Chemistry on Rotating Grain Surface: Ro-Thermal Desorption of Molecules
  from Ice Mantles
  It is widely believed that water and complex organic molecules (COMs) first
form in the ice mantle of dust grains and are subsequently returned into the
gas due to grain heating by intense radiation of protostars. Previous research
on the desorption of molecules from the ice mantle assumed that grains are at
rest which is contrary to the fact that grains are suprathermally rotating as a
result of their interaction with an anisotropic radiation or gas flow. {To
clearly understand how molecules are released in to the gas phase, the effect
of grain suprathermal rotation on surface chemistry must be quantified}. In
this paper, we study the effect of suprathermal rotation of dust grains spun-up
by radiative torques on the desorption of molecules from icy grain mantles
around protostars. We show that centrifugal potential energy due to grain
rotation reduces the potential barrier of molecules and significantly enhances
their desorption rate. We term this mechanism {\it rotational-thermal} or {\it
ro-thermal} desorption. We apply the ro-thermal mechanism for studying the
desorption of molecules from icy grains which are simultaneously heated to high
temperatures and spun-up to suprathermal rotation by an intense radiation of
protostars. We find that ro-thermal desorption is much more efficient than
thermal desorption for molecules with high binding energy such as water and
COMs. Our results have important implications for understanding the origin of
COMs detected in star-forming regions and call for attention to the effect of
suprathermal rotation of icy grains to use molecules as a tracer of physical
conditions of star-forming regions.
