Modeling the H$\alpha$ Emission Surrounding Spica using the Lyman
  Continuum from a Gravity-darkened Central Star
  The large, faint H$\alpha$ emission surrounding the early B-star binary Spica
has been used to constrain the total hydrogen recombination rate of the nebula
and indirectly probe the Lyman continuum luminosity of the primary star. Early
analysis suggested that a stellar atmosphere model, consistent with Spica A's
spectral type, has a Lyman continuum luminosity about two times lower than
required to account for the measured H$\alpha$ surface brightness within the
nebula. To more consistently model both the stellar and nebular emission, we
have used a model atmosphere for Spica A which includes the effects of gravity
darkening as input to photoionization models to produce synthetic H$\alpha$
surface brightness distributions for comparison to data from the Southern
$H\alpha$ Sky Survey Atlas (SHASSA).
  This paper presents a method for the computation of projected surface
brightness profiles from 1D volume emissivity models and constrains both
stellar and nebular parameters. A mean effective temperature for Spica A of
$\simeq$ 24,800 K is sufficient to match both the observed absolute
spectrophotometry, from the far-UV to the near-IR, and radial H$\alpha$ surface
brightness distributions. Model hydrogen densities increase with the distance
from the star, more steeply and linearly towards the southeast. The northwest
matter-bounded portion of the nebula is predicted to leak $\sim$17% of Lyman
continuum photons. Model H II region column densities are consistent with
archival observations along the line of sight.
