Molecular Oxygen Abundance in Galactic Massive Star Formation Regions Based on SWAS Observations

Abstract

Molecular oxygen abundance is a key parameter in understanding the chemical network of the interstellar medium. We estimate the molecular oxygen column density and abundance for a sample of Galactic massive star formation regions based on observations from the Submillimiter Wave Astronomy Satellite ({\it SWAS}) survey. We obtained an averaged O$_2$ spectrum based on this sample using the ({\it SWAS}) survey data (O$_2$, 487.249 GHz, $N$ = 3-1, $J$ = 3-2). No emission or absorption feature is seen around the supposed central velocity with a total integration time of $t$$_{\rm{total}}$ = 8.67 $\times 10^3$ \ hours and a rms noise per channel of 1.45\ mK. Assuming \add{an kinetic temperature} T$_{\rm{kin}}$ = 30\ K, we derive the 3$\sigma$ upper limit of the O$_2$ column density to be 3.3 $\times 10^{15}$\ cm$^{-2}$, close to the lowest values reported in Galactic massive star formation regions in previous studies. The corresponding O$_2$ abundance upper limit is 7.6 $\times 10^{-8}$, lower than all previous results based on {\it SWAS} observations and is close to the lowest reported value in massive star formation regions. On a galactic scale, our statistical results confirm a generally low O$_2$ abundance for Galactic massive star formation regions. This abundance is also lower than results reported in extragalactic sources.