ODIN OBSERVATIONS OF H_2O AND O_2 IN COMETS AND INTERSTELLAR CLOUDS

摘要

We here report on results from single-position observations, and in some cases also mapping, of the 557 GHz ortho-H_2O line in several comets and in many interstellar molecular clouds by the Odin sub-millimetre wave spectroscopy satellite. The H_2O production rates have been accurately determined in four comets, C/2001 A2 (LINEAR), 19P/Borrelly, C/2000 WM, (LINEAR), and 153P/ 2002 C1 (Ikeya-Zhang). In comet Ikeya-Zhang our detection at a low level of the corresponding H_2 ~(18)O emission line verifies the H_2O production rate (which depends upon the assumed radiative and collisional excitation and also upon radiative transfer modelling) and is consistent with a nearly terrestrial ~(16)O/~(18)O-isotope ratio. In an astro-biological context, the cometary H_2O production rates are especially important as reference levels for comparison with abundances of other molecules simultaneously observed with growid-based telescopes. In interstellar clouds the observed gas-phase H_2O abundances (vs H_2) range from 5 x 10~(-4) in the Orion KL outflow/shock region (where essentially all oxygen is locked up in H_2O) to circa 10~(-8) in quiescent cloud regions (where H_2O is just one of many trace molecules). From an astrobiological point of view, the molecular abundances in star forming clouds are important in terms of initial conditions for the chemistry in proto-planetary disks ("proto-solar nebulae"), the formation sites of new planetary systems. In simultaneous observations, Odin has also detected the 572 GHz ortho- NH_3 line in cold and warm clouds as well as in the Orion outflow and Bar/PDR regions (an area of increased ionisation caused by the intense UV flux from newly born massive stars). In other simultaneous observations, we have performed sensitive searches for O_2 at 119 GHz. Although no detection can be reported as yet, the resulting very low abundance limits (< 10~(-7)) are very intriguing when they are compared with current "standard" model expectations, which fall in the range 10~(-5)-10~(-4).