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

Comet C/2006 W3 (Christensen) was observed in November 2009 at 3.3 AU from the Sun with <i>Herschel<i/>.The PACS instrument acquired images of the dust coma in 70-<i>μ<i/>m and 160-<i>μ<i/>m filters and spectra covering several H<sub>2<sub/>O rotational lines. Spectra in the range 450–1550 GHz were acquired with SPIRE. The comet emission continuum from 70 to 672 <i>μ<i/>m 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, CH<sub>3<sub/>OH, HCN, H<sub>2<sub/>S, OH) measured with the IRAM 30-m and Nançay 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 H<sub>2<sub/>O production rate ratio exceeds 220%. The dust-to-gas production rate ratio is on the order of 1.