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See detailCoRoT Observations of O Stars: Diverse Origins of Variability
Blomme, R.; Briquet, Maryline ULg; Degroote, P. et al

in Astronomical Society of the Pacific Conference Series (2013, January 01)

Six O-type stars were observed continuously by the CoRoT satellite during a 34.3-day run. The unprecedented quality of the data allows us to detect even low-amplitude stellar pulsations in some of these ... [more ▼]

Six O-type stars were observed continuously by the CoRoT satellite during a 34.3-day run. The unprecedented quality of the data allows us to detect even low-amplitude stellar pulsations in some of these stars (HD 46202 and the binaries HD 46149 and Plaskett's star). These cover both opacity-driven modes and solar-like stochastic oscillations, both of importance to the asteroseismological modeling of O stars. Additional effects can be seen in the CoRoT light curves, such as binarity and rotational modulation. Some of the hottest O-type stars (HD 46223, HD 46150 and HD 46966) are dominated by the presence of red-noise: we speculate that this is related to a sub-surface convection zone. [less ▲]

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See detailSpectral Modelling of Massive Binary Systems: The Example of LZ Cep
Palate, Matthieu ULg; Rauw, Grégor ULg; Mahy, Laurent ULg

in Central European astrophysical bulletin (2013)

Despite their importance for many astrophysical processes, massive stars are still not fully understood. Massive binaries offer an attractive way to improve our knowledge of the fundamental properties of ... [more ▼]

Despite their importance for many astrophysical processes, massive stars are still not fully understood. Massive binaries offer an attractive way to improve our knowledge of the fundamental properties of these objects. However, some secondary effects are known to generate variations in the spectra of massive binaries, rendering their analyses more difficult. We present here a new approach to the computation of synthetic spectra of massive binaries at different phases of their orbital cycle. Our model starts with the Roche potential modified by radiation pressure and accounts for the influence of the companion star on the shape and physical properties of the stellar surface. We further account for gravity darkening and reflection effects to compute the surface temperature. Once the local gravity and temperature are determined, we interpolate in a grid of NLTE plan-parallel atmosphere model spectra to obtain the local contribution to the spectrum at each surface points. Then we sum all the contributions, accounting for the Doppler shift, and limb-darkening to obtain the total spectrum. The computation is repeated for different orbital phases and can be compared to the observations to determine the best parameters. We illustrate our method through the example of the LZ Cep system (O9III + ON9.7V). [less ▲]

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See detailThe 2.35 year itch of Cygnus OB2 #9. I. Optical and X-ray monitoring
Nazé, Yaël ULg; Mahy, Laurent ULg; Damerdji, Yassine ULg et al

in Astronomy and Astrophysics (2012), 546

Context. Nonthermal radio emission in massive stars is expected to arise in wind-wind collisions occurring inside a binary system. One such case, the O-type star Cyg OB2 #9, was proven to be a binary only ... [more ▼]

Context. Nonthermal radio emission in massive stars is expected to arise in wind-wind collisions occurring inside a binary system. One such case, the O-type star Cyg OB2 #9, was proven to be a binary only four years ago, but the orbital parameters remained uncertain. The periastron passage of 2011 was the first one to be observable under good conditions since the discovery of binarity. <BR /> Aims: In this context, we have organized a large monitoring campaign to refine the orbital solution and to study the wind-wind collision. <BR /> Methods: This paper presents the analysis of optical spectroscopic data, as well as of a dedicated X-ray monitoring performed with Swift and XMM-Newton. <BR /> Results: In light of our refined orbital solution, Cyg OB2 #9 appears as a massive O+O binary with a long period and high eccentricity; its components (O5-5.5I for the primary and O3-4III for the secondary) have similar masses and similar luminosities. The new data also provide the first evidence that a wind-wind collision is present in the system. In the optical domain, the broad Hα line varies, displaying enhanced absorption and emission components at periastron. X-ray observations yield the unambiguous signature of an adiabatic collision, because as the stars approach periastron, the X-ray luminosity closely follows the 1/D variation expected in that case. The X-ray spectrum appears, however, slightly softer at periastron, which is probably related to winds colliding at slightly lower speeds at that time. <BR /> Conclusions: It is the first time that such a variation has been detected in O+O systems, and the first case where the wind-wind collision is found to remain adiabatic even at periastron passage. [less ▲]

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See detail9 Sagittarii: uncovering an O-type spectroscopic binary with an 8.6 year period
Rauw, Grégor ULg; Sana, H.; Spano, M. et al

in Astronomy and Astrophysics (2012), 542

Context. The O-type object 9 Sgr is a well-known synchrotron radio emitter. This feature is usually attributed to colliding-wind binary systems, but 9 Sgr was long considered a single star. <BR /> Aims ... [more ▼]

Context. The O-type object 9 Sgr is a well-known synchrotron radio emitter. This feature is usually attributed to colliding-wind binary systems, but 9 Sgr was long considered a single star. <BR /> Aims: We have conducted a long-term spectroscopic monitoring of this star to investigate its multiplicity and search for evidence for wind-wind interactions. <BR /> Methods: Radial velocities are determined and analysed using various period search methods. Spectral disentangling is applied to separate the spectra of the components of the binary system. <BR /> Results: We derive the first ever orbital solution of 9 Sgr. The system is found to consist of an O3.5 V((f[SUP]+[/SUP])) primary and an O5-5.5 V((f)) secondary moving around each other on a highly eccentric (e = 0.7), 8.6 year orbit. The spectra reveal no variable emission lines that could be formed in the wind interaction zone in agreement with the expected properties of the interaction in such a wide system. <BR /> Conclusions: Our results provide further support to the paradigm of synchrotron radio emission from early-type stars being a manifestation of interacting winds in a binary system. Based on observations collected at the European Southern Observatory (La Silla, Chile and Cerro Paranal, Chile) and the San Pedro Mártir observatory (Mexico).Appendix A is available in electronic form at <A href="http://www.aanda.org">http://www.aanda.org</A>The reduced spectra are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via <A href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/542/A95">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/542/A95</A> [less ▲]

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See detailEvidence for a physically bound third component in HD 150136
Mahy, Laurent ULg; Gosset, Eric ULg; Sana, H et al

in Astronomy and Astrophysics (2012), 540

Context. HD 150136 is one of the nearest systems harbouring an O3 star. Although this system was considered for a long time as a binary, more recent investigations have suggested the possible existence of ... [more ▼]

Context. HD 150136 is one of the nearest systems harbouring an O3 star. Although this system was considered for a long time as a binary, more recent investigations have suggested the possible existence of a third component. <BR /> Aims: We present a detailed analysis of HD 150136 to test its triple nature. In addition, we investigate the physical properties of the individual components of this system. <BR /> Methods: We analysed high-resolution, high signal-to-noise data collected through multi-epoch runs spread over ten years. We applied a disentangling program to refine the radial velocities and to obtain the individual spectra of each star. With the radial velocities, we computed the orbital solution of the inner system, and we describe the main properties of the orbit of the outer star such as the preliminary mass ratio, the eccentricity, and the orbital-period range. With the individual spectra, we determined the stellar parameters of each star by means of the CMFGEN atmosphere code. <BR /> Results: We offer clear evidence that HD 150136 is a triple system composed of an O3V((f[SUP]∗[/SUP]))-3.5V((f[SUP]+[/SUP])), an O5.5-6V((f)), and an O6.5-7V((f)) star. The three stars are between 0-3 Myr old. We derive dynamical masses of about 64, 40, and 35 M[SUB]&sun;[/SUB] for the primary, the secondary and the third components by assuming an inclination of 49° (sin[SUP]3[/SUP]i = 0.43). It currently corresponds to one of the most massive systems in our galaxy. The third star moves with a period in the range of 2950 to 5500 d on an outer orbit with an eccentricity of at least 0.3. However, because of the long orbital period, our dataset is not sufficient to constrain the orbital solution of the tertiary component with high accuracy. <BR /> Conclusions: We confirm there is a tertiary star in the spectrum of HD 150136 and show that it is physically bound to the inner binary system. This discovery makes HD 150136 the first confirmed triple system with an O3 primary star. Table 1 is available in electronic form at <A href="http://www.aanda.org">http://www.aanda.org</A> [less ▲]

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See detailA quantitative study of O stars in NGC 2244 and the Monoceros OB2 association
Martins, F.; Mahy, Laurent ULg; Hillier, D. J. et al

in Astronomy and Astrophysics (2012), 538

<BR /> Aims: Our goal is to determine the stellar and wind properties of seven O stars in the cluster NGC 2244 and three O stars in the OB association Mon OB2. These properties give us insight into the ... [more ▼]

<BR /> Aims: Our goal is to determine the stellar and wind properties of seven O stars in the cluster NGC 2244 and three O stars in the OB association Mon OB2. These properties give us insight into the mass loss rates of O stars. They allow us to both check the validity of rotational mixing in massive stars and to better understand the effects of the ionizing flux and wind mechanical energy release on the surrounding interstellar medium and its influence on triggered star formation. <BR /> Methods: We collected optical and UV spectra of the target stars that we analyzed by means of atmosphere models computed with the code CMFGEN. The spectra of binary stars were disentangled and the components studied separately. <BR /> Results: All stars have an evolutionary age less than 5 million years, with the most massive stars being among the youngest. Nitrogen surface abundances show no clear relation with projected rotational velocities. Binaries and single stars show the same range of enrichment. This is attributed to the youth and/or wide separation of the binary systems in which the components have not (yet) experienced strong interaction. A clear trend toward greater enrichment in higher luminosity objects is observed, consistent with what evolutionary models with rotation predict for a population of O stars at any given age. We confirm the weakness of winds in late O dwarfs. In general, mass loss rates derived from UV lines are lower than mass loss rates obtained from Hα. The UV mass loss rates are even lower than the single-line driving limit in the latest type dwarfs. These issues are discussed in the context of the structure of massive stars winds. The evolutionary and spectroscopic masses are in agreement above 25 M[SUB]&sun;[/SUB], but the uncertainties are large. Below this threshold, the few late-type O stars studied here indicate that the mass discrepancy still seems to hold. Appendix A is available in electronic form at <A href="http://www.aanda.org">http://www.aanda.org</A> [less ▲]

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See detailAstrophysical parameters and multiplicity of massive stars: a mathematical approach
Mahy, Laurent ULg

Doctoral thesis (2011)

Often quoted for their crucial role in the ecology of galaxies, massive stars remain mysterious. The exact process leading to their formation, their stability or their stellar winds are so many questions ... [more ▼]

Often quoted for their crucial role in the ecology of galaxies, massive stars remain mysterious. The exact process leading to their formation, their stability or their stellar winds are so many questions which are still unanswered. Since the lifetime of these objects is too short to allow them to move far away from their birth places, the large majority of massive stars are located in young open clusters or in OB associations. The observations of massive stars in these locations thus provide with promising information on their formation mechanisms. One of the most interesting properties, in this context, is their multiplicity. Indeed, the proportion of multiple systems, the values of orbital periods, of eccentricities, or of mass ratios are parameters directly linked to the formation mechanism and to the dynamical interactions occurring during the earliest stages of their existence. Moreover, constraining this multiplicity with a sufficient accuracy allows to obtain a better determination of the nature and physical parameters of massive stars. The present work is devoted to the study of the O star populations located in the young open cluster NGC2244 and in the surrounding Mon OB2 association as well as in several OB associations of the Cygnus complex. We establish not only the multiplicity of about thirty stars but also their fundamental parameters such as their masses or their N content. However, the complexity of observed spectra of multiple systems makes these objects difficult to investigate. Their analysis thus requires the development and the utilization of mathematical tools such as the "disentangling" which allows to separate the spectra of the components from the observed ones or such as the Doppler tomography which aims at mapping the wind interactions seen in some binary systems. The first part of this dissertation is dedicated to the development and to the characterization of these mathematical methods whilst the second part focuses on the study of populations of massive stars. We refine the orbital and physical parameters of systems already known as multiple. In this context, the analysis of the two components of LZ Cep system reveals modified abundances for the secondary star, confirming a mass transfer from the secondary towards to primary. We also detect for the first time a third star in the HD150136 system. This object, composed of an O3, an O5.5 and an O6.5, constitutes a new test to apply these mathematical methods and thus to better constrain the physical properties of the system as well as those of each component. Moreover, we find, in the population of NGC2244, of Mon OB2 and of the Cygnus OB associations, six new binary systems and derive the orbital solutions for five of them. These results show that NGC2244 hosts only one long-period binary and none with a short period whilst four short-period and no long-period systems are detected in the Cygnus complex. This obvious lack of short-period system in NGC2244 contrasts with the O star populations in other young open clusters such as NGC6231 or IC 2944. Besides the multiplicity, the fundamental parameters such as the N content also allows us to, notably, adapt the so-called Hunter-diagram to galactic O stars. Moreover, we have obtained photometric data for six O-type stars in NGC2244 and Mon OB2 with the CoRoT satellite. These data of unprecedented quality allow us to detect the presence, in the O star light curves, of red noise which is supposed to originate from either the sub-surface convection zone, the granulation or an onset of clumping in the winds of the stars. We also highlight the existence of non-radial pulsations in the light curve of Plaskett’s Star as well as a hot spot located between the two components of that system. In addition, numerous frequencies, extracted from the CoRoT light curves, reveal, for some of the observed stars, solar-like oscillations or beta Cephei-like pulsations. This analysis thus gives a first observational constraint on the bright end of the massive star instability strip. [less ▲]

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See detailVariability in the CoRoT photometry of three hot O-type stars. HD 46223, HD 46150, and HD 46966
Blomme, R.; Mahy, Laurent ULg; Catala, C. et al

in Astronomy and Astrophysics (2011), 533

Context. The detection of pulsational frequencies in stellar photometry is required as input for asteroseismological modelling. The second short run (SRa02) of the CoRoT mission has provided photometric ... [more ▼]

Context. The detection of pulsational frequencies in stellar photometry is required as input for asteroseismological modelling. The second short run (SRa02) of the CoRoT mission has provided photometric data of unprecedented quality and time-coverage for a number of O-type stars. <BR /> Aims: We analyse the CoRoT data corresponding to three hot O-type stars, describing the properties of their light curves and search for pulsational frequencies, which we then compare to theoretical model predictions. <BR /> Methods: We determine the amplitude spectrum of the data, using the Lomb-Scargle and a multifrequency HMM-like technique. Frequencies are extracted by prewhitening, and their significance is evaluated under the assumption that the light curve is dominated by red noise. We search for harmonics, linear combinations, and regular spacings among these frequencies. We use simulations with the same time sampling as the data as a powerful tool to judge the significance of our results. From the theoretical point of view, we use the MAD non-adiabatic pulsation code to determine the expected frequencies of excited modes. <BR /> Results: A substantial number of frequencies is listed, but none can be convincingly identified as being connected to pulsations. The amplitude spectrum is dominated by red noise. Theoretical modelling shows that all three O-type stars can have excited modes, but the relation between the theoretical frequencies and the observed spectrum is not obvious. <BR /> Conclusions: The dominant red noise component in the hot O-type stars studied here clearly points to a different origin than the pulsations seen in cooler O stars. The physical cause of this red noise is unclear, but we speculate on the possibility of sub-surface convection, granulation, or stellar wind inhomogeneities being responsible. The CoRoT space mission was developed and is operated by the French space agency CNES, with participation of ESA's RSSD and Science Programmes, Austria, Belgium, Brazil, Germany and Spain.Tables 2-4 are available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via <A href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/533/A4">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/533/A4</A> [less ▲]

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See detailThe two components of the evolved massive binary LZ Cephei. Testing the effects of binarity on stellar evolution
Mahy, Laurent ULg; Martins, F.; Machado, C. et al

in Astronomy and Astrophysics (2011)

Aims. We present an in-depth study of the two components of the binary system LZCep to constrain the effects of binarity on the evolution of massive stars. Methods. We analyzed a set of high-resolution ... [more ▼]

Aims. We present an in-depth study of the two components of the binary system LZCep to constrain the effects of binarity on the evolution of massive stars. Methods. We analyzed a set of high-resolution, high signal-to-noise ratio optical spectra obtained over the orbital period of the system to perform a spectroscopic disentangling and derive an orbital solution. We subsequently determine the stellar properties of each component by means of an analysis with the CMFGEN atmosphere code. Finally, with the derived stellar parameters, we model the Hipparcos photometric light curve using the program NIGHTFALL to obtain the orbit inclination and the stellar masses. Results. LZ Cep is a O9III+ON9.7V binary. It is as a semi-detached system in which either the primary or the secondary star almost fills up its Roche lobe. The dynamical masses are about 16.0 M (primary) and 6.5 M (secondary). The latter is lower than the typical mass of late-type O stars. The secondary component is chemically more evolved than the primary (which barely shows any sign of CNO processing), with strong helium and nitrogen enhancements as well as carbon and oxygen depletions. These properties (surface abundances and mass) are typical ofWolf-Rayet stars, although the spectral type is ON9.7V. The luminosity of the secondary is consistent with that of core He-burning objects. The preferred, tentative evolutionary scenario to explain the observed properties involves mass transfer from the secondary – which was initially more massive- towards the primary. The secondary is now almost a core He-burning object, probably with only a thin envelope of H-rich and CNO processed material. A very inefficient mass transfer is necessary to explain the chemical appearance of the primary. Alternative scenarios are discussed but they are affected by greater uncertainties. [less ▲]

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See detailHD 150136: towards one of the most massive systems?
Mahy, Laurent ULg; Gosset, Eric ULg; Sana, Hugues et al

in Neiner, Coralie; Wade, G.; Meynet, G. (Eds.) et al Active OB stars: structure, evolution, mass loss and critical limits (2011, July 01)

We present the preliminary results of an intensive monitoring devoted to HD 150136. Already quoted as an O3+O6 binary, we detected a third O-type component physically linked to the system, making it one ... [more ▼]

We present the preliminary results of an intensive monitoring devoted to HD 150136. Already quoted as an O3+O6 binary, we detected a third O-type component physically linked to the system, making it one of the nearest (1.3 kpc) most massive systems known until now (~134M[SUB]solar[/SUB]). To determine the physical parameters of this system, we applied a disentangling program to study individually the three components. It allows us to constrain their spectral types and to derive a new orbital solution for the short-period system. [less ▲]

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See detailThe multiplicity of O-type stars in NGC 2244
Mahy, Laurent ULg; Rauw, Grégor ULg; Martins, F. et al

in Bulletin de la Societe Royale des Sciences de Liege (2011), 80

The investigation of the multiplicity of massive stars is crucial to determine a robust binary fraction but also for understanding the physical properties of these objects. In this contribution, we will ... [more ▼]

The investigation of the multiplicity of massive stars is crucial to determine a robust binary fraction but also for understanding the physical properties of these objects. In this contribution, we will present the main results from our long-term spectroscopic survey devoted to the young open cluster NGC 2244. We discuss the spectral classification, the projected rotational velocity (v sin{i}) and the multiplicity of O-stars. The stellar and wind parameters of each star, obtained using the CMFGEN atmosphere code, help us to better constrain the individual properties of these objects. Several of these stars were observed by the CoRoT satellite (SRa02) in the Asteroseismology channel. This intensive monitoring and the unprecedented quality of the light curves allow us to shed a new light on these objects. [less ▲]

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See detailA first orbital solution for the non-thermal radio emitter Cyg OB2 #9
Nazé, Yaël ULg; Damerdji, Yassine ULg; Rauw, Grégor ULg et al

in Bulletin de la Société Royale des Sciences de Liège (2011), 80

We reported in 2008 the first detection of the binary nature of Cyg OB2 #9. Since then, we have continued our spectroscopic monitoring of this object, doubling the number of available spectra of the star ... [more ▼]

We reported in 2008 the first detection of the binary nature of Cyg OB2 #9. Since then, we have continued our spectroscopic monitoring of this object, doubling the number of available spectra of the star while covering a second periastron passage. Using a variety of techniques, the radial velocities were estimated and a first, preliminary orbital solution was derived (P=2.4 yrs). The mass ratio appears close to unity and the eccentricity is large, 0.7-0.75. [less ▲]

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See detailPlaskett's star: analysis of the CoRoT photometric data
Mahy, Laurent ULg; Gosset, Eric ULg; Baudin, F. et al

in Astronomy and Astrophysics (2011), 525

Context. The second short run (SRa02) of the CoRoT space mission for asteroseismology was partly devoted to stars belonging to the Mon OB2 association. An intense monitoring has been performed on Plaskett ... [more ▼]

Context. The second short run (SRa02) of the CoRoT space mission for asteroseismology was partly devoted to stars belonging to the Mon OB2 association. An intense monitoring has been performed on Plaskett's star (HD 47129) and the unprecedented quality of the light curve allows us to shed new light on this very massive, non-eclipsing binary system. <BR /> Aims: We particularly aimed at detecting periodic variability that might be associated with pulsations or interactions between both components. We also searched for variations related to the orbital cycle that could help to constrain the inclination and the morphology of the binary system. <BR /> Methods: We applied an iterative Fourier-based prewhitening and a multiperiodic fitting procedure to analyse the time series and extract the frequencies of variations from the CoRoT light curve. We describe the noise properties to tentatively define an appropriate significance criterion and, in consequence, to only point out the peaks at a certain significance level. We also detect the variations related to the orbital motion and study them with the NIGHTFALL programme. <BR /> Results: The periodogram computed from Plaskett's star CoRoT light curve mainly exhibits a majority of peaks at low frequencies. Among these peaks, we highlight a list of 43 values, notably including two different sets of harmonic frequencies whose fundamental peaks are located at about 0.07 and 0.82 d[SUP]-1[/SUP]. The former represents the orbital frequency of the binary system, whilst the latter could probably be associated with non-radial pulsations. The study of the 0.07 d[SUP]-1[/SUP] variations reveals a hot spot most probably situated on the primary star and facing the secondary. <BR /> Conclusions: The investigation of this unique dataset constitutes a further step in the understanding of Plaskett's star. These results provide a first basis for future seismic modelling and put forward the probable existence of non-radial pulsations in Plaskett's star. Moreover, the fit of the orbital variations confirms the problem of the distance of this system which was already mentioned in previous works. A hot region between both components renders the determination of the inclination ambiguous. The CoRoT space mission was developed and is operated by the French space agency CNES, with participation of ESA's RSSD and Science Programmes, Austria, Belgium, Brazil, Germany and Spain.Table 2 is only available in electronic form at <A href="http://www.aanda.org">http://www.aanda.org</A> [less ▲]

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See detailDetection of frequency spacings in the young O-type binary HD 46149 from CoRoT photometry
Degroote, P.; Briquet, Maryline ULg; Auvergne, M. et al

in Astronomy and Astrophysics (2010), 519

<BR /> Aims: Using the CoRoT space based photometry of the O-type binary HD 46149, stellar atmospheric effects related to rotation can be separated from pulsations, because they leave distinct signatures ... [more ▼]

<BR /> Aims: Using the CoRoT space based photometry of the O-type binary HD 46149, stellar atmospheric effects related to rotation can be separated from pulsations, because they leave distinct signatures in the light curve. This offers the possibility of characterising and exploiting any pulsations seismologically. <BR /> Methods: Combining high-quality space based photometry, multi-wavelength photometry, spectroscopy and constraints imposed by binarity and cluster membership, the detected pulsations in HD 46149 are analyzed and compared with those for a grid of stellar evolutionary models in a proof-of-concept approach. <BR /> Results: We present evidence of solar-like oscillations in a massive O-type star, and show that the observed frequency range and spacings are compatible with theoretical predictions. Thus, we unlock and confirm the strong potential of this seismically unexplored region in the HR diagram. The CoRoT space mission was developed and is operated by the French space agency CNES, with participation of ESA's RSSD and Science Programmes, Austria, Belgium, Brazil, Germany, and Spain.Based on observations made with the ESO telescopes at La Silla Observatory under the ESO Large Programme LP182.D-0356.Based on observations made with the Mercator Telescope, operated on the island of La Palma by the Flemish Community, at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. [less ▲]

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See detailFirst Orbital Solution for the Non-thermal Emitter Cyg OB2 No. 9
Nazé, Yaël ULg; Damerdji, Yassine ULg; Rauw, Grégor ULg et al

in Astrophysical Journal (2010), 719

After the first detection of its binary nature, the spectroscopic monitoring of the non-thermal radio emitter Cyg OB2 No. 9 (P = 2.4 yr) has continued, doubling the number of available spectra of the star ... [more ▼]

After the first detection of its binary nature, the spectroscopic monitoring of the non-thermal radio emitter Cyg OB2 No. 9 (P = 2.4 yr) has continued, doubling the number of available spectra of the star. Since the discovery paper of 2008, a second periastron passage has occurred in 2009 February. Using a variety of techniques, the radial velocities could be estimated and a first, preliminary orbital solution was derived from the He I 5876 line. The mass ratio appears close to unity and the eccentricity is large, i.e., 0.7-0.75. X-ray data from 2004 and 2007 are also analyzed in quest of peculiarities linked to binarity. The observations reveal neither large overluminosity nor strong hardness, but it must be noted that the high-energy data were taken after the periastron passage, at a time where colliding wind emission may be low. Some unusual X-ray variability is however detected, with a 10% flux decrease between 2004 and 2007. To clarify their origin and find a more obvious signature of the wind-wind collision, additional data, taken at periastron and close to it, are needed. Based on observations collected at the Haute-Provence Observatory and with XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member States and the USA (NASA). [less ▲]

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See detailA New Investigation of the Binary HD 48099
Mahy, Laurent ULg; Rauw, Grégor ULg; Martins, F. et al

in Astrophysical Journal (2010), 708

With an orbital period of about 3.078 days, the double-lined spectroscopic binary HD 48099 is, until now, the only short-period O+O system known in the Mon OB2 association. Even though an orbital solution ... [more ▼]

With an orbital period of about 3.078 days, the double-lined spectroscopic binary HD 48099 is, until now, the only short-period O+O system known in the Mon OB2 association. Even though an orbital solution has already been derived for this system, few information are available about the individual stars. We present, in this paper, the results of a long-term spectroscopic campaign. We derive a new orbital solution and apply a disentangling method to recover the mean spectrum of each star. To improve our knowledge concerning both components, we determine their spectral classifications and their projected rotational velocities. We also constrain the main stellar parameters of both stars by using the CMFGEN atmosphere code and provide the wind properties for the primary star through the study of International Ultraviolet Explorer spectra. This investigation reveals that HD 48099 is an O5.5 V ((f)) + O9 V binary with M [SUB]1[/SUB]sin[SUP]3[/SUP] i = 0.70 M [SUB]sun[/SUB] and M [SUB]2[/SUB]sin[SUP]3[/SUP] i = 0.39 M [SUB]sun[/SUB], implying a rather low orbital inclination. This result, combined with both a large effective temperature and log g, suggests that the primary star (vsin i sime 91 km s[SUP]â 1[/SUP]) is actually a fast rotator with a strongly clumped wind and a nitrogen abundance of about 8 times the solar value. [less ▲]

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See detailHD 150136: towards one of the most massive systems?
Mahy, Laurent ULg; Gosset, Eric ULg; Rauw, Grégor ULg et al

Poster (2010)

The improvement of the astronomical instrumentation allowed scientists to develop new analysis techniques to investigate binary and multiple stars. In this context, we present the preliminary results of ... [more ▼]

The improvement of the astronomical instrumentation allowed scientists to develop new analysis techniques to investigate binary and multiple stars. In this context, we present the preliminary results of an intense monitoring devoted to the triple system, HD 150136. Supposed to be the nearest O3 star (1.3 kpc), this object is also composed of two other early O-type stars, making it one of the most massive systems known until now (~ 134Mʘ). This multiple star is a non-thermal radio emitter and presents a colliding wind interaction zone. To determine the physical parameters of this system, we applied a disentangling program to study individually the three components. It allows us to revise their spectral classification and to derive a new orbital solution for the short-period system but also the first one for the entire system. [less ▲]

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See detailEarly-type stars in the young open cluster NGC 2244 and in the Monoceros OB2 association. I. The multiplicity of O-type stars
Mahy, Laurent ULg; Nazé, Yaël ULg; Rauw, Grégor ULg et al

in Astronomy and Astrophysics (2009), 502

Aims. We present the results obtained from a long-term spectroscopic campaign to study the multiplicity of O-type stars in both the young open cluster NGC 2244 and the Mon OB2 association. Methods: Our ... [more ▼]

Aims. We present the results obtained from a long-term spectroscopic campaign to study the multiplicity of O-type stars in both the young open cluster NGC 2244 and the Mon OB2 association. Methods: Our spectroscopic monitoring was performed over several years, allowing us to investigate different timescales. For each star, several spectral diagnostic tools were applied to search for line shifts and profile variations. We also measured the projected rotational velocity and revisited the spectral classification. Results: Several stars in our sample have been previously considered to be spectroscopic binaries, although only a few scattered observations were available. Our results now have identified a more complex situation for two new spectroscopic binaries (HD 46 149 in NGC 2244 and HD 46 573 in Mon OB2). The first object is a long-period double-lined spectroscopic binary, although the exact value of its period remains uncertain and the second object is classified as an SB1 system with a period of about 10.67 days but the time series of our observations do not enable us to derive a unique orbital solution for this system. We also find another star to be variable in radial velocity (HD 46 150) and detect line profile variations in two rapid rotators (HD 46 056 and HD 46 485). Conclusions: This spectroscopic investigation places a firm lower limit (17%) on the binary fraction of O-stars in NGC 2244 and reveals the lack of short-period O+OB systems in this cluster. In addition, a comparison of these new results with two other well-studied clusters (NGC 6231 and IC 1805) puts forward possible hints of a relation between stellar density and binarity, which could provide constraints on the theories of the formation and early evolution of hot stars. Table 2 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/502/937 Based on observations collected at Observatoire de Haute-Provence (France), San Pedro Màrtir Observatory (Mexico), La Silla Observatory (European Southern Observatory), and Asiago Observatory (Italy). [less ▲]

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See detailMultiplicity of O-type stars in NGC 2244
Mahy, Laurent ULg; Nazé, Yaël ULg; Rauw, Grégor ULg et al

Poster (2008)

We present the main results from our long‐term spectroscopic campaign devoted to the O‐type stars in the young open cluster NGC 2244. Previous works by our team on the O‐stars multiplicity in other young ... [more ▼]

We present the main results from our long‐term spectroscopic campaign devoted to the O‐type stars in the young open cluster NGC 2244. Previous works by our team on the O‐stars multiplicity in other young open clusters (IC 1805 and NGC 6231) showed that the number of binary systems in such clusters was larger than 40%. Until now, only two stars (HD 47129 and HD 48099) in the surroundings of NGC 2244 have been confirmed as spectroscopic binaries (SB) although Garcia & Mermilliod (2001) estimated the O‐type binary fraction at 50%. In this context, we revisited the spectral classification, the projected rotational velocity and the multiplicity of O‐type stars in NGC 2244. [less ▲]

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