Mass constraints for 15 protoplanetary disks from HD 1-0

Abstract

Hydrogen deuteride (HD) rotational line emission can provide reliable protoplanetary disk gas mass measurements, but it is difficult to observe and detections have been limited to three T-Tauri disks. No new data have been available since the \emph{Herschel} Space Observatory mission ended in 2013. We set out to obtain new disk gas mass constraints by analysing upper limits on HD 1-0 emission in \emph{Herschel}/PACS archival data from the DIGIT key programme. With a focus on the Herbig Ae/Be disks, whose stars are more luminous than T Tauris, we determine upper limits for HD in data previosly analysed for its line detections. Their significance is studied with a grid of models run with the DALI physical-chemical code, customised to include deuterium chemistry. Nearly all the disks are constrained to $M_{\mathrm{gas}}\le 0.1$M${}_{\odot }$, ruling out global gravitational instability. A strong constraint is obtained for the HD 163296 disk mass, $M_{\mathrm{gas}}\le 0.067$M${}_{\odot }$, implying $\hat{I}{”}_{g/d}\le 100$. This HD-based mass limit is towards the low end of CO-based mass estimates for the disk, highlighting the large uncertainty in using only CO and suggesting that gas-phase CO depletion in HD 163296 is at most a factor of a few. The $M_{\mathrm{gas}}$ limits for HD 163296 and HD 100546, both bright disks with massive candidate protoplanetary systems, suggest disk-to-planet mass conversion efficiencies of $M_{\mathrm{p}}/(M_{\mathrm{gas}}+M_{\mathrm{p}})\approx 10$ to $40$% for present-day values. Near-future observations with SOFIA/HIRMES will be able to detect HD in the brightest Herbig~Ae/Be disks within $150$pc with $\approx 10$h integration time.

Accepted for publication in A&A