Herschel and IRAM-30m Observations of Comet C/2012 S1 (ISON) at 4.5 AU from the Sun

Abstract

Our goal was to characterize the distant gaseous and dust activity of comet C\2012 S1 (ISON), inbound, from observations of H2O, CO and the dust coma in the far-infrared and submillimeter domains. In this paper, we report observations undertaken with the Herschel Space Observatory on 8 & 13 March 2013 (rh = 4.54 - 4.47AU) and with the 30m telescope of Institut de Radioastronomie Millimétrique (IRAM) in March and April 2013 (rh = 4.45 - 4.18 AU). The HIFI instrument aboard Herschel was used to observe the H${}_2$O $1_{10}-1_{01}$ line at 557 GHz, whereas images of the dust coma at 70 and 160 μm were acquired with the PACS instrument. Spectra acquired at the IRAM 30m telescope cover the CO J(2-1) line at 230.5 GHz. The spectral observations were analysed with excitation and radiative transfer models. A model of dust thermal emission taking into account a range of dust sizes is used to analyse the PACS maps. While H${}_2$O was not detected in our 8 March 2013 observation, we derive a sensitive 3 $\sigma$ upper limit of QH${}_2$O < 3.5 x 10${}^{26}$ molecules/s for this date. A marginal 3.2 $\sigma$ detection of CO is found, corresponding to a CO production rate of QCO = 3.5 x 10${}^{27}$ molecules/s. The Herschel PACS measurements show a clear detection of the coma and tail in both the 70 μm and 160 μm maps. Under the assumption of a 2 km radius nucleus, we infer dust production rates in the range 10 - 13 kg/s or 40 - 70 kg/s depending on whether a low or high gaseous activity from the nucleus surface is assumed. We constrain the size distribution of the emitted dust by comparing PACS 70 and 160 μm data, and considering optical data. Size indices between -4 and -3.6 are suggested. The morphology of the tail observed on 70 μm images can be explained by the presence of grains with ages older than 60 days.

6 pages, 5 figures, 3 tables. Accepted by A&A

References in corpus (6)

• A Comprehensive Study of Gamma-Ray Burst Optical Emission: II. Afterglow Onset and Late Re-Brightening Components
• TNOs are Cool: A survey of the trans-Neptunian region. VIII. Combined Herschel PACS and SPIRE observations of 9 bright targets at 70--500 micron
• A study of the distant activity of comet C/2006 W3 (Christensen) using Herschel and ground-based radio telescopes
• Physical Properties of OSIRIS-REx Target Asteroid (101955) 1999 RQ36 derived from Herschel, ESO-VISIR and Spitzer observations
• HIFI Observations of Water in the Atmosphere of Comet C/2008 Q3 (Garradd)
• Extinction in the Coma of Comet 17P/Holmes

Cited by in corpus (4)

• The Main-Belt Comets: The Pan-STARRS1 Perspective
• Molecular observations of comets C/2012 S1 (ISON) and C/2013 R1 (Lovejoy): HNC/HCN ratios and upper limits to PH3
• Observations of Comet ISON (C/2012 S1) from Lowell Observatory
• Herschel observations of gas and dust in comet C/2006 W3 (Christensen) at 5 AU from the Sun