K-band High-resolution Spectroscopy of Embedded High-mass Protostars

Abstract

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 cm −3 ) at a single temperature of typically 100–200 K, and located within 200–600 au of the star.