Far-Ultraviolet Observations of the Spica Nebula and the Interaction
  Zone
  We report the analysis results of far ultraviolet (FUV) observations, made
for a broad region around $\alpha$ Vir (Spica) including the interaction zone
of Loop I and the Local Bubble. The whole region was optically thin and a
general correlation was seen between the FUV continuum intensity and the dust
extinction, except in the neighborhood of the bright central star, indicating
the dust scattering nature of the FUV continuum. We performed Monte-Carlo
radiative transfer simulations to obtain the optical parameters related to the
dust scattering as well as the geometrical structure of the region. The albedo
and asymmetry factor were found to be 0.38$\pm$0.06 and 0.46$\pm$0.06,
respectively, in good agreement with the Milky Way dust grain models. The
distance to and the thickness of the interaction zone were estimated to be
70$^{+4}_{-8}$ pc and 40$^{+8}_{-10}$ pc, respectively. The diffuse FUV
continuum in the northern region above Spica was mostly the result of
scattering of the starlight from Spica, while that in the southern region was
mainly due to the background stars. The \ion{C}{4} $\lambda\lambda$1548, 1551
emission was found throughout the whole region, in contrast to the \ion{Si}{2}*
$\lambda$1532 emission which was bright only within the \ion{H}{2} region. This
indicates that the \ion{C}{4} line arises mostly at the shell boundaries of the
bubbles, with a larger portion likely from the Loop I than from the Local
Bubble side, whereas the \ion{Si}{2}* line is from the photoionized Spica
nebula.
