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See detailSpectroscopic study of the O-type runaway supergiant HD 195592
De Becker, Michaël ULg; Linder, Natacha ULg; Rauw, Grégor ULg

in New Astronomy (2010), 15

The scope of this paper is to perform a detailed spectroscopic study of the northern O-type supergiant HD195592. We use a large sample of high quality spectra in order to investigate its multiplicity, and ... [more ▼]

The scope of this paper is to perform a detailed spectroscopic study of the northern O-type supergiant HD195592. We use a large sample of high quality spectra in order to investigate its multiplicity, and to probe the line profile variability. Our analysis reveals a clear spectroscopic binary signature in the profile of the He i λ 6678 line, pointing to a probable O + B system. We report on low amplitude radial velocity variations in every strong absorption line in the blue spectrum of HD 195592. These variations are ruled by two time-scales respectively of 5.063 and about 20 days. The former is firmly established, whilst the latter is poorly constrained. We report also on a very significant line profile variability of the H β line, with time scales strongly related to those of the radial velocities. Our results provide significant evidence that HD 195592 is a binary system, with a period that might be the variability time-scale of about 5 days. The second time scale may be the signature of an additional star moving along a wider orbit provided its mass is low enough, even though direct evidence for the presence of a third star is still lacking. Alternatively, the second time-scale may be the signature of a variability intrinsic to the stellar wind of the primary, potentially related to the stellar rotation. [less ▲]

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See detailA multiwavelength investigation of the massive eclipsing binary Cyg OB2 #5
Linder, Natacha ULg; Rauw, Grégor ULg; Manfroid, Jean ULg et al

in Astronomy and Astrophysics (2009), 495

Context. The properties of the early-type binary Cyg OB2 #5 have been debated for many years and spectroscopic and photometric investigations yielded conflicting results. Aims. We have attempted to ... [more ▼]

Context. The properties of the early-type binary Cyg OB2 #5 have been debated for many years and spectroscopic and photometric investigations yielded conflicting results. Aims. We have attempted to constrain the physical properties of the binary by collecting new optical and X-ray observations. Methods. The optical light curves obtained with narrow-band continuum and line-bearing filters are analysed and compared. Optical spectra are used to map the location of the He II $\lambda $ 4686 and H$\alpha $ line-emission regions in velocity space. New XMM-Newton as well as archive X-ray spectra are analysed to search for variability and constrain the properties of the hot plasma in this system. Results. We find that the orbital period of the system slowly changes though we are unable to discriminate between several possible explanations of this trend. The best fit solution of the continuum light curve reveals a contact configuration with the secondary star being significantly brighter and hotter on its leading side facing the primary. The mean temperature of the secondary star turns out to be only slightly lower than that of the primary, whilst the bolometric luminosity ratio is found to be 3.1. The solution of the light curve yields a distance of $925 \pm 25$ pc much lower than the usually assumed distance of the Cyg OB2 association. Whilst we confirm the existence of episodes of higher X-ray fluxes, the data reveal no phase-locked modulation with the 6.6 day period of the eclipsing binary nor any clear relation between the X-ray flux and the 6.7 yr radio cycle. Conclusions. The bright region of the secondary star is probably heated by energy transfer in a common envelope in this contact binary system as well as by the collision with the primary's wind. The existence of a common photosphere probably also explains the odd mass-luminosity relation of the stars in this system. Most of the X-ray, non-thermal radio, and possibly $\gamma$-ray emission of Cyg OB2 #5 is likely to arise from the interaction of the combined wind of the eclipsing binary with at least one additional star of this multiple system. [less ▲]

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See detailEvidence for short-term variations in two O-type stars
De Becker, Michaël ULg; Linder, Natacha ULg; Rauw, Grégor ULg

in Information Bulletin on Variable Stars (2008), 5841

Only a few O-type stars are known to display short-term (a fraction of a day) variations. Intense spectroscopic monitoring of the ON8V star HD 13268 revealed low amplitude variations with periods of ... [more ▼]

Only a few O-type stars are known to display short-term (a fraction of a day) variations. Intense spectroscopic monitoring of the ON8V star HD 13268 revealed low amplitude variations with periods of several hours. In addition, observations of the SB1 system HD 15137 revealed variations on a time scale of a few hours. We consider these stars to be a good candidates for non radial pulsations, even though variations related to inhomogeneities in a circumstellar disk can not be rejected. [less ▲]

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See detailThe massive star binary fraction in young open clusters - I. NGC 6231 revisited
Sana, H.; Gosset, Eric ULg; Nazé, Yaël ULg et al

in Monthly Notices of the Royal Astronomical Society (2008), 386(1), 447-460

We present the results of a long-term high-resolution spectroscopy campaign on the O-type stars in NGC 6231. We revise the spectral classification and multiplicity of these objects and we constrain the ... [more ▼]

We present the results of a long-term high-resolution spectroscopy campaign on the O-type stars in NGC 6231. We revise the spectral classification and multiplicity of these objects and we constrain the fundamental properties of the O-star population. Almost three quarters of the O-type stars in the cluster are members of a binary system. The minimum binary fraction is 0.63, with half the O-type binaries having an orbital period of the order of a few days. The eccentricities of all the short-period binaries are revised downward, and henceforth match a normal period-eccentricity distribution. The mass ratio distribution shows a large preference for O + OB binaries, ruling out the possibility that, in NGC 6231, the companion of an O-type star is randomly drawn from a standard initial mass function. Obtained from a complete and homogeneous population of O-type stars, our conclusions provide interesting observational constraints to be confronted with the formation and early evolution theories of O-stars. [less ▲]

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See detailHigh-resolution optical spectroscopy of Plaskett's star
Linder, Natacha ULg; Rauw, Grégor ULg; Martins, Fabrice et al

in Astronomy and Astrophysics (2008), 489(2), 713-723

Context. Plaskett's star (HD 47 129) is a very massive O + O binary that belongs to the Mon OB2 association. Previous work suggests that this system displays the Struve-Sahade effect although the ... [more ▼]

Context. Plaskett's star (HD 47 129) is a very massive O + O binary that belongs to the Mon OB2 association. Previous work suggests that this system displays the Struve-Sahade effect although the measurements of the secondary radial velocities are very difficult and give controversial results. Both components have powerful stellar winds that collide and produce a strong X-ray emission. Aims. Our aim is to study the physical parameters of this system in detail and to investigate the relation between its spectral properties and its evolutionary status. Methods. We present here analysis of an extensive set of high-resolution optical spectra of HD 47 129. We used a disentangling method to separate the individual spectra of each star. We derived a new orbital solution and discuss the spectral classification of both components. A Doppler tomography technique applied to the emission lines H alpha and He II gamma 4686 yields a Doppler map that illustrates the wind interactions in the system. Finally, an atmosphere code is used to determine the different chemical abundances of the system components and the wind parameters. Results. HD 47 129 appears to be an O8 III/I + O7.5 III binary system in a post RLOF evolutionary stage, where matter has been transferred from the primary to the secondary star. The He overabundance of the secondary supports this scenario. In addition, the N overabundance and C underabundance of the primary component confirm previous results based on X-ray spectroscopy and indicate that the primary is an evolved massive star. We also determined a new orbital solution, with M-P sin(3) i = 45.4 +/- 2.4 M-circle dot and M-S sin(3) i = 47.3 +/- 0.3 M-circle dot. Furthermore, the secondary star has a high rotational velocity (nu sin i similar to 300 km s(-1)) that deforms its surface, leading to a non-uniform distribution in effective temperature. This could explain the variations in the equivalent widths of the secondary lines with phase. We suggest that the wind of the secondary star is confined near the equatorial plane because of its high rotational velocity, affecting the ram pressure equilibrium in the wind interaction zone. [less ▲]

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See detailSpectroscopic and photometric variability of the O9.5Vp star HD93521
Rauw, Grégor ULg; De Becker, Michaël ULg; van Winckel, Hans et al

in Astronomy and Astrophysics (2008), 487(2), 659-670

Aims. The line profile variability and photometric variability of the O9.5Vp star HD93521 are examined in order to establish the properties of the non-radial pulsations in this star. Methods. Fourier ... [more ▼]

Aims. The line profile variability and photometric variability of the O9.5Vp star HD93521 are examined in order to establish the properties of the non-radial pulsations in this star. Methods. Fourier techniques are used to characterize the modulations of the He I lambda lambda 5876, 6678 and H alpha lines in several spectroscopic time series and to search for variations in a photometric time series. Results. Our spectroscopic data confirm the existence of two periods of 1.75 and 2.89 h. The line profiles, especially those affected by emission wings, exhibit also modulations on longer time scales, but these are epoch-dependent and change from line to line. Unlike previous claims, we find no unambiguous signature of the rotational period in our data, nor of a third pulsation period (corresponding to a frequency of 2.66 d(-1)). Conclusions. HD93521 very likely exhibits non-radial pulsations with periods of 1.75 and 2.89 h with l similar or equal to 8 +/- 1 and l similar or equal to 4 +/- 1 respectively. No significant signal is found in the first harmonics of these two periods. The 2.89 h mode is seen at all epochs and in all lines investigated, while the visibility of the 1.75 h mode is clearly epoch-dependent. Whilst light variations are detected, their connection to these periodicities is not straightforward. [less ▲]

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See detailThe Struve-Sahade effect in the optical spectra of O-type binaries - I. Main-sequence systems
Linder, Natacha ULg; Rauw, Grégor ULg; Sana, H. et al

in Astronomy and Astrophysics (2007), 474(1), 193-204

Aims. We present a spectroscopic analysis of four massive binary systems that are known or are good candidates to display the Struve-Sahade effect (defined as the apparent strengthening of the secondary ... [more ▼]

Aims. We present a spectroscopic analysis of four massive binary systems that are known or are good candidates to display the Struve-Sahade effect (defined as the apparent strengthening of the secondary spectrum of the binary when the star is approaching, and the corresponding weakening of the lines when it is receding). Methods. We use high resolution optical spectra to determine new orbital solutions and spectral types of HD 165 052, HD 100 213, HD 159 176 and DH Cep. As good knowledge of the fundamental parameters of the considered systems is necessary to examine the Struve-Sahade effect. We then study equivalent width variations in the lines of both components of these binaries during their orbital cycle. Results. In the case of these four systems, variations appear in the equivalent widths of some lines during the orbital cycle, but the definition given above can any longer be valid, since it is now clear that the effect modifies the primary spectrum as much as the secondary spectrum. Furthermore, the lines affected, and the way in which they are affected, depend on the considered system. For at least two of them (HD 100 213 and HD 159 176) these variations probably reflect the ellipsoidal variable nature of the system. [less ▲]

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See detailThe XMM-Newton view of Plaskett's star and its surroundings
Linder, Natacha ULg; Rauw, Grégor ULg; Pollock, A. M. T. et al

in Monthly Notices of the Royal Astronomical Society (2006), 370(4), 1623-1632

XMM-Newton data of Plaskett's star (HD 47129) are used in order to analyse its X-ray spectrum and variability and hence to derive further constraints on the wind interaction in this early-type binary (O6 ... [more ▼]

XMM-Newton data of Plaskett's star (HD 47129) are used in order to analyse its X-ray spectrum and variability and hence to derive further constraints on the wind interaction in this early-type binary (O6 I + O7.5 I) system. Conventional models fail to provide a consistent fit of the European Photon Imaging Camera (EPIC) and Reflexion Grating Spectrometer (RGS) spectra. The lines seen in the RGS spectrum have a temperature of maximum emissivity between 0.18 and 1.4 keV. The EPIC and RGS spectra are best fitted by a non-equilibrium model consisting of a bremsstrahlung continuum at 2.2 +/- 0.1 keV and a number of independent emission lines. Our tests also suggest that an overabundance in nitrogen by a factor of similar to 6 might be indicated to best represent the RGS spectrum. On the other hand, a short-term variability study of the light curves of the system indicates that the X-ray flux of Plaskett's star did not display any significant variability during our observation. This result holds for all time-scales investigated here (from a few minutes to about one hour). Combining our XMM-Newton data with ROSAT archival observations, we find, however, a significant variability on the orbital time-scale. If this behaviour is indeed phase locked, it suggests a minimum in the X-ray flux when the primary star is in front. This might be attributed to an occultation of the colliding wind region by the body of the primary. Finally, 71 other X-ray sources have been detected in the field around Plaskett's star and most of them have a near-infrared (near-IR) counterpart with colours that are consistent with those of slightly reddened main-sequence objects. Actually, a sizeable fraction of the X-ray sources in the EPIC images could be either foreground or background sources with no direct connection to HD 47129. [less ▲]

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See detailXMM-Newton observations of the Cyg OB2 association
Rauw, Grégor ULg; De Becker, Michaël ULg; Linder, Natacha ULg

in Massive Stars and High-Energy Emission in OB Associations (2005, November)

We present the first results of an observing campaign of the Cyg OB2 association with the XMM-Newton observatory. The brightest OB-type stars exhibit rather hard spectra suggesting that at least part of ... [more ▼]

We present the first results of an observing campaign of the Cyg OB2 association with the XMM-Newton observatory. The brightest OB-type stars exhibit rather hard spectra suggesting that at least part of their X-ray emission arises in a wind-wind interaction. The EPIC images reveal a large number of fainter X-ray sources most of which are probably low-mass pre-main sequence stars belonging to Cyg OB2. [less ▲]

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