Tracing the gas masses of Herbig disks

Lucas Stapper, Michiel Hogerheijde, Ewine van Dishoeck

One of the driving parameters for the evolution of protoplanetary disks is the total mass of the disk. However, determining this mass is no easy feat. Given that the most direct mass tracer H2 is difficult to detect, especially given the cold environments of the disk, CO is often used as the second-best option. However, in the disks around T Tauri stars CO undergoes freeze-out onto the grains, subsequently reducing the abundance of gas-phase CO. The intermediate mass pre-main sequence stars called Herbig stars are much warmer than T Tauri stars, which increases the temperature of the disk and decreases the amount of CO freeze-out from the CO emitting layer. This work compiled a sample of 35 Herbig disks, containing 30 ALMA archival data sets and 5 new NOEMA observations, covering 12CO, 13CO and/or C18O. Using a thermo-chemical code, we obtained mass estimates from the luminosity of the 13CO and C18O lines, and find that for most Herbig disks even C18O is optically thick. Comparing the gas mass estimates with the masses from continuum observations, we find that all disks except the largest ones are consistent with a gas-to-dust ratio of 100. In addition, for all detected disks we obtain a measure of the size of the disk. Given their dust disk size, we find that 4 disks are consistent with being radial drift dominated, and we find in general a larger difference between the gas and dust disk sizes than for the disks in the Lupus star forming region. In addition, we explore other methods to derive the gas surface density profile and mass using a suite of CO isotopologues.

[Poster PDF File]