Hot Salty Disks around Massive Protostars

Massive stars play pivotal roles in various dynamical and chemical aspects of the Universe, and it is of prime importance to understand their formation process. A key question is whether or not massive stars form via disk accretion similarly to low-mass stars. One difficulty in disk search around massive protostars is the lack of knowledge of which lines can trace the innermost region and separate the disk from the envelope. Based on the recent high-resolution ALMA observations (e.g., Zhang+Tanaka et al. 2019a,b,c; Tanaka, Zhang et al. 2020), we propose the new sets of emission lines as the massive disk tracers at the scale of 100 au: (1) lines of refractory molecules, e.g., silicon compounds and alkali metal halides, (2) high-energy molecular lines of Eu/k>1,000K, and (3) hydrogen recombination lines from ionized gas of 10,000K. The presence of gaseous refractory molecules, which originate from the destruction of dust grains, and high-energy lines suggests the dynamic and hot nature of disks around massive protostars. Developing from the hot-core chemistry induced by the ice sublimation (~100K), "hot disks" will open a new avenue of astrochemistry with a unique connection to planetary science. We also note that the lower-energy transitions of refractory molecules and the recombination lines are excellent targets for future radio observations by ngVLA.