molecular emission features for comet Austin normalized a perihelion. The points are means from nine spectral features of C2, NH2, and [0 l]. The error bars are the rms deviations of the individual features from the mean. The curve drawn through the data varies as r 2.2.
dust ratio. The n u m b e r of lines in the c o m e t Austin spectra m a k e s the continuum level impossible to m e a s u r e directly in the wavelength region 5000-6300 A. H o w e v e r , the continuum color of both c o m e t s Austin and CG (Section III) suggests that the assumption of a solar continuum is valid; a solar s p e c t r u m therefore was used that matches the c o m e t Austin continuum longward of 6300 A. Readily identifiable features of the molecules C2 (5170, 5630, 6110 ,&) and NH2 (5710, 5980, 6340, 6620 A), and atomic oxygen (6300, 6370 A) have been m e a s u r e d for equivalent width. M e a s u r e m e n t s of a single feature at several heliocentric distances can indicate variations of the gas production. Assuming that each feature sampled the gas production rate at the same instant, the equivalent widths of the sets of lines listed a b o v e were averaged to obtain a gas/dust ratio for each species. The equivalent widths determined for sets of C2 features and NH2 features were found to have essentially the same de-
437
pendence on heliocentric distance, r 2.3 for C2 and r 2.1 for NH2. Figure 4 shows the grand mean of normalized equivalent widths of all three species. The error bars show the dispersion about the mean of the equivalent widths for individual features. If the data for C2 only and NH2 only were plotted in Fig. 4, all points would be within the error bars of the means and the separation of the C2 and NH2 points on each night would be less than half of the error bar of the mean. Combining data f r o m these different molecules therefore appears to be a valid a s s e s s m e n t of the gas/dust ratio. A curve with an r - 2 2 d e p e n d e n c e is drawn through the data of Fig. 4. A formal uncertainty on the exponent probably would be an underestimate, so it is more appropriate to state the dependence as r -2 and not r t or r 3.
When the continuum flux is constant, the equivalent width variations reflect the gas production rate. Gas production is expected to vary as r -2 (Delsemme, 1982) and the second half of the c o m e t Austin observations verifies the r -2 dependence. For the first half of the observations, the equivalent widths are roughly constant even though the continuum flux m o r e than doubles. The gas production therefore must have increased in the same proportion as the dust production. The equivalent width provides an indicator of the n u m b e r of molecules c o m p a r e d to the n u m b e r of dust particles. The F i n s o n Probstein analysis of dust tail p h o t o g r a p h s provides a ratio of the masses of gas and dust. An application of both method would yield c o m p l e m e n t a r y information on the dust, but the equivalent width is much easier to obtain in that no dust tail is required. The mass gas/dust ratio for c o m e t West (Delsemme, 1982) varies by a factor of 5 around a dust outburst. Although the continuum flux increase of c o m e t Austin was one-fourth that of c o m e t West, the c o m e t Austin equivalent width gave no evidence for variation of the n u m b e r ratio of gas/ dust.
438
MYERS AND NORDSIECK Vl. CONCLUSIONS
1. T h e c o n t i n u u m flux o f both c o m e t s A u s t i n a n d C G i n c r e a s e d significantly n e a r p e r i h e l i o n : c o m e t A u s t i n i n c r e a s e d its flux by 60% o v e r a few days. T h e a l b e d o c o l o r d e p e n d e n c e was n e u t r a l to within 30% for c o m e t A u s t i n a n d to within 5% for c o m e t C G , e v e n d u r i n g the rapid c o n t i n u u m flux i n c r e a s e of c o m e t A u s t i n . 2. T h e e q u i v a l e n t width m a y b c a useful d i a g n o s t i c o f the n u m b e r gas/dust ratio. T h e e q u i v a l e n t w i d t h s of f e a t u r e s in spectra of c o m e t A u s t i n varied with h e l i o c e n t r i c d i s t a n c e as r ~-. T h e a d h e r e n c e to the c u r v e is r e m a r k a b l e d u r i n g large v a r i a t i o n s of c o n t i n u u m flux, i m p l y i n g that the n u m b e r ratio of g a s / d u s t r e m a i n e d u n c h a n g e d . 3. T h e p o l a r i z a t i o n of c o m e t s A u s t i n and CG was e i t h e r p e r p e n d i c u l a r or parallel to the s c a t t e r i n g p l a n e within u n c e r t a i n t i e s a n d the d e g r e e o f p o l a r i z a t i o n was w a v e length i n d e p e n d e n t to within +-10c~ for c o m e t A u s t i n . T h e p o l a r i z a t i o n was i n s e n sitive to the dust p r o d u c t i o n rate and to c o m e t d i f f e r e n c e s that are reflected by the gas c o n t e n t , since the p o l a r i z a t i o n v e r s u s p h a s e angle c u r v e a p p e a r s to be c o n t i n u o u s for the g a s s y c o m e t A u s t i n and the d u s t y c o m e t CG. 4. N e g a t i v e p o l a r i z a t i o n at small phase angles is c o n f i r m e d by the o b s e r v a t i o n s of c o m e t C G a n d is s u s p e c t e d to o c c u r in c o m e t A u s t i n by j u d g i n g the t r e n d of the p o l a r i z a t i o n p o i n t s . T h e p o l a r i z a t i o n as a f u n c t i o n o f p h a s e a n g l e for c o m e t CG imitates the c u r v e of c o m e t W e s t and resembles c u r v e s c h a r a c t e r i s t i c of a s t e r o i d s : the m i n i m u m p o l a r i z a t i o n is - 1 . 9 + 0.3%,, the c r o s s o v e r p h a s e angle is 22 _+ 1°, a n d the p o l a r i m e t r i c slope is 0.31 -+ 0.03% per degree. T h e ability to o b t a i n a r e a s o n a b l e c o m e t a l b e d o ( < 0 . 0 8 ) from c o r r e l a t i o n s of a s t e r o i d a l b e d o to p o l a r i z a t i o n p a r a m e t e r s l e n d s s u p p o r t to the s u g g e s t i o n that a multiple s c a t t e r i n g m e c h a n i s m used to e x p l a i n the b a c k s c a t t e r e d p o l a r i z a t i o n in a s t e r o i d s m a y be valid for c o m e t s . H o w e v e r , if the larger n e g a t i v e p o l a r i z a t i o n s a n d p o l a r i m e t -
ric slopes seen in earlier o b s e r v a t i o n s of o t h e r c o m e t s arc verified, Mie m o d e l s m a y be required to explain the p o l a r i z a t i o n . If the multiple s c a t t e r i n g e x p l a n a t i o n for the n e g a t i v e p o l a r i z a t i o n p r o v e s valid, the polarization of b a c k s c a t t e r i n g could p r o v e to be a sensitive way of o b s e r v i n g grain agg l o m e r a t i o n in the i n t e r p l a n e t a r y and interstellar m e d i u m . REFERENCES BOWEIA, E., AN[) B. ZEI I.NER(1974). Polarization of asteroids and satellites. In Planels, Slar.~ and Nebulae Studied with PhotopolarimetJ3' (T. Gehrels, Ed.L pp. 381-404. Univ. of Arizona Press, Tucson. BREedER, M. (1976). Catalog of spectrophotometric scans of stars. Astrophy,~. J. Suppl. 32, 7-87. BROWNI Ek. l). E.. R. S. RAJAN, AND D. A. T¢)MANDI
(1977). A chemical and textural comparison between carbonaceous chondrites and interplanetary dust. In ('ornery, Asteroid,~, Meteorile,~--lnlerrelation.~, Evolution and Ori~,,in,~ (A. H. Delsemme, Ed.), pp.
137-141. University of Toledo, Toledo. ('AMPINS, H., J. GRADIE, M. LEBOESKY. AND G . RIEU.k(1981). Infrared obserwltions of faint comets. In Modern Observational Techniques.[br Comets (J, (7. Brandl et al., Eds.), pp. 83-89. USGPO, Washington, I).C. DELSEMM~, A. H. (1982). Chemical composition of cometary nuclei. In Comers' (L. L. Wilkening, Ed.). pp. 85-130. Univ. of Arizona Press, Tucson. DOl IFUS, A., J. E. GEAKE, J. C. MANDEVIIL, AND B. Z~I.I NER (1977L The nature of asteroid surfaces, from optical polarimetry. In Comets. Asteroids, Meteorites-Interrelation,s, Evolution and Ori~eins (A. H. Delsemme, Ed.), pp. 243-251. University of Toledo, Toledo. DotJ~FUS, A., ',ND B. ZEt ENER(1979). Optical polarimetry of asleroids and laboratory samples. In A,steroids (T. Gehrels, Ed.t, pp. 170-183. Univ. of Arizona Press. Tucson. FELDMAN, P. D., M. F. A'HEARN, D. G. SCHLEI('HER, M. G. FEs'rou. M. K. WALLIS.W. M. BURION, D. W. HUGHES, H. U. KELLER, AND P. BENVENUn (1983). Evolution of the ultraviolet coma of comet Austin (1982g). Preprint. GEHRH S, T. (1977). The physical basis of the polarimetric method for deriving asteroid albedos. In Comets. A steroids. Meteorites--Interrelations. Evolution and Origins (A. H. Delsemme, Ed.t, pp.
253-256. University of Toledo, Toledo. HANNER, M. S. (1980L Physical characteristics of cometary dust from optical studies. In IA U Symposium No. 90 (1. Halliday and B. A. Mclntosh, Ed.), pp. 223-236. Reidel, Dordrecht. KfSE1EV. N. N., AND G. P. CHERNOVA(1978). Polar-
COMET SPECTROPOLAR1METRY ization of the radiation of comet West, 1975n. Soy. Astron. 22, 607-61 I. KISELEV, N. N., AND G. P. CHERNOVA (1981). Phase functions of polarization and brightness and the nature of cometary atmosphere particles. Icarus 48, 473-48 I. LUP~E, O. L., AND K. H. NORDSlECK (1983). Visible and infrared continuum spectropolarimetric observations of ten OB supergiant and O emission line stars. Wisconsin Astrophysics No. 183. Submitted for publication. MICHALSKY, J. J. (1981). Optical polarimetry of comet West 1976V1. Icarus 47, 388-396.
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NEY, E. P. (1982). Optical and infrared observations of bright comets in the range 0.5/.Lm to 20 p~m, In Comets (L. L. Wilkening, Ed.), pp. 323-340. Univ. of Arizona Press, Tucson. OISHI, M., H. OKUDA, AND N. C. WICKRAMASINGHE (1978). Infrared observations of comet West (1975n). lI. A model of the cometary dust. Publ. Astron. Soc. Japan 30, 161-171. TAYLOR, D. (1963). Ph.D. thesis, Univesity of Wisconsin. WEINBERG, J. L., AND BEESON, D. E. (1976). Polarization reversal in the tail of comet Ikeya-Seki (1965 VIII). Astron. Astrophys. 48, 151-153.