Issue |
A&A
Volume 463, Number 3, March I 2007
|
|
---|---|---|
Page(s) | 1111 - 1123 | |
Section | Stellar atmospheres | |
DOI | https://doi.org/10.1051/0004-6361:20053838 | |
Published online | 13 November 2006 |
A new paradigm for the X-ray emission of O stars from XMM-Newton observations of the O9.7 supergiant ζ Orionis *
European Space Agency XMM-Newton Science Operations Centre, European Space Astronomy Centre, Apartado 50727, Villafranca del Castillo, 28080 Madrid, Spain e-mail: Andy.Pallock@esa.int
Received:
15
July
2005
Accepted:
6
November
2006
XMM-Newton observations of the O supergiant ζ Orionis (O9.7 Ib)
extend knowledge of its high-resolution spectrum beyond the line at 33.7 Å
and suggest a new framework for the interpretation of the X-ray spectra of single hot stars.
All the lines are broad and asymmetric with similar velocity profiles.
X-rays probably originate in the wind's terminal velocity regime in collisionless shocks
controlled by magnetic fields rather than in cooling shocks
in the acceleration zone. During post-shock relaxation,
exchange of energy between ions and electrons is so slow that electron
heating does not take place before hot gas is quenched by the majority cool gas.
The observed plasma is not in equilibrium and the electron bremsstrahlung
continuum is weak. Charge exchange, ionization and excitation are likely to be produced
by protons. Fully thermalized post-shock velocities ensure high cross-sections
and account for the observed line widths, with some allowance probably necessary
for non-thermal particle acceleration.
In general, the form of X-ray spectra in both single and binary stars is likely
to be determined principally by the amount of post-shock electron heating:
magnetically confined X-ray plasma in binary systems can evolve further
towards the higher electron temperatures of equilibrium
while in single stars this does not take place.
The long mean-free path for Coulomb energy exchange between fast-moving ions
may also inhibit the development of line-driven instabilities.
Key words: X-rays: stars / stars: early-type / stars: individual : ζ Orionis / stars: winds, outflows / shock waves
© ESO, 2007
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.