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 .