The models described above were all calculated by assuming a point-like
parent photon source. More generally,
the OVI photon source may be an
extended region, with the size of the region depending on the
characteristic temperature and luminosity of the ionizing radiation
field and the density structure of the nebula surrounding the hot
component.
We have carried out calculations for
emission from an optically thin, homogeneous sphere of radius
. This may not be a physically realistic
geometry in general, but was chosen for
simplicity (other geometries are easily treated).
The normalized
probability of
photon emission within radius r is
where 
is a uniform random deviate in the range 0-1.
The radial offset of a parent photon packet from the (point)
hot source is then
and the initial
location in the Cartesian frame is
where
the direction cosines of the position vector of the initial
photon packet
direction are obtained from two uniform random deviates:
with 
and 
.
The reference model was recalculated with an extended source
(
); in addition, models were calculated with and
without an extended source for 
and for
. The polarization spectra of these models are
plotted in Fig. 22 (click here).
The polarization spectra of the extended-source models generally have a similar shape to their point-source counterparts, but a much reduced magnitude. As a result, the polarized-flux spectra of the extended-source models are simply lower-intensity versions of the point-source models. This reduction in polarization is straightforwardly understood. The radiation field of an extended source is less `forward peaked' than in the point-source case; that is, photons incident on the first scatterer will come from a finite cone angle. Thus the nett polarization resulting from the first scattering will be much reduced. Subsequent Rayleigh scattering will dilute the polarization signal from the first scattering, but the resulting polarization will still be reduced.