HST observation of the inner coma of 2060 chiron

The analysis of persistent comae of very large and distant comets allows us to infer the poorly known physical properties of dust and icy grains produced beyond Jupiter. Chiron is particularly interesting because Fulle (Astron. Astrophys.282, 980–988; 1994) has shown that the strong asymmetricity of its coma (West, Astron. Astrophys.241, 635–645 1991) allows us to infer rotational properties of Chironʹs 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 during February/March 1993, when Ch iron was 9.3 AU from the Sun and 8.4 AU from the Earth. From the available set we extracted 26 Chiron images and four star images which were placed on the same location of the Planetary Camera, in order to avoid changes of the position-dependent HST PSF. These images were averaged in order to increase the signal-to-noise ratio. We deconvolved both Chiron and star images with the new two-channel Lucy image restoration algorithm (Hook et al, ST-ECF Newsletter21, 16, 1994), which provides corrected images containing delta functions representing point-like sources in the corrected field, and an underlying background (Chironʹs coma in our case). Then we subtracted from Chironʹs deconvolved average image the star deconvolved averaged image, thus obtaining an image which contains Chironʹs coma alone. All the images were taken through the F555W HST filter. so that the coma image was finally calibrated in the Johnson V band. It concerns a coma about 4 × 104 km wide with isophote levels ranging from 18 to 23 V-mag arcsec−2. The isophotes show a significant asymmetricity, oriented 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 orientation during the last three years, in agreement with Chironʹs large distance from the Sun and the steady dust production building up the persistent coma.