A new paradigm for the X-ray emission of O stars from XMM-Newton observations of the O9.7 supergiant zeta Orionis

Abstract

XMM-Newton observations of the O supergiant zeta Orionis (O9.7 Ib) extend knowledge of its high-resolution spectrum beyond the CVI line at 33.7 Angstroms 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.

arXiv comment34 pages, 10 figures, 7 tables, accepted by Astronomy & Astrophysics

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• X-Ray Spectroscopy of Stars
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• The weak magnetic field of the O9.7 supergiant zeta Orionis A
• Plasma code for astrophysical charge exchange emission at X-ray wavelengths
• XMM-Newton X-ray study of early type stars in the Carina OB1 association
• Discovery of X-ray Emission from the Wolf-Rayet star WR142 of oxygen subtype
• The Correlation between X-Ray Line Ionization and Optical Spectral Types of the OB Stars
• XMM-Newton observations of Zeta Orionis (O9.7 Ib): A Collisional Ionization Equilibrium model
• X-ray diagnostics of massive star winds
• High-resolution X-ray spectroscopy of Theta Car
• Identification of Galactic Bulge Survey X-ray Sources with Tycho-2 Stars
• X-ray Emission Line Profiles from Wind Clump Bow Shocks in Massive Stars
• High-resolution Charge Exchange Spectra with L-shell Nickel Show Striking Differences from Models
• The X-ray spectrum of delta Orionis observed by LETGS aboard Chandra
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