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 H_2O and dust production rates in C/2006 W3 (Christensen) with the Herschel Space Observatory at a heliocentric distance of ~5 AU and compared these data with previous post-perihelion Herschel and ground-based observations at ~3.3 AU from the Sun. Methods. We have searched for emission in the HO and NH_3 ground-state rotational transitions, J_K_(a)K_(c) (1_(10)–1_(01)) at 557 GHz and JK (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 μm and 160 μm channels were acquired with the Photodetector Array Camera and Spectrometer (PACS) instrument on UT 26.5 August 2010. Results. A tentative 4σ H_2O 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) × 1027 molec s^(-1). A 3σ upper limit for the ammonia production rate of