A study of the distant activity of comet C/2006 W3 (Christensen) using Herschel and ground-based radio telescopes

TL;DR

This study investigates the activity of comet C/2006 W3 (Christensen) at 3.3 AU from the Sun using Herschel and ground-based telescopes, revealing a dust coma, volatile enrichment, and high CO to water ratio.

Contribution

It provides new measurements of dust and gas emissions of a distant comet, highlighting its volatile enrichment and dust-to-gas ratio at large heliocentric distance.

Findings

No water lines detected in spectra.

Comet is enriched in volatile species like CO.

High CO to H2O production ratio (>220%).

Abstract

Comet C/2006 W3 (Christensen) was observed in November 2009 at 3.3 AU from the Sun with Herschel. The PACS instrument acquired images of the dust coma in 70- and 160-micrometers filters, and spectra covering several H2O rotational lines. Spectra in the range 450-1550 GHz were acquired with SPIRE. The comet emission continuum from 70 to 672 micrometers was measured, but no lines were detected. The spectral energy distribution indicates thermal emission from large particles and provides a measure of the size distribution index and dust production rate. The upper limit to the water production rate is compared to the production rates of other species (CO, CH3OH, HCN, H2S, OH) measured with the IRAM 30-m and Nancay telescopes. The coma is found to be strongly enriched in species more volatile than water, in comparison to comets observed closer to the Sun. The CO to H2O production rate ratio exceeds 220%. The dust to gas production rate ratio is on the order of 1.