HST OBSERVATION OF THE INNER COMA OF 2060-CHIRON

The analysis of persistent comae of very large and distant comets allo ws us to infer the poorly known physical properties of dust and icy gr ains produced beyond Jupiter. Chiron is particularly interesting becau se Fulle (Astron. Astrophys. 282, 980-988, 1994) has shown that the st rong asymmetricity of its coma (West, Astron. Astrophys. 241, 635-644 1991) allows us to infer rotational properties of Chiron's nucleus. In this paper we report the observation of the asymmetric inner coma of Chiron from the analysis of 44 images of Chiron and eight images of a standard star available from the HST archive. The images were taken du ring February/March 1993, when Chiron was 9.3 AU from the Sun and 8.4 AU from the Earth. From the available set we extracted 26 Chiron image s and four star images which were placed on the same location of the P lanetary Camera, in order to avoid changes of the position-dependent H ST PSF. These images were averaged in order to increase the signal-to- noise ratio. We deconvolved both Chiron and star images with the new t wo-channel Lucy image restoration algorithm (Hook et al., ST-ECF Newsl etter 21, 16, 1994), which provides corrected images containing delta functions representing point-like sources in the corrected field, and an underlying background (Chiron's coma in our case). Then we subtract ed from Chiron's deconvolved average image the star deconvolved averag ed image, thus obtaining an image which contains Chiron's coma alone. All the images were taken through the F555W HST filter, so that the co ma image was finally calibrated in the Johnson V band. It concerns a c oma about 4 x 10(4) km wide with isophote levels ranging from 18 to 23 V-mag arcsec(-2). The isophotes show a significant asymmetricity, ori ented like those obtained by Luu and Jewitt (Astron. J. 100, 913-933, 1990) and West (1991). This fact may indicate no changes of the spin o rientation during the last three years, in agreement with Chiron's lar ge distance from the Sun and the Steady dust production building up th e persistent coma.