2023ApJ...956..135W

The dominant form of oxygen in cold molecular clouds is gas-phase carbon monoxide (CO) and ice-phase water (H₂O). Yet, in planet-forming disks around young stars, gas-phase CO and H₂O are less abundant relative to their interstellar medium values, and no other major oxygen-carrying molecules have been detected. Some astrochemical models predict that gas-phase molecular oxygen (O₂) should be a major carrier of volatile oxygen in disks. We report a deep search for emission from the isotopologue ¹⁶O¹⁸O (N_J = 2₁ - 0₁ line at 233.946 GHz) in the nearby protoplanetary disk around TW Hya. We used imaging techniques and matched filtering to search for weak emission but do not detect ¹⁶O¹⁸O. Based on our results, we calculate upper limits on the gas-phase O₂ abundance in TW Hya of (6.4-70) × 10^–7 relative to H, which is 2-3 orders of magnitude below solar oxygen abundance. We conclude that gas-phase O₂ is not a major oxygen carrier in TW Hya. Two other potential oxygen-carrying molecules, SO and SO₂, were covered in our observations, which we also do not detect. Additionally, we report a serendipitous detection of the C¹⁵N N_J = 2_5/2 - 1_3/2 hyperfine transitions, F = 3 - 2 and F = 2 - 1, at 219.9 GHz, which we found via matched filtering and confirm through imaging.