2022ApJ...926..148F

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.