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 ν LSR = +3.5km s -1 with Δν 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 118750.343 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. © ESO 2007.