Multi-line detection of O₂towardρOphiuchi A

Abstract

Context. Models of pure gas-phase chemistry in well-shielded regions of molecular clouds predict relatively high levels of molecular oxygen, O 2 , and water, H 2 O. These high abundances imply high cooling rates, leading to relatively short timescales for the evolution of gravitationally unstable dense cores, forming stars and planets. Contrary to expectations, the dedicated space missions SWAS and Odin typically found only very small amounts of water vapour and essentially no O 2 in the dense star-forming interstellar medium. Aims. Only toward ρ Oph A did Odin detect a very weak line of O 2 at 119 GHz in a beam of size 10 arcmin. The line emission of related molecules changes on angular scales of the order of some tens of arcseconds, requiring a larger telescope aperture such as that of the Herschel Space Observatory to resolve the O 2 emission and pinpoint its origin. Methods. We use the Heterodyne Instrument for the Far Infrared (HIFI) aboard Herschel to obtain high resolution O 2 spectra toward selected positions in the ρ Oph A core. These data are analysed using standard techniques for O 2 excitation and compared to recent PDR-like chemical cloud models.Results. The N J = 3 3 −1 2 line at 487.2 GHz is clearly detected toward all three observed positions in the ρ Oph A core. In addition, an oversampled map of the 5 4 −3 4 transition at 773.8 GHz reveals the detection of the line in only half of the observed area. On the basis of their ratios, the temperature of the O 2 emitting gas appears to vary quite substantially, with warm gas ( > ∼ 50 K) being adjacent to a much colder region, of temperatures lower than 30 K. Conclusions. The exploited models predict that the O 2 column densities are sensitive to the prevailing dust temperatures, but rather insensitive to the temperatures of the gas. In agreement with these models, the observationally determined O 2 column densities do not seem to depend strongly on the derived gas temperatures, but fall into the range N(O 2 ) = 3 to > ∼ 6 × 10 15 cm −2 . Beam-averaged O 2 abundances are about 5 × 10 −8 relative to H 2 . Combining the HIFI data with earlier Odin observations yields a source size at 119 GHz in the range of 4 to 5 arcmin, encompassing the entire ρ Oph A core. We speculate that one of the reasons for the generally very low detection rate of O 2 is the short period of time during which O 2 molecules are reasonably abundant in molecular clouds.

The complexity of Orion: an ALMA view

“…But owing to the high sensitivity of this survey, one could have hoped to detect some of those species already known elsewhere in the interstellar medium, in comets, or mysteriously absent such as the first two we discuss now. (Goldsmith et al 2000) and Odin (Pagani et al 2003) spacecrafts, counterbalanced by a single detection in ρ Ophiuchi (Larsson et al 2007) with Odin, we detected O 2 towards Orion (Goldsmith et al 2011) and confirmed the detection towards ρ Ophiuchi, (Liseau et al 2012). However, the location of the emission in Orion is ill-defined and that made the interpretation of the observations difficult (Goldsmith et al 2011;Chen et al 2014;Melnick & Kaufman 2015).…”

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 .…”

Search for HOOH in Orion

“…Goldsmith et al 2000;Pagani et al 2003), with the definite detection of the molecule in merely two sources, viz. ρ Oph A (Larsson et al 2007;Liseau et al 2012) and Orion A (Goldsmith et al 2011;Chen et al 2014). Some cases have been either resolved or remained undecided (e.g.…”

“…Chen et al (2014) were able to pinpoint the location of the 9 O 2 source, near the position identified as H 2 -Peak 1 and somewhat offset from the hot core centre. The non-detection of the O 2 line at 1121 GHz led the authors to conclude that gas temperatures do not exceed 50 K, with best-fit model values more like 30 K. The excitation conditions thus resemble those in ρ Oph A (Liseau et al 2012). Du et al (2012) developed models for grain surface chemistry, and as an example, they considered the particular case of ρ Oph A.…”