Detection of HC18O+ in a Protoplanetary Disk: Exploring Oxygen Isotope Fractionation of CO

Abstract The oxygen isotope fractionation scenario, which has been developed to explain the oxygen isotope anomaly in solar system materials, predicts that CO gas is depleted in 18O in protoplanetary disks, where segregation between solids and gas inside disks has already occurred. Based on Atacama Large Millimeter/submillimeter Array observations, we report the first detection of HC18O+(4–3) in a Class II protoplanetary disk (TW Hya). This detection allows us to explore the oxygen isotope fractionation of CO in the disk from optically thin HCO+ isotopologues as a proxy of optically thicker CO isotopologues. Using the H13CO+(4–3) data previously obtained with the SMA, we find that the H13CO+/HC18O+ ratio in the central ≲100 au regions of the disk is 10.3 ± 3.2. We construct a chemical model of the TW Hya disk with carbon and oxygen isotope fractionation chemistry, and estimate the conversion factor from H13CO+/HC18O+ to 13CO/C18O. With the conversion factor (=0.8), the 13CO/C18O ratio is estimated to be 8.3 ± 2.6, which is consistent with the elemental abundance ratio in the local interstellar medium (8.1 ± 0.8) within the error margin. Therefore, there is no clear evidence of 18O depletion in CO gas in the central ≲100 au regions of the disk, although we could not draw a robust conclusion due to uncertainties. In conclusion, optically thin lines of HCO+ isotopologues are useful tracers of CO isotopic ratios, which are not very constrained directly from optically thick lines of CO isotopologues. Future higher sensitivity observations of H13CO+ and HC18O+ would allow us to better constrain the oxygen fractionation in the disk.