Mass constraints for 15 protoplanetary discs from HD 1-0

Context. Hydrogen deuteride (HD) rotational line emission can provide reliable protoplanetary disc gas mass measurements, but this molecule is difficult to observe and detections have been limited to three T Tauri discs. No new data have been available since the Herschel Space Observatory mission ended in 2013.
Aims: We set out to obtain new disc gas mass constraints by analysing upper limits on HD 1-0 emission in Herschel/PACS archival data from the DIGIT key programme.
Methods: With a focus on the Herbig Ae/Be discs, whose stars are more luminous than T Tauris, we determined upper limits for HD in data previously analysed for its line detections. We studied the significance of these limits with a grid of models run with the DALI physical-chemical code, customised to include deuterium chemistry.
Results: Nearly all the discs are constrained to M_gas ≤ 0.1 M_⊙, ruling out global gravitational instability. A strong constraint is obtained for the HD 163296 disc mass, M_gas ≤ 0.067 M_⊙, implying Δ_g/d ≤ 100. This HD-based mass limit is towards the low end of CO-based mass estimates for the disc, 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_gas limits for HD 163296 and HD 100546, both bright discs with massive candidate protoplanetary systems, suggest disc-to-planet mass conversion efficiencies of M_p/(M_gas + M_p) ≈ 10-40% for present-day values. Near-future observations with SOFIA/HIRMES will be able to detect HD in the brightest Herbig Ae/Be discs within 150 pc with ≈ 10 h integration time.