Determining protoplanetary disk gas masses from CO isotopologues line observations

Abstract

Context. Despite intensive studies of protoplanetary disks, there is still no reliable way to determine their total (gast+dust) mass and their surface density distribution, quantities that are crucial for describing both the structure and the evolution of disks up to the formation of planets. Aims. The goal of this work is to use less abundant CO isotopologues, such as 13 CO, C 18 O and C 17 O, whose detection is routine for ALMA, to infer the gas mass of disks. Isotope-selective effects need to be taken into account in the analysis, because they can significantly modify CO isotopologues line intensities. Methods. CO isotope-selective photodissociation has been implemented in the physical-chemical code DALI (Dust And LInes) and more than 800 disk models have been run for a range of disk and stellar parameters. Dust and gas temperature structures have been computed self-consistently, together with a chemical calculation of the main atomic and molecular species. Both disk structure and stellar parameters have been investigated by varying the parameters in the grid of models. Total fluxes have been ray-traced for different CO isotopologues and for various low J− transitions for different inclinations. Results. A combination of 13 CO and C 18 O total intensities allows inference of the total disk mass, although with non-negligible uncertainties. These can be overcome by employing spatially resolved observations, i.e. the disk's radial extent and inclination. Comparison with parametric models by Williams & Best (2014) shows differences at the factor of a few level, especially for extremely low and high disk masses. Finally, total line intensities for different CO isotopologue and for various low-J transitions are provided and are fitted to simple formulae. The effects of a lower gas-phase carbon abundance and different gas-to-dust ratios are investigated as well, and comparison with other tracers is made. Conclusions. Disk masses can be determined within a factor of a few by comparing CO isotopologue lines observations with the simulated line fluxes provided in this paper, modulo the uncertainties in the volatile elemental abundances.

Far-infrared HD emission as a measure of protoplanetary disk mass

“…In addition, high mass disks have larger densities, which decreases the gas temperature in the disk. Miotello et al (2016) found a similar decrease in line luminosity for 13 CO and C 18 O for high mass disks, which they attributed to optical depth for 13 CO and increased freeze-out for C 18 O.…”

“…A possible explanation can be found in the gas mass of the disk. Miotello et al (2014Miotello et al ( , 2016 show that for disks with M disk ≥ 7 · 10 −3 M freeze-out is the dominant process affecting the C 18 O flux, whereas isotope-selective photodissociation is more important for low mass disks (see, e.g., Fig. 3 in Miotello et al 2016).…”

Apparent Disk-mass Reduction and Planetisimal Formation in GravitationallyUnstable Disks in Class 0/I Young Stellar Objects

“…Note, however, that the gas mass is estimated from the CO line emissions and as the authors noted, whether the large dust-togas mass ratio indicates rapid gas loss or depletion of CO by chemical evolution is unclear (see also Miotello et al 2016). Note also that the age of the objects are 1 3 Myr -and they are not in the Class 0/I phase.…”

Alma Measurements of Circumstellar Material in the Gq Lup System

“…A significant caveat to our derived gas mass is that it depends inversely on the CO/H 2 abundance ratio, which we assume to be ISM-like ∼10 −4 . Furthermore, recent work by Miotello et al (2016) suggests that a more complex analysis is required to accurately determine disk gas masses.…”