An upper limit for the water outgassing rate of the main-belt comet 176P/LINEAR observed with Herschel/HIFI^⋆

¹ Max Planck Institute for Solar System Research, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
² LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France
³ Rosetta Science Operations Centre, ESAC, European Space Agency, 28691 Villanueva de la Cañada, Madrid, Spain
⁴ California Institute of Technology, Pasadena, CA 91125, USA
⁵ Space Research Centre, Polish Academy of Sciences, Warsaw, Poland
⁶ Institute d’Astrophysique et de Geophysique, Université de Liège, Belgium
⁷ Herschel Science Centre, ESAC, European Space Agency, 28691 Villanueva de la Cañada, Madrid, Spain
⁸ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain

Received: 5 August 2012
Accepted: 25 August 2012

Abstract

176P/LINEAR is a member of the new cometary class known as main-belt comets (MBCs). It displayed cometary activity shortly during its 2005 perihelion passage, which may be driven by the sublimation of subsurface ices. We have therefore searched for emission of the H₂O 1₁₀–1₀₁ ground state rotational line at 557 GHz toward 176P/LINEAR with the Heterodyne Instrument for the Far Infrared (HIFI) onboard the Herschel Space Observatory on UT 8.78 August 2011, about 40 days after its most recent perihelion passage, when the object was at a heliocentric distance of 2.58 AU. No H₂O line emission was detected in our observations, from which we derive sensitive 3-σ upper limits for the water production rate and column density of <4 × 10²⁵ mol s^-1 and of <3 × 10¹⁰ cm^-2, respectively. From the peak brightness measured during the object’s active period in 2005, this upper limit is lower than predicted by the relation between production rates and visual magnitudes observed for a sample of comets at this heliocentric distance. Thus, 176P/LINEAR was most likely less active at the time of our observation than during its previous perihelion passage. The retrieved upper limit is lower than most values derived for the H₂O production rate from the spectroscopic search for CN emission in MBCs.