High-Resolution SOFIA/EXES Spectroscopy of $\mathrm{SO}_2$ Gas in the
  Massive Young Stellar Object MonR2 IRS3: Implications for the Sulfur Budget
  Sulfur has been observed to be severely depleted in dense clouds leading to
uncertainty in the molecules that contain it and the chemistry behind their
evolution. Here, we aim to shed light on the sulfur chemistry in young stellar
objects (YSOs) by using high-resolution infrared spectroscopy of absorption by
the $\nu_3$ rovibrational band of SO$_2$ obtained with the
Echelon-Cross-Echelle Spectrograph on the Stratospheric Observatory for
Infrared Astronomy. Using local thermodynamic equilibrium models we derive
physical parameters for the SO$_2$ gas in the massive YSO MonR2 IRS3. This
yields a SO$_2$/$\mathrm{H}$ abundance lower limit of $5.6\pm0.5\times10^{-7}$,
or $>\!4\%$ of the cosmic sulfur budget, and an intrinsic line width (Doppler
parameter) of $b<3.20\;\mathrm{km\;s}^{-1}$. The small line widths and high
temperature ($T_\mathrm{ex}=234\pm15\;\mathrm{K}$) locate the gas in a
relatively quiescent region near the YSO, presumably in the hot core where ices
have evaporated. This sublimation unlocks a volatile sulfur reservoir (e.g.,
sulfur allotropes as detected abundantly in comet 67P/Churyumov--Gerasimenko),
which is followed by SO$_2$ formation by warm, dense gas-phase chemistry. The
narrowness of the lines makes formation of SO$_2$ from sulfur sputtered off
grains in shocks less likely toward MonR2 IRS3.
