Thermal-Infrared High-Resolution Imaging of Comet Austin

Abstract

We report postperihelion imaging of the dynamically new Comet Austin (1989c1 = 1990V) obtained with the UCSD mid-infrared astronomical camera (the "Golden Gopher") during five nights in May 1990. The 64 × 16 array format of the camera covered 53 × 13 arcsec of the sky. The images were taken through a wide 9- to 12-μm filter and a narrower 8- to 9-μm filter. During the observing run, the heliocentric distances ranged from 0.8 to 0.9 AU and the geocentric distance ranged from 0.5 to 0.4 AU. The inner (16 × 4) × 103 km of the coma was sampled with a linear resolution of 290-220 km per pixel. The images obtained on the same night looked similar and were combined together to enhance the signal-to-noise ratio in the coadded images. No jet- or shell-like structures are observed. The coma is slightly elongated along the projected Sun-Comet direction, but there is no systematically higher intensity sunward over anti-sunward. Radial brightness profiles correspond to the steady-state model approximation of r-1 up to an angular distance of 4-5 arcsec (1-1.5 × 103 km) from the nucleus but steepen to r-1.25 larger distances. This is consistent with the presence of fading grains in the close vicinity of the cometary nucleus. The thermal infrared flux measured in the images decreases as R-5.5 with the heliocentric distance, suggesting a R-3 dependence of the dust production rate. The estimated mass loss rate was 1 × 106 g/sec on May 6 and 5.7 × 105 g/sec on May 12, giving a dust-to-gas ratio of 0.25. A map of the spatial behavior of the silicate emission in the coma was generated by comparison of the images obtained on May 12 through the two different filters. This shows a peak in the sunward direction at a distance of 1.5-2 × 103 km from the nucleus. The results presented here are the first application of the Golden Gopher camera in cometary research and they demonstrate the potential of mid-infrared array technology, enabling further insight into the nature, properties, and evolution of comets.