2010A&A...510A..98L

Contrary to theoretical expectation, surprisingly low concentrations of molecular oxygen, O₂, have been found in the interstellar medium. Telluric absorption makes ground based O₂ observations essentially impossible and observations had to be done from space. Millimetre-wave telescopes on space platforms were necessarily small, which resulted in large, several arcminutes wide, beam patterns. Observations of the (N_J=1₁-1₀) ground state transition of O₂ with the Odin satellite resulted in a >5σ detection toward the dense core ρ Oph A. At the frequency of the line, 119GHz, the Odin telescope has a beam width of 10', larger than the size of the dense core. The precise nature of the emitting source and its exact location and extent are therefore unknown. The current investigation is intended to remedy this. Although the Earth's atmosphere is entirely opaque to low-lying O₂ transitions, it allows ground based observations of the much rarer ¹⁶O¹⁸O in favourable conditions and at much higher angular resolution with larger telescopes. In addition, ρ Oph A exhibits both multiple radial velocity systems and considerable velocity gradients. Extensive mapping of the region in the proxy C¹⁸O (J=3-2) line can be expected to help identify the O₂ source on the basis of its line shape and Doppler velocity. Line opacities were determined from observations of optically thin ¹³C¹⁸O (J=3-2). During several observing periods, two C¹⁸O intensity maxima in ρ Oph A were searched for O¹⁸O in the (2₁-0₁) line at 234GHz with the 12m APEX telescope. These positions are associated also with peaks in the mm-continuum emission from dust. Our observations resulted in an upper limit on the integrated O¹⁸O intensity of ∫T^*_Adυ<0.01K.km/s (3σ) into the 26.5" beam. Together with the C¹⁸O data, this leads to a ratio of N(C¹⁸O)/N(O¹⁸O)>16. Combining Odin's O₂ with the present O¹⁸O observations we infer an O₂ abundance 5x10^–7<X(O₂)≲2.5x10^–6. Examining the evidence, which is based primarily on observations in lines of O¹⁸O and C¹⁸O, leads us to conclude that the source of observed O₂ emission is most likely confined to the central regions of the ρ Oph A core. In this limited area, implied O₂ abundances could thus be higher than inferred on the basis of Odin observations (5x10^–8) by up to two orders of magnitude.