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Title: How much gas do protoplanetary disks contain?
Plain: Most of a disk's mass is hydrogen gas, which is nearly invisible, so astronomers weigh it through rarer molecules such as carbon monoxide. Those molecules give masses far below expectations, either because the disks are gas-poor or because the carbon monoxide has frozen onto grains or been destroyed.
Precise: Gas masses inferred from CO isotopologue lines are often $10\text{-}100$ times below $100\,M_{\mathrm{dust}}$, the value for an interstellar gas-to-dust ratio. Determine whether disks are truly gas-poor or CO is depleted by freeze-out and chemical conversion, by calibrating CO against independent tracers (HD rotational lines, N2H+, dynamical masses from rotation curves). The answer is the disk gas-mass distribution versus stellar mass and age with uncertainty below a factor of 3.
Settled by: HD 112 micron line measurements for a sample of disks by a far-infrared spectrometer, or dynamical disk masses from deviations from Keplerian rotation, compared with CO-based masses for the same disks.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: open
Status note: (empty)
Posed since: (empty)