SiO Outflows as Tracers of Massive Star Formation in Infrared Dark
  Clouds
  To study the early phases of massive star formation, we present ALMA
observations of SiO(5-4) emission and VLA observations of 6 cm continuum
emission towards 32 Infrared Dark Cloud (IRDC) clumps, spatially resolved down
to $\lesssim 0.05$ pc. Out of the 32 clumps, we detect SiO emission in 20
clumps, and in 11 of them the SiO emission is relatively strong and likely
tracing protostellar outflows. Some SiO outflows are collimated, while others
are less ordered. For the six strongest SiO outflows, we estimate basic outflow
properties. In our entire sample, where there is SiO emission, we find 1.3 mm
continuum and infrared emission nearby, but not vice versa. We build the
spectral energy distributions (SEDs) of cores with 1.3 mm continuum emission
and fit them with radiative transfer (RT) models. The low luminosities and
stellar masses returned by SED fitting suggest these are early stage
protostars. We see a slight trend of increasing SiO line luminosity with
bolometric luminosity, which suggests more powerful shocks in the vicinity of
more massive YSOs. We do not see a clear relation between the SiO luminosity
and the evolutionary stage indicated by $L/M$. We conclude that as a protostar
approaches a bolometric luminosity of $\sim 10^2 \: L_{\odot}$, the shocks in
the outflow are generally strong enough to form SiO emission. The VLA 6 cm
observations toward the 15 clumps with the strongest SiO emission detect
emission in four clumps, which is likely shock ionized jets associated with the
more massive ones of these protostellar cores.
