Molecular oxygen in the ρ  Ophiuchi cloud

Abstract

Context. Molecular oxygen, O 2 , has been expected historically to be an abundant component of the chemical species in molecular clouds and, as such, an important coolant of the dense interstellar medium. However, a number of attempts from both ground and from space have failed to detect O 2 emission. Aims. The work described here uses heterodyne spectroscopy from space to search for molecular oxygen in the interstellar medium. Methods. The Odin satellite carries a 1.1 m sub-millimeter dish and a dedicated 119 GHz receiver for the ground state line of O 2 . Starting in 2002, the star forming molecular cloud core ρ Oph A was observed with Odin for 34 days during several observing runs. Results. We detect a spectral line at v LSR = +3.5 km s −1 with ∆v FWHM = 1.5 km s −1 , parameters which are also common to other species associated with ρ Oph A. This feature is identified as the O 2 (N J = 1 1 −1 0 ) transition at 118 750.343 MHz. Conclusions. The abundance of molecular oxygen, relative to H 2 , is 5 × 10 −8 averaged over the Odin beam. This abundance is consistently lower than previously reported upper limits.

Shocks in dense clouds

“…8 and 9) the gas phase abundance of SiO diminishes when SiO starts to deplete into mantles on the surfaces of grains in the high density postshock gas. In our work we have adopted an initial gas-phase fractional abundance for O 2 of X(O 2 ) = 10 −7 , in accordance with the recent SWAS results (Larsson et al 2007). This initial value has no impact on our results because O 2 has to be reformed in dissociative shocks (Fig.…”

Astrochemistry of dust, ice and gas: introduction and overview

“…The success story of combined laboratory and observational studies became complete with the detection of gas-phase HO 2 199 Interestingly, ρ Oph A is also one of only two positions where interstellar O 2 gas has been firmly detected through multi-line observations with the Odin and Herschel satellites 202,203 . Deep limits in other cold cores confirm the scenario in which most of the O and O 2 must be converted into H 2 O ice on the grains before the protostar starts to heat its surroundings 204,205 .…”

Oxygen depletion in dense molecular clouds: a clue to a low O2abundance?

“…The ODIN satellite also initially gave negative results (Pagani et al 2003) with improved upper limits of ≈(1−2) × 10 −7 for nine sources. However, a reanalysis by Larsson et al (2007), using more precise knowledge of the telescope behavior, resulted in a detection of O 2 in ρ Ophiuchi cloud, with a beam-averaged abundance of 5 × 10 −8 relative to H 2 . Using ground-based observations of 16 O 18 O and C 18 O lines, Liseau et al (2010) argue that the emitting region may be much smaller than the beam of ODIN thus the O 2 abundance could be larger by one or two orders of magnitude.…”