Physical Model of Dust Polarization by Radiative Torque Alignment and
  Disruption and Implications for Grain Internal Structures
  Dust polarization depends on the physical and mechanical properties of dust,
as well as the properties of local environments. To understand how dust
polarization varies with grain mechanical properties and the local environment,
in this paper, we model the wavelength-dependence polarization of starlight and
polarized dust emission by aligned grains by simultaneously taking into account
grain alignment and rotational disruption by radiative torques (RATs). We
explore a wide range of the local radiation field and grain mechanical
properties characterized by tensile strength. We find that the maximum
polarization and the peak wavelength shift to shorter wavelengths as the
radiation strength $U$ increases due to the enhanced alignment of small grains.
Grain rotational disruption by RATs tends to decrease the optical-near infrared
polarization but increases the ultraviolet polarization of starlight due to the
conversion of large grains into smaller ones. In particular, we find that the
submillimeter (submm) polarization degree at $850~\mu \rm m$ ($P_{850}$) does
not increase monotonically with the radiation strength or grain temperature
($T_{d}$), but it depends on the tensile strength of grain materials. Our
physical model of dust polarization can be tested with observations toward
star-forming regions or molecular clouds irradiated by a nearby star, which
have higher radiation intensity than the average interstellar radiation field.
Finally, we compare our predictions of the $P_{850}-T_{d}$ relationship with
{\it Planck} data and find that the observed decrease of $P_{850}$ with $T_{d}$
can be explained when grain disruption by RATs is accounted for, suggesting
that interstellar grains unlikely to have a compact structure but perhaps a
composite one. The variation of the submm polarization with U (or $T_{d}$) can
provide a valuable constraint on the internal structures of cosmic dust.
