  The oxygen isotope fractionation scenario, which has been developed to
explain the oxygen isotope anomaly in the solar system materials, predicts that
CO gas is depleted in 18O in protoplanetary disks, where segregation between
solids and gas inside disks had already occurred. Based on ALMA 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 TW Hya disk from optically thin HCO+ isotopologues as a proxy of
optically thicker CO isotopologues. Using the H13CO+(4-3) data previously
obtained with 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 ISM (8.1 +- 0.8) within error
margin. Then there is no clear evidence of 18O depletion in CO gas of the disk,
although we could not draw any robust conclusion due to large uncertainties. In
conclusion, optically thin lines of HCO+ isotopologues are useful tracers of CO
isotopic ratios, which are hardly constrained directly from optically thick
lines of CO isotopologues. Future higher sensitivity observations of H13CO+ and
HC18O+ would be able to allow us to better constrain the oxygen fractionation
in the disk.
