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

Context. The sungrazer comet C/2012 S1 (ISON) (perihelion at r_h = 0.0125 AU from the Sun) was bright and active when discovered in September 2012 at 6.3 AU from the Sun.
Aims: Our goal was to characterize the distant gaseous and dust activity of this comet, inbound, from observations of H₂O, CO and the dust coma in the far-infrared and submillimeter domains.
Methods: We report observations undertaken with the Herschel space observatory on 8 and 13 March 2013 (r_h = 4.54-4.47AU) and with the 30 m telescope of Institut de Radioastronomie Millimétrique (IRAM) in March and April 2013 (r_h = 4.45-4.18 AU). The HIFI instrument aboard Herschel was used to observe the H₂O 1₁₀ - 1₀₁ line at 557 GHz, whereas images of the dust coma at 70 μm and 160 μm were acquired with the PACS instrument. Spectra acquired at the IRAM 30 m 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.
Results: While H₂O was not detected in our 8 March 2013 observation, we derive a sensitive 3σ upper limit of Q_H2O < 3.5 × 10²⁶ molecules s^-1 for this date. A marginal 3.2σ detection of CO is found, corresponding to a CO production rate of Q_CO = 3.5 × 10²⁷ molecules s^-1. 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^-1 or 40-70 kg s^-1, 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.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. Based on observations carried out with the 30 m telescope, operated by the Institut de RadioAstronomie Millimétrique (IRAM), which is funded by a partnership of INSU/CNRS (France), MPG (Germany), and IGN (Spain).