Inner coma imaging of Comet Levy (1990c) with the Hubble space telescope

Abstract

Comet Levy (1990c) was observed with the Hubble Space Telescope (HST) on UT 27 September 1990 when both the heliocentric and geocentric distances were ∼1 AU. Two sets of images of the comet were taken 6.5 hr apart with the Wide-Field Camera (WFC) through a broadband red filter that was selected to isolate continuum emission peaking sharply at the nucleus. A single WFC pixel projected to a distance of ∼78 km at the comet, and deconvolution techniques were used to recover virtually the full spatial resolution capability of the HST. The images show a highly asymmetrical coma in which the sunward- facing hemisphere is more than a factor of 2 brighter than the tailward hemisphere, consistent with volatile sublimation occurring primarily on the dayside of the nucleus. The azimuthal dependence of the spatial brightness distribution on the sunward side is roughly Gaussian with FWHM ∼ 135° and an axis of symmetry that is nearly coincident with the projected Sun-comet line. While the azimuthal profile is clearly not consistent with isotropic emission of dust into the sunward hemisphere, the profile width is significantly larger than the widths of dust jets observed by the Halley Multicolor Camera on the Giotto spacecraft (H.J. Reitsema et al. 1989, Icarus 81, 31-40). Radial brightness profiles perpendicular to the Sun-comet line are very symmetric about the nucleus and follow approximately a ϱ^-1 law (where ϱ is the projected distance to the nucleus). For the time-averaged radial brightness profile, the relative I-band luminosity increases linearly with aperture size over the entire range 0″.1 ≤ ϱ ≤ 10″. However, the coma of Comet Levy is definitely not in steady state. Detailed analysis of the images appears to show a hemispherical arc of dust propagating through the coma with an average projected velocity of ∼0.16 km sec ^-1. Periodic occurrences of similar dust arcs could be responsible for the temporal variability in the continuum photometry observed from the International Ultraviolet Explorer (P.D. Feldman et al. Icarus 95, 65-72.) and from the ground (D.G. Schleicher et al. 1991, Icarus 94, 511-523).