2021ApJ...912..108H

A classical paradox in high-mass star formation is that powerful radiation pressure can halt accretion, preventing further growth of a central star. Disk accretion has been proposed to solve this problem, but the disks and the accretion process in high-mass star formation are poorly understood. We executed high-resolution (R = 35,000-70,000) iSHELL spectroscopy in K-band for 11 high-mass protostars. Br-γ emission was observed toward eight sources, and the line profiles for most of these sources are similar to those of low-mass PMS stars. Using an empirical relationship between the Br-γ and accretion luminosities, we tentatively estimate disk accretion rates ranging from <=10^–8 and ∼10^–4 M_☉ yr^–1. These low-mass-accretion rates suggest that high-mass protostars gain more mass via episodic accretion as proposed for low-mass protostars. Given the detection limits, CO overtone emission (v = 2-0 and 3-1), likely associated with the inner disk region (r ≪ 100 au), was found toward two sources. This low-detection rate compared with Br-γ emission is consistent with previous observations. Ten out of the 11 sources show absorption at the v = 0-2 R(7) - R(14) CO R-branch. Most of them are either blueshifted or redshifted, indicating that the absorption is associated with an outflow or an inflow with a velocity of up to ∼50 km s^–1. Our analysis indicates that the absorption layer is well thermalized (and therefore n_H2_≳10⁶ cm^–3) at a single temperature of typically 100-200 K, and located within 200-600 au of the star.