Physical and Chemical Conditions of the Protostellar Envelope and the Protoplanetary Disk in HL Tau

We report our Submillimeter Array (SMA) observations of the Class I–II protostar HL Tau in 13 CO (2–1), C 18 O (2–1), SO(5 6 –4 5 ), 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 × 0.″5) and extended (∼6.″5 × 4.″3) components, tracing the protoplanetary disk and the protostellar envelope, respectively. The 13 CO, C 18 O, 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 13 CO and C 18 O 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) × 10 –4 M ⊙ and M ⊙ , respectively. The disk dust mass has been estimated to be (1–3) × 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.