Survey of cometary co2, co, and particulate emissions using the spitzer space telescope

摘要

We surveyed 23 comets using the Infrared Array Camera on the Spitzer Space Telescope in wide filters centered at 3.6 and 4.5μm. Emission in the 3.6μm filter arises from sunlight scattered by dust grains; the 3.6μm images generally have a coma near the nucleus and a tail in the antisolar direction due to dust grains swept back by solar radiation pressure. The 4.5μm filter contains scattered sunlight by, and thermal emission from, the same dust grains, as well as strong emission lines from CO2 and CO gas. The 4.5μm images are often much brighter than could be explained by dust grains, and they show sometimes distinct morphologies, in which cases we infer they are dominated by gas. Based on the ratio of 4.5-3.6μm brightness, we classify the survey comets as CO_2+CO 'rich' and 'poor'. This classification is correlated with previous classifications by A'Hearn based on carbon-chain molecule abundance, in the sense that comets classified as 'depleted' in carbon-chain molecules are also 'poor' in CO_2+CO. The gas emission in the IRAC 4.5μm images is characterized by a smooth morphology, typically a fan in the sunward hemisphere with a radial profile that varies approximately as the inverse of projected distance from the nucleus, as would apply for constant production and free expansion. There are very significant radial and azimuthal enhancements in many of the comets, and these are often distinct between the gas and dust, indicating that ejection of solid material may be driven either by H_2O or CO_2. Notable features in the images include the following. There is a prominent loop of gas emission from 103P/Hartley 2, offset toward the sunward direction; the loop could be due to an outburst of CO_2 before the Spitzer image. Prominent, double jets are present in the image of 88P/Howell, with one directed nearly toward the Sun and the other closer to the terminator (but still on the daytime hemisphere). A prominent single jet is evident for C/2002 T7 (LINEAR), 22P/Kopff and 81P/Wild 2. Spirals are apparent in 29P/Schwassmann-Wachmann 1 and C/2006 W3 (Christensen); we measure a rotation rate of 21h for the latter comet. Arcs (possibly parts of a spiral) are apparent in the images of 10P/Tempel 2, and 2P/Encke.