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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$0.11 in the FUV at the 90% confidence level. The corresponding limits on the distance and optical depth ($τ$) of the dust sheet is 3.65$\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 $τ$ to 0.04$\pm$0.01.