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Paper: From Molecular Oxygen to Primordial Molecules with the Odin Satellite
Volume: 395, Frontiers of Astrophysics: A Celebration of NRAO's 50th Anniversary
Page: 378
Authors: Persson, C.; Encrenaz, P.; Hjalmarson, Å .; Sandqvist, Aa.; Odin Team
Abstract: Odin is a mm/submm-wave-spectroscopy Astronomy and Aeronomy satellite. Its design life time was two years, but it has now operated successfully for more than six years. Its 1.1 m high-precision telescope, with a beam efficiency of 90%, has beamwidths of 10 arcmin and 2 arcmin at mm and submm wavelengths, respectively. It is equipped with a cryogenic receiver package of four tunable, SSB, submm Schottky mixers covering the 486–504 and 541–581 GHz frequency range, and a fixed-tuned HEMT receiver at a frequency of 118–119 GHz.

Odin has discovered the elusive oxygen (O2) molecule at 119 GHz—the detection was made in the ρ Oph A molecular-cloud core. The O2 abundance, X(O2), is ≈ 5 × 10−8—Larsson et al. (2007: A&A, 466, 999). Odin has also discovered water (H2O) at 557 GHz in the core of the Milky Way Galaxy—the Sgr A CircumNuclear Disk (CND)—and mapped its distribution in the Sgr A Complex including the +20 and +50 km s−1 Molecular Clouds—Sandqvist et al. (2006: J.Phys. Conf.Ser., 54, 72). Furthermore, devoting about 1,100 orbits (each of which contains 1 hour of observable astronomy time) Odin has performed a spectral-line survey of Orion KL in the ranges 487–492 and 542–577 GHz. There are 280 identified spectral lines from 38 species and 64 unidentified lines [Olofsson et al. (2007: A&A, 476, 791) and Persson et al. (2007: A&A, 476, 807)].

Odin has searched for emission from the 557GHz ortho-H2O line in six nearby starburst galaxies yielding three-sigma upper limits to the water abun- dance relative to H2 (NGC253 – < 2.0 × 10−9; IC342 - < 2.6 × 10−9; M82 – < 1.7 × 10−9; NGC4258 - < 1.3 × 10−8; CenA – < 7.8 × 10−9 and M51 – < 2.4 × 10−9) [Wilson et al. (2007: A&A, 469, 121)].

Odin has an ongoing project for studying the first phases of structure formation in the early universe. The presence of primordial molecules (e.g., HD, H+2 , LiH, HeH+) is investigated by searching for spectral lines produced by resonant scattering of the cosmic microwave background (CMB). During the summer of 2004, Odin performed a spectral scan (547–578 GHz, 1 MHz resolution) towards two Wilkinson Microwave Anisotropy Probe CMB Hot Spots using 350 orbits (5 orbits per LO setting and point). No lines were detected down to an rms level of 65mK. A new observing strategy was developed for a second observing session (430 orbits) during the winter of 2006/7 to accommodate for the spatial uncertainty of primordial clouds. The spectral range is 486.5-492.0 and 542.0-547.5 GHz (10 orbits per LO setting and point, 1MHz resolution yielding an rms of about 30mK). Analysis of these data is ongoing. This is a pilot programme for GTO time on ESA’s Herschel Space Observatory, planned for launch in late 2008.

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