Low upper limits on the O₂abundance from the Odin satellite

Abstract

Abstract. For the first time, a search has been conducted in our Galaxy for the 119 GHz transition connecting to the ground state of O 2 , using the Odin satellite. Equipped with a sensitive 3 mm receiver (T sys (SSB) = 600 K), Odin has reached unprecedented upper limits on the abundance of O 2 , especially in cold dark clouds where the excited state levels involved in the 487 GHz transition are not expected to be significantly populated. Here we report upper limits for a dozen sources. In cold dark clouds we improve upon the published SWAS upper limits by more than an order of magnitude, reaching N(O 2 )/N(H 2 ) ≤ 10 −7 in half of the sources. While standard chemical models are definitively ruled out by these new limits, our results are compatible with several recent studies that derive lower O 2 abundances. Goldsmith et al. (2002) recently reported a SWAS tentative detection of the 487 GHz transition of O 2 in an outflow wing towards ρ Oph A in a combination of 7 beams covering approximately 10 ×14 . In a brief (1.3 hour integration time) and partial covering of the SWAS region (≈65% if we exclude their central position), we did not detect the corresponding 119 GHz line. Our 3 sigma upper limit on the O 2 column density is 7.3 × 10 15 cm −2 . We presently cannot exclude the possibility that the SWAS signal lies mostly outside of the 9 Odin beam and has escaped our sensitive detector.

Molecular Oxygen in the Nearest QSO Mrk 231

“…The nondetection of O 2 emission in the central 2 kpc of Mrk 231 with our NOEMA observations gave the best upper limit at the of extragalactic O 2 N J =1 1 -1 0 to CO 1-0 line ratio of 1×10 −3 (1σ) , much lower than the 1.2×10 −2 value for NGC 6240, which was used previously for estimating the O 2 /H 2 abundance ratio (Combes et al 1991). Using the same CO J=1-0 flux to H 2 conversion factor as that in NGC 6240 and similar excitation conditions of the O 2 molecules (Combes et al 1991), the O 2 to H 2 abundance ratio should be less than 8×10 −8 in the host galaxy of Mrk 231, which is consistent with that found the vast majority of that in dense molecular gas in the Milky Way (Goldsmith et al 2000;Pagani et al 2003). The only exceptions are Orion (Goldsmith et al 2011) (having significantly higher abundance in 10 −6 in a very small region) and ρ Oph (Larsson et al 2007;Liseau et al 2012) (with comparable abundance 5×10 −8 ).…”

How micron-sized dust particles determine the chemistry of our Universe

“…O 2 remained unseen allowing to derive some upper limits (X(O 2 ) ≤ 10 −7 2728). However, while recent Hershel observations confirmed the general trend that O 2 has very low abundances (detection of O 2 towards ρ Oph, X(O 2 )~10 −8 29), one isolated high abundance of O 2 has been reported towards ORION26.…”

Modelling CO formation in the turbulent interstellar medium

“…However, observations of a number of local star-forming regions with the Submillimeter Wave Astronomy Satellite (SWAS) and the Odin satellite find H 2 O and O 2 abundances that are more than a factor of 10 smaller (see e.g. Bergin et al 2000;Goldsmith et al 2000;Pagani et al 2003;Larsson et al 2007). This discrepancy is probably due to the neglect of freeze-out processes in our current study.…”