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

O’Rourke, L.; Bockelée-Morvan, D.; Biver, N.; Altieri, B.; Teyssier, D.; Jorda, L.; Debout, V.; Snodgrass, C.; Küppers, M.; A’Hearn, M.; Müller, T. G. and Farnham, T. (2013). Herschel and IRAM-30m observations of comet C/2012 S1 (ISON) at 4.5 AU from the Sun. Astronomy & Astrophysics, 560, article no. A101.

DOI: https://doi.org/10.1051/0004-6361/201322756


Context. The sungrazer comet C/2012 S1 (ISON) (perihelion at rh = 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 (rh = 4.54–4.47AU) and with the 30 m 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 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 QH2O < 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 QCO = 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.

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