Dust properties from GALEX observations of a UV halo around Spica
  GALEX has detected ultraviolet halos extending as far as 5$^{\circ}$ around
four bright stars (Murthy et al. (2011)). These halos are produced by
scattering of starlight by dust grains in thin foreground clouds that are not
physically associated with the star. Assuming a simple model consisting of a
single layer of dust in front of the star, Murthy et al.(2011) have been able
to model these halo intensities and constrain the value of the phase function
asymmetry factor $g$ of the scattering grains in the FUV and NUV. However due
to the uncertainty in the dust geometry they could not constrain the albedo. In
this work we have tried to constrain the optical constants and dust geometry by
modeling the UV halo of Spica. Since the halo emission is not symmetric, we
have modeled the Northern and Southern parts of the halo separately. To the
North of Spica, the best-fit albedo is 0.26$\pm$0.1 and $g$ is 0.58$\pm$0.11 in
the FUV at the 90% confidence level. The corresponding limits on the distance
and optical depth ($\tau$) of the dust sheet is 3.65$\pm$1.05 pc and
0.047$\pm$0.006 respectively. However, owing to a complicated dust distribution
to the South of Spica, we were unable to uniquely constrain the dust parameters
in that region. Nevertheless, by assuming the optical constants of the Northern
region and assuming a denser medium, we were able to constrain the distance of
the dust to 9.5$\pm$1.5 pc and the corresponding $\tau$ to 0.04$\pm$0.01.
