Modeling accretion shocks at the disk-envelope interface -- Sulfur
  chemistry
  As material from an infalling protostellar envelope hits the forming disk, an
accretion shock may develop which could (partially) alter the envelope material
entering the disk. Observations with the Atacama Large Millimeter/submillimeter
Array (ALMA) indicate that emission originating from warm SO and SO$_2$ might
be good tracers of such accretion shocks. The goal of this work is to test
under what shock conditions the abundances of gas-phase SO and SO$_2$ increase
in an accretion shock at the disk-envelope interface. Detailed shock models
including gas dynamics are computed using the Paris-Durham shock code for
non-magnetized J-type accretion shocks in typical inner envelope conditions.
The effect of pre-shock density, shock velocity, and strength of the
ultraviolet (UV) radiation field on the abundance of warm SO and SO$_2$ is
explored. Warm gas-phase chemistry is efficient in forming SO under most J-type
shock conditions considered. In lower-velocity (~3 km/s) shocks, the abundance
of SO is increased through subsequent reactions starting from thermally
desorbed CH$_4$ toward H$_2$CO and finally SO. In higher velocity (>4 km/s)
shocks, both SO and SO$_2$ are formed through reactions of OH and atomic S. The
strength of the UV radiation field is crucial for SO and in particular SO$_2$
formation through the photodissociation of H$_2$O. Thermal desorption of SO and
SO$_2$ ice is only relevant in high-velocity (>5 km/s) shocks at high densities
($>10^7$ cm$^{-3}$). Warm emission from SO and SO$_2$ is a possible tracer of
accretion shocks at the disk-envelope interface as long as a local UV field is
present. Additional observations with ALMA at high-angular resolution could
provide further constraints. Moreover, the James Webb Space Telescope will give
access to other possible slow, dense shock tracers such as H$_2$, H$_2$O, and
[S I] 25$\mu$m.
