Beyond 3 au from the Sun: The Hypervolatiles CH 4 , C 2 H 6 , and CO in the Distant Comet C/2006 W3 (Christensen) ∗

Abstract

Comet C/2006 W3 (Christensen) remained outside a heliocentric distance ( R h ) of 3.1 au throughout its apparition, but it presented an exceptional opportunity to directly sense a suite of molecules released from its nucleus. The Cryogenic Infrared Echelle Spectrograph at ESO-VLT detected infrared emissions from the three “hypervolatiles” (CO, CH 4 , and C 2 H 6 ) that have the lowest sublimation temperatures among species that are commonly studied in comets by remote sensing. Even at R h  = 3.25 au, the production rate of each molecule exceeded those measured for the same species in a number of other comets, although these comets were observed much closer to the Sun. Detections of CO at R h  = 3.25, 4.03, and 4.73 au constrained its post-perihelion decrease in production rate, which most likely dominated the outgassing. At 3.25 au, our measured abundances scaled as CO/CH 4 /C 2 H 6  ≈ 100/4.4/2.1. The C 2 H 6 /CH 4 ratio falls within the range of previously studied comets at R h < 2 au, while CO/CH 4 is comparatively high and similar to in situ measurements from Rosetta at ∼10 km from the nucleus of 67P/Churyumov-Gerasimenko conducted at a very similar R h (3.15 au). The independent detections of ( Herschel Space Observatory ) and CO (this work) imply a coma abundance in C/2006 W3 near R h  = 5 au. All these measurements are of high value for constraining models of nucleus sublimation (plausibly CO-driven) beyond R h  = 3 au, where molecular detections in comets are still especially sparse.