Surface photometry of comet P/Encke

Abstract

We have developed a scheme to clean cometary digital images from offending background-star trails, and applied this technique to a pair of deep Kitt Peak 4-m plates of comet P/Encke, taken in October 1980. Simultaneous and subsequent digital spectra have been obtained at Lick Observatory. The non-polluted coma images show a strong asymmetric sunward-oriented fan/jet, and an extended and rounder (mostly gaseous) main coma, out to ~ 10^5 km radius. The stellar-trail point-spread function has a narrow width (σ7~0.6 arcsec), so that spatial resolution better than ~300 km is achieved at the comet. The photometric gradient near the nucleus is very steep, strongly suggesting an icy-grain component which evaporates quickly (at radii ⩽ 500 km) in the sunlight. Further from the nucleus, the profile becomes shallower, bluer, and more gas dominated. The effect of solar radiation pressure on C_2, CN, and other molecules is probably responsible for the rounding of the outer, faint isophotes. The source of the molecules is likely to be larger than the nucleus itself, and a substantial fraction may originate in the jet. The technique described here may also be applicable in surface photometry of galaxies, in cases where the heavy image pollution by foreground stars is present.

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