Measurements and Analysis of Helium‐like Triplet Ratios in the X‐Ray Spectra of O‐Type Stars

Leutenegger, Maurice A.; Paerels, F.; Kahn, S. M.; Cohen, David H.

doi:10.1086/507147

Cited by 83 publications

(144 citation statements)

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“…Below Ra, shocks cool primarily by radiation, while above it, adiabatic expansion dominates the cooling. The bulk of the X-ray emission from EWS in massive stars comes from the first several stellar radii of the wind according to both theoretical calculations (Feldmeier et al 1997) and observational constraints from line profiles and forbidden-tointercombination line intensity ratios measured via X-ray spectroscopy (Leutenegger et al 2006). Thus the values of Repolust et al (2004); g Searle et al (2008); all terminal velocities from h Haser (1995), all mass-loss rates from Cohen et al (2014), and all ISM column densities from Fruscione et al (1994) Ra listed in the table justify the assumption of radiatively cooled shocks in the programme stars.…”

Section: Theory and Data Analysismentioning

confidence: 99%

Measuring the shock-heating rate in the winds of O stars using X-ray line spectra

Cohen

Gayley

et al. 2014

Monthly Notices of the Royal Astronomical Society

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We present a new method for using measured X-ray emission line fluxes from O stars to determine the shock-heating rate due to instabilities in their radiation-driven winds. The high densities of these winds means that their embedded shocks quickly cool by local radiative emission, while cooling by expansion should be negligible. Ignoring for simplicity any non-radiative mixing or conductive cooling, the method presented here exploits the idea that the cooling post-shock plasma systematically passes through the temperature characteristic of distinct emission lines in the X-ray spectrum. In this way, the observed flux distribution among these X-ray lines can be used to construct the cumulative probability distribution of shock strengths that a typical wind parcel encounters as it advects through the wind. We apply this new method to Chandra grating spectra from five O stars with X-ray emission indicative of embedded wind shocks in effectively single massive stars. The results for all the stars are quite similar: the average wind mass element passes through roughly one shock that heats it to at least 10 6 K as it advects through the wind, and the cumulative distribution of shock strengths is a strongly decreasing function of temperature, consistent with a negative power-law of index n ≈ 3, implying a marginal distribution of shock strengths that scales as T −4 , and with hints of an even steeper decline or cut-off above 10 7 K.

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Section: Theory and Data Analysismentioning

confidence: 99%

Measuring the shock-heating rate in the winds of O stars using X-ray line spectra

Cohen

Gayley

et al. 2014

Monthly Notices of the Royal Astronomical Society

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“…While for late type stars the collisions and therewith the electron density are the dominant processes, for hot, early type stars the strong UV radiation field (the location relative to the stellar surface) is most important. This has become a wellproven diagnostic tool for establishing the radial location of the X-ray source in the stellar wind of hot stars (Miller et al 2002;Leutenegger et al 2006;Waldron & Cassinelli 2007). The appropriate features in the He-like ions of C v, N vi, O vii, Ne ix, Mg xi, and Si xiii lie in the range from 5 to 45 Å, which is covered by LETGS.…”

Section: He-like Line Ratiosmentioning

confidence: 99%

The X-ray spectrum ofδOrionis observed by LETGS aboardChandra

Raassen

Pollock

2013

A&A

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Aims. We analyze the high-resolution X-ray spectrum of the supergiant O-star δ Orionis (O9.5II) with line ratios of He-like ions and a thermal plasma model, and we examine its variability. Methods. The O-supergiant δ Ori was observed in the wavelength range 5-175 Å by the X-ray detector HRC-S in combination with the grating LETG aboard Chandra. We studied the He-like ions in combination with the UV-radiation field to determine local plasma temperatures and to establish the distance of the X-ray emitting ions to the stellar surface. We measured individual lines by means of Gaussian profiles, folded through the response matrix, to obtain wavelengths, line fluxes, half widths at half maximum (HWHM) and line shifts to characterize the plasma. We consider multitemperature models in collisional ionization equilibrium (CIE) to determine temperatures, emission measures, and abundances.Results. Analysis of the He-like triplets extended to N vi and C v implies ionization stratification with the hottest plasma to be found within a few stellar radii 3 R * (Mg xi) and the coolest farther out, far beyond the acceleration zone, up to 49 R * (N vi) and 75 R * (C v).The observed temperatures cover a range from about 0.1 to 0.7 keV, i.e., 1-8 MK. The X-ray luminosity (L x ) is ∼1.5 × 10 32 erg/s in the range from 0.07 to 3 keV covered by LETGS. Velocity widths of about 1040 km s −1 have been determined.

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“…Leutenegger et al 2006;Cohen 2008). Analyses of HETG spectra of other stars in the same general region of the H-R Diagram, for example of τ Sco (B0.2 V), β Cru (B0.5 III), and ζ Oph (O9.5 V) by Cohen (2003), Cohen (2008), and Waldron (2005), respectively, paint a complex picture.…”

Section: Velocity Broadening and Shifts In Warm Plasma Linesmentioning

confidence: 99%

γCassiopeiae: an X-ray Be star with personality

Oliveira

Smith

Motch

2010

A&A

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An exciting unsolved problem in the study of high energy processes of early type stars concerns the physical mechanism for producing X-rays near the Be star γ Cassiopeiae. By now we know that this source and several "γ Cas analogs" exhibit an unusual hard thermal Xray spectrum, compared both to normal massive stars and the non-thermal emission of known Be/X-ray binaries. Also, its light curve is variable on almost all conceivable timescales. In this study we reanalyze a high dispersion spectrum obtained by Chandra in 2001 and combine it with the analysis of a new (2004) spectrum and light curve obtained by XMM-Newton. We find that both spectra can be fit well with 3-4 optically thin, thermal components consisting of a hot component having a temperature kT Q ∼ 12-14 keV, perhaps one with a value of ∼2.4 keV, and two with well defined values near 0.6 keV and 0.11 keV. We argue that these components arise in discrete (almost monothermal) plasmas. Moreover, they cannot be produced within an integral gas structure or by the cooling of a dominant hot process. Consistent with earlier findings, we also find that the Fe abundance arising from K-shell ions is significantly subsolar and less than the Fe abundance from L-shell ions. We also find novel properties not present in the earlier Chandra spectrum, including a dramatic decrease in the local photoelectric absorption of soft X-rays, a decrease in the strength of the Fe and possibly of the Si K fluorescence features, underpredicted lines in two ions each of Ne and N (suggesting abundances that are ∼1.5-3× and ∼4× solar, respectively), and broadening of the strong Ne X Lyα and O VIII Lyα lines. In addition, we note certain traits in the γ Cas spectrum that are different from those of the fairly well studied analog HD 110432 -in this sense the stars have different "personalities." In particular, for γ Cas the hot X-ray component remains nearly constant in temperature, and the photoelectric absorption of the X-ray plasmas can change dramatically. As found by previous investigators of γ Cas, changes in flux, whether occurring slowly or in rapidly evolving flares, are only seldomly accompanied by variations in hardness. Moreover, the light curve can show a "periodicity" that is due to the presence of flux minima that recur semiregularly over a few hours, and which can appear again at different epochs.

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Measurements and Analysis of Helium‐like Triplet Ratios in the X‐Ray Spectra of O‐Type Stars

Cited by 83 publications

References 58 publications

Measuring the shock-heating rate in the winds of O stars using X-ray line spectra

Measuring the shock-heating rate in the winds of O stars using X-ray line spectra

The X-ray spectrum ofδOrionis observed by LETGS aboardChandra

γCassiopeiae: an X-ray Be star with personality

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