Herschel observations of gas and dust in comet C/2006 W3 (Christensen) at 5 AU from the Sun

Context. Cometary activity at large heliocentric distances is thought to be driven by outgassing of molecular species more volatile than water that are present in the nucleus. The long-period comet C/2006 W3 (Christensen) was an exceptional target for a detailed study of its distant gaseous and dust activity. Aims. We aimed to measure the H2O and dust production rates in C/2006 W3 (Christensen) with the Herschel Space Observatory at a heliocentric distance of similar to 5 AU and compared these data with previous post-perihelion Herschel and ground-based observations at similar to 3.3 AU from the Sun. Methods. We have searched for emission in the H2O and NH3 ground-state rotational transitions, J(KaKc) (1(10)-1(01)) at 557 GHz and J(K) (1(0)-0(0)) at 572 GHz, simultaneously, toward comet C/2006 W3 (Christensen) with the Heterodyne Instrument for the Far Infrared (HIFI) onboard Herschel on UT 1.5 September 2010. Photometric observations of the dust coma in the 70 mu m and 160 mu m channels were acquired with the Photodetector Array Camera and Spectrometer (PACS) instrument on UT 26.5 August 2010. Results. A tentative 4 sigma H2O line emission feature was found in the spectra obtained with the HIFI wide-band and high-resolution spectrometers, from which we derive a water production rate of (2.0 +/- 0.5) x 10(27) molec s(-1). A 3 sigma upper limit for the ammonia production rate of <1.5 x 10(27) molec s(-1) is obtained taking into account the contribution from all hyperfine components. The dust thermal emission was detected in the 70 mu m and 160 mu m filters, with a more extended emission in the blue channel. We fit the radial dependence of the surface brightness with radially symmetric profiles for the blue and red bands. The dust production rates, obtained for a dust size distribution index that explains the fluxes at the photocenters of the 70 mu m and 160 mu m PACS images, lie in the range 70 kg s(-1) to 110 kg s(-1). Scaling the CO production rate measured post-perihelion at 3.20 AU and 3.32 AU, these values correspond to a dust-to-gas production rate ratio in the range 0.3-0.4. Conclusions. The blueshift of the water line detected by HIFI suggests preferential emission from the subsolar point. However, it is also possible that water sublimation occurs in small ice-bearing grains that are emitted from an active region on the nucleus surface at a speed of similar to 0.2km s(-1). The dust production rates derived in August 2010 are roughly one order of magnitude lower than in September 2009, suggesting that the dust-to-gas production rate ratio remained approximately constant during the period when the activity became increasingly dominated by CO outgassing.