K-band High-Resolution Spectroscopy of Embedded Massive Protostars
  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-$\gamma$ 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-$\gamma$
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-$\gamma$ emission is consistent with
previous observations. Ten out of the eleven sources show absorption at the
$v$=0-2 ${\rm R(7)-R(14)}$ 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 $\sim50$ 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.
