Photoelectic lightcurve and phase relation of the asteroids 505 Cava

tCAmJS 64, 528--530 (1985)

Photoelectric Lightcurve and Phase Relation of the Asteroid 505 Cava J. W. Y O U N G AND A. W. H A R R I S Jet Propulsion Laboratory, Calif)>rnia Institute ~/" Technology, Pasadena, Califi~rnia 91109 Received March 18, 1985: revised September 24. 1985 Photoelectric observations of 505 Cava taken on 8 nights at the Table Mountain Observatory in the fall of 1982 give a period of rotation of 8.1796 _+ 0.0010 hr with an amplitude of 0."125. The absolute magnitude vs solar phase angle (phase relation) was measured over the range of phase angle of 10 to 25 °. The resulting very steep phase relation is suggestive of a dark object (cf. K. L u m m e and E. Bowell. 1981, Astron. J. 86, 1705). This is consistent with the - F C " classification suggested by D. Tholen (1984. Asteroid Taxonomy from Cluster Analysis ~)f"Photometry, PhD dissertation, University of Arizona. Tucson) based on eight color photometry. ~, 1985Academic P, ess. Inc.

Table 1 is a s u m m a r y of the aspect and c o m p a r i s o n star data for each night of observation. The " P A W ' coordinates which are listed are the ecliptic coordinates of the phase angle bisector, the point which bisects the geocentric and heliocentric positions of the asteroid (Harris et al., 1984). The c o m p a r i s o n stars used each night are identified by SAO catalog n u m b e r or by a shortened form of the 1950 coordinates: i.e., 226.3-0247 refers to a star at ~+~gst) -: 2h26.~3, 8199(1 -- --2047 '. The comparison stars used in 1979 and on N o v e m b e r 21, 1982, were not standardized nor were their identifications recorded, thus no values for magnitudes are given. Small corrections were applied to the individual observations each night to r e m o v e the effects of changing phase angle and distances to the Earth and Sun, so that each individual lightcurve represents the variation in absolute magnitude at the tabulated value of phase angle.

INTRODUCTION

The asteroid 505 C a v a was observed by photographic p h o t o m e t r y in March 1975 by Lagerkvist (1978), who suggested a period greater than 0d32. 505 C a v a was also observed photoelectrically by Harris and Young (1980) on one night in 1979 (Fig. I). T h e y suggested a period of 7 < I hr. The T R I A D classification of 505 C a v a is "'U'" (Bowell et al., 1979), based on UBV colors, but more recently, Tholen (1984) assigns it a classification of " ' F C . " This classification denotes an object with a generally " C " type spectrum, but with a w e a k e r than normal absorption feature at +0.4 /xm. Although no direct measure of albedo has been obtained for this asteroid+ other members of the F - C class range are known to be of low albedo (Tholen, 1984). OBSERVATIONS

The 1982 observations were made through a V filter with an S-20 PMT cooled with dry ice, attached to the 61-cm reflector of the Table Mountain O b s e r v a t o r y . The details of the physical observations are discussed by Harris and Young (1983). The 1979 o b s e r v a t i o n s were made unfiltered with the same PMT.

RESULTS

The lightcurve presented in Fig. 2 is a c o m p o s i t e of all 8 nights of observations in 1982. Subcomposites were prepared from each m o n t h ' s observations, which were then c o m b i n e d to produce a single lightcurve. The resulting period of rotation of 528

0019-1035185 $3.00 Copyright ~" 1985by Academic Press, Inc. All rightsof reproduction in any form reserved.

PHOTOMETRY

Am

I

o.Io

0.1

I

O F 505 C A V A

I

I

529

I

i

1

I|

I I •

•

I

4

I°

5

•

6

I

1

7

I

I

8 9 UT l/I/79

I0

I

I II

I

I

12

13

FIG. 1. The 1979 lightcurv e of 505 Cava.

TABLE 1 ASPECT

Data

RA

(1950)

Dec

AND

COMPARISON

START

DATA

PAB

r

z~

c~

Vo(a)

C o m p Star

C omp V

2.137 2.126 2.125 2.123 2.079 2.072 2.068 2.044 2.040

1.182 1.171 1.166 1.162 1.142 I. 162 1.174 1.324 t.366

8.33 10.49 10.14 9.80 11.85 14.32 15.55 23.44 24.70

---9.29 -9.53 --9.94

-226.3-0247 226.3 0247 226.3-0247 200.9 0251 200.9-0251 -110202 151.4+0152

-11.71 11.71 11.71 11.58 I 1.58 -8.83 11.25

Long (1950) Lat 01/01/79 10/12/82 10/13/82 10/14/82 11/12/82 11/18/82 11/21/82 12/14/82 12/19/82

8h 2 2 2 2 1 1 I I

01."13 29.2 28.5 27.8 0 2.7 58.2 56.3 50.7 51.7

+26°18 ' - 0 2 24 - 0 2 28 - 0 2 32 - 0 2 56 02 35 - 0 2 21 +00 36 +01 27

113.°1 30.3 30.4 30.4 32.6 33.2 33.6 37.7 38.9

+04.04 -12.6 12.6 -12.5 -11.2 10.8 -10.5 -08.5 08.0

8.1796 -+ 0.0010 hr does agree within the uncertainties of the previously published values. Note that the 1982 data alone might be satisfied by a period of 4.1 hr, but this value is incompatible with the 1979 lightcurve. Likewise, the 1979 lightcurve by itself is inconclusive. It is only when the two are considered together that an unambiguous result can be inferred. The difficulty in determining the correct period of an asteroid from a single opposition has been emphasized by Zappalb. et al. (1983). I

I

I

Since the amplitude of the lightcurve in 1979 was much less than in 1982, it can be inferred that the pole position of 505 Cava lies more or less in the direction of the 1979 observation position, h = 113 °,/3 = 4 °. This conclusion is further strengthened by the fact that in 1982, when the asteroid was - 9 0 ° away from the 1979 position, the lightcurve was quite unusual (perhaps even quadrupally periodic), but contained only even harmonics of significant amplitude; i.e., the lightcurve can be acceptably composited

I

I

I

I

I

I

@ x

9.3

..A" ~.+

;9.5

"p

O 0 ~x

9.4

bi

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LolI~ ÷

9,

a+

o

c~

x

9F . ° A

9.6--

+ 10112182 o )0/13/82 • 10/14/82 l I 3 4

• 11/12/82 A I I / 18182 • 11/21/82 I I 5 6

x 12/14/82

Psy - 8 ~ 1 7 9 6

o 12119182

I I 7 8 U T I0114182

I 9

I I0

FIG. 2. The 1982 c o m p o s i t e lightcurve of 505 Cava.

I

I I

l 12

530

Y O U N G A N D HARRIS 9.2 9.5 9.4 9,5

~ 9.6

>* 9,7 9.8

v (o °) = 8 . 5 0 Q =-0.024

9,9

I0.0 0°

I 5°

I I I I0" 15* 20* PHASE ANGLE

I \ 2: 5*

FIG. 3. The phase relation of 505 Cava. The three points are best-fit averages for October. N o v e m b e r , and D e c e m b e r 1982 (see text).

The absolute magnitude at maximum light and linear phase coefficient can be computed from V0(0°) and Q (Lumme and Bowell, 1981; Harris and Young, 1983). The values for 505 Cava are V0(l, 01 = 8.84, /3v = 0.045. The magnitude at mean light is 0.m12 fainter in all of the magnitude systems, or V(1, 0) = 8.95. Assuming a nominal albedo of 0.040, the average for dark C - F class asteroids (Tholen, 1984), and using the formula given by Zellner (1979), we estimate a diameter for 505 Cava to be 107 km. ACKNOWLEDGMENT

with a period of one-half the correct rotation period. This characteristic is indicative of an equatorial view (Russell, 1906; Oslro and Connelly, 1984). The absolute V magnitude as a function of solar phase angle at the rotational phase of maximum brightness is plotted in Fig. 3. The three points represent the best fit averaged for each of the three (grouped) observational periods; i.e., October, November, D e c e m b e r 1982, The numerical values are also tabulated in Table I. The curve plotted is the L u m m e / B o w e l l phase relation ( L u m m e and Boweil, 1981) with a value of Q = -0.024, which best fits the data. This procedure is outlined in Harris and Young (1983). No observations were possible during the 1982 opposition at significantly lower phase angles than reported here. This very steep phase relation indicates a low albedo surface ( L u m m e and Bowell, 19811. The absolute magnitude at maximum light and 0 ° angle, V0(0°) is 8.50 -+ 0.05. The data can also be fit to the magnitude function proposed by L u m m e et al. (in preparation), which is being used for reductions of IRAS data (Tedesco, in preparation), and has been suggested for use by the IAU for magnitude predictions of asteroids. The constants in this system are (V(0 °) 8.55 and F - 0.005. The fitted curve is indistinguishable from the one plotted in Fig. 3.

This work was supported by the Lunar and Planetary Program of N A S A under Contract NAS7-400 to the Jet Propulsion Laboratory. REFERENCES B o w H J . , E.. q'. GEHRELS, AND B. ZEIt.NER (19791. Magnitudes, colors, types, and adopted diameters of the asteroids. In Asteroids (T. Gehrels, Ed.), pp. 1108-1129. Univ. of Arizona Press, T u c s o n . HARRIS, A. W., ,',ND J. W. YOUNG (19841). Asteroid rotation, II1. Icarus 43, 20-32. H a r m s . A. W., AND I. W. YOUN(; (19831. Asteroid rotation, IV. Icarus 54, 59-109. HARRIS, A. W., J. W. YOUNG, F. SCAI TRITI, AND V. ZAPPALA (1984). Lightcurves and phase relations of the asteroids 82 A l k m e n e and 444 Gyptis. h arus 57, 251-258. L,XGERKVlS3, C.-I. (19781. Photographic photometry of II0 main-belt asteroids. Astr