Water Production Rate of the Jupiter-Family Comet 46P/Wirtanen in the 2008 Apparition with the Subaru Telescope/IRCS

Abstract

Abstract We present the water production rate of comet 46P/Wirtanen in its 2008 apparition, determined from high-dispersion near-infrared spectroscopic observations. Comet 46P/Wirtanen, one of Jupiter-family comets, was the target of the ROSETTA mission in the past. Observations of comet 46P were carried out in the middle of 2008 February with the Subaru telescope and Infrared Camera and Spectrograph (IRCS) instrument. We detected three water emission lines, and the water production rate was determined to be (1.7$\ \pm\ $0.2) $\times$ 10$^{28}\ $molecules s$^{-1}$ on 2008 February 19 near the perihelion passage of the comet. This was the first direct detection of water in the coma of comet 46P. We compared our result with the water production rate determined from radio observations near the perihelion passage in the same apparition, and we found that the rate of radio observations is consistent with our value within $\ \pm\ $2$\ \sigma$ errors. We also compared the water production rates observed in the 2008 apparition with other results observed in the 1997 apparition. We found that the water production rates in 2008 are consistent with those in 1997. There was no secular change of water production rates around its perihelion passage in recent decades for comet 46P.