Wind signatures in the X-ray emission-line profiles of the late-O supergiant zeta Orionis (original) (raw)

Wind signatures in the X-ray emission line proflles of the late O supergiant ‡ Orionis

2006

X-ray line profile analysis has proved to be the most direct diagnostic of the kinematics and spatial distribution of the very hot plasma around O stars. The Dopplerbroadened line profiles provide information about the velocity distribution of the hot plasma, while the wavelength-dependent attenuation across a line profile provides information about the absorption to the hot plasma, thus providing a strong constraint on its physical location. In this paper we apply several analysis techniques to the emission lines in the Chandra HETGS spectrum of the late-O supergiant ζ Ori (O9.7 Ib), including the fitting of a simple line-profile model. We show that there is distinct evidence for blue shifts and profile asymmetry, as well as broadening in the X-ray emission lines of ζ Ori. These are the observational hallmarks of a wind-shock X-ray source, and the results for ζ Ori are very similar to those for the earlier O star, ζ Pup, which we have previously shown to be well-fit by the same wind-shock line-profile model. The more subtle effects on the line-profile morphologies in ζ Ori, as compared to ζ Pup, are consistent with the somewhat lower density wind in this later O supergiant. In both stars, the wind optical depths required to explain the mildly asymmetric X-ray line profiles imply reductions in the effective opacity of nearly an order of magnitude, which may be explained by some combination of mass-loss rate reduction and large-scale clumping, with its associated porosity-based effects on radiation transfer. In the context of the recent reanalysis of the helium-like line intensity ratios in both ζ Ori and ζ Pup, and also in light of recent work questioning the published mass-loss rates in OB stars, these new results indicate that the X-ray emission from ζ Ori can be understood within the framework of the standard wind-shock scenario for hot stars.

Wind Signatures in the X-ray Emission Line Profiles of the O Supergiant zeta Orionis

2005

The profiles of X-ray emission lines from several hot stars are resolved by the Chandra grating spectrometers. These profile shapes provide information about the kinematics of the hot plasma around these stars and, via signatures of continuum attenuation, also provide information about the overlying cold stellar wind. We present a quantitative analysis of roughly one dozen emission lines in the Chandra HETGS spectrum of zeta Ori (O 9.7 Ib), and analyze the results in the context of the line-force instability shock model. The distribution in velocity space of the emission, along with the separate helium-like line ratio results, are consistent with the wind-shock scenario. But, although there is statistically significant evidence for some wind attenuation, the degree of wind attenuation is a factor of five to ten less than what would be expected in the context of a smooth, spherically symmetric wind and the UV-derived wind mass-loss rate. These results are consistent with those derived from the X-ray emission line profiles of the hotter O star, zeta Pup, and also with recent, independent evidence from UV spectroscopy, that O star mass-loss rates have been overestimated. We acknowledge support from NASA contract NAS8-03060 and from the National Science Foundation, through their REU program at the Keck Northeast Astronomy Consortium, via contract AST-0353997.

Wind signatures in the X-ray emission line pro¯les of the late O supergiant Orionis

2006

X-ray line pro¯le analysis has proved to be the most direct diagnostic of the kine-matics and spatial distribution of the very hot plasma around O stars. The Doppler-broadened line pro¯les provide information about the velocity distribution of the hot plasma, while the wavelength-dependent attenuation across a line pro¯le provides in-formation about the absorption to the hot plasma, thus providing a strong constraint on its physical location. In this paper we apply several analysis techniques to the emission lines in the Chandra HETGS spectrum of the late-O supergiant Ori (O9.7 Ib), including the ¯tting of a simple line-pro¯le model. We show that there is distinct evidence for blue shifts and pro¯le asymmetry, as well as broadening in the X-ray emission lines of Ori. These are the observational hallmarks of a wind-shock X-ray source, and the results for Ori are very similar to those for the earlier O star, Pup, which we have previously shown to be well-¯t by the same wind-shock line...

Wind Signatures in the X-ray Emission Line Pro¯les of the Late

2014

X-ray line pro¯le analysis has proved to be the most direct diagnostic of the kinematics and spatial distribution of the very hot plasma around O stars. The Doppler-broadened line pro¯les provide information about the velocity distribu-tion of the hot plasma, while the wavelength-dependent attenuation across a line pro¯le provides information about the absorption to the hot plasma, thus pro-viding a strong constraint on its physical location. In this paper we apply several analysis techniques to the emission lines in the Chandra HETGS spectrum of the late-O supergiant Ori (O9.7 Ib), including the ¯tting of a simple line-pro¯le model. We show that there is distinct evidence for blue shifts and pro¯le asym-metry, as well as broadening in the X-ray emission lines of Ori. These are the observational hallmarks of a wind-shock X-ray source, and the results for Ori are very similar to those for the earlier O star, Pup, which we have previously shown to be well-¯t by the same wind-shock lin...

Analysis of the X-ray emission of OB stars: O stars

Research in Astronomy and Astrophysics, 2018

We investigate the global properties of X-ray emission from O stars, analyzing the X-ray spectra of 32 O stars from archival data of the XMM-Newton space observatory. We examine two hypotheses about of the origin of X-ray emission from O stars. The first is a paradigm proposed by Pollock, that was revealed from an analysis of the ζ Ori X-ray observation. The second is the magnetically confined wind-shock (MCWS) model. For checking Pollock's hypothesis, we determine the distribution of the ratio of half width at half maximum (HWHM) to the wind terminal velocity for lines in spectra of all examined stars. In addition, we check three probable consequences from the MCWS model. We analyze if a correlation exists between the spectral hardness and such stellar parameters as the wind terminal velocity, stellar magnetic field and mass loss rate. The result showed that Pollock's hypothesis is not correct. We also established that not all consequences of the MCWS model considered by us are confirmed. In addition, our spectral analysis method indicated that O stars probably have clumped stellar winds with spherical clumps.

X-rays from massive OB stars: thermal emission from radiative shocks

Monthly Notices of the Royal Astronomical Society, 2007

Chandra gratings spectra of a sample of 15 massive OB stars were analyzed under the basic assumption that the X-ray emission is produced in an ensemble of shocks formed in the winds driven by these objects. Shocks develop either as a result of radiation-driven instabilities or due to confinement of the wind by relatively strong magnetic field, and since they are radiative, a simple model of their X-ray emission was developed that allows a direct comparison with observations. According to our model, the shock structures (clumps, complete or fractional shells) eventually become 'cold' clouds in the X-ray sky of the star. As a result, it is expected that for large covering factors of the hot clumps, there is a high probability for X-ray absorption by the 'cold' clouds, resulting in blue-shifted spectral lines. Our analysis has revealed that such a correlation indeed exists for the considered sample of OB stars. As to the temperature characteristics of the X-ray emission plasma, the studied OB stars fall in two groups: (i) one with plasma temperature limited to ∼0.1-0.4 keV; (ii) the other wtih X-rays produced in plasmas at considerably higher temperatures. We argue that the two groups correspond to different mechanisms for the origin of X-rays: in radiative-driven instability shocks and in magnetically-confined wind shocks, respectively.

X-ray emission line profile modeling of hot stars

Review of Scientific Instruments, 2003

The launch of high-spectral-resolution x-ray telescopes (Chandra, XMM ) has provided a host of new spectral line diagnostics for the astrophysics community. In this paper we discuss Dopplerbroadened emission line profiles from highly supersonic outflows of massive stars. These outflows, or winds, are driven by radiation pressure and carry a tremendous amount of kinetic energy, which can be converted to x rays by shock-heating even a small fraction of the wind plasma. The unshocked, cold wind is a source of continuum opacity to the x rays generated in the shock-heated portion of the wind. Thus the emergent line profiles are affected by transport through a two-component, moving, optically thick medium. While complicated, the interactions among these physical effects can provide quantitative information about the spatial distribution and velocity of the x-ray-emitting and absorbing plasma in stellar winds. We present quantitative models of both a sphericallysymmetric wind and a wind with hot plasma confined in an equatorial disk by a dipole magnetic field.

X-ray emission from O-type stars: DH Cep and HD 97434

New Astronomy, 2010

We present X-ray emission characteristics of the massive O-type stars DH Cep and HD 97434 using archival XMM-Newton observations. There is no convincing evidence for short term variability in the X-ray intensity during the observations. However, the analysis of their spectra reveals X-ray structure being consistent with two-temperature plasma model. The hydrogen column densities derived from X-ray spectra of DH Cep and HD 97434 are in agreement with the reddening measurements for their corresponding host clusters NGC 7380 and Trumpler 18, indicating that the absorption by stellar wind is negligible. The X-ray emission from these hot stars is interpreted in terms of the standard instability-driven wind shock model.

Mass Loss and Magnetospheres: X-rays from Hot Stars and Young Stellar Objects

arXiv (Cornell University), 2001

High-resolution X-ray spectra of high-mass stars and lowmass T-Tauri stars obtained during the first year of the Chandra mission are providing important clues about the mechanisms which produce Xrays on very young stars. For ζ Pup (O4 If) and ζ Ori (O9.5 I), the broad, blue-shifted line profiles, line ratios, and derived temperature distribution suggest that the X-rays are produced throughout the wind via instabilitydriven wind shocks. For some less luminous OB stars, like θ 1 Ori C (O7 V) and τ Sco (B0 V), the line profiles are symmetric and narrower. The presence of time-variable emission and very high-temperature lines in θ 1 Ori C and τ Sco suggest that magnetically confined wind shocks may be at work. The grating spectrum of the classical T-Tauri star TW Hya is remarkable because the forbidden-line emission of He-like Ne IX and O VII is very weak, implying that the X-ray emitting region is very dense, n e ≈ 6 × 10 12 cm −3 , or that the X-rays are produced very close to the ultraviolet hotspot at the base of an accretion funnel. ACIS light curves and spectra of flares and low-mass and high-mass young stellar objects in Orion and ρ Ophiuchus further suggest that extreme magnetic activity is a general property of many very young stars.

Analysis of the X-ray emission of OB stars II: B stars

Research in Astronomy and Astrophysics, 2019

This paper is the second part of an investigation into the mechanism for the origin of X-rays in early-type stars. Archival X-ray observations of 25 B stars, obtained by the XMM-Newton satellite, are analysed. We check two hypotheses on the origin of X-ray emission: the Magnetically Confined Wind Shock Model (MCWS) and Pollock’s paradigm. For all studied stars, the mean ratio of the half widths at half maximum to the terminal velocities appears to be R ≈ 0.15 − 0.20 in contradiction to Pollock’s hypothesis that R ≈ 0.5 . We checked three possible consequences of the MCWS model: correlations between the hardness of the X-ray spectra for B stars and terminal wind velocities, mass loss rates and magnetic fields. It was shown that such correlations are marginal or even absent both for magnetic and non-magnetic B stars.