1 Introduction

The HST/FGS astrometer has already been successfully used in the past to derive angular diameters and flattenings of Mira stars (Lattanzi et al. 1997) with typical sensitivities around the $\sim$1 mas level. An application of FGS to the measurement of Solar System bodies (minor planets in particular) has been suggested in the past, and the results presented here are the first successful attempts that prove that the HST/FGS is valuable in measuring the apparent angular sizes of minor bodies and in providing important information on their shapes. Five main-belt asteroids and one Jupiter Trojan have been observed as part of an approved program devoted to search for duplicity among asteroids during HST Cycle 8 (Zappalà et al. 1998; Hestroffer et al. 2002a). The targets ((15) Eunomia (43) Ariadne; (44) Nysa, (63) Ausonia, (216) Kleopatra and (624) Hektor) were selected on the basis of peculiar photometric properties suggesting a possible binarity (see Leone et al. 1984; Cellino et al. 1985). Apart from their large maximum amplitudes, the light-curves of the selected targets are generally compatible with the behavior expected for couples of bodies having overall rubble pile internal structures, for which equilibrium shapes can be expected (Weidenschilling 1980; Zappalà et al. 1983). In particular, the observed rotation periods and light-curves could be consistent with contact - or nearly-contact - binaries that could form as outcomes of catastrophic collisions with large angular momentum transfer. Of course, this is not the only possible interpretation of the available data, since purely shape effects can be responsible for light-curve morphologies like those observed for these objects (Cellino et al. 1989). As shown in a previous paper (Hestroffer et al. 2002b, called hereafter Paper I), the HST/FGS instrument is powerful enough to resolve such binary systems or to find constraints for alternative shape models.

Our observations show that there is no compelling evidence for well separated or nearly-contact binaries among the objects of our sample. The HST/FGS nevertheless provides important and accurate results on the pole orientation, size, shape, and brightness distribution of these asteroids, hence enabling the computation of a physical ephemeris. The goodness of fit obtained assuming a single triaxial ellipsoid shape varies among the different objects of our sample. While (44) Nysa and (63) Ausonia are very well modeled by prolate spheroids, the data obtained for (15) Eunomia, (43) Ariadne and (624) Hektor show slight but appreciable departures from this ideal shape. (216) Kleopatra is confirmed to have a bi-lobated dumbbell shape (see also Tanga et al. 2001) as suggested by previous observations (Ostro et al. 2000; Marchis et al. 1999).

A methodological discussion on the basic strategy was presented in Paper I, together with the principles employed in the data reduction. In this second paper we present the detailed results obtained for each of the observed bodies. In Sect. 2 the observing circumstances are given. The results are developed in Sect. 3, together with the physical parameters (pole orientation, shape and size estimate) that have been derived.

   

Table 1: Selected targets ephemeris and observation logs.

		UTC time of visit		Mag.	Sidereal	Geoc.	Solar	SEP $^{\rm (a)}$	Scans
Name	Date	first	last	V	period $^{\rm (b)}$	Dist.	phase	$\lambda\quad \beta$	step	#	roll $^{\rm (c)}$
		[h]	[h]		[h]	[AU]	[deg]	[deg]	[mas]		[deg]
(15) Eunomia $^{\rm (d)}$	30 Sep. 98	23:30:03	00:05:10	8.6	6.0828	1.360	20.7	126 -8.4	1.5	4	42.1
(43) Ariadne	22 Aug. 98	17:14:45	17:51:47	10.3	5.7620	0.958	18.6	145 +44.4	1.0	4	273.0
(44) Nysa	05 Sep. 98	18:40:09	19:17:11	10.6	6.4214	1.638	17.2	221 -6.5	1.0	4	52.9
(63) Ausonia	02 Apr. 98	16:07:08	16:40:32	11.7	9.2976	1.824	19.7	304 +57.1	1.0	4	287.6
(216) Kleopatra	13 Jan. 00	13:32:59	14:11:17	10.8	5.3853	1.529	22.4	$\phantom{0}57$ -43.0	1.0	2	248.7
(624) Hektor	23 Oct. 98	12:03:52	12:42:43	15.0	6.9205	4.618	9.5	313 -32.5	2.4	6	55.1

(a) Coordinates of the Sub-Earth Point (SEP) with respect to the asteroid equator, following the pole coordinates given in Table 2. The longitude
origin is assumed to be at a semi-meridian lying on the plane that contains the major axis, and it is given for the first visit.
(b) From P. Magnusson's on-line database: http://www.astro.uu.se/~per
(c) Position angle of the FGS-X axis (also called "roll angle'') with respect to the North direction and counted positive toward the East.
(d) Last visit on October 1st.