Can we measure protoplanetary disk masses with CO observations?
Abstract
Gas in protostellar disks provides the raw material for giant planet formation and controls the dynamics of the planetesimal-building dust grains. Accurate gas-mass measurements provide insight into disk evolution and also help us to understand the formation environments of planets. While carbon monoxide (CO) is usually too optically thick to probe the entire mass budget in giant-planet forming regions, rare isotopologues of CO have been used as gas mass tracers for disks, assuming an interstellar CO/H2 abundance ratio. However, our chemical models of T-Tauri disks show that CO beyond 20 AU around a solar-type star is dissociated by He+, with the carbon becoming sequestered in complex organic molecules. Over million-year time scale, CO dissociation leads to a CO/H2 ratio that decreases both with distance from the star and as a function of time.In this dissertation talk, I will present radiative transfer simulations that assess the accuracy of CO-based disk mass measurements. The combination of CO chemical depletion in the outer disk and optically thick emission from the inner disk leads to gas mass estimates that are too low by over an order of magnitude, given the standard assumptions of interstellar CO/H2 ratio and optically thin emission. Furthermore, the million-year timescale of CO depletion introduces an age/mass degeneracy into observations. To reach factor of a few accuracy for CO-based disk mass measurements, we suggest that observers and modelers adopt the following strategies: (1) select the low-J transitions; (2) observe multiple CO isotopologues and use either intensity ratios or normalized line profiles to diagnose CO depletion; and (3) use spatially resolved observations to measure the CO abundance distribution.
- Publication:
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American Astronomical Society Meeting Abstracts #230
- Pub Date:
- June 2017
- Bibcode:
- 2017AAS...23040205Y