Understanding polarized dust emission from $\rho$ Ophiuchi A in light of
  grain alignment and disruption by radiative torques
  The alignment of dust grains with the ambient magnetic field produces
polarization of starlight as well as thermal dust emission. Using the archival
SOFIA/HAWC+ polarimetric data observed toward $\rho$ Ophiuchus (Oph) A cloud
hosted by a B association star at 89 $\mu$m and 154 $\mu$m, we find that the
fractional polarization of thermal dust emission first increases with the grain
temperature and then decreases once the grain temperature exceeds $\simeq$
25-32 K. The latter trend differs from the prediction of the popular RAdiative
Torques (RATs) alignment theory which implies a monotonic increase of the
polarization fraction with the grain temperature. We perform numerical modeling
of polarized dust emission for the $\rho$ Oph-A cloud and calculate the degree
of dust polarization by simultaneously considering the dust grain alignment and
rotational disruption by RATs. Our modeling results could successfully
reproduce both the rising and declining trends of the observational data.
Moreover, we show that the alignment of only silicate grains or a mixture of
silicate-carbon grains within a composite structure can reproduce the
observational trends, assuming that all dust grains follow a power-law size
distribution. Although there are a number of simplifications and limitations to
our modeling, our results suggest grains in $\rho$ Oph-A cloud have a composite
structure, and the grain size distribution has steeper slope than the standard
size distribution for the interstellar medium. Combination of SOFIA/HAWC+ data
with JCMT observations 450 $\mu$m and 850 $\mu$m would be useful to test the
proposed scenario based on grain alignment and disruption by RATs.
