Measurements and analysis of helium-like triplet ratios in the X-ray spectra of O-type stars

Maurice A. Leutenegger$^1$, Frits B. S. Paerels$^1$, Steven M. Kahn$^2$, David H. Cohen$^3$

1 - Columbia University, 550 West 120th Street, New York, NY 10027, USA;
2 - Kavli Institute for Particle Astrophysics and Cosmology, Stanford Linear Accelerator Center and Stanford University;
3 - Swarthmore College, 500 College Avenue, Swarthmore, PA 19081

We discuss new methods of measuring and interpreting the
forbidden-to-intercombination line ratios of helium-like triplets in the X-ray
spectra of O-type stars, including accounting for the spatial distribution of
the X-ray emitting plasma and using the detailed photospheric UV
spectrum. Measurements are made for four O stars using archival {\it Chandra}
HETGS data. We assume an X-ray emitting plasma spatially distributed in the
wind above some minimum radius \(R_0\). We find minimum radii of formation
typically in the range of \(1.25 < R_0 / R_* < 1.67\), which is consistent
with results obtained independently from line profile fits. We find no
evidence for anomalously low \(f/i\) ratios and we do not require the
existence of X-ray emitting plasmas at radii that are too small to generate
sufficiently strong shocks.

Reference: Accepted in ApJ.
Status: Manuscript has been accepted