Icy grains as a source of CO in comet 29P/Schwassmann-Wachmann 1

Astronomiska Observatoriet, Box 515, 75120 Uppsala, Sweden

Corresponding author: marcus@astro.uu.se

Received: 22 March 2002
Accepted: 5 November 2002

Abstract

A model of micron-sized icy grains is used to simulate the behaviour of solid material ejected from comet 29P/Schwassmann-Wachmann 1, which resides at ~6 AU from the Sun. It is demonstrated that a population of grains composed primarily of water ice can release their ice content on timescales of order 10⁶ s. This justifies the idea that icy grains can serve as an extended source of more volatile species, as was assumed when evidence for a coma source of CO was found in radio observations of this comet. The model also predicts short lifetimes for the icy grains inside 3 AU from the Sun, removing the extended appearance of the CO production at smaller heliocentric distances, as seen in comet Hale-Bopp. A grain population sublimating in a time interval similar to that derived for the coma source of CO in comet 29P/Schwassmann-Wachmann 1 must have most of its mass concentrated to small (with radii <5 μm) or porous grains. Those grains must carry a significant ice content, with an refractory to ice ratio of less than ~30%. By introducing continuous grain fragmentation the extended source activity can be reproduced for a grain population that includes larger particles. This would call for grains to split into smaller pieces on a timescale of ~ s.