2018ApJ...869...59W

We report our Submillimeter Array (SMA) observations of the Class I-II protostar HL Tau in ¹³CO (2-1), C¹⁸O (2-1), SO(5₆-4₅), and 1.3 mm dust-continuum emission and our analyses of the ALMA long baseline data of HCO⁺ (1-0) emission. The 1.3 mm continuum emission observed with the SMA shows compact (∼0.''8 x 0.''5) and extended (∼6.''5 x 4.''3) components, tracing the protoplanetary disk and the protostellar envelope, respectively. The ¹³CO, C¹⁸O, and HCO⁺ show a compact (∼200 au) component at velocities higher than 3 km s^–1 from the systemic velocity and an extended (∼1000 au) component at lower velocities. The high-velocity component traces the Keplerian rotating disk, and the low-velocity component traces the infalling envelope. The HCO⁺ high-velocity component is fitted with a Keplerian disk model with a central stellar mass of 1.4 M_☉. The radial intensity profiles of ¹³CO and C¹⁸O along the disk major axis are fitted with a disk+envelope model, and the gas masses of the disk and envelope are estimated to be (2-40) x 10^–4 M_☉ and 2.9×10^–3M_☉, respectively. The disk dust mass has been estimated to be (1-3) x 10^–3 M_☉ in the literature. Thus, our estimated disk gas mass suggests that the gas-to-dust mass ratio in the disk is <10, a factor of 10 lower than the estimated ratio in the envelope. We discuss possible gas depletion or CO depletion in the planet-forming candidate HL Tau in the context of disk and envelope evolution.