Molecular oxygen in the ρ Ophiuchi cloud ***

Context.Molecular oxygen, O2, 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 O2 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 O2. Starting in 2002, the star forming molecular cloud core  was observed with Odin for 34 days during several observing runs. Results.We detect a spectral line at vLSR km s-1 with km s-1, parameters which are also common to other species associated with . This feature is identified as the O2 () transition at 118 750.343 MHz. Conclusions.The abundance of molecular oxygen, relative to H2 , is averaged over the Odin beam. This abundance is consistently lower than previously reported upper limits.