Signal-to-noise ratios for possible stellar occultations by pluto

ICARUS 66, 556-560 (1986)

Signal-to-Noise Ratios for Possible Stellar Occultations by Pluto A. S. BOSH*, J. L. ELLIOT,*'? S. E. KRUSE,* R. L. BARON,* E. W. DUNHAM,* AND L. M. FRENCH* *Department of Earth, Atmospheric, and Planetary Sciences, and ?Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received June 24, 1985; revised March 5, 1986 Signal-to-noise ratios and magnitudes in the J o h n s o n B V R s y s t e m are presented for nine stars that might be occulted by Pluto during the period 1985-1990. F r o m these calculations of the signalto-noise ratio that could be achieved with a l - m telescope, we find that each star (if occulted) is sufficiently bright to give useful information about a possible a t m o s p h e r e of Pluto. © 1986Academic Press, Inc.

observational strategy if a star is particulady red or blue. Furthermore, their estimated rms error of 0.5 mag is uncomfortably large for deciding whether or not to attempt observations of occultations of the faintest starts on their list. In this paper we present BVR photometry of the occultation candidate stars and calculate the S/N that could be achieved with a 1-m telescope. This objective does not require the ultimate photometric accuracy that is possible with present techniques, but can be attained with photometry that is accurate to 0.1-0.2 magnitudes.

I. I N T R O D U C T I O N

Recently Mink and Klemola (1985) presented a list of ten stars that might be occulted by Pluto during the period 19851990. Successful observation of a stellar occultation by Pluto would tell us whether or not this planet has an atmosphere; if Pluto does have an atmosphere, these observations would allow the determination of its structure (see Elliot, 1979). With two or more occultation chords, we could also obtain the precise diameter of Pluto. Stellar occultation data would be complementary to that obtained from the series of mutual occultations and eclipses between Pluto and Charon that has just begun (Binzel et al., 1985). Of course, these goals can be achieved only if the occultation data have a sufficient signal-to-noise ratio (S/N), which will depend on (1) the photometric conditions at the time of the event, (2) the throughput of the photometer used, (3) the size of the telescope, and (4) the magnitudes and colors of Pluto and the occulted star. Although the magnitudes presented by Mink and Klemola (1985) are helpful in identifying which are likely to be the most promising events, they refer to a single wavelength band and are therefore not useful for determining the color of the star--which may alter one's

II. O B S E R V A T I O N S A N D D A T A R E D U C T I O N

On four nights in April 1985 and two nights in January 1986, nine program stars from the list of Mink and Klemola (1985) and Pluto were observed with the Lowell Observatory's 31-in. and 42-in. telescopes at Anderson Mesa, Arizona. The first star on their list was omitted because the date of its possible occultation had passed with no reported observations. We have retained in this paper our photometry of the second star on their list, however, since the possible observation of its occultation by Pluto has been reported by Brosch and Mendelson (1985a,b). The April 1985 observations were carried

556 0019-1035/86 $3.00 Copyright © 1986by AcademicPress, Inc. All rights of reproduction in any form reserved.

PLUTO OCCULTATION CANDIDATES

557

TABLE I FILTERS AND PHOTOMETRIC COEFFICIENTS Date

Band

Center wavelength (/~)

Bandpass (FWHM, a ,~)

Apr 85

B' V' R' Open B V B V

4300 5500 7000

980 1100 1000

4400 5500 4400 5500

980 890 980 890

11 Jan 86 12 Jan 86

Extinction, k

0.29 0.17 0.16 0.14 0.20 0.13 0.22 0.13

± 0.06 ± 0.03 ± 0.07 ± 0.02 ± 0.01 ± 0.01 -+ 0.01 ± 0.01

Transformation, b

0.212 -+ 0.004 0.103 ± 0.003 0.008___ 0.010 0.072 -0.019 0.087 -0.021

± ± ± ±

0.006 0.006 0.010 0.004

a Full width at half maximum. b As defined by H e n d e n and Kaitchuck (1982).

out on the 31-in. telescope with the SNAPSHOT (a dual-chip CCD photometer; see Dunham et al. (1985) for description). The chip in the direct beam was used with a reducing lens that gave an effective scale of 0.7 arcseconds per pixel. Table I lists wavelengths and bandpasses of the filters, where the primes indicate that the filters do not exactly correspond to the Johnson B, V, and R. The observation of each object consisted of placing its image on a standard region of the CCD and then recording data within a frame (320 rows by 393 columns) surrounding this region. The following sequence of exposures was used: 3 open chip, 3 with each of the three filters, 3 open chip. Consistent results for the two series of open chip observations was used in our later analysis to ensure that atmospheric conditions were the same at the beginning and end of the data recording for a particular object. The exposure time for the standard star was 0.5 sec and that for the program stars and Pluto was 70 sec. For each exposure, the signal from the object of interest was obtained by adding the total number of electrons produced by the CCD within a circle of diameter 30 pixels surrounding the object and then subtracting the background, which was determined by averaging the recorded electrons

within an annulus 5 pixels wide that surrounded the circle. In a few cases a bright, nearby star forced us to use smaller areas. Since our CCD chip is highly uniform within the region used for the present photometry, bias and fiat frame corrections were not necessary. The extinction coefficients used for reducing the data for all nights in April 1985 were determined on the night of 10 April and are given in Table I, along with their errors. Since the maximum airmass difference between any program star and the standard star (or Pluto) was one airmass, uncertainty in the extinction coefficients should not be a major source of error. The transformation coefficients for converting the magnitudes obtained for our filters to the Johnson BVR system were determined as part of another program and are also given in Table I. The B, V, and R magnitudes of the stars observed are given in Table II, where the errors in the V magnitudes are the propagated random errors that were determined from the internal consistency of the individual values for the star and background, along with the formal errors in the extinction coefficients. The B - V and V - R colors should be more reliable than the V magnitudes; perhaps the errors in these will be only a few hundredths of a magnitude. Comparison of our mean V magnitudes

558

BOSH ET AL. T A B L E II OCCULTATION CANDIDATE

Star a

Measurement

MAGNITUDES

V

Random error in V

B- V

V-R

0.05 0,03 0.03 0,17 0,05

1.24 1.20 1.21 0.89 0.64 0.71 0.33 0.49 0.62 0.70 0.72 0,68 0.67 0.49 0.59 0,70 0.67 0.65

0.86

09:27

11.33 11.40 11.43 15.77 14.74

Date

UT 07:44 10:45 11:52

06:25 11:35

14.91

0.09

P5

9 A p t 85 l l Jan 86 12 Jan 86 9Apr85 12Apr 85 12 Jan 86 7 A p r 85

07:23

14.72

P6

9 A p r 85

10:08

15.17

85 07:02 85 06:06 86 11:13 86 11:58 85 06:43 86 11:23 86 12:10 85 07:58 86 11:52 86 12:19

15.29 12,26 12,27 12.32 14.10 14,15 14.08 13,10 13,15 13,13

0,06 0.13 0,08 0.06 0.04 0.03 0.06 0.05 0.03 0.06 0.04 0.04

P2 b

P3 P4

P7 P8

P9

P10

9Apr llApr 11 Jan 12 Jan 11Apr 11 Jan 12 Jan 11 Apt 11 Jan 12 Jan

0,62 0,49 0.29 0.38 0,41 0.49

0.43

0.49

After Mink and Klemola (1985). b Magnitudes include contribution of neighboring star, which is approximately 2 magnitudes fainter than P2 itself.

(photoelectric used when available) with those given by Mink and Klemola (1985) shows their values to be 0m40 fainter than ours, with an rms scatter of 0.m53 about this mean offset. As an independent check on our CCD photometry, we have compared our Pluto magnitudes from April 1985 with those that would be predicted from the standard Pluto light curve of Binzel and Mulholland (1984). The two nights' data for Pluto were reduced in the same manner as were the data for the program stars, and these are listed in Table

III. We calculated the rotational phases (planetocentric) at the times of our observations using a period of 6.3874 days and JD 2,444,240.59 as the zero phase point (Binzel and Mulholland 1984). The light-time correction was approximately 4 hr. The phase coefficient of 0.041 magnitudes per degree determined by Binzel and Mulholland (1984) was also included. Table III gives results of these calculations, where comparison of our magnitudes for Pluto with those of Binzel and Mulholland (1984) shows agreement to well within _0.ml. The January 1986 observations were carfled out on the 42-in. telescope, using the Lowell Observatory's single channel photometer with EMI6256 phototube. Four photometric standard stars from area 100 (Landolt, •983) were used to determine extinction and transformation coefficients for each night. These coefficients are included in Table I. Photometry in B and V was performed on the five brightest occultation candidates: P2, P4, P8, P9, and P10. Results of this photoelectric photometry are included in Table II. Table IV summarizes the circumstances of the Pluto occultations. Columns one through five list star name, date and time of occultation, impact parameter, and region of visibility, as given by Mink and Klemola (1985). The sixth column gives lunar phase, as fraction full. The seventh and eighth columns give the planetocentric rotational phase of Pluto and the maximum duration of an occultation. For direct comparison

T A B L E llI PLUTO MAGNITUDES

Date

JD

Rot'l phase

7Apr85

2,446,162.75

0.93

1 2 A p r 85

2,446,167.79

0.72

Magnitudes

Present work Predicted ~ Present work Predicted a

V

B-V

V-R

13.95 14.02 13.71 13.77

0.88 0.83 0.85 0.83

0.67 -0.67 --

a Calculated from Pluto photometry given by Binzel and Mulholland (1984).

PLUTO OCCULTATION CANDIDATES

559

TABLE IV POSSIBLE Star a

Occultation a

OCCULTATIONS

BY PLUTO

Impact parameter a

Region of visibility a

Lunar phase

Rot'l phase

Max dur. (sec)

M, - Me

V , -- V~

Signal drop

S/N b

Scale ht. error c

Date

UT

P2

1985 19 A u g

18:20

0.25s

N. Africa, Eur.

0.25

0.96

149

-2.25

-2.79

0.93

154

0.01

P3 P4 P5 P6 P7

1986 21 M a r 30 A p r 22 J u n 18 Aug 9 Sep

I1:00 11:42 01:05 20:00 05:05

0,80n 0.22n 0.18s 0.82s fi.59n

E. Pacific C. Pacific N, Atlantic W. Africa, Eur. W. Pacific

0.70 0.55 1.00 0.95 0.35

0.42 0.69 0,91 0.96 0.31

164 128 233 160 113

1.67 0.92 0.63 1.07 1.28

1.87 1.11 0.72 0.97 1.29

0.15 0.27 0.34 0.29 0.23

7 14 19 15 12

0.11 0.05 0.04 0.05 0.08

P8

1988 9 Jun

10:06

0.17s

N. Pacific

0.35

0.38

168

-1.55

-1.60

0.81

83

0.01

P9

1989 14 J u n

01:56

0.25s

N. Atlantic

0.70

0.26 d

171

0.16

0.22

0.45

27

0.03

PI0

1990 10 J a n

00:17

0.86s

India, E. Asia

0.90

0.12

142

-1.08

-0.96

0.71

58

0.01

a b c a

After M i n k and K l e m o l a (1985). F o r l - m telescope. See text for other assumptions. Fractional error in scale height, er(H)/H. W e note that P9 m a y be occulted w h e n C h a r o n is transiting Pluto (Binzel et al., 1985).

with each occultation candidate, we calculated the apparent magnitude of Pluto at the time of each predicted occultation (i.e., the magnitude of Pluto as would actually be seen by the occultation observer) and have tabulated the magnitude difference between Pluto and the occulted star in the ninth column for M, the "open chip" magnitude difference for our CCD, and in the tenth column for V. The spectral response of our chip is given by Dunham et al. (1985). Photoelectric V magnitudes were used when available. The next column gives the fractional drop in the combined light of Pluto and the star (within the V passband) that would occur for a total occultation of the star (again using photoelectric V magnitudes when available). This fraction does not include contributions from background sources, such as dark counts, skylight, or scattered moonlight (which could be significant). The twelfth column of Table IV gives the signalto-noise ratio for detecting the starlight in a 1-sec integration. In calculating the signalto-noise ratio we used the method of Elliot (1977), assuming the observations were

made (i) with a 1-m telescope, (ii) through the passband of a V filter, and (iii) with an overall quantum efficiency of the system of 0.05 (typical of what can be achieved with a photomultiplier). Although these signal-tonoise ratios would apply to ideal conditions, even greater signal-to-noise ratios could be achieved by using a larger telescope, a wavelength passband broader than V, and a system that achieves a quantum efficiency greater than 0.05. Finally, the last column of Table IV gives the expected rms error in the scale height determination for Pluto's atmosphere (if it has one) that could be derived from the data. This estimate was obtained using Eq. BI3 of French et al. (1977), under the assumptions that the scale height of Pluto's atmosphere would be about 75 km and that the occultation occurs normal to the limb. III. CONCLUSIONS From a photometric standpoint, the best potential occultation (after that of P2) would be that of P8. According to the astrometry presented by Mink and Klemola (1985), the occultation of this star should be

560

BOSH ET AL.

visible from Earth, since the impact parameter for this event (the angle subtended by the sum of the radii of the Earth and Pluto at the distance of Pluto) does not exceed 0.37 arcseconds. However, further astrometry will be needed to improve the accuracy of the prediction. Even the faintest stars on the list should give interesting results if their occultations by Pluto can be observed with a 1-m telescope and sensitive photometric equipment. ACKNOWLEDGMENTS We thank A. H. Hoag for telescope time and are grateful to the Lowell Observatory staff, R. L. Millis, and S. E. Levine for their help and encouragment. We also thank G. G. Menchaca and D. M. Kramer for assisting with the January 1986 observations, and gratefully aknowledge the support of the MIT Undergraduate Research Opportunities Program. This work was supported, in part, by NASA Grant NSG-7526 and NSF Grant AST-8209825.

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determination of the phase coefficient. Astron. J. 89, 1759-1761. BINZEL, R. P., D. J. THOLEN, E. F. TEDESCO, B. J. BURATTI, AND R. M. NELSON (1985). The detection of eclipses in the Pluto-Charon system. Science (Washington, D.C.) 288, 1193-1195. BROSCH, N., AND MENDELSON, H. (1985a). Occultation by Pluto on 1985 August 19. IAUC No. 4097. BROSCH, N., AND MENDELSON, H. (1985b). Occultation by Pluto on 1985 August 19. IAUC No. 4117. DUNHAM, E. W., R. L. BARON, J. L. ELLIOT, J. V. VALLERGA, J. P. DOTY, AND G. R. RICKER (1985). A high speed dual-CCD imaging photometer. P.A.S.P. 97, 1196-1204. ELLIOT, J. L. (1977). Signal-to-noise ratios for stellar occultations by the rings of Uranus, 1977-1980. Astron. J. 82, 1036-1038. ELLIOT, J. L. (1979). Stellar occultation studies of the Solar System. Ann. Rev. Astron. Astrophys. 17, 445-475. FRENCH, R. G., ELLIOT, J. L., AND GIERASCH, P. G. (1977). Analysis of stellar occultation data: Effects of photon noise and initial conditions. Icarus 33, 186-202. HENDEN, A. J., AND R. H. KAITCHUCK (1982). Astronomical Photometry. Van Nostrand-Reinhold, New York. LANDOLT, A. U. (1983). UBVRI photometric standard stars around the celestial equator. Astron. J. 88, 439-460. MINK, D. J., AND A. KLEMOLA (1985). Predicted occultations by Uranus, Neptune, and Pluto: 19851990. Astron. J. 90, 1894-1899.