Disk gas masses

 A Parametric Approach   Just as we use CO as a proxy for molecular gas in star forming clouds and cores, so we can use it in circumstellar disks where the bulk of the material is too cold for H2 to emit. Further, we can measure gas masses more accurately by observing the more optically thin tracers 13CO and C18O.   The difficulty in disks is that they contain a wide range of densities and temperatures and the CO emission only comes from a "warm molecular layer" between freeze-out in the midplane and photodissociation at the surface. An additional problem to date has been that the CO isotopologue lines are generally quite weak. Rather than model individual disks in detail, which will become impractical to do in the era of large ALMA surveys, we parameterize the disk structure and show that a combination of 13CO and C18O (which can be observed simultaneously) constrains disk gas masses to within a factor of 3-10. Details (and caveats) are in this ApJ paper that I wrote with Will Best during his second year research project. We provide a lookup table of CO and CO isotopologue integrated line fluxes for ~18000 disk models that can be used to compare with observations to estimate gas masses quickly and simply. An example of its use (and accompanying code) is shown below.   Our finding of low gas masses and gas/dust ratios may be an important clue to understanding the ubiquity of Neotunes and super-Earths (both of which should rapidly grow to Jupiters in the presence of abundant gas) in the exoplanet zoo.

 Look-up table Download Table 3 (comma separated values) of Williams & Best (2014) and a python script to read it and show the models that fit your 13CO and C18O integrated line intensities.   Usage: \$ ipython [1] from gasmass import * [2] gasmass(f13=4.0, f18=1.0, d=100, Jup=3, fcont=50)   Example shown to the left. Contact me if you have any questions.