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

Abstract

Context. Protoplanetary disks around young stars are the sites of planet formation. While the dust mass can be estimated using standard methods, determining the gas mass -and thus the amount of material available to form giant planets -has proven to be very difficult. Hydrogen deuteride (HD) is a promising alternative to the commonly-used gas mass tracer, carbon monoxide. However, its potential has not yet been investigated with models incorporating both HD and CO isotopologue-specific chemistry, and its sensitivity to uncertainties in disk parameters has not yet been quantified. Aims. To examine the robustness of HD as tracer of the disk gas mass, specifically the effect of gas mass on the HD far infrared emission and its sensitivity to the vertical structure. Also, to provide requirements for future far-infrared missions such as SPICA. Methods. Deuterium chemistry reactions relevant for HD were implemented in the thermochemical code DALI and more than 160 disk models were run for a range of disk masses and vertical structures. Results. The HD J=1-0 line intensity depends directly on the gas mass through a sublinear power law relation with a slope of ∼0.8. Assuming no prior knowledge about the vertical structure of a disk and using only the HD 1-0 flux, gas masses can be estimated to within a factor of two for low mass disks (M disk ≤ 10 −3 M ). For more massive disks, this uncertainty increases to more than an order of magnitude. Adding the HD 2-1 line or independent information about the vertical structure can reduce this uncertainty to a factor of ∼ 3 for all disk masses. For TW Hya, using the radial and vertical structure from Kama et al. (2016b) the observations constrain the gas mass to 6 · 10 −3 M ≤ M disk ≤ 9 · 10 −3 M . Future observations require a 5σ sensitivity of 1.8 · 10 −20 W m −2 (2.5 · 10 −20 W m −2 ) and a spectral resolving power R ≥ 300 (1000) to detect HD 1-0 (HD 2-1) for all disk masses above 10 −5 M with a line-to-continuum ratio ≥ 0.01. Conclusions. These results show that HD can be used as an independent gas mass tracer with a relatively low uncertainty and should be considered as an important science goal for future far-infrared missions.

Detectability of embedded protoplanets from hydrodynamical simulations

“…This system is the closest known protoplanetary disc to us (60.2 AU, Gaia Collaboration et al 2018), with a central star of 0.7-0.8 M (Andrews et al 2012;Herczeg & Hillenbrand 2014) relatively old (7-10 Myr, Ruane et al 2017), and with an estimated total disc mass of 0.05 M (Bergin et al 2013). The surface gas density has been modelled from ALMA line emission observations (Kama et al 2016;Trapman et al 2017). From their unperturbed models, we used a fiducial surface density of 10.9 g · cm −2 at 40 AU for the re-scaling (a surface density factor ×0.24 of the simulated disc).…”

Temperature, Mass, and Turbulence: A Spatially Resolved Multiband Non-LTE Analysis of CS in TW Hya

“…Modelling of the HD J = 1 − 0 transition, Bergin et al (2013) concluded that the mass of the disk must be M disk > 0.05 M sun . More recently, Trapman et al (2017) used additional observations of the J = 2 − 1 transition to find a mass of between 6 × 10 −3 and 9 × 10 −3 M sun . This large range is due primarily to the sensitivity of the HD emission to the assumed thermal structure and differences in assumed the cosmic D/H ratio.…”

Super-Earths in the TW Hya disc

“…At this mass the self-gravity of the disc is significant and gravitational instability may lead to disc fragmentation (Kratter & Lodato 2016). Trapman et al (2017), using additional constraints on the vertical structure from Kama et al (2016) and adding HD 2-1 line observations, suggest a gas mass, in daniel.mentiplay@monash.edu between these two extremes, of (6-9) × 10 −3 M . Recent carbon sulfide (CS) molecular observations find a minimum disc mass of 3 × 10 −4 M (Teague et al 2018).…”