Chandra grating spectroscopy of embedded wind shock X-ray emission from O stars shows low plasma temperatures and significant wind absorption

Abstract We present a uniform analysis of six examples of embedded wind shock (EWS) O star X-ray sources observed at high resolution with the Chandra grating spectrometers. By modeling both the hot plasma emission and the continuum absorption of the soft X-rays by the cool, partially ionized bulk of the wind we derive the temperature distribution of the shock-heated plasma and the wind mass-loss rate of each star. We find a similar temperature distribution for each star’s hot wind plasma, consistent with a power-law differential emission measure, $\frac{d\log EM}{d\log T}$, with a slope a little steeper than -2, up to temperatures of only about 107 K. The wind mass-loss rates, which are derived from the broadband X-ray absorption signatures in the spectra, are consistent with those found from other diagnostics. The most notable conclusion of this study is that wind absorption is a very important effect, especially at longer wavelengths. More than 90 per cent of the X-rays between 18 and 25 Å produced by shocks in the wind of ζ Pup are absorbed, for example. It appears that the empirical trend of X-ray hardness with spectral subtype among O stars is primarily an absorption effect.

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Variability in X-ray line ratios in helium-like ions of massive stars: the wind-driven case

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834752 ◽

2019 ◽

Vol 625 ◽

pp. A86 ◽

Cited By ~ 1

Author(s):

R. Ignace ◽

Z. Damrau ◽

K. T. Hole

Keyword(s):

Mass Loss ◽

Massive Stars ◽

Mass Loss Rate ◽

Variable Mass ◽

Line Ratio ◽

High Spectral Resolution ◽

X Rays ◽

Wind Integration ◽

X Ray ◽

Wind Variability

Context. High spectral resolution and long exposure times are providing unprecedented levels of data quality of massive stars at X-ray wavelengths. Aims. A key diagnostic of the X-ray emitting plasma are the fir lines for He-like triplets. In particular, owing to radiative pumping effects, the forbidden-to-intercombination line luminosity ratio, R = f∕i, can be used to determine the proximity of the hot plasma to the UV-bright photospheres of massive stars. Moreover, the era of large observing programs additionally allows for investigation of line variability. Methods. This contribution is the second to explore how variability in the line ratio can provide new diagnostic information about distributed X-rays in a massive star wind. We focus on wind integration for total line luminosities, taking account of radiative pumping and stellar occultation. While the case of a variable stellar radiation field was explored in the first paper, the effects of wind variability are emphasized in this work. Results. We formulate an expression for the ratio of line luminosities f∕i that closely resembles the classic expression for the on-the-spot result. While there are many ways to drive variability in the line ratio, we use variable mass loss as an illustrative example for wind integration, particularly since this produces no variability for the on-the-spot case. The f∕i ratio can be significantly modulated owing to evolving wind properties. The extent of the variation depends on how the timescale for the wind flow compares to the timescale over which the line emissivities change. Conclusions. While a variety of factors can ellicit variable line ratios, a time-varying mass-loss rate serves to demonstrate the range of amplitude and phased-dependent behavior in f∕i line ratios. Importantly, we evaluate how variable mass loss might bias measures of f∕i. For observational exposures that are less than the timescale of variable mass loss, biased measures (relative to the time-averaged wind) can result; if exposures are long, the f∕i ratio is reflective of the time-averaged spherical wind.

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Massive star mass-loss revealed by X-ray observations of young supernovae

Proceedings of the International Astronomical Union ◽

10.1017/s1743921318008438 ◽

2018 ◽

Vol 14 (S346) ◽

pp. 83-87

Author(s):

Vikram V. Dwarkadas

Keyword(s):

Mass Loss ◽

Massive Stars ◽

Ambient Medium ◽

Mass Loss Rate ◽

Stellar Mass ◽

X Rays ◽

X Ray ◽

The Common ◽

Almost All ◽

Loss Rates

AbstractMassive stars lose a considerable amount of mass during their lifetime. When the star explodes as a supernova (SN), the resulting shock wave expands in the medium created by the stellar mass-loss. Thermal X-ray emission from the SN depends on the square of the density of the ambient medium, which in turn depends on the mass-loss rate (and velocity) of the progenitor wind. The emission can therefore be used to probe the stellar mass-loss in the decades or centuries before the star’s death.We have aggregated together data available in the literature, or analysed by us, to compute the X-ray lightcurves of almost all young supernovae detectable in X-rays. We use this database to explore the mass-loss rates of massive stars that collapse to form supernovae. Mass-loss rates are lowest for the common Type IIP supernovae, but increase by several orders of magnitude for the highest luminosity X-ray SNe.

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Colliding stellar wind modelling of the X-ray emission from WR 140

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3591 ◽

2020 ◽

Vol 500 (4) ◽

pp. 4837-4848

Author(s):

Svetozar A Zhekov

Keyword(s):

Mass Loss ◽

Effective Mass ◽

Stellar Wind ◽

Mass Loss Rate ◽

Emission Measure ◽

Strong Line ◽

Stellar Winds ◽

Line Profiles ◽

X Ray ◽

Binary Orbit

ABSTRACT We modelled the Chandra and Rossi X-ray Timing Explorer X-ray spectra of the massive binary WR 140 in the framework of the standard colliding stellar wind (CSW) picture. Models with partial electron heating at the shock fronts are a better representation of the X-ray data than those with complete temperature equalization. Emission measure of the X-ray plasma in the CSW region exhibits a considerable decrease at orbital phases near periastron. This is equivalent to variable effective mass-loss rates over the binary orbit. At orbital phases near periastron, a considerable X-ray absorption in excess to that from the stellar winds in WR 140 is present. The standard CSW model provides line profiles that in general do not match well the observed line profiles of the strong line features in the X-ray spectrum of WR 140. The variable effective mass-loss rate could be understood qualitatively in CSW picture of clumpy stellar winds where clumps are efficiently dissolved in the CSW region near apastron but not at periastron. However, future development of CSW models with non-spherically symmetric stellar winds might be needed to get a better correspondence between theory and observations.

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X-ray spectral diagnostics of activity in massive stars

Proceedings of the International Astronomical Union ◽

10.1017/s1743921311010763 ◽

2010 ◽

Vol 6 (S272) ◽

pp. 348-353 ◽

Cited By ~ 1

Author(s):

David H. Cohen ◽

Emma E. Wollman ◽

Maurice A. Leutenegger

Keyword(s):

Mass Loss ◽

Loss Rate ◽

Mass Loss Rate ◽

Broad Band ◽

Variable Structure ◽

Spectral Energy ◽

X Rays ◽

X Ray ◽

Spectral Diagnostics ◽

Loss Rates

AbstractX-rays give direct evidence of instabilities, time-variable structure, and shock heating in the winds of O stars. The observed broad X-ray emission lines provide information about the kinematics of shock-heated wind plasma, enabling us to test wind-shock models. And their shapes provide information about wind absorption, and thus about the wind mass-loss rates. Mass-loss rates determined from X-ray line profiles are not sensitive to density-squared clumping effects, and indicate mass-loss rate reductions of factors of 3 to 6 over traditional diagnostics that suffer from density-squared effects. Broad-band X-ray spectral energy distributions also provide mass-loss rate information via soft X-ray absorption signatures. In some cases, the degree of wind absorption is so high, that the hardening of the X-ray SED can be quite significant. We discuss these results as applied to the early O stars ζ Pup (O4 If), 9 Sgr (O4 V((f))), and HD 93129A (O2 If*).

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X-rays From Centrifugal Magnetospheres in Massive Stars

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314007352 ◽

2014 ◽

Vol 9 (S307) ◽

pp. 449-450 ◽

Cited By ~ 1

Author(s):

Christopher Bard ◽

Richard Townsend

Keyword(s):

Mass Loss ◽

Loss Rate ◽

Massive Stars ◽

Mass Loss Rate ◽

X Rays ◽

Stellar Rotation ◽

Hydrodynamic Simulations ◽

X Ray ◽

Ob Stars ◽

Magnetically Confined

AbstractIn the subset of massive OB stars with strong global magnetic fields, X-rays arise from magnetically confined wind shocks (Babel & Montmerle 1997). However, it is not yet clear what the effect of stellar rotation and mass-loss rate is on these wind shocks and resulting X-rays. Here, we present results from a grid of Arbitrary Rigid-Field Hydrodynamic simulations (ARFHD) of a B-star centrifugal magnetosphere with an eye towards quantifying the effect of stellar rotation and mass-loss rate on the level of X-ray emission. The results are also compared to a generalized XADM model for X-rays in dynamical magnetospheres (ud-Doula et al. 2014).

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Wind inhibition by X-ray irradiation in HMXBs: the influence of clumping and the final X-ray luminosity

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833419 ◽

2018 ◽

Vol 620 ◽

pp. A150 ◽

Cited By ~ 7

Author(s):

J. Krtička ◽

J. Kubát ◽

I. Krtičková

Keyword(s):

Mass Loss ◽

Stellar Wind ◽

Loss Rate ◽

Mass Loss Rate ◽

Terminal Velocity ◽

High Mass ◽

Small Scale ◽

X Rays ◽

X Ray ◽

X Ray Binaries

Context. In wind-powered X-ray binaries, the radiatively driven stellar wind from the primary may be inhibited by the X-ray irradiation. This creates the feedback that limits the X-ray luminosity of the compact secondary. Wind inhibition might be weakened by the effect of small-scale wind inhomogeneities (clumping) possibly affecting the limiting X-ray luminosity. Aims. We study the influence of X-ray irradiation on the stellar wind for different radial distributions of clumping. Methods. We calculate hot star wind models with external irradiation and clumping using our global wind code. The models are calculated for different parameters of the binary. We determine the parameters for which the X-ray wind ionization is so strong that it leads to a decrease of the radiative force. This causes a decrease of the wind velocity and even of the mass-loss rate in the case of extreme X-ray irradiation. Results. Clumping weakens the effect of X-ray irradiation because it favours recombination and leads to an increase of the wind mass-loss rate. The best match between the models and observed properties of high-mass X-ray binaries (HMXBs) is derived with radially variable clumping. We describe the influence of X-ray irradiation on the terminal velocity and on the mass-loss rate in a parametric way. The X-ray luminosities predicted within the Bondi-Hoyle-Lyttleton theory agree nicely with observations when accounting for X-ray irradiation. Conclusions. The ionizing feedback regulates the accretion onto the compact companion resulting in a relatively stable X-ray source. The wind-powered accretion model can account for large luminosities in HMXBs only when introducing the ionizing feedback. There are two possible states following from the dependence of X-ray luminosity on the wind terminal velocity and mass-loss rate. One state has low X-ray luminosity and a nearly undisturbed wind, and the second state has high X-ray luminosity and exhibits a strong influence of X-rays on the flow.

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Testing the Wind-shock Paradigm for B-Type Star X-Ray Production with θ Car

Proceedings of the International Astronomical Union ◽

10.1017/s174392131700206x ◽

2016 ◽

Vol 12 (S329) ◽

pp. 395-395

Author(s):

T. F. Doyle ◽

V. Petit ◽

D. Cohen ◽

M. Leutenegger

Keyword(s):

Mass Loss Rate ◽

Low Density ◽

X Rays ◽

Wind Regime ◽

X Ray ◽

Wind Theory ◽

Stellar Photosphere ◽

Hydrodynamical Simulations ◽

Grating Spectroscopy ◽

Weak Wind

AbstractWe present Chandra X-ray grating spectroscopy of the B0.2V star, θ Carina. θ Car is in a critical transition region between the latest O-type and earliest B-type stars, where some stars are observed to have UV-determined wind densities much lower than theoretically expected (e.g., Marcolino et al. 2009). In general, X-ray emission in this low-density wind regime should be less prominent than for O-stars (e.g., Martins et al. 2005), but observations suggest a higher than expected X-ray emission filling factor (Lucy 2012; Huenemoerder et al. 2012); if a larger fraction of the wind is shock-heated, it could explain the weak UV wind signature seen in weak wind stars, but this might severely challenge predictions of radiatively-driven wind theory.We measured the line widths of several He-, H-like and Fe ions and the f/i ratio of He-like ions in the X-ray spectrum, which improves upon the results from Nazé et al. (2008) (XMM-Newton RGS) with additional measurements (Chandra HETG) of Mgxi and Sixiii by further constraining the X-ray emission location. The f/i ratio is modified by the proximity to the UV-emitting stellar photosphere, and is therefore a diagnostic of the radial location of the X-ray emitting plasma. The measured widths of X-ray lines are narrow, <300 km s−1 and the f/i ratios place the X-rays relatively close to the surface, both implying θ Car is a weak wind star. The measured widths are also consistent with other later-type stars in the weak wind regime, β Cru (Cohen et al. 2008), for example, and are smaller on average than earlier weak wind stars such as μ Col (Huenemoerder et al. 2012). This could point to a spectral type divide, where one hypothesis, low density, works for early-B type stars and the other hypothesis, a larger fraction of shock-heated gas, explains weak winds in late-O type stars. Archival IUE data still needs to be analyzed to determine the mass loss rate and hydrodynamical simulations will be compared with observations to determine which hypothesis works for θ Car.

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Gas shells and magnetic fields in the Orion-Eridanus superbubble

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936239 ◽

2019 ◽

Vol 631 ◽

pp. A52 ◽

Cited By ~ 4

Author(s):

T. Joubaud ◽

I. A. Grenier ◽

J. Ballet ◽

J. D. Soler

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Expansion Velocity ◽

X Rays ◽

X Ray ◽

Stellar Feedback ◽

Hot Plasma ◽

Formation History ◽

X Ray Absorption ◽

The Magnetic Field

Aims. The Orion-Eridanus superbubble has been blown by supernovae and supersonic winds of the massive stars in the Orion OB associations. It is the nearest site at which stellar feedback on the interstellar medium that surrounds young massive clusters can be studied. The formation history and current structure of the superbubble are still poorly understood, however. It has been pointed out that the picture of a single expanding object should be replaced by a combination of nested shells that are superimposed along the line of sight. We have investigated the composite structure of the Eridanus side of the superbubble in the light of a new decomposition of the atomic and molecular gas. Methods. We used H I 21 cm and CO (J = 1−0) emission lines to separate coherent gas shells in space and velocity, and we studied their relation to the warm ionised gas probed in Hα emission, the hot plasma emitting X-rays, and the magnetic fields traced by dust polarised emission. We also constrained the relative distances to the clouds using dust reddening maps and X-ray absorption. We applied the Davis–Chandrasekhar–Fermi method to the dust polarisation data to estimate the plane-of-sky components of the magnetic field in several clouds and along the outer rim of the superbubble. Results. Our gas decomposition has revealed several shells inside the superbubble that span distances from about 150–250 pc. One of these shells forms a nearly complete ring filled with hot plasma. Other shells likely correspond to the layers of swept-up gas that is compressed behind the expanding outer shock wave. We used the gas and magnetic field data downstream of the shock to derive the shock expansion velocity, which is close to ~20 km s−1. Taking the X-ray absorption by the gas into account, we find that the hot plasma inside the superbubble is over-pressured compared to plasma in the Local Bubble. The plasma comprises a mix of hotter and cooler gas along the lines of sight, with temperatures of (3–9) and (0.3 − 1.2) × 106 K, respectively. The magnetic field along the western and southern rims and in the approaching wall of the superbubble appears to be shaped and compressed by the ongoing expansion. We find plane-of-sky magnetic field strengths from 3 to 15 μG along the rim.

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Diagnostic of the temperature and differential emission measure (DEM) based on <I>Hinode</I>/XRT data

Annales Geophysicae ◽

10.5194/angeo-26-2999-2008 ◽

2008 ◽

Vol 26 (10) ◽

pp. 2999-3005 ◽

Cited By ~ 7

Author(s):

M. Siarkowski ◽

R. Falewicz ◽

A. Kępa ◽

P. Rudawy

Keyword(s):

Genetic Algorithm ◽

Temperature Distribution ◽

Emission Measure ◽

Ratio Method ◽

X Rays ◽

Differential Emission Measure ◽

X Ray ◽

Isothermal Plasma ◽

Coupling hydrodynamics with comoving frame radiative transfer

Astronomy and Astrophysics ◽

10.1051/0004-6361/201731575 ◽

2018 ◽

Vol 610 ◽

pp. A60 ◽

Cited By ~ 23

Author(s):

A. A. C. Sander ◽

F. Fürst ◽

P. Kretschmar ◽

L. M. Oskinova ◽

H. Todt ◽

...

Keyword(s):

Radiative Transfer ◽

Mass Loss ◽

Wind Velocity ◽

Loss Rate ◽

Mass Loss Rate ◽

High Mass ◽

Detailed Knowledge ◽

X Rays ◽

X Ray ◽

Atmosphere Model

Context. Vela X-1, a prototypical high-mass X-ray binary (HMXB), hosts a neutron star (NS) in a close orbit around an early-B supergiant donor star. Accretion of the donor star's wind onto the NS powers its strong X-ray luminosity. To understand the physics of HMXBs, detailed knowledge about the donor star winds is required. Aims. To gain a realistic picture of the donor star in Vela X-1, we constructed a hydrodynamically consistent atmosphere model describing the wind stratification while properly reproducing the observed donor spectrum. To investigate how X-ray illumination affects the stellar wind, we calculated additional models for different X-ray luminosity regimes. Methods. We used the recently updated version of the Potsdam Wolf–Rayet code to consistently solve the hydrodynamic equation together with the statistical equations and the radiative transfer. Results. The wind flow in Vela X-1 is driven by ions from various elements, with Fe iii and S iii leading in the outer wind. The model-predicted mass-loss rate is in line with earlier empirical studies. The mass-loss rate is almost unaffected by the presence of the accreting NS in the wind. The terminal wind velocity is confirmed at v∞≈ 600 km s−1. On the other hand, the wind velocity in the inner region where the NS is located is only ≈100 km s−1, which is not expected on the basis of a standard β-velocity law. In models with an enhanced level of X-rays, the velocity field in the outer wind can be altered. If the X-ray flux is too high, the acceleration breaks down because the ionization increases. Conclusions. Accounting for radiation hydrodynamics, our Vela X-1 donor atmosphere model reveals a low wind speed at the NS location, and it provides quantitative information on wind driving in this important HMXB.

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