The Effects of Clumps in Explaining X-ray Emission Lines from Hot Stars

J. P. Cassinelli^1, R. Ignace^2, W. L. Waldron^3, J. Cho^4, N. A. Murphy^1, A. Lazarian^1

^1 University of Wisconsin
^2 East Tennessee State University
^3 Eureka Scientific Inc
^4 Chungnam National University

It is now well established that stellar winds of hot stars are fragmentary
and that the X-ray emission from stellar winds has a strong contribution
from shocks in winds. Chandra high spectral resolution observations of
line profiles of O and B stars have shown numerous properties that had
not been expected. Here we suggest explanations by considering the X-rays
as arising from bow shocks that occur where the stellar wind impacts on
spherical clumps in the winds. We use an accurate and stable numerical
hydrodynamical code to obtain steady-state physical conditions for the
temperature and density structure in a bow shock. We use these solutions
plus analytic approximations to interpret some major X-ray features:
the simple power-law distribution of the observed emission measure
derived from many hot star X-ray spectra and the wide range of ionization
stages that appear to be present in X-ray sources throughout the winds.
Also associated with the adiabatic cooling of the gas around a clump is
a significant transverse velocity for the hot plasma flow around the clumps,
and this can help to understand anomalies associated with observed line
widths, and the differences in widths seen in stars with high and low
mass-loss rates. The differences between bow shocks and the planar shocks
that are often used for hot stars are discussed. We introduce an ``on
the shock'' (OTSh) approximation that is useful for interpreting the
X-rays and the consequences of clumps in hot star winds and elsewhere
in astronomy.

Reference: to appear in the Astrophysical Journal (available at astroph/0804.4680)
Status: Manuscript has been accepted