Dust Rotational Dynamics in C-shocks: Rotational Disruption of
  Nanoparticles by Stochastic Mechanical Torques and Spinning Dust Emission
  Polycyclic aromatic hydrocarbons (PAHs) and nanoparticles are expected to
play an important role in many astrophysical processes due to its dominant
surface area, including gas heating, chemistry, star formation , and anomalous
microwave emission. In dense magnetized molecular clouds where C-shocks are
present, PAHs and nanoparticles are widely believed to originate from grain
shattering due to grain-grain collisions. The remaining question is whether
these nanoparticles can survive in the dense and hot shocked regions, and how
to constrain their size and abundance with observations. In this paper, we
present a new mechanism to destroy nanoparticles in C-shocks based on
centrifugal stress within rapidly spinning nanoparticles spun-up by stochastic
atomic bombardment, which is termed rotational disruption. We find that, due to
supersonic neutral gas-charged grain drift in C-shocks, nanoparticles can be
spun-up to suprathermal rotation by stochastic torques exerted by supersonic
neutral flow. The resulting centrifugal stress within suprathermally rotating
nanoparticles can exceed the maximum tensile strength of grain material
($S_{\max}$), resulting in rapid disruption of nanoparticles smaller than
$a\sim 1$ nm for $S_{\max}\sim 10^{9}\erg\cm^{-3}$. The proposed disruption
mechanism is shown to be more efficient than thermal sputtering in controlling
the lower cutoff of grain size distribution in C-shocks. We model microwave
emission from spinning nanoparticles in C-shocks subject to supersonic neutral
drift and rotational disruption. We find that suprathermally rotating
nanoparticles can emit strong microwave radiation, and both peak flux and peak
frequency increase with increasing the shock velocity. We suggest spinning dust
as a new method to constrain nanoparticles and trace shock velocities in dense,
shocked regions.
