K-band High-Resolution Spectroscopy of Embedded Massive 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 eleven high-mass protostars. Br-${\hat{I}}^3$ 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-${\hat{I}}^3$ and accretion luminosities, we tentatively estimate disk accretion rates ranging from $\lesssim$10${}^{-8}$ and $\sim$10${}^{-4}$ $M_{\odot }$ 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 ($r\ll 100$ au), was found towards two sources. This low-detection rate compared with Br-${\hat{I}}^3$ emission is consistent with previous observations. Ten out of the eleven sources show absorption at the $v$=0-2 $\mathrm{R}(7)-\mathrm{R}(14)$ 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 $\sim 50$ km s${}^{-1}$. Our analysis indicates that the absorption layer is well thermalized (and therefore $n_{{\mathrm{H}}_2}\gtrsim 10^6$ cm${}^{-3}$) at a single temperature of typically 100-200 K, and located within 200-600 au of the star.

18 page, 7 figures, accepted to ApJ