Magnetic field geometry of an unusual cometary cloud Gal 110-13
  We carried out optical polarimetry of an isolated cloud, Gal 110-13, to map
the plane-of-the-sky magnetic field geometry. The main aim of the study is to
understand the most plausible mechanism responsible for the unusual cometary
shape of the cloud in the context of its magnetic field geometry. When
unpolarized starlight passes through the intervening interstellar dust grains
that are aligned with their short axes parallel to the local magnetic field, it
gets linearly polarized. The plane-of-the-sky magnetic field component can
therefore be traced by doing polarization measurements of background stars
projected on clouds. Because the light in the optical wavelength range is most
efficiently polarized by the dust grains typically found in the outer layers of
the molecular clouds, optical polarimetry enables us to trace the magnetic
field geometry of the outer layers of the clouds. We made R-band polarization
measurements of 207 stars in the direction of Gal 110-13. The distance of Gal
110-13 was determined as $\sim450\pm80$ pc using our polarization and 2MASS
near-infrared data. The foreground interstellar contribution was removed from
the observed polarization values by observing a number of stars located in the
vicinity of Gal 110-13 which has Hipparcos parallax measurements. The
plane-of-the-sky magnetic field lines are found to be well ordered and aligned
with the elongated structure of Gal 110-13. Using structure function analysis,
we estimated the strength of the plane-of-the-sky component of the magnetic
field as $\sim25\mu$G. Based on our results and comparing them with those from
simulations, we conclude that compression by the ionization fronts from 10 Lac
is the most plausible cause of the comet-like morphology of Gal 110-13 and of
the initiation of subsequent star formation.
