Photometry of occultation candidate stars

ICARUS 67, 540--542 (1986)

NOTE Photometry of Occultation Candidate Stars II. Uranus 1 9 8 5 - 1 9 9 0 and Saturn 1986-1991 LINDA M. FRENCH,* GUARIONEX MORALES,* STEVEN L. GAISER,* AND JAY A. FROGELt *Department o f Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute o f Technology, Cambridge, Massachusetts 02139; and tKitt Peak National Observatory, National Optical Astronomy Observatories, P.O. Box 26732, Tucson, Arizona 85726 Received M a r c h 27, 1986; revised May 2, 1986 V a n d I m a g n i t u d e s are p r e s e n t e d for 20 stars to be occulted by the Uranian rings during the period 1986-1990 a n d for 15 stars to be occulted by Saturn' s rings during 1986-1991. K m a g n i t u d e s for s e v e n U r a n u s stars are presented. T h e occultation stars are up to one magnitude brighter in the K b a n d p a s s t h a n one would predict on the basis of V - I colors alone. This effect can be explained by typical a m o u n t s o f interstellar reddening. © 1986AcademicPress, Inc.

Introduction. In an earlier paper (L. F r e n c h et al., 1985) we p r e s e n t e d V a n d I m a g n i t u d e s for stars which were occulted by the U r a n i a n rings during 1985 and for four o f the brighest stars to be occulted by S a t u r n ' s rings t h r o u g h 1989. In this work we present photometry of 20 additional U r a n u s stars from the list of Mink and K l e m o l a (1985) a n d 15 Saturn stars from that of Killian and Dalton (1985). K - b a n d (hcentral ~ 2.2 /xm) o b s e r v a t i o n s are preferred for occultation studies of the U r a n i a n rings b e c a u s e of the planet's strong methane absorption at the K b a n d p a s s . Earlier we predicted K m a g n i t u d e s by extrapolating the V - I color index to infrared wavelengths. W e have obtained direct infrared p h o t o m e t r y for several of the U r a n u s stars; in this paper we c o m p a r e the m e a s u r e d ( J - K ) color indices to t h o s e which one would predict from V and I o b s e r v a t i o n s alone. Observations and data reduction. For the V and I o b s e r v a t i o n s , p r o g r a m stars and equatorial U B V R I s t a n d a r d s from Landolt (1983) were o b s e r v e d on 5 nights in June 1985, using the 1.3-m telescope at the M c G r a w - H i l l O b s e r v a t o r y . A C C D detector with a T e x a s I n s t r u m e n t s virtual p h a s e chip was used; the detector properties h a v e been described elsewhere (Meyer and Ricker, 1980; Ricker et al., 1981). Landolt s t a n d a r d s o f widely varying V - I color indices were u s e d to d e t e r m i n e first-order extinction coefficients, t r a n s f o r m a t i o n coefficients, and zero point values. Values o f the coefficients were in good a g r e e m e n t from night to night; m e a n values for each coefficient were u s e d in the final data analysis. T h e adopted ex-

tinction a n d transformation coefficients are given in Table I. T h e results of the visual b a n d p h o t o m e t r y are given in Tables II a n d III. Uncertainties (ltr) have been calculated by propagating the effects of Poisson noise in the star signal, the uncertainty in determining the sky b a c k g r o u n d level, a n d uncertainties in the transformation a n d extinction coefficients and the zero point. During one night d e v o t e d to observing the 4 M a y 1985 occultation by the Uranian rings (R. French et al., 1985), we obtained K m a g n i t u d e s of the stars from the list o f Mink a n d K l e m o l a to be occulted during 1985 and 1986. T h e Cerro Tololo Inter-American Obs e r v a t o r y 4-m telescope with infrared p h o t o m e t e r and InSb detector w a s used. T h e standard infrared photometric s y s t e m is that o f Elias et al. (1982). T h e quoted error bars for the 1986 stars are large b e c a u s e only a few standard stars were o b s e r v e d and conditions were

TABLE I C C D PHOTOMETRIC COEFFICIENTS Extinction," k

tr(k)

Transformation,b e

tr(e)

V 1

0.22 0.13

0.04 0.04

0.07 0.02

0.04 0.04

First-order extinction coefficient, in magnitudes/air mass. b As defined by the equation V = e x (V - I) + (k x air mass) + constant. "

540 0019-1035/86 $3.00 Copyright © 1986by AcademicPress. Inc. All rights of reproduction in any form reserved.

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NOTE

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TABLE II

TABLE IV

SATURN OCCULTATION STARS 1986--1991

K

M A G N I T U D E S O F U R A N U S O C C U L T A T I O N ST A RS

Star ID

Date

V mag

o-(V)

I mag

o-(1)

Star ID

Date

K mag

Gr(K)

S18 $24 $28 $29

26 June 1986 17 August 1986 15 September 1986 22 September 1986

11.66 11.66 15.81 15.24

0.08 0.05 0.09 0.06

10.22 I 1.06 14.35 14.39

0.07 0.04 0.07 0.05

U23

4 May 1985 24 May 1985

9.30 6.48

0.05 0.05

$30 $45

28 January 1987 I 1 July 1987

13.54 12.73

0.11 0.07

11.40 11.62

0.09 0.06

U29

$66 $67 $69 $72 $73 $90

17 February 1989 24 February 1989 10 March 1989 16 April 1989 29 April 1989 24 October 1989

10.51 12.16 12.53 10.63 10.63 13.41

0.05 0.06 0.08 0.05 0.05 0.09

10.95 11.53 11.26 10.89 10.89 11.63

0.04 0.04 0.06 0.04 0.04 0.08

U31 032

Sl02 Sll9

12 March 1990 27 October 1990

11.48 ll.12

0.09 0.08

9.86 9.62

0.08 0.07

(1985).

S127

15 March 1991

12.76

0.07

11.77

0.05

Note. Star ID numbers are those of Killian and Dalton (1985).

not totally photometric throughout the entire night. However, the two 1985 Uranus stars were rechecked at a later date and gave K magnitudes which agree with the previous values to better than 0.1 mag. Thus, we feel that the error bars assigned to the 1986 stars are realistic. In any case, this level of accuracy is adequate for the purpose of identifying occultations which are likely to give high signal-to-noise ratios. We include magnitudes of 1985 and early 1986 candidate stars for the information of those who may have observed the events. Results of the infrared photometry are summarized in Table IV.

TABLE III

U25 U28

26 April 1986 15 May 1986 25 June 1986 2 July 1986 23 July 1986 24 July 1986

030

033

8.5 9.9 7.4 8.9 8.3 11.6

0.2 0.2 0.2 0.2 0.2 0.2

Note. Star numbers are those of Mink and Klemola

Comparison of visual and infrared photometry. If there were no interstellar extinction, one could easily compute K magnitudes for main sequence stars from (V-I) colors. In reality, we are looking toward the general direction of the galactic center, where extinction is extensive and inhomogeneous. Further, we have no knowledge of the spectral or luminosity classes of the stars and hence of their intrinsic color indices. Figure 1 shows the (V-I) and (J-K) colors for the 14 Uranus stars for which these data are available. In addition to the CTIO infrared photometry reported here, some of the infrared magnitudes are from Covault and French (1986). The solid line indicates the expected colors for unreddened main sequence stars (Johnson, 1966); the observations show that a majority of the stars studied here have redder (J-K) c o l o r s - 2.0

i

i

i

I

i

i

i

URANUS OCCULTATION STARS 1986--1990 Star ID

Date

V mag

~r(V)

1 mag

o-(1)

1.5 [

1.oi-

U28 U29 U32 U33

26 April 1986 15 May 1986 23 July 1986 24 July 1986

14.01 14.04 13.80 14.29

0.11 0.09 0.12 0.10

11.76 12.46 I 1.57 12.52

0.10 0.07 0.09 0.08

U37 U39 U42

16 April 1987 29 May 1987 21 June 1987

11.94 13.72 12.31

0.14 0.09 0.08

9.54 12.46 11.16

0.12 0.08 0.06

U43 U45

8 March 1988 27 April 1988

13.06 13.54

0.06 0.07

12.38 12.39

0.05 0.06

U47 U49 U50 U51 U53

6 March 1989 13 March 1989 14 March 1989 21 May 1989 10 June 1989

12.42 12.66 12.52 12.43 13.81

0.08 0.11 0.08 0.06 0.12

11.01 10.64 11.28 ll.60 11.44

0.07 0.09 0.06 0.05 0.10

U57 U68

3 April 1990 12 July 1990

12.61 13.25

0.10 0.07

10.77 12.26

0.08 0.06

Note. Star numbers are those of Mink and Klemola (1985).

-+-

+

0.5

0

0

+ +

-I-

m

"~o

%

K ~

0.5

1.0

1.5

2.0

I 2.5

I 3.0

I 3.5

4.0

(v-0 FIG. I. Color-color plot for Uranus occultation stars 1985-1990. ( V - l ) colors are from this paper; (JK) values are from the present work and Covanlt and French (1986). The line connecting circles represents the position of unreddened Hyades and field stars (Johnson, 1966) of various spectral types.

542

FRENCH ET AL.

they are brighter at the K b a n d - - t h a n one would predict from (V-I) photometry alone. We have estimated the colors one would observe for main sequence stars at various distances, assuming typical values of the ratio of total-to-selective absorption and the amount of differential extinction between the bandpasses (Johnson, 1968; Humphreys and Ney, 1974; Lee, 1970). As noted above, we cannot determine the spectral classes or the extinction parameters uniquely. However, we find that the observed magnitudes can be reproduced with reasonable values of the interstellar extinction parameters. Thus, we consider it unlikely that either set of photometric measurements is in error or that these objects are anything other than normal reddened stars. On the basis of these results, we anticipate that the observed K magnitudes for Uranus occultation stars will be brighter than those which one would predict from their (V-I) colors. The effect can exceed 1 mag for stars of (V-I) ~ 3. Discussion. From the (V-I) photometry alone, the best events for Uranus from 1986 through 1990 are those of 16 April 1987, 6 March and l0 June 1989, and 3 April 1990. The brightest Saturn stars at visible wavelengths are occulted on 17 February and 16 April 1989; we note that on 3 July 1989 the Saturnian rings will occult a 5.4-mag SAO star (see Taylor, 1983; Killian and Dalton, 1985; L. French et al., 1985). ACKNOWLEDGMENTS This work was supported by NASA Grant NSG7526 and NSF Grant AST-8209825. We thank Oscar Rivera for assistance at the telescope at CTIO, and Gabriel Menchaca for the preparation of finder charts. We also gratefully acknowledge the support of the MIT Undergraduate Research Opportunities Program. REFERENCES COVAULT, C. E., AND L. M. FRENCH (1986). JHK magnitudes of occultation candidate stars: Neptune 1986-1990, Uranus 1987-1990, and Pluto 1986. Icarus, in press. ELIAS, J. H., J. A. FROGEL, K. MATTHEWS, AND G. NEUGEBAUER (1982). Infrared Standard Stars. Astron. J. 87, 1029-1034.

FRENCH, L. M., G. MORALES, A. S. DALTON, J. J. KLAVETTER, AND S. R. CONNER (1985). Photometry of occultation candidate stars. I. Uranus 1985 and Saturn 1986-1990. Astron. J. 90, 668-669. FRENCH, R. G., R. L. BARON, J. L. ELLIOT, L. M. FRENCH, J. A. KANGAS, K. J. MEECH, M. E. RESSLER, J. FROGEL, J. JIMENEZ, M. JoY, E. F. ERICKSON, K. MATTHEWS, B. T. SO1FER, G. NEUGEBAUER, AND P. NICHOLSON (1985). The 4 May and 24 May 1985 occultations by the Uranian tings. Bull. Amer. Astron. Soc. 17, 718. HUMPHREYS, R. M., AND E. P. NEY (1974). Visual and infrared observations of late-type supergiants in the southern sky. Astrophys. J. 194, 623-628. JOHNSON, H. L. (1966). Astronomical measurements in the infrared. Annu. Reo. Astron. Astrophys. 4, 193-206. JOHNSON, H. L. (1968). Interstellar extinction. In Stars and Stellar Systems, Vol. 7, Nebulae and Interstellar Matter (L. H. Aller and B. M. Middlehurst, Eds.), pp. 167-216. Univ. Chicago Press, Chicago. KILLIAN, A. M., AND A. S. DALTON (1985). Stellar occultations by Saturn's rings for 1985-1991. Astron. J. 90, 2372-2376. LANDOLT, A. U. (1983). UBVRI photometric standard stars around the celestial equator. Astron. J. 88, 439-460. LEE, T. A. (1970). Photometry of high-luminosity Mtype stars. Astrophys. J. 162, 217-238. MEYER, S. S., AND G. R. R1CKER (1980). A dual charge-coupled device (CCD) astronomical spectrometer and direct imaging camera. I. Optical and detector systems. SPIE J. 264, 38-41. MINK, D. J., AND A. KLEMOLA (1985). Predicted occultations by Uranus, Neptune, and Pluto: 19851990. Astron. J. 90, 1894-1899. RICKER, G. R., M. W. BAUTZ, D. DEWEY, AND S. S. MEYER (1981). Optical studies of X-ray sources with the M A S C O T - - A charge-coupled device (CCD)based astronomical instrument. SPIE J. 290, 190201. TAYLOR, G. (1983). IAU Commission 20 Working Group on Prediction o f Occultations by Satellites and Minor Planets, Bulletin 30.