Deep Search For Molecular Oxygen in TW Hya

Abstract: The dominant form of oxygen in cold molecular clouds is gas-phase carbon monoxide (CO) and ice-phase water (H$2$O). Yet, in planet-forming disks around young stars, gas-phase CO and H$_2$O are less abundant relative to their ISM values, and no other major oxygen-carrying molecules have been detected. Some astrochemical models predict that gas-phase molecular oxygen (O$_2$) should be a major carrier of volatile oxygen in disks. We report a deep search for emission from the isotopologue $^{16}$O$^{18}$O ($N_J=2_1-0_1$ 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 $^{16}$O$^{18}$O. Based on our results, we calculate upper limits on the gas-phase O$_2$ abundance in TW Hya of $(6.4-70)\times10^{-7}$ relative to H, which is $2-3$ orders of magnitude below solar oxygen abundance. We conclude that gas-phase O$_2$ is not a major oxygen-carrier in TW Hya. Two other potential oxygen-carrying molecules, SO and SO$_2$, were covered in our observations, which we also do not detect. Additionally, we report a serendipitous detection of the C$^{15}$N $N_J = 2{5/2}-1_{3/2}$ hyperfine transitions, $F = 3 - 2$ and $F = 2 - 1$, at 219.9 GHz, which we found via matched filtering and confirm through imaging.