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

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