Molecular oxygen in the ρ Ophiuchi cloud

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 1.1m 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 rho OphA 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 = vFWHM = 1.5 km s(-1), parameters which are also common to other species associated with rho OphA. 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 x 10(-8) averaged over the Odin beam. This abundance is consistently lower than previously reported upper limits.