Submillimetre observations of comets with Odin: 2001–2005

Abstract

The Odin satellite, launched in February 2001, is equipped with a 1.1-m submillimetre telescope. Odin was used to observe the 557 GHz line of water with high spectral resolution in 12 comets between 2001 and 2005. Line shapes and spatial mapping provide information on the anisotropy of the outgassing and constraints on water excitation, enabling accurate measurements of the water production rate. Five comets were regularly observed over periods of more than one month to monitor the variation of their water outgassing rate with heliocentric distance. Observing campaigns have been generally coordinated with ground-based observations of molecular lines at Nançay, CSO or IRAM 30-m telescopes to obtain molecular abundances relative to water. Thanks to Odin's frequency coverage, it was also possible to detect the H 2 18O 548 GHz line, first in comet 153P/Ikeya–Zhang in April 2002 [Lecacheux, A., Biver, N., Crovisier, J. et al., 2003, Observations of water in comets with Odin. Astron. Astrophys. 402, L55–L58.] and then in comets C/2002 T7 (LINEAR), C/2001 Q4 (NEAT) and C/2004 Q2 (Machholz). The 16O/ 18O isotopic ratio ( ≈ 450 ) is consistent with the terrestrial value. Ammonia has been searched for in three comets through its J K = 1 0 – 0 0 line at 572 GHz and was tentatively detected in C/2001 Q4 and C/2002 T7. The derived abundances of NH 3 relative to water are 0.5% and 0.3%, respectively, similar to values obtained in other comets with different techniques.