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The brightness and lightcurve of Triton in 1987
ICARUS 79, 15--22 (1989)
The Brightness and Lightcurve of Triton in 1987 NElL L. LARK Physics Department, University of the Pacific, Stockton, Califi~rnia 95211 AND
H. B. HAMMEL, l DALE P. CRUIKSHANK, 2 DAVID J. THOLEN, AND MICHAEL A. RIGLER Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822 Received April 26, 1988; revised August 12, 1988
We report C C D and photoelectric photometry of Triton obtained during the apparition of 1987. At 8900 .~, a wavelength which might be expected to show a strong contrast because of methane absorption on Triton, no rotational lightcurve is seen at a level of < 0 . 0 2 mag. This result is in contrast to measurements of the V lightcurve a decade earlier which indicated a variation of 0.06 mag, and other data suggesting a variation of as much as 0.25 mag. The V magnitude of Triton measured photoelectrically in 1987 is reasonably consistent with the most reliable earlier measurements and shows no convincing evidence for long-term change. We find V(1, a) = - 1.244 ___ 0.023. © 1989AcademicPress, Inc.
INTRODUCTION
The brightness of Triton is difficult to measure by conventional photoelectric photometric techniques because of its proximity to Neptune and its current location in the Milky Way. Imaging detectors such as CCDs are well suited to this task because they permit rigorous subtraction of the background light. In this paper we report CCD photometry of Triton obtained over a full cycle of its orbit in June 1987. Triton orbits Neptune with a period of 5.877 days, very likely in locked synchronism, such that the same hemisphere faces Neptune at all times (Peale 1977). The orbit is retrograde, inclined 159° to Neptune's equatorial plane, and is very nearly circular (eccentricity <0.01). The apparent angular t Present affiliation: Jet Propulsion Laboratory, MS 169-237, 4800 Oak Grove Dr., Pasadena, CA 91109. 2 Present affiliation: N A S A Ames Research Center, MS 245-6, Moffett Field, CA 94035.
separation between Triton and Neptune ranges between 11 and 17 arcsec. Early photoelectric photometry of Triton was reported by Harris (1961) from measurements in standard U B V filters on five occasions in 1950, 1951, 1953, and 1956. Harris noted with reservation that there was some indication that the satellite's leading face (the hemisphere facing the direction of the orbital motion) was brighter than the trailing face by about 0.25 mag in the V filter. Subsequently Andersson (1974) and Degewij et al. (1980) reported photoelectric photometric observations of Triton in U B V filters in 1972 and 1977. The 1977 data of Degewij et al. gave marginal evidence for a leading/trailing asymmetry, amounting to 0.1.-0.2 mag, but in the opposite sense as that suspected by Harris. Franz (1981) also observed in 1977 with a photoelectric area-scanning photometer that permitted rigorous subtraction of the background scattered light from Neptune. In addition to obtaining the brightness of 15 0019-1035/89 $3.00 Copyright © 1989by Academic Press, Inc. All rights of reproduction in any form reserved.
16
LARK ET AL. TABI,E I SUMMARY
OF PHOTOMETRIC
MEASUREMENTS
OF TRITON [)ate
V(1. a)
AV
U-B
B-V
Relercncc
195(t-1956 1972
1.16 1.28
+ ? + 0.11
0.25
(I.4(I (I.25
0.77 I).77
Harris (1961~ Andersson(19741
1977 1977
1.12 1.22
+ 0.12 ~ 0.05
ILl-0.2 0.06
0.30 0.~0
0.70 I).74
I)egewij (1980) t.Tanz (1981~
1987
1.244 * 11.tl23
,0.02"
---
This paper
ture or estimated by us. The quantity V(i, a) is the Johnson V magnitude referenced to 1 A U from the Sun and from the Earth with no solar phase function correction applied. If in fact/3 = 0, the data are directly comparable with one another. Table i also gives the results of the observations reported in this paper, details of which are in the following sections.
" This is ~,891R)A. not AV.
CCD PHOTOMETRY OF TRITON Triton in U B V filters, measured relative to Neptune, Franz found -+0.03 mag variation in V synchronous with the revolution, and in the sense of the leading face being brighter than the trailing. Triton is always observed at very small solar phase angles, well within the range in which airless asteroids, planets, and satellites with regoliths show a very strong surge in brightness at opposition. To c o m p a r e the brightness of Triton measured in different years and at different solar phase angles, and to establish a rotational lightcurve, it is necessary first to apply a solar phase function to the data. The phase function for Triton has been measured by Goguen et al. (1988), who found 13 = 0.022 -+ 0.035 mag deg-1; a typical value for an airless asteroid or planetary satellite is about 0. i mag deg o v e r phase angles of 0.0 to 2.0 deg. The Triton data are also consistent with /3 = 0.00, as evidenced by the large error bar on the determination. The small measured value of/3 for Triton is consistent with the presence of an a t m o s p h e r e or a surface of some material other than a particulate regolith (e.g., Cruikshank et al. 1988). Harris (1961) did not measure or correct for the opposition effect in his data, nor did Franz (1981), whose data were taken in the range a = 1.8-1.9 deg. Degewij et al. (1980) reported sufficient details of their data so that comparison can be made with the other data sets with an a s s u m e d / 3 = 0. Table I summarizes the photometric results of the investigators referenced above, using error values from the litera-
We used the University of Hawaii's 2.24m telescope on Mauna K e a to take CCD images including both Neptune and Triton on each frame. The field of the 800 × 800pixel Texas Instrument CCD, at the i l l 0 Cassegrain focus, was nearly 2 min of arc across, with an image scale of 0.140 arcsec pixel ~. We recorded images on 9 nights, June 16-24, 1987, but obtained the necessary combination of good observing conditions and suitable m e a s u r e m e n t s and calibrations only on June 18-23, giving uniform coverage around one entire orbit. During these observations the solar phase angle decreased from 0.34 to 0.18 deg. The observations reported here were taken through a filter centered at 8900 ( F W H M = 200 ,~). This bandpass coincides with a strong absorption feature in the spectrum of methane. Since Neptune absorbs strongly at this wavelength, use of this filter reduced the brightness of Neptune in comparison with Triton by more than a factor of 10. Our images of Neptune show the strongest contrast and disk-integrated photometry of Neptune shows the greatest temporal variation at this wavelength (Hammel 1988). Weak absorption at this wavelength due to methane on Triton has been observed (Apt et al. 1983). We have not analyzed our partial sets of observations at 6190 ,~ Ca w e a k e r methane absorption band) and at 6340 and 8260 ,~ (nearby continuum wavelength regions) since these Triton images are about 10 times fainter. The C C D exposures were limited by the brightness of Neptune.
PHOTOMETRY OF TRITON A typical night's m e a s u r e m e n t s consisted of several sets of three or four images including both Neptune and Triton. The exposure times varied because of the changing brightness of Neptune; most were between 100 and 180 sec, with a few as short as 50 sec. These image sets were spread o v e r about 5 hr, and were interspersed between observations of the standard stars SAO 083543 (= BD+26°2606) (Oke and G u n n 1983)and H D 170493 (= B D - l ° 3 5 0 0 ) (Landolt 1983). The standard stars were used for p h o t o m e t r i c calibration and atmospheric extinction correction. In addition, internal checks on the p h o t o m e t r y were made by measuring s o m e of the stars which were visible near the east edge of the field late on one night and near the west edge of the field early on the following night. (Neptune was in retrograde motion, very near its June 28 opposition date.) T h e s e internal checks increased our confidence in the validity of the results, but b e c a u s e of the difficulties in reproducing p h o t o m e t r y close to the edge of the CCD, this technique did not i m p r o v e the overall accuracy. Processing and analysis of the C C D images required that each frame be given individual attention. The fiat-fielding process did not completely remove all the nonuniformities and flaws in the C C D images. Triton was near the galactic plane and consequently the field was very crowded, with about 300 stars visible in a typical image. It was often difficult to find suitable blank sky for background subtraction, and often there were stars too near Triton and Neptune to be avoided. The p h o t o m e t r y was done using areas of about 8 sec of arc square (55 x 55 pixels for the stars and 57 x 57 pixels for Triton due to its finite angular size). T h e s e areas included 97% of the total flux from the object of interest. When possible, four different background areas were measured around each star or Triton image, and the standard deviation of the mean of the four was taken as the uncertainty in the sky background subtraction. This uncertainty was always
17
considerably larger than the statistical uncertainty due to the limited n u m b e r of counts (data numbers) from the Triton image. The sky background was typically about 35% and light from Neptune was less than 2% of the total flux in the photometric area centered on Triton. When nearby field stars or CCD flaws interfered, the photometry was done in more than one way, and the difference between the results was taken as a measure of the uncertainty. Photometry was done for individual interfering stars up to 5 mag fainter than Triton. The photometric results are expressed as relative magnitudes of Triton calculated from the expression mag = 22 - 2.5 * log(counts/sec), where the 22 is an arbitrary constant chosen to give relative magnitudes in rough accord with standard V photometric values. The atmospheric extinction coefficient was 0.0269 -4- 0.0010 mag per unit air mass through this filter on 21 June 1987 and 0.0227 ± 0.0015 for the other five dates. We determined the relative magnitude of the photometrically calibrated standard star SAO 083543 as 9.733 at zero air mass through this filter; Oke and Gunn (1983) provide a flux curve for this star. Absolute calibration of similar p h o t o m e t r y of Neptune is discussed in detail by Hammel (1988). No systematic variation is evident in the 69 individual magnitude measurements. The simple mean of the m e a s u r e m e n t s is 13.340 with the standard deviation of the distribution being 0.016 mag. The uncertainties of the individual observations are typically between 0.004 and 0,016 mag, with a mean of 0.010 mag. The good correspondence between individual uncertainty estimates and the observed scatter in the results is one indication that there is no strong time variation in Triton's magnitude. It is also an indication that the major sources of error have been identified. As a further check for any systematic variation, the 69 observations were subdi-
18
LARK ET AL. TABLE II
given in Table !I and are plotted in Fig. 1.
AVERAGESOF T H E 21 SETS OF SEQUENTIAL MEASUREMENTSOF TRITON'SREI.ATIVE MAGNITUDEAT 8900 /~ . . . . . . . . . . . . UT date Mean Std. UT date Mean Sld, dev. of mean mag dev. of mean mag
Since the points in Fig. 1 seem to give some hint o f regular variation, a search for periodic variation on time scales o f a few hours was made. N o statistically significant periodic variation was found. As a n o t h e r c h e c k for possible systematic effects, the means o f these 21 sets o f measurements were plotted on a time scale folded at 17 hr, the period o f variation in the disk-integrated magnitude o f N e p t u n e measured from these same images ( H a m m e l 1988). Although the image o f N e p t u n e was only 12 to 15 sec o f arc a w a y f r o m the image o f Triton, and N e p t u n e ' s magnitude varied by 0.6 mag f r o m 2.1 to 2,7 mag brighter than Triton, no significant variation o f Triton is evident. For the clearest and m o s t sensitive search for variation in T r i t o n ' s magnitude associated with its 5.9-day orbital period, the individual o b s e r v a t i o n s for each night were averaged. T h e s e nightly mean magnitudes and the standard deviations o f the m e a n s are s h o w n in Table III and plotted in Fig. !. The decimal dates are the means o f the equally weighted times o f the midpoints
mid-exp, 18.482 18.549 18.586 19.319 19.375 19.419 19.561} 19.586 21).397 20.532 21.368
mid-exp. 13.372 13.324 13.318 13.360 13.349 13.340 13,343 13.319 13.324 13.353 13.331
+- (I.1)12 ± 0.001 ± 1).(~)2
13.334 + 0.0~2 13.346 11.0~ 13.350 0.1)17 13.346 + 0.007 13,336 , o.o11 13.335 o.ois 13.336 + O.(X)6 13.340 0.0u) 13.342 ' 0.010 13.342 , o.o11
21.436 21.532 21.591 22.407 22.489 22.578 23,419 23.470 23.561 23.601
+ O.(X)6
+- 0.003 ± 0.0(H ± O.O~H ± 0.009 ± 0.012 ± 0.0~k5 +-- 0.020
vided into 21 sets o f m e a s u r e m e n t s , with the images in any one set taken in s e q u e n c e o v e r a short time period, usually 10-15 min, but o c c a s i o n a l l y as long as one hour. For each o f these 21 sets, the standard deviation o f the three or four o b s e r v a t i o n s from the mean for that set was calculated. T h e s e deviations range from 0.001 to 0.020 mag, with a mean o f 0.009 mag and a standard deviation o f 0.005. The 21 mean values are 13.28
I
I
I
I
I
13.30
13.32
1,,3.34
13.36
I
13.38
~
o
short-term
•
daily a v e r a g e s
a . . . . ges
1,3.40
13.42 18
I
I
I
I
i
19
20
21
22
23
24
UT of m i d - e x p o s u r e , June 1 9 8 7 FIG. 1. L i g h t c u r v e o f T r i t o n at 8900 ,~. E a c h o f t h e o p e n c i r c l e s r e p r e s e n t s the a v e r a g e o f s e v e r a l ( u s u a l l y t h r e e o r f o u r ) c l o s e l y s p a c e d i n d i v i d u a l m e a s u r e m e n t s . T h e filled c i r c l e s c o n n e c t e d b y lines represent nightly averages.
PHOTOMETRY OF TRITON T A B L E 11I DAILY AVERAGES OF TmTON'S RELATIVE MAGNITUDE AT 8900 ,~ 1987 Date June June June June June June
18.53 19.44 20.46 21.48 22.51 23.50
Orbital phase 0.245 0.399 0.573 0.746 0.922 1.090
Mean magnitude 13.340 13.344 13.339 13.340 13.338 13.339
± + ± ± ± ±
0.009 0.004 0.007 0.005 0.004 0.002
Note. The second column shows Triton's orbital phase as the fraction of one orbital period from greatest eastern elongation. Greatest eastern elongation occurred at 1987 June 17.0917 UT.
of the individual observations for each night. The mean of the six daily mean magnitudes is 13.340 with a standard deviation of the distribution of only 0.004 mag. No statistically significant variation is evident here. T a k e n at face value, these data seem to indicate that any variation in magnitude with orbital position is less than 0.01 mag. Because of the slightly larger variation seen within the m e a s u r e m e n t s of any single night, we claim only that the variation is less than 0.02 mag. The leading face of Triton was toward Earth midway between our observations of June 19 and June 20, and the face of Triton which faces Neptune was toward Earth during our observations of June 18. P H O T O E L E C T R I C P H O T O M E T R Y OF TRITON
The V magnitude for Triton was obtained with the 2.24-m telescope and a single channel photoelectric p h o t o m e t e r equipped with an RCA C31034A G a A s photomultiplier tube. T w o different techniques were employed to perform background subtraction, neither of which is perfect. H o w e v e r , given N e p t u n e ' s p l a c e m e n t near the galactic center, we feel that contamination by field stars below the limit of visibility is probably the limiting factor in the a c c u r a c y of our p h o t o m e t r y . F o r the first technique we utilized a 6.7-arcsec focal plane diaphragm
19
centered on Triton, at a time when its offset from the center of Neptune was 5 arcsec east and 10 arcsec north. Background measurements were made on opposite sides of Triton at approximately the same radial distance from Neptune. For the first pair of readings, the background offsets from Triton were 5 arcsec west and 2 arcsec north, followed by 5 arcsec east and 2 arcsec south. These background readings differed by +-15%, with the higher reading being about one-quarter the signal from Triton. In an attempt to achieve a better balance between the background readings, a second pair of m e a s u r e m e n t s were made using the same e a s t - w e s t offset but a n o r t h - s o u t h offset of 3 arcsec. For the second pair, the b a c k g r o u n d readings differed by only -+ 1.5%. The mean of the two background m e a s u r e m e n t s was assumed to represent the background at the position centered on Triton. This assumption would be valid if the background varied linearly from one side of Triton to the other, but in reality, the sky background around Neptune is not azimuthally symmetric due to the diffraction pattern produced by the secondary mirror support, making the assumption of linearity only an approximation. For the second technique, two focal plane diaphragms of different sizes (6.7 and 9.6 arcsec) were utilized, both centered on Triton. The difference between the two signal levels is due primarily to the difference in sky area covered by the diaphragms, but the signal from Triton is also a weak function of the diaphragm size. Both effects were calibrated by examining one of the bright standard stars and a blank sky field through both diaphragms. Algebraically, /'/small = nobject +
nbackground -1- ndark
nbig ~ O/F/object "~- flF/background d- F/dark
in which n refers to the photon count rate, and small and big refer to the two diaphragm sizes. The calibration measurements yielded O/= 1.0147 _+ 0.0013 and fl = 2.167 --- 0.034. Dark m e a s u r e m e n t s were
20
LARK ET AL. T A B L E IV V PHOTOMETRY OF TRITON, AUGUST [3, [987
UT
11):23 1t):27 10:31 10:33 Mean Note. ~
V
Background subtraction technique
13.504+ 0.015 13.483± 0.005 13.539± 0.013 13.517-+ 0.018 13.508-+ I).1)23
Opposite sides Opposite sides Concentric diaphragms Concentric diaphragms
1735,,5 = 29.504 AU, r = 30.233 AU.
made only at the beginning 1.7 sec ~) and end (3.6 sec - ~) o f the night. F o r the reduction o f the Triton data, we a d o p t e d nd~,,k -2.7 +-- 1.0 sec ~. Given the signal level from Triton o f about 3600 sec J , the uncertainty in the dark c o u n t rate is far f r o m being the limiting f a c t o r in the a c c u r a c y o f the Triton p h o t o m e t r y . N o t e that this t e c h n i q u e implicitly a s s u m e s that the m e a n brightness c o n t a i n e d within the a n n u l u s interior to the 9.6-arcsec d i a p h r a g m but e x t e r i o r to the 6.7-arcsec d i a p h r a g m also represents the m e a n o f the b a c k g r o u n d brightness interior to the 6.7-arcsec d i a p h r a g m . T h e actual sitttation p r o b a b l y r e p r e s e n t s only a close app r o x i m a t i o n to this ideal one. For the reduction to J o h n s o n V, the atmospheric extinction and zero-point quantities were determined from the leastsquares fit to 12 m e a s u r e m e n t s o f two standard stars (Aquila 07 and 12, taken from T e d e s c o et al. (1982)) spanning an airmass range o f 1.35 to 2.38. The resulting extinction is kv - 0.1 122 -+ 0.0014 mag airmass ~, which is quite nominal for M a u n a Kea. The formal uncertainty in the zero point is only 0.0006 mag, which indicates g o o d internal c o n s i s t e n c y b e t w e e n the two standard stars, but the a c c u r a c y o f those stars" magnitudes relative to the J o h n s o n U B V s y s t e m is actually the limiting f a c t o r here. For T r i t o n ' s color term correction, we a s s u m e d B - V = 0.74 ( F r a n z 1981): we did not attempt any o f o u r o w n B obserwltions. The resulting J o h n s o n V magnitudes for
Triton are s h o w n in Table IV. For the final a d o p t e d magnitude o f Triton, we first took a weighted m e a n o f the two magnitudes derived f r o m each technique and then c o m puted the equal-weight mean o f these two results. The stated formal uncertainty is p r o b a b l y optimistic, given the u n k n o w n contribution from field stars, but the limiting magnitude for these o b s e r v a t i o n s is such that the error due to invisible field stars should be less than 0.06 mag. DISCUSSION Lightcurve
O b s e r v a t i o n s of Triton prior to those o f F r a n z (1981) were intended primarily to determine the absolute brightness of the satellite, and it is possible that systematic errors in the data were wrongly ascribed to the p r e s e n c e o f a real lightcurve. Harris ( 1961 ) reported his results " w i t h r e s e r v a t i o n " and Degewij et al, (1980) noted that their data gave " m a r g i n a l e v i d e n c e " for orbital variation. F r a n z (1981) o b s e r v e d at several points in the orbit at one apparition and derived his result from a data set that was m o r e c o m p a c t in time and therefore better suited to a determination o f the rotational lightcurve. T h e data reported here were obtained in an even more c o m p a c t time frame, uniformly c o v e r i n g a single orbit. As noted a b o v e , these data show that any possible variation in the lightcurve is less than 0.02 mag at 8900 A. This wavelength was c h o s e n b e c a u s e it c o r r e s p o n d s to a weak absorption o f m e t h a n e (gas a n d / o r ice) in the spectrum o f Triton. If the m e t h a n e on Triton consists of a surface deposit of ice, an uneven distribution on that surface might be e x p e c t e d to result in a rotational lightcurve with m a x i m u m amplitude at the c h o s e n wavelength. If, h o w e v e r , the m e t h a n e on Triton in 1987 was p r e d o m i n a n t l y in the satellite's a t m o s p h e r e , the distribution should be more or less uniform around the b o d y and the rotational c o n t r a s t resulting from the m e t h a n e itself should disappear. Fur-
PHOTOMETRY OF TRITON thermore, if the methane atmosphere of Triton was sufficiently dense in 1987 to render the solid surface invisible, any lightcurve resulting from surface variegation would likewise disappear. Cruikshank et al. (1988) have given some evidence for a decrease in the near-infrared methane band strength (2.3 /zm) between 1980 and 1986, but there is no corresponding information on the evolution of the 8900-A band. The only measurement of the 8900-A methane band on Triton is the very weak detection by Apt et al. (1983). The state of methane on Triton is not known. While its presence has been established from both infrared (e.g., Cruikshank and Apt 1984) and photovisual region (Apt et al. 1983) spectroscopy, there are major ambiguities as to the relative contributions of gas and ice or frost to the spectral signature. Cruikshank et al. (1988) raise the possibility that the infrared spectral signature of methane on Triton is most consistent with that of methane dissolved in liquid nitrogen. In any event, the fact that Triton experiences seasonal extremes and is presently approaching "maximum summer" suggests that the volatile inventory of the satellite may be mobile on the time scale of the interval covered by the observations referenced here, and that major changes in the state and distribution of methane (and perhaps nitrogen) might affect both the spectroscopic and photometric properties (Trafton 1984). Realizing that our data did not confirm the lightcurve measured by Franz (1981), we undertook the photoelectric measurement of Triton described above to examine the hypothesis that changes in the volatile inventory may have altered the surface and atmosphere in such as way that not only was the lightcurve quenched between 1977 and 1987, but that the brightness might have changed as well. Absolute Brightness
The photometric standards used by Harris (1961) are not given in his paper. De-
21
gewij et al. (1980) give details of their comparison stars, noting that the brightnesses are uncertain by about 0.05 mag. Franz (1981) made photometric scans across Neptune and Triton and computed the brightness of the satellite relative to the planet. He then used Harris' (1961) V magnitude of Neptune to calculate the brightness of Triton. This technique is satisfactory in terms of the short-term brightness of Neptune, which appears to be stable against diurnal variations caused by uneven distribution of clouds in the planet's atmosphere (large diurnal variations do appear in the infrared, e.g., at 8900 A). In the longer term, however, Neptune is observed to change in brightness in synchronism with the solar activity cycle (Lockwood and Thompson 1986) in narrower Stromgren filters near the broadband V wavelength. Therefore, some correction of unknown amount may be needed to compare rigorously the V mag of Triton obtained by Franz (1981) with that reported in this paper. Despite that correction factor, the V(I, a) reported here falls within the uncertainties of Franz' (1981) value obtained a decade earlier. O. G. Franz (private communication, 1987) estimates that his V(I, ~) of Triton is uncertain by +-0.05 mag, taking into account the random errors of measurement and the expected uncertainty in the brightness of Neptune from Harris' data. SUMMARY In 1987, Triton did not vary in brightness at 8900 ,~ over its 6-day orbital cycle by as much as 0.02 mag. Furthermore, its V magnitude was consistent with the most reliable previous measurements. Thus the apparent lightcurve of 0.06 mag observed by Franz (1981) in 1977 seems to have been quenched by 1987, but any change in the overall brightness of the satellite was below our detection limit. It is also possible, although it seems unlikely, that the shorter wavelengths (Johnson B and V filters) in which Franz observed are more sensitive to the
22
LARK
rotational lightcurve than the methaneband wavelength that we used. ACKNOWLEDGMENTS This research of the Planetary A s t r o n o m y group of the Institute for A s t r o n o m y was supported by N A S A Grant N G L 12-001-057. C C D research at the IFA is supported in part by N S F Grants AST-8615631 and AST-8514575. N . L . L . gratefully acknowledges the hospitality and support of the IFA. We thank Otto Franz for a discussion of his 1977 photometry. J. Elliot, J. Harrington, and O. K u h n assisted with the observations on June 21-23.
REFERENCES ANDERSSON, L. E. 1974. A Photometric Study o f Pluto and Satellites o f the Outer Planets. Ph.D. dissertation, Indiana University. APT, J., N. P. CARLETON, AND C. D. MACKAY 1983. Methane on Triton and Pluto: N e w C C D spectra. Astrophys. J. 270, 342-350. CRUIKSHANK, D. P., AND J. APT 1984. Methane on Triton: Physical state and distribution, h'arns 58, 306-31 I. CRUIKSHANK, D. P., R. H. BROWN, A. T. TOKUNAGA, R. G. SMITH, AND J. R. PISC1TELLI 1988. Volatiles on Triton: The infrared spectra evidence, 2.0-2.5 p,m. h'arus 74, 413-423. DEGEWIJ, J., L. E. ANDERSSON, AND B. ZELLNER
ET AL. 1980. Photometric properties of outer planetary satellites. Icarus 44, 520-540. FRANZ, 0 . G. 1981. UBV Photometry of Triton. h'arus 45, 602-606. GOGUEN. J., H. B. HAMMEL, AND R. H. BROWN 1988. Photometry of Titania, Oberon and Triton. Icarus, in press. HAMMEL, H. B. 1988. The Atmosphere o f Neptune Studied with CCD Imaging at Methane-Band and Continuum Wavelengths. Doctoral dissertation, University of Hawaii. HARRIS, D. L. 1961. Photometry and colorimetry of planets and satellites. In Planets and Satellites (G. P. Kuiper and B. M. Middlehurst, Eds.), pp. 272342. Univ. Chicago Press, C h i c a g o / L o n d o n . LANDOLT, A. 1983. UBVR! photometric standards around the celestial equator. Astron. J. 88, 439-460. LOCKWOOD, G. W., AND D. T. THOMPSON 1986. L o n g - t e r m brightness variations of N e p t u n e and the solar cycle modulation of its albedo. Science 234, 1543-1545. OKE, J. B., AND J. E. GUNN 1983. Secondary standards for absolute spectrophotometry. Astrophys. J. 266, 713-717. P~ALE, S. J. 1977. Rotational histories of the natural satellites. In Planetary Satellites (J. A. Burns, Ed.), pp. 87-112. Univ. of Arizona Press, Tucson. TFDESCO, E. F., D. J. THOLEN, AND B. ZELLNER 1982. The eight-color asteroid survey: Standard stars. Astron. J. 87, 1585-1592. TRAFTON, L. 1984. Large seasonal variations in Trit o n ' s atmosphere. Icarus 58, 312-324.
The Brightness and Lightcurve of Triton in 1987 NElL L. LARK Physics Department, University of the Pacific, Stockton, Califi~rnia 95211 AND
H. B. HAMMEL, l DALE P. CRUIKSHANK, 2 DAVID J. THOLEN, AND MICHAEL A. RIGLER Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822 Received April 26, 1988; revised August 12, 1988
We report C C D and photoelectric photometry of Triton obtained during the apparition of 1987. At 8900 .~, a wavelength which might be expected to show a strong contrast because of methane absorption on Triton, no rotational lightcurve is seen at a level of < 0 . 0 2 mag. This result is in contrast to measurements of the V lightcurve a decade earlier which indicated a variation of 0.06 mag, and other data suggesting a variation of as much as 0.25 mag. The V magnitude of Triton measured photoelectrically in 1987 is reasonably consistent with the most reliable earlier measurements and shows no convincing evidence for long-term change. We find V(1, a) = - 1.244 ___ 0.023. © 1989AcademicPress, Inc.
INTRODUCTION
The brightness of Triton is difficult to measure by conventional photoelectric photometric techniques because of its proximity to Neptune and its current location in the Milky Way. Imaging detectors such as CCDs are well suited to this task because they permit rigorous subtraction of the background light. In this paper we report CCD photometry of Triton obtained over a full cycle of its orbit in June 1987. Triton orbits Neptune with a period of 5.877 days, very likely in locked synchronism, such that the same hemisphere faces Neptune at all times (Peale 1977). The orbit is retrograde, inclined 159° to Neptune's equatorial plane, and is very nearly circular (eccentricity <0.01). The apparent angular t Present affiliation: Jet Propulsion Laboratory, MS 169-237, 4800 Oak Grove Dr., Pasadena, CA 91109. 2 Present affiliation: N A S A Ames Research Center, MS 245-6, Moffett Field, CA 94035.
separation between Triton and Neptune ranges between 11 and 17 arcsec. Early photoelectric photometry of Triton was reported by Harris (1961) from measurements in standard U B V filters on five occasions in 1950, 1951, 1953, and 1956. Harris noted with reservation that there was some indication that the satellite's leading face (the hemisphere facing the direction of the orbital motion) was brighter than the trailing face by about 0.25 mag in the V filter. Subsequently Andersson (1974) and Degewij et al. (1980) reported photoelectric photometric observations of Triton in U B V filters in 1972 and 1977. The 1977 data of Degewij et al. gave marginal evidence for a leading/trailing asymmetry, amounting to 0.1.-0.2 mag, but in the opposite sense as that suspected by Harris. Franz (1981) also observed in 1977 with a photoelectric area-scanning photometer that permitted rigorous subtraction of the background scattered light from Neptune. In addition to obtaining the brightness of 15 0019-1035/89 $3.00 Copyright © 1989by Academic Press, Inc. All rights of reproduction in any form reserved.
16
LARK ET AL. TABI,E I SUMMARY
OF PHOTOMETRIC
MEASUREMENTS
OF TRITON [)ate
V(1. a)
AV
U-B
B-V
Relercncc
195(t-1956 1972
1.16 1.28
+ ? + 0.11
0.25
(I.4(I (I.25
0.77 I).77
Harris (1961~ Andersson(19741
1977 1977
1.12 1.22
+ 0.12 ~ 0.05
ILl-0.2 0.06
0.30 0.~0
0.70 I).74
I)egewij (1980) t.Tanz (1981~
1987
1.244 * 11.tl23
,0.02"
---
This paper
ture or estimated by us. The quantity V(i, a) is the Johnson V magnitude referenced to 1 A U from the Sun and from the Earth with no solar phase function correction applied. If in fact/3 = 0, the data are directly comparable with one another. Table i also gives the results of the observations reported in this paper, details of which are in the following sections.
" This is ~,891R)A. not AV.
CCD PHOTOMETRY OF TRITON Triton in U B V filters, measured relative to Neptune, Franz found -+0.03 mag variation in V synchronous with the revolution, and in the sense of the leading face being brighter than the trailing. Triton is always observed at very small solar phase angles, well within the range in which airless asteroids, planets, and satellites with regoliths show a very strong surge in brightness at opposition. To c o m p a r e the brightness of Triton measured in different years and at different solar phase angles, and to establish a rotational lightcurve, it is necessary first to apply a solar phase function to the data. The phase function for Triton has been measured by Goguen et al. (1988), who found 13 = 0.022 -+ 0.035 mag deg-1; a typical value for an airless asteroid or planetary satellite is about 0. i mag deg o v e r phase angles of 0.0 to 2.0 deg. The Triton data are also consistent with /3 = 0.00, as evidenced by the large error bar on the determination. The small measured value of/3 for Triton is consistent with the presence of an a t m o s p h e r e or a surface of some material other than a particulate regolith (e.g., Cruikshank et al. 1988). Harris (1961) did not measure or correct for the opposition effect in his data, nor did Franz (1981), whose data were taken in the range a = 1.8-1.9 deg. Degewij et al. (1980) reported sufficient details of their data so that comparison can be made with the other data sets with an a s s u m e d / 3 = 0. Table I summarizes the photometric results of the investigators referenced above, using error values from the litera-
We used the University of Hawaii's 2.24m telescope on Mauna K e a to take CCD images including both Neptune and Triton on each frame. The field of the 800 × 800pixel Texas Instrument CCD, at the i l l 0 Cassegrain focus, was nearly 2 min of arc across, with an image scale of 0.140 arcsec pixel ~. We recorded images on 9 nights, June 16-24, 1987, but obtained the necessary combination of good observing conditions and suitable m e a s u r e m e n t s and calibrations only on June 18-23, giving uniform coverage around one entire orbit. During these observations the solar phase angle decreased from 0.34 to 0.18 deg. The observations reported here were taken through a filter centered at 8900 ( F W H M = 200 ,~). This bandpass coincides with a strong absorption feature in the spectrum of methane. Since Neptune absorbs strongly at this wavelength, use of this filter reduced the brightness of Neptune in comparison with Triton by more than a factor of 10. Our images of Neptune show the strongest contrast and disk-integrated photometry of Neptune shows the greatest temporal variation at this wavelength (Hammel 1988). Weak absorption at this wavelength due to methane on Triton has been observed (Apt et al. 1983). We have not analyzed our partial sets of observations at 6190 ,~ Ca w e a k e r methane absorption band) and at 6340 and 8260 ,~ (nearby continuum wavelength regions) since these Triton images are about 10 times fainter. The C C D exposures were limited by the brightness of Neptune.
PHOTOMETRY OF TRITON A typical night's m e a s u r e m e n t s consisted of several sets of three or four images including both Neptune and Triton. The exposure times varied because of the changing brightness of Neptune; most were between 100 and 180 sec, with a few as short as 50 sec. These image sets were spread o v e r about 5 hr, and were interspersed between observations of the standard stars SAO 083543 (= BD+26°2606) (Oke and G u n n 1983)and H D 170493 (= B D - l ° 3 5 0 0 ) (Landolt 1983). The standard stars were used for p h o t o m e t r i c calibration and atmospheric extinction correction. In addition, internal checks on the p h o t o m e t r y were made by measuring s o m e of the stars which were visible near the east edge of the field late on one night and near the west edge of the field early on the following night. (Neptune was in retrograde motion, very near its June 28 opposition date.) T h e s e internal checks increased our confidence in the validity of the results, but b e c a u s e of the difficulties in reproducing p h o t o m e t r y close to the edge of the CCD, this technique did not i m p r o v e the overall accuracy. Processing and analysis of the C C D images required that each frame be given individual attention. The fiat-fielding process did not completely remove all the nonuniformities and flaws in the C C D images. Triton was near the galactic plane and consequently the field was very crowded, with about 300 stars visible in a typical image. It was often difficult to find suitable blank sky for background subtraction, and often there were stars too near Triton and Neptune to be avoided. The p h o t o m e t r y was done using areas of about 8 sec of arc square (55 x 55 pixels for the stars and 57 x 57 pixels for Triton due to its finite angular size). T h e s e areas included 97% of the total flux from the object of interest. When possible, four different background areas were measured around each star or Triton image, and the standard deviation of the mean of the four was taken as the uncertainty in the sky background subtraction. This uncertainty was always
17
considerably larger than the statistical uncertainty due to the limited n u m b e r of counts (data numbers) from the Triton image. The sky background was typically about 35% and light from Neptune was less than 2% of the total flux in the photometric area centered on Triton. When nearby field stars or CCD flaws interfered, the photometry was done in more than one way, and the difference between the results was taken as a measure of the uncertainty. Photometry was done for individual interfering stars up to 5 mag fainter than Triton. The photometric results are expressed as relative magnitudes of Triton calculated from the expression mag = 22 - 2.5 * log(counts/sec), where the 22 is an arbitrary constant chosen to give relative magnitudes in rough accord with standard V photometric values. The atmospheric extinction coefficient was 0.0269 -4- 0.0010 mag per unit air mass through this filter on 21 June 1987 and 0.0227 ± 0.0015 for the other five dates. We determined the relative magnitude of the photometrically calibrated standard star SAO 083543 as 9.733 at zero air mass through this filter; Oke and Gunn (1983) provide a flux curve for this star. Absolute calibration of similar p h o t o m e t r y of Neptune is discussed in detail by Hammel (1988). No systematic variation is evident in the 69 individual magnitude measurements. The simple mean of the m e a s u r e m e n t s is 13.340 with the standard deviation of the distribution being 0.016 mag. The uncertainties of the individual observations are typically between 0.004 and 0,016 mag, with a mean of 0.010 mag. The good correspondence between individual uncertainty estimates and the observed scatter in the results is one indication that there is no strong time variation in Triton's magnitude. It is also an indication that the major sources of error have been identified. As a further check for any systematic variation, the 69 observations were subdi-
18
LARK ET AL. TABLE II
given in Table !I and are plotted in Fig. 1.
AVERAGESOF T H E 21 SETS OF SEQUENTIAL MEASUREMENTSOF TRITON'SREI.ATIVE MAGNITUDEAT 8900 /~ . . . . . . . . . . . . UT date Mean Std. UT date Mean Sld, dev. of mean mag dev. of mean mag
Since the points in Fig. 1 seem to give some hint o f regular variation, a search for periodic variation on time scales o f a few hours was made. N o statistically significant periodic variation was found. As a n o t h e r c h e c k for possible systematic effects, the means o f these 21 sets o f measurements were plotted on a time scale folded at 17 hr, the period o f variation in the disk-integrated magnitude o f N e p t u n e measured from these same images ( H a m m e l 1988). Although the image o f N e p t u n e was only 12 to 15 sec o f arc a w a y f r o m the image o f Triton, and N e p t u n e ' s magnitude varied by 0.6 mag f r o m 2.1 to 2,7 mag brighter than Triton, no significant variation o f Triton is evident. For the clearest and m o s t sensitive search for variation in T r i t o n ' s magnitude associated with its 5.9-day orbital period, the individual o b s e r v a t i o n s for each night were averaged. T h e s e nightly mean magnitudes and the standard deviations o f the m e a n s are s h o w n in Table III and plotted in Fig. !. The decimal dates are the means o f the equally weighted times o f the midpoints
mid-exp, 18.482 18.549 18.586 19.319 19.375 19.419 19.561} 19.586 21).397 20.532 21.368
mid-exp. 13.372 13.324 13.318 13.360 13.349 13.340 13,343 13.319 13.324 13.353 13.331
+- (I.1)12 ± 0.001 ± 1).(~)2
13.334 + 0.0~2 13.346 11.0~ 13.350 0.1)17 13.346 + 0.007 13,336 , o.o11 13.335 o.ois 13.336 + O.(X)6 13.340 0.0u) 13.342 ' 0.010 13.342 , o.o11
21.436 21.532 21.591 22.407 22.489 22.578 23,419 23.470 23.561 23.601
+ O.(X)6
+- 0.003 ± 0.0(H ± O.O~H ± 0.009 ± 0.012 ± 0.0~k5 +-- 0.020
vided into 21 sets o f m e a s u r e m e n t s , with the images in any one set taken in s e q u e n c e o v e r a short time period, usually 10-15 min, but o c c a s i o n a l l y as long as one hour. For each o f these 21 sets, the standard deviation o f the three or four o b s e r v a t i o n s from the mean for that set was calculated. T h e s e deviations range from 0.001 to 0.020 mag, with a mean o f 0.009 mag and a standard deviation o f 0.005. The 21 mean values are 13.28
I
I
I
I
I
13.30
13.32
1,,3.34
13.36
I
13.38
~
o
short-term
•
daily a v e r a g e s
a . . . . ges
1,3.40
13.42 18
I
I
I
I
i
19
20
21
22
23
24
UT of m i d - e x p o s u r e , June 1 9 8 7 FIG. 1. L i g h t c u r v e o f T r i t o n at 8900 ,~. E a c h o f t h e o p e n c i r c l e s r e p r e s e n t s the a v e r a g e o f s e v e r a l ( u s u a l l y t h r e e o r f o u r ) c l o s e l y s p a c e d i n d i v i d u a l m e a s u r e m e n t s . T h e filled c i r c l e s c o n n e c t e d b y lines represent nightly averages.
PHOTOMETRY OF TRITON T A B L E 11I DAILY AVERAGES OF TmTON'S RELATIVE MAGNITUDE AT 8900 ,~ 1987 Date June June June June June June
18.53 19.44 20.46 21.48 22.51 23.50
Orbital phase 0.245 0.399 0.573 0.746 0.922 1.090
Mean magnitude 13.340 13.344 13.339 13.340 13.338 13.339
± + ± ± ± ±
0.009 0.004 0.007 0.005 0.004 0.002
Note. The second column shows Triton's orbital phase as the fraction of one orbital period from greatest eastern elongation. Greatest eastern elongation occurred at 1987 June 17.0917 UT.
of the individual observations for each night. The mean of the six daily mean magnitudes is 13.340 with a standard deviation of the distribution of only 0.004 mag. No statistically significant variation is evident here. T a k e n at face value, these data seem to indicate that any variation in magnitude with orbital position is less than 0.01 mag. Because of the slightly larger variation seen within the m e a s u r e m e n t s of any single night, we claim only that the variation is less than 0.02 mag. The leading face of Triton was toward Earth midway between our observations of June 19 and June 20, and the face of Triton which faces Neptune was toward Earth during our observations of June 18. P H O T O E L E C T R I C P H O T O M E T R Y OF TRITON
The V magnitude for Triton was obtained with the 2.24-m telescope and a single channel photoelectric p h o t o m e t e r equipped with an RCA C31034A G a A s photomultiplier tube. T w o different techniques were employed to perform background subtraction, neither of which is perfect. H o w e v e r , given N e p t u n e ' s p l a c e m e n t near the galactic center, we feel that contamination by field stars below the limit of visibility is probably the limiting factor in the a c c u r a c y of our p h o t o m e t r y . F o r the first technique we utilized a 6.7-arcsec focal plane diaphragm
19
centered on Triton, at a time when its offset from the center of Neptune was 5 arcsec east and 10 arcsec north. Background measurements were made on opposite sides of Triton at approximately the same radial distance from Neptune. For the first pair of readings, the background offsets from Triton were 5 arcsec west and 2 arcsec north, followed by 5 arcsec east and 2 arcsec south. These background readings differed by +-15%, with the higher reading being about one-quarter the signal from Triton. In an attempt to achieve a better balance between the background readings, a second pair of m e a s u r e m e n t s were made using the same e a s t - w e s t offset but a n o r t h - s o u t h offset of 3 arcsec. For the second pair, the b a c k g r o u n d readings differed by only -+ 1.5%. The mean of the two background m e a s u r e m e n t s was assumed to represent the background at the position centered on Triton. This assumption would be valid if the background varied linearly from one side of Triton to the other, but in reality, the sky background around Neptune is not azimuthally symmetric due to the diffraction pattern produced by the secondary mirror support, making the assumption of linearity only an approximation. For the second technique, two focal plane diaphragms of different sizes (6.7 and 9.6 arcsec) were utilized, both centered on Triton. The difference between the two signal levels is due primarily to the difference in sky area covered by the diaphragms, but the signal from Triton is also a weak function of the diaphragm size. Both effects were calibrated by examining one of the bright standard stars and a blank sky field through both diaphragms. Algebraically, /'/small = nobject +
nbackground -1- ndark
nbig ~ O/F/object "~- flF/background d- F/dark
in which n refers to the photon count rate, and small and big refer to the two diaphragm sizes. The calibration measurements yielded O/= 1.0147 _+ 0.0013 and fl = 2.167 --- 0.034. Dark m e a s u r e m e n t s were
20
LARK ET AL. T A B L E IV V PHOTOMETRY OF TRITON, AUGUST [3, [987
UT
11):23 1t):27 10:31 10:33 Mean Note. ~
V
Background subtraction technique
13.504+ 0.015 13.483± 0.005 13.539± 0.013 13.517-+ 0.018 13.508-+ I).1)23
Opposite sides Opposite sides Concentric diaphragms Concentric diaphragms
1735,,5 = 29.504 AU, r = 30.233 AU.
made only at the beginning 1.7 sec ~) and end (3.6 sec - ~) o f the night. F o r the reduction o f the Triton data, we a d o p t e d nd~,,k -2.7 +-- 1.0 sec ~. Given the signal level from Triton o f about 3600 sec J , the uncertainty in the dark c o u n t rate is far f r o m being the limiting f a c t o r in the a c c u r a c y o f the Triton p h o t o m e t r y . N o t e that this t e c h n i q u e implicitly a s s u m e s that the m e a n brightness c o n t a i n e d within the a n n u l u s interior to the 9.6-arcsec d i a p h r a g m but e x t e r i o r to the 6.7-arcsec d i a p h r a g m also represents the m e a n o f the b a c k g r o u n d brightness interior to the 6.7-arcsec d i a p h r a g m . T h e actual sitttation p r o b a b l y r e p r e s e n t s only a close app r o x i m a t i o n to this ideal one. For the reduction to J o h n s o n V, the atmospheric extinction and zero-point quantities were determined from the leastsquares fit to 12 m e a s u r e m e n t s o f two standard stars (Aquila 07 and 12, taken from T e d e s c o et al. (1982)) spanning an airmass range o f 1.35 to 2.38. The resulting extinction is kv - 0.1 122 -+ 0.0014 mag airmass ~, which is quite nominal for M a u n a Kea. The formal uncertainty in the zero point is only 0.0006 mag, which indicates g o o d internal c o n s i s t e n c y b e t w e e n the two standard stars, but the a c c u r a c y o f those stars" magnitudes relative to the J o h n s o n U B V s y s t e m is actually the limiting f a c t o r here. For T r i t o n ' s color term correction, we a s s u m e d B - V = 0.74 ( F r a n z 1981): we did not attempt any o f o u r o w n B obserwltions. The resulting J o h n s o n V magnitudes for
Triton are s h o w n in Table IV. For the final a d o p t e d magnitude o f Triton, we first took a weighted m e a n o f the two magnitudes derived f r o m each technique and then c o m puted the equal-weight mean o f these two results. The stated formal uncertainty is p r o b a b l y optimistic, given the u n k n o w n contribution from field stars, but the limiting magnitude for these o b s e r v a t i o n s is such that the error due to invisible field stars should be less than 0.06 mag. DISCUSSION Lightcurve
O b s e r v a t i o n s of Triton prior to those o f F r a n z (1981) were intended primarily to determine the absolute brightness of the satellite, and it is possible that systematic errors in the data were wrongly ascribed to the p r e s e n c e o f a real lightcurve. Harris ( 1961 ) reported his results " w i t h r e s e r v a t i o n " and Degewij et al, (1980) noted that their data gave " m a r g i n a l e v i d e n c e " for orbital variation. F r a n z (1981) o b s e r v e d at several points in the orbit at one apparition and derived his result from a data set that was m o r e c o m p a c t in time and therefore better suited to a determination o f the rotational lightcurve. T h e data reported here were obtained in an even more c o m p a c t time frame, uniformly c o v e r i n g a single orbit. As noted a b o v e , these data show that any possible variation in the lightcurve is less than 0.02 mag at 8900 A. This wavelength was c h o s e n b e c a u s e it c o r r e s p o n d s to a weak absorption o f m e t h a n e (gas a n d / o r ice) in the spectrum o f Triton. If the m e t h a n e on Triton consists of a surface deposit of ice, an uneven distribution on that surface might be e x p e c t e d to result in a rotational lightcurve with m a x i m u m amplitude at the c h o s e n wavelength. If, h o w e v e r , the m e t h a n e on Triton in 1987 was p r e d o m i n a n t l y in the satellite's a t m o s p h e r e , the distribution should be more or less uniform around the b o d y and the rotational c o n t r a s t resulting from the m e t h a n e itself should disappear. Fur-
PHOTOMETRY OF TRITON thermore, if the methane atmosphere of Triton was sufficiently dense in 1987 to render the solid surface invisible, any lightcurve resulting from surface variegation would likewise disappear. Cruikshank et al. (1988) have given some evidence for a decrease in the near-infrared methane band strength (2.3 /zm) between 1980 and 1986, but there is no corresponding information on the evolution of the 8900-A band. The only measurement of the 8900-A methane band on Triton is the very weak detection by Apt et al. (1983). The state of methane on Triton is not known. While its presence has been established from both infrared (e.g., Cruikshank and Apt 1984) and photovisual region (Apt et al. 1983) spectroscopy, there are major ambiguities as to the relative contributions of gas and ice or frost to the spectral signature. Cruikshank et al. (1988) raise the possibility that the infrared spectral signature of methane on Triton is most consistent with that of methane dissolved in liquid nitrogen. In any event, the fact that Triton experiences seasonal extremes and is presently approaching "maximum summer" suggests that the volatile inventory of the satellite may be mobile on the time scale of the interval covered by the observations referenced here, and that major changes in the state and distribution of methane (and perhaps nitrogen) might affect both the spectroscopic and photometric properties (Trafton 1984). Realizing that our data did not confirm the lightcurve measured by Franz (1981), we undertook the photoelectric measurement of Triton described above to examine the hypothesis that changes in the volatile inventory may have altered the surface and atmosphere in such as way that not only was the lightcurve quenched between 1977 and 1987, but that the brightness might have changed as well. Absolute Brightness
The photometric standards used by Harris (1961) are not given in his paper. De-
21
gewij et al. (1980) give details of their comparison stars, noting that the brightnesses are uncertain by about 0.05 mag. Franz (1981) made photometric scans across Neptune and Triton and computed the brightness of the satellite relative to the planet. He then used Harris' (1961) V magnitude of Neptune to calculate the brightness of Triton. This technique is satisfactory in terms of the short-term brightness of Neptune, which appears to be stable against diurnal variations caused by uneven distribution of clouds in the planet's atmosphere (large diurnal variations do appear in the infrared, e.g., at 8900 A). In the longer term, however, Neptune is observed to change in brightness in synchronism with the solar activity cycle (Lockwood and Thompson 1986) in narrower Stromgren filters near the broadband V wavelength. Therefore, some correction of unknown amount may be needed to compare rigorously the V mag of Triton obtained by Franz (1981) with that reported in this paper. Despite that correction factor, the V(I, a) reported here falls within the uncertainties of Franz' (1981) value obtained a decade earlier. O. G. Franz (private communication, 1987) estimates that his V(I, ~) of Triton is uncertain by +-0.05 mag, taking into account the random errors of measurement and the expected uncertainty in the brightness of Neptune from Harris' data. SUMMARY In 1987, Triton did not vary in brightness at 8900 ,~ over its 6-day orbital cycle by as much as 0.02 mag. Furthermore, its V magnitude was consistent with the most reliable previous measurements. Thus the apparent lightcurve of 0.06 mag observed by Franz (1981) in 1977 seems to have been quenched by 1987, but any change in the overall brightness of the satellite was below our detection limit. It is also possible, although it seems unlikely, that the shorter wavelengths (Johnson B and V filters) in which Franz observed are more sensitive to the
22
LARK
rotational lightcurve than the methaneband wavelength that we used. ACKNOWLEDGMENTS This research of the Planetary A s t r o n o m y group of the Institute for A s t r o n o m y was supported by N A S A Grant N G L 12-001-057. C C D research at the IFA is supported in part by N S F Grants AST-8615631 and AST-8514575. N . L . L . gratefully acknowledges the hospitality and support of the IFA. We thank Otto Franz for a discussion of his 1977 photometry. J. Elliot, J. Harrington, and O. K u h n assisted with the observations on June 21-23.
REFERENCES ANDERSSON, L. E. 1974. A Photometric Study o f Pluto and Satellites o f the Outer Planets. Ph.D. dissertation, Indiana University. APT, J., N. P. CARLETON, AND C. D. MACKAY 1983. Methane on Triton and Pluto: N e w C C D spectra. Astrophys. J. 270, 342-350. CRUIKSHANK, D. P., AND J. APT 1984. Methane on Triton: Physical state and distribution, h'arns 58, 306-31 I. CRUIKSHANK, D. P., R. H. BROWN, A. T. TOKUNAGA, R. G. SMITH, AND J. R. PISC1TELLI 1988. Volatiles on Triton: The infrared spectra evidence, 2.0-2.5 p,m. h'arus 74, 413-423. DEGEWIJ, J., L. E. ANDERSSON, AND B. ZELLNER
ET AL. 1980. Photometric properties of outer planetary satellites. Icarus 44, 520-540. FRANZ, 0 . G. 1981. UBV Photometry of Triton. h'arus 45, 602-606. GOGUEN. J., H. B. HAMMEL, AND R. H. BROWN 1988. Photometry of Titania, Oberon and Triton. Icarus, in press. HAMMEL, H. B. 1988. The Atmosphere o f Neptune Studied with CCD Imaging at Methane-Band and Continuum Wavelengths. Doctoral dissertation, University of Hawaii. HARRIS, D. L. 1961. Photometry and colorimetry of planets and satellites. In Planets and Satellites (G. P. Kuiper and B. M. Middlehurst, Eds.), pp. 272342. Univ. Chicago Press, C h i c a g o / L o n d o n . LANDOLT, A. 1983. UBVR! photometric standards around the celestial equator. Astron. J. 88, 439-460. LOCKWOOD, G. W., AND D. T. THOMPSON 1986. L o n g - t e r m brightness variations of N e p t u n e and the solar cycle modulation of its albedo. Science 234, 1543-1545. OKE, J. B., AND J. E. GUNN 1983. Secondary standards for absolute spectrophotometry. Astrophys. J. 266, 713-717. P~ALE, S. J. 1977. Rotational histories of the natural satellites. In Planetary Satellites (J. A. Burns, Ed.), pp. 87-112. Univ. of Arizona Press, Tucson. TFDESCO, E. F., D. J. THOLEN, AND B. ZELLNER 1982. The eight-color asteroid survey: Standard stars. Astron. J. 87, 1585-1592. TRAFTON, L. 1984. Large seasonal variations in Trit o n ' s atmosphere. Icarus 58, 312-324.