Simultaneous X-ray and optical spectroscopy of the Oef supergiant<i>λ</i>Cephei

Rauw, Grégor; Hervé, Anthony; Nazé, Yaël; González‐Pérez, J. N.; Hempelmann, A.; Mittag, M.; Schmitt, J. H. M. M.; Schröder, K.-P.; Gosset, Éric; Eenens, P.; Uuh-Sonda, J. M.

doi:10.1051/0004-6361/201526057

Cited by 47 publications

(58 citation statements)

References 91 publications

(163 reference statements)

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“…In terms of a stationary description, a simple model (e.g., Hillier et al 1993;Cassinelli et al 1994) assumes randomly distributed shocks above a minimum radius, R min ≈ 1.5 R * . This is consistent with X-ray line diagnostics ( e.g., Leutenegger et al 2013b; see also Rauw et al 2015) in which the hot shocked gas (with temperatures of a few million Kelvin and a volume filling factor on the order of 10 −3 to a few 10 −2 ) is collisionally ionized/excited and emits X-ray/EUV photons due to spontaneous decay, radiative recombinations, and bremsstrahlung. The ambient, cool wind then reabsorbs part of the emission, mostly via K-shell processes.…”

Section: Introductionsupporting

confidence: 87%

“…Hillier et al (1993) analyzed the sensitivity to R min , pointing to indistinguishable X-ray flux differences when the onset is varied between 1.5 and 2 R * . Recent analyses of X-ray line emission from hot star winds also point to values around 1.5 R * (e.g., Leutenegger et al 2006;Oskinova et al 2006;Hervé et al 2013;Cohen et al 2014b), although Rauw et al (2015) derived a value of 1.2 R * for the wind of λ Cep.…”

Section: X-ray Emissionmentioning

confidence: 99%

“…X-ray emission from hot stars has been measured at soft (0.1 to > ∼ 2 keV) and harder energies, either at low resolution in the form of a quasi-continuum, or at high resolution allowing the investigation of individual lines (e.g., Oskinova et al 2006;Owocki & Cohen 2006;Hervé et al 2013;Leutenegger et al 2013b;Cohen et al 2014b;Rauw et al 2015). The first X-ray satellite observatory, EINSTEIN, has already revealed that O stars are soft X-ray sources (Harnden et al 1979;Seward et al 1979), and Cassinelli & Swank (1983) were the first to show that the observed X-ray emission is due to thermal emission, dominated by lines.…”

Section: Introductionmentioning

confidence: 99%

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Atmospheric NLTE models for the spectroscopic analysis of blue stars with winds

Carneiro¹,

Puls²,

Sundqvist³

et al. 2016

A&A

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Context. Extreme ultraviolet (EUV) and X-ray radiation emitted from wind-embedded shocks in hot, massive stars can affect the ionization balance in their outer atmospheres and can be the mechanism responsible for producing highly ionized atomic species detected in stellar wind UV spectra. Aims. To allow for these processes in the context of spectral analysis, we have implemented the emission from wind-embedded shocks and related physics into our unified, NLTE model atmosphere/spectrum synthesis code FASTWIND. Methods. The shock structure and corresponding emission is calculated as a function of user-supplied parameters (volume filling factor, radial stratification of shock strength, and radial onset of emission). We account for a temperature and density stratification inside the postshock cooling zones, calculated for radiative and adiabatic cooling in the inner and outer wind, respectively. The highenergy absorption of the cool wind is considered by adding important K-shell opacities, and corresponding Auger ionization rates have been included in the NLTE network. To test our implementation and to check the resulting effects, we calculated a comprehensive model grid with a variety of X-ray emission parameters. Results. We tested and verified our implementation carefully against corresponding results from various alternative model atmosphere codes, and studied the effects from shock emission for important ions from He, C, N, O, Si, and P. Surprisingly, dielectronic recombination turned out to play an essential role for the ionization balance of O iv/O v (particularly in dwarfs with T eff ∼ 45 000 K).Finally, we investigated the frequency dependence and radial behavior of the mass absorption coefficient, κ ν (r), which is important in the context of X-ray line formation in massive star winds. Conclusions. In almost all of the cases considered, direct ionization is of major influence because of the enhanced EUV radiation field, and Auger ionization only affects N vi and O vi significantly. The approximation of a radially constant κ ν is justified for r 1.2 R * and λ 18 Å and also for many models at longer wavelengths. To estimate the actual value of this quantity, however, the He ii opacities need to be calculated from detailed NLTE modeling, at least for wavelengths longer than 18 to 20 Å, and information on the individual CNO abundances has to be present.

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Section: Introductionsupporting

confidence: 87%

Section: X-ray Emissionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Atmospheric NLTE models for the spectroscopic analysis of blue stars with winds

Carneiro¹,

Puls²,

Sundqvist³

et al. 2016

A&A

View full text Add to dashboard Cite

show abstract

“…Sundqvist et al (2011) argued that the photospheric contribution of the He ii blend with Hα is negligible. Rauw et al (2015) found the dominant frequency to be ν 2 (our adopted rotation frequency) in this line; it is most likely also present in our data two months later, but this is difficult to confirm as ν 2 is close to the lower frequency limit (set by the temporal coverage) in our Fourier analysis. Marchenko et al (1998), which is remarkably present in 2012.…”

Section: Power Spectramentioning

confidence: 50%

Multiple short-lived stellar prominences on O stars: The O6.5I(n)fp starλCephei

Sudnik

Henrichs

2016

A&A

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Context. Most O-type stars and many B stars show unexplained cyclical variability in their spectral lines, i.e., modulation on the rotational timescale, but not strictly periodic. The variability occurs in the so-called discrete absorption components (DACs) that accelerate through the UV-wind line profiles and also in many optical lines. For such OB stars no dipolar magnetic fields have been detected with upper limits of ∼300 G. Aims. We investigate whether multiple magnetic loops on the surface rather than non-radial pulsations or a dipolar magnetic field can explain the observed cyclical UV and optical spectral line variability. Methods. We present time-resolved, high-resolution optical spectroscopy of the O6.5I(n)fp star λ Cephei. We apply a simplified phenomenological model in which multiple spherical blobs attached to the surface represent magnetic-loop structures, which we call stellar prominences, by analogy with solar prominences. We compare the calculated line profiles as a function of rotational phase, adopting a rotation period of 4.1 d, with observed relative changes in subsequent quotient spectra. Results. We identify many periodicities in spectral lines, almost none of which is stable over timescales from months to years. We show that the relative changes in various optical absorption and emission lines are often very similar. Our proposed model applied to the He ii λ4686 line can typically be fitted with 2−5 equatorial blobs with lifetimes between ∼1 and 24 h.Conclusions. Given the irregular timescales involved, we propose that the azimuthal distribution of DACs correspond to the locations of stellar prominences attached to the surface. This could explain the observed variability of optical and UV lines, and put constraints on the strength and lifetime of these structures, which can be compared with recent theoretical predictions, in which bright magnetic surface spots are formed by the action of the subsurface convection zone.

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“…With the fourfold exposure time available now, the new combined RGS spectrum is of significantly better quality, and we can use it to further characterize the soft X-ray emission. Unfortunately, a detailed modelling such as done for ζ Pup (Hervé et al 2013) or λ Cep (Rauw et al 2015b) cannot be done in this case. Even though the emission in the soft band is probably only very little affected by the wind interaction, 9 Sgr hosts two stars, both of which are likely to contribute to the X-ray emission.…”

Section: High-resolution X-ray Spectroscopymentioning

confidence: 99%

Testing the theory of colliding winds: the periastron passage of 9 Sagittarii

Rauw

Blomme

Nazé

et al. 2016

A&A

Self Cite

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Context. The long-period, highly eccentric O-star binary 9 Sgr, known for its non-thermal radio emission and its relatively bright X-ray emission, went through its periastron in 2013. Aims. Such an event can be used to observationally test the predictions of the theory of colliding stellar winds over a broad range of wavelengths. Methods. We conducted a multi-wavelength monitoring campaign of 9 Sgr around the 2013 periastron. In this paper, we focus on X-ray observations and optical spectroscopy. Results. The optical spectra allow us to revisit the orbital solution of 9 Sgr and to refine its orbital period to 9.1 years. The X-ray flux is maximum at periastron over all energy bands, but with clear differences as a function of energy. The largest variations are observed at energies above 2 keV, whilst the spectrum in the soft band (0.5−1.0 keV) remains mostly unchanged, indicating that it arises far from the collision region, in the inner winds of the individual components. The level of the hard emission at periastron clearly deviates from the 1/r relation expected for an adiabatic wind-interaction zone, whilst this relation seems to hold at the other phases that are covered by our observations. The spectra taken at phase 0.946 reveal a clear Fe xxv line at 6.7 keV, but no such line is detected at periastron (φ = 0.000), although a simple model predicts a strong line that should be easily visible in the data. Conclusions. The peculiarities of the X-ray spectrum of 9 Sgr could reflect the effect of radiative inhibition as well as a phasedependent efficiency of particle acceleration on the shock properties.

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Simultaneous X-ray and optical spectroscopy of the Oef supergiantλCephei

Cited by 47 publications

References 91 publications

Atmospheric NLTE models for the spectroscopic analysis of blue stars with winds

Atmospheric NLTE models for the spectroscopic analysis of blue stars with winds

Multiple short-lived stellar prominences on O stars: The O6.5I(n)fp starλCephei

Testing the theory of colliding winds: the periastron passage of 9 Sagittarii

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