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

Abstract

We report our SMA observations of the Class \RN{1}-\RN{2} protostar HL Tau in the ${}^{13}$CO (2--1), C${}^{18}$O (2--1), SO(5${}_6$--4${}_5$), and the 1.3 mm dust-continuum emission and our analyses of the ALMA long baseline data of the HCO${}^{+}$ (1--0) emission. The 1.3 mm continuum emission observed with the SMA shows compact ($\sim$0$\farcs$8 $\times$ 0$\farcs$5) and extended ($\sim$6$\farcs$5 $\times$ 4$\farcs$3) components, tracing the protoplanetary disk and the protostellar envelope, respectively. The ${}^{13}$CO, C${}^{18}$O, and HCO${}^{+}$ show a compact ($\sim$ 200 AU) component at velocities higher than 3 km s${}^{-1}$ from the systemic velocity and an extended ($\sim$ 1000 AU) component at the 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 $M_{\odot }$. The radial intensity profiles of the ${}^{13}$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 $2\mbox{--}40\times10^{-4}$ $M_{\odot }$ and $2.9\times 10^{-3}$ $M_{\odot }$, respectively. The disk dust mass has been estimated to be $1\mbox{--}3 \times 10^{-3}$ $M_{\odot }$ 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 ten lower than the estimated ratio in the envelope. We discuss the possible gas depletion or CO depletion in the planet-forming candidate HL Tau in the context of disk and envelope evolution.

27 pages, 11 figures, Accepted for publication in the ApJ