DiskMINT: Self-Consistent Thermochemical Disk Models with Radially Varying Gas and Dust -- Application to the Massive, CO-Rich Disk of IM Lup

Abstract

Disks around young stars are the birthplaces of planets, and the spatial distribution of their gas and dust masses is critical for understanding where and what types of planets can form. We present self-consistent thermochemical disk models built with DiskMINT, which extends its initial framework to allow for spatially decoupled gas and dust distributions. DiskMINT calculates the gas temperature based on thermal equilibrium with dust grains, solves vertical gas hydrostatic equilibrium, and includes key processes for the CO chemistry, specifically selective photodissociation, and freeze-out with conversion CO/CO$_2$ ice. We apply DiskMINT to study the IM Lup disk, a large massive disk, yet with an inferred CO depletion of up to 100 based on earlier thermochemical models. By fitting the multi-wavelength SED along with the millimeter continuum, ${\rm C^{18}O}$ radial emission profiles, we find $0.02-0.08\,{\rm M_\odot}$ for the gas disk mass, which are consistent with the dynamical-based mass within the uncertainties. We further compare the derived surface densities for dust and gas and find that the outer disk is drift-dominated, with a dust-to-gas mass ratio of approximately 0.01-0.02, which is likely insufficient to meet the conditions for the streaming instability to occur. Our results suggest that when interpreted with self-consistent thermochemical models, ${\rm C^{18}O}$ alone can serve as a reliable tracer of both the total gas mass and its radial distribution. This approach enables gas mass estimates in lower-mass disks, where dynamical constraints are not available, and in fainter systems where rare species like ${\rm N_2H^+}$ are too weak to detect.