Continuum photons can be absorbed by photoionization. This is regarded as a
photon destruction process since the converse, radiative recombination, produces
an 'RRC' spectrum which has a different energy dependnce than the absorption,
and so the two are treated separately. Xstar includes the photoabsorption
associated with every bound-free transition in the atomic database; there
are of them.
Photons can scatter via Thomson scattering or resonance scattering in lines.
Xstar currently includes resonance scattering in lines in the final spectrum
seen by a distant observer (as described in the following section).
Thomson scattering does not strongly affect the shape of the spectrum unless
the photon energy or temperature are a significant fraction of .
Furthermore, similar arguments apply to Thomson scattering as apply to the
radiative excitation in section 9.D.2. In the case of a spherical stationary
cloud where all the photons from the cloud are observed, Thomson scattering will
have no net effect. For this reason, version of xstar prior to 2.54a omitted
Thomson scattering. Beginning with verion 2.54a, Thomson scattering is
included with the same multiplicative factor 1-cfrac as is used for radiative
excitation. Thus, when cfrac=1 the classical results without Thomson
scattering are recovered, and when cfrac=0 Thomson scattering is included in
the continuum opacity. Of cours, Thomson scattering is unimportant when
the cloud column is less than
cm
. And, when the cloud is
optically thick to Thomson scattering, xstar does not adequately treat the
effects of multiple scatterings.