Oral Presentation

Nonthermal Velocity Dispersion in the Outer Disk of HL Tau

Author(s): Jinshi Sai (Kagoshima Univ.), Shigehisa Takakuwa (Kagoshima Univ.), Hsi-Wei Yen (ASIAA), Ysuke Tsukamoto (Kagoshima Univ./Konan Univ.), Yuya Fukuhara (ASIAA)

Presenter: Jinshi Sai (Kagoshima University)

Turbulence in protoplanetary disks plays a key role in disk evolution and planet formation, yet its strength remains poorly constrained in embedded Class 0/I systems. In this talk, I will present the first direct measurement of the nonthermal velocity dispersion in the late Class I disk of HL Tau using ALMA archival data of H2CO emission. With parametric model fitting, the nonthermal velocity dispersion is measured to be ~60% of sound speed near the disk midplane (z/R<~0.1) at outer radii (r>100 au). In contrast, previous studies inferred much weaker turbulence—below a few percent of the sound speed—in the inner disk (r<100 au) from the dust-ring scale heights. The large nonthermal motions in the outer disk may be explained by turbulence driven by gravitational instability or by infall from the envelope. Our results suggest a pronounced radial variation in the turbulence strength across the disk.