Retrievals of Protoplanetary Disk Parameters Using Thermochemical Models. I. Disk Gas Mass from Hydrogen Deuteride Spectroscopy

Abstract

Abstract We discuss statistical relationships between the mass of protoplanetary disks, the hydrogen deuteride (HD) line flux, and the dust spectral energy distribution (SED) determined using 3000 ProDiMo disk models. The models have 15 free parameters describing the disk physical properties, the central star, and the local radiation field. The sampling of physical parameters is done using a Monte Carlo approach to evaluate the probability density functions of observables as a function of physical parameters. We find that the mass-averaged HD fractional abundance is almost constant even though the ultraviolet flux varies by several orders of magnitude. Probing the statistical relation between the physical quantities and the HD flux, we find that low-mass (optically thin) disks display a tight correlation between the average disk gas temperature and HD line flux, while massive disks show no such correlation. We demonstrate that the central star luminosity, disk size, dust size distribution, and HD flux may be used to determine the disk gas mass within a factor of 3. We also find that the far-infrared and submillimeter/millimeter SEDs and the HD flux may serve as strong constraints for determining the disk gas mass to within a factor of 2. If the HD lines are fully spectrally resolved (R ≳ 1.5 × 106, Δv = 0.2 km s−1), the 56 and 112 μm HD line profiles alone may constrain the disk gas mass to within a factor of 2.