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

TL;DR

This study sets an upper limit on water outgassing from main-belt comet 176P/LINEAR using Herschel/HIFI, indicating it was less active than during its previous perihelion and providing constraints on its sublimation activity.

Contribution

First sensitive upper limit on water production rate for 176P/LINEAR using Herschel/HIFI, improving understanding of activity levels in main-belt comets.

Findings

No H2O emission detected, upper limit < 4e25 molecules/sec.

176P/LINEAR was less active at observation time than during previous perihelion.

Upper limit lower than most values from CN emission searches in MBCs.

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 that may be driven by the sublimation of sub-surface 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) on board 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-sigma upper limits for the water production rate and column density of < 4e25 molec/s and of < 3e10 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 by Jorda et al. (2008) at this heliocentric distance. Thus, 176P/LINEAR was 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.