tokens: This observational evidence makes HL Tau an example of a young (∼1 Myr; Briceño et al. 2002 ), embedded, disc-hosting star. HL Tau has therefore become a prime target for studies aiming to characterize young discs (Greaves et al. 2008 ; Carrasco-González et al. 2009 ). In particular, HL Tau was the first circumstellar disc observed with the long baselines of Atacama Large Millimeter/submillimeter Array (ALMA). This revealed an ordered series of concentric rings and gaps in the disc across multiple millimetre wavelengths (2.9, 1.3, and 0.9 mm; ALMA Partnership et al. 2015 ). Many theories have been put forward to explain the origin of these structures (e.g. Zhang, Blake Bergin 2015 ; Okuzumi et al. 2016 ), but one of the most persistent involves the growth of planets within the disc (Dipierro et al. 2015b ). While efforts to detect thermal emission from young planets in the gaps have been unsucessful, the corresponding upper limits on their masses still lie at the higher end of the giant-planet regime (10–15 M Jup ; Testi et al. 2015 ). The combination of the potential for massive planets at large radii, coupled with evidence of a high disc mass (∼0.1–0.13 M ⊙ ; Guilloteau et al. 2011 ; Kwon, Looney Mundy 2011 ), has seen gravitational instability (GI; Boss 1997 ) being invoked to explain both the observed dust structures in HL Tau (Akiyama et al. 2016 ; Takahashi Inutsuka 2016 ), along with the origin of any planetary companions (Nero Bjorkman 2009 ). However, recent measurements of the gas mass in the HL Tau disc using have shown appears to be too low for GI to occur ( M gas = 2–40 × 10 −3 M ⊙ ; Wu et al. 2018 ). In this Letter, we present a detection of the J = 3–2 transition towards the disc around HL Tau. We have demonstrated this line to be a robust tracer of disc gas mass (Booth et al. 2019 ). We use these data to derive a lower limit to the total gas disc mass and discuss how this impacts the gravitational stability of the HL Tau star–disc system. 2 OBSERVATIONS HL Tau was observed by ALMA in Band 7 on 2017 November 24 for 1.46 h with 49 antennae in configuration C43-8 under project code 2017.1.01178.S (PI: E. Humphreys). Baseline lengths ranged from 92 to 8547 m, and the spectral window (SPW) covering the J = 3–2 transition (321.852 GHz) had a native spectral resolution of 0.908 km s −1 (which we note does not resolve the J = 3–2 hyperfine structure). Data (self-)calibration and imaging were performed with casa version 5.1.1 (McMullin et al. 2007 ). Continuum subtraction was performed in the uv -plane using a constant baseline (fitorder 0). Line imaging was performed with the clean algorithm, using a Keplerian mask with the measured position and inclination angles of the disc (e.g. PA = 138° and i = 46°; ALMA Partnership et al. 2015 ). The native resolution of the data resulted in a natural beam size of 0.14 × 0.11 arcsec 2 (−13°). We used a 0.2-arcsec uv -taper, which was found to maximize the signal-to-noise ratio (S/N) of the line
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