Tracing the CO “ice line'' in an MRI-active protoplanetary disk with rare CO isotopologues



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The properties of planet-forming midplanes of protostellar disks remain largely unprobed by observations due to the high optical depth of common molecular lines and continuum. However, rotational emission lines from rare isotopologues may have optical depth near unity in the vertical direction, so that the lines are strong enough to be detected, yet remain transparent enough to trace the disk midplane. In this thesis, we present a chemical model of an MRI-active protoplanetary disk including different C, O isotopes and detailed photochemical reactions. The CO condensation front is found to be at 1.5 AU on the disk midplane around a solar like star, and its location remains almost unchanged during 3Myr of evolution. The optical depth of low-order rotational lines of C¹⁷O are around unity, which suggests it may be possible to see into the disk midplane using C¹⁷O. Such ALMA observations would provide estimates of the disk midplane temperature if the CO ice lines were spatially or spectrally resolved. With our computed C¹⁷O/H₂ abundance ratio, one would also be able to measure the disk masses by measuring the intensity of gas emission.