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 Submillimeter 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 ttotal = 8.67 ×103 hours and a rms noise per channel of 1.45 mK. Assuming an kinetic temperature Tkin = 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 7.6 ×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.