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

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 (SWAS) survey. We obtained an averaged O₂ spectrum based on this sample using the (SWAS) survey data (O₂, 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 _total = 8.67 × 10³ hr and an rms noise per channel of 1.45 mK. Assuming a kinetic temperature T _kin = 30 K, we derive the 3σ upper limit of the O₂ column density to be 3.3 × 10¹⁵ cm⁻², close to the lowest values reported in Galactic massive star formation regions in previous studies. The corresponding O₂ abundance upper limit is 6.7 × 10⁻⁸, lower than all previous results based on 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 O₂ abundance for Galactic massive star formation regions. This abundance is also lower than results reported in extragalactic sources.