References of "Rauw, Grégor"
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See detailConstraining the fundamental parameters of the O-type binary CPD-41 7733
Sana, H.; Rauw, Grégor ULg; Gosset, Eric ULg

in Astrophysical Journal (2007), 659(2, Part 1), 1582-1591

Using a set of high-resolution spectra, we studied the physical and orbital properties of the O-type binary CPD -41 7733, located in the core of NGC 6231. We report the unambiguous detection of a ... [more ▼]

Using a set of high-resolution spectra, we studied the physical and orbital properties of the O-type binary CPD -41 7733, located in the core of NGC 6231. We report the unambiguous detection of a secondary spectral signature and we derive the first SB2 orbital solution of the system. The period is 5.6815 + 0.0015 days, and the orbit has no significant eccentricity. CPD -41 7733 probably consists of stars of spectral types O8.5 and B3. As for other objects in the cluster, we observe discrepant luminosity classifications while using spectroscopic or brightness criteria. Still, the present analysis suggests that both components display physical parameters close to those of typical O8.5 and B3 dwarfs. We also analyze the X-ray light curves and spectra obtained during six 30 ks XMM-Newton pointings spread over the 5.7 day period. We find no significant variability between the different pointings, nor within the individual observations. The CPD - 41 7733 X- ray spectrum is well reproduced by a three-temperature thermal mekal model with temperatures of 0.3, 0.8, and 2.4 keV. No X-ray overluminosity, resulting, e. g., from a possible wind interaction, is observed. The emission of CPD -41 7733 is thus very representative of typical O-type star X- ray emission. [less ▲]

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See detailEarly-type stars in the core of the young open cluster Westerlund 2
Rauw, Grégor ULg; Manfroid, Jean ULg; Gosset, Eric ULg et al

in Astronomy and Astrophysics (2007), 463(3), 981-991

Aims. The properties of the early-type stars in the core of the Westerlund 2 cluster are examined in order to establish a link between the cluster and the very massive Wolf-Rayet binary WR 20a as well as ... [more ▼]

Aims. The properties of the early-type stars in the core of the Westerlund 2 cluster are examined in order to establish a link between the cluster and the very massive Wolf-Rayet binary WR 20a as well as the H II complex RCW 49. Methods. Photometric monitoring as well as spectroscopic observations of Westerlund 2 are used to search for light variability and to establish the spectral types of the early-type stars in the cluster core. Results. The first light curves of the eclipsing binary WR 20a in B and V filters are analysed and a distance of 8 kpc is inferred. Three additional eclipsing binaries, which are probable late O or early B-type cluster members, are discovered, but none of the known early O-type stars in the cluster displays significant photometric variability above 1% at the 1-sigma level. The twelve brightest O-type stars are found to have spectral types between O3 and O6.5, significantly earlier than previously thought. Conclusions. The distance of the early-type stars in Westerlund 2 is established to be in excellent agreement with the distance of WR20a, indicating that WR20a actually belongs to the cluster. Our best estimate of the cluster distance thus amounts to 8.0 +/- 1.4 kpc. Despite the earlier spectral types, the currently known population of early-type stars in Westerlund 2 does not provide enough ionizing photons to account for the radio emission of the RCW49 complex. This suggests that there might still exist a number of embedded early O-stars in RCW49. [less ▲]

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See detailTowards an understanding of the Of ? p star HD 191612: phase-resolved multiwavelength observations
Nazé, Yaël ULg; Rauw, Grégor ULg; Pollock, A. M. T. et al

in Monthly Notices of the Royal Astronomical Society (2007), 375(1), 145-153

We present the analysis of phase-resolved X-ray and optical observations of the peculiar hot star HD 191612 (Of ? p). This star is known to display line-profile variations that are recurrent with a period ... [more ▼]

We present the analysis of phase-resolved X-ray and optical observations of the peculiar hot star HD 191612 (Of ? p). This star is known to display line-profile variations that are recurrent with a period of 538 d and its spectrum was found to present the signature of a magnetic field. In the X-rays, it is slightly overluminous compared to the canonical L-X/L-BOL) relation and appears brighter when the optical lines are strongest. Our XMM-Newton observations further reveal that the X-ray spectrum of HD 191612 exhibits rather broad lines and is dominated by a 'cool' (0.2-0.6 keV) thermal component, two characteristics at odds with the proposed magnetic rotator model. We also report for the first time the low-level variability of the metallic (absorption/emission) lines and He ii absorptions that appear to be associated with radial-velocity shifts. Finally, we compare our results with observations of the early-type stars and discuss several possible scenarios. [less ▲]

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See detailNGC6231: X-ray Properties of the Early-Type Star Population
Sana, H.; Nazé, Yaël ULg; Gosset, Eric ULg et al

in Massive Stars in Interactive Binaries (2007)

Based on a deep XMM-Newton observation of the young open cluster NGC6231, we derive the main X-ray properties of its early-type star population. Among the 610 X-ray sources detected in the field, 42 are ... [more ▼]

Based on a deep XMM-Newton observation of the young open cluster NGC6231, we derive the main X-ray properties of its early-type star population. Among the 610 X-ray sources detected in the field, 42 are associated with early-type stars. We investigate their L[SUB]X[/SUB]/L[SUB]bol[/SUB] relationship and confirm the clear dichotomy between O- and B-type stars. The cut-off line between the two behaviours occurs at L[SUB]bol[/SUB] ~ 10[SUP]38[/SUP] erg s[SUP]-1[/SUP] as previously proposed by Berghöfer et al. (1997). The distinction between single and binary stars is not clear cut, except for the colliding wind system HD152248. The X-ray detected B-stars in NGC6231 appear to be more luminous than predicted from the Berghöfer et al. relation. Though this suggests a bimodal distribution of the B-star X-ray emission, we caution however that these results might be biased by detection limits. Finally we investigate the X-ray variability of the detected sources and we find that about 40% of the X-ray emitters in the field of view present consistent signs of variability in the EPIC instruments. This fraction is much larger than previously thought. About one third of the early-type star population, either singles or binaries, further show variability. These variations do not seem to be exclusively related to binarity and could thus not be systematically interpreted in the framework of a wind-wind collision phenomenon. [less ▲]

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See detailNew Colliding-Wind Massive Binaries
De Becker, Michaël ULg; Rauw, Grégor ULg

in Massive Stars in Interactive Binaries (2007)

We report on the recent discovery of two new massive binary systems: WR20a and CygOB2#8A. For both systems, we briefly present the orbital solutions and we discuss the strong phase-locked line profile ... [more ▼]

We report on the recent discovery of two new massive binary systems: WR20a and CygOB2#8A. For both systems, we briefly present the orbital solutions and we discuss the strong phase-locked line profile variability of the Halpha and HeII lambda 4686 lines for WR20a, and the HeII &; 4686 line in the case of CygOB2#8A. In both cases, we show that the profile variability reveals the signature of a wind interaction. The prospects for future high-energy observations are also discussed. [less ▲]

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See detailXMM-Newton Studies of the Wolf-Rayet Colliding-Wind Binaries WR 25 (WN6h+O4f) and WR 11 (WC8+O7.5III)
van der Hucht, K. A.; Raassen, A. J. J.; Mewe, R. et al

in Massive Stars in Interactive Binaries (2007)

We report the analysis of high- and medium-resolution X-ray spectra of the Wolf-Rayet (WR) objects WR25 (HD93162, WN6h+O4f) and WR11 (gamma[SUP]2[/SUP] Velorum, WC8+O7.5III, P =78.53 d), obtained with the ... [more ▼]

We report the analysis of high- and medium-resolution X-ray spectra of the Wolf-Rayet (WR) objects WR25 (HD93162, WN6h+O4f) and WR11 (gamma[SUP]2[/SUP] Velorum, WC8+O7.5III, P =78.53 d), obtained with the reflection grating spectrometers (rgs) and the european photon imaging cameras (epicmos and <small>PN</small>) <small>CCD</small> spectrometers on board the XMM-Newton satellite. [less ▲]

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See detailThe first orbital solution for the massive colliding-wind binary HD 93162 ( WR 25)
Gamen, R.; Gosset, Eric ULg; Morrell, N. et al

in Astronomy and Astrophysics (2006), 460(3), 777-782

Context. Since the discovery, with the EINSTEIN satellite, of strong X-ray emission associated with HD 93162 (equivalent to WR 25), this object has been predicted to be a colliding-wind binary system ... [more ▼]

Context. Since the discovery, with the EINSTEIN satellite, of strong X-ray emission associated with HD 93162 (equivalent to WR 25), this object has been predicted to be a colliding-wind binary system. However, radial-velocity variations that would prove the suspected binary nature have yet to be found. Aims. We spectroscopically monitored this object to investigate its possible variability to address this discordance. Methods. We compiled the largest available radial-velocity data set for this star to look for variations that might be due to binary motion. We derived radial velocities from spectroscopic data acquired mainly between 1994 and 2006, and searched these radial velocities for periodicities using different numerical methods. Results. For the first time, periodic radial-velocity variations are detected. Our analysis definitively shows that the Wolf-Rayet star WR 25 is an eccentric binary system with a probable period of about 208 days. [less ▲]

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See detailAn XMM-Newton view of the young open cluster NGC 6231 - II. The OB star population
Sana, H.; Rauw, Grégor ULg; Nazé, Yaël ULg et al

in Monthly Notices of the Royal Astronomical Society (2006), 372(2), 661-678

In this second paper of the series, we pursue the analysis of the 180-ks XMM-Newton campaign towards the young open cluster NGC 6231 and we focus on its rich OB star population. We present a literature ... [more ▼]

In this second paper of the series, we pursue the analysis of the 180-ks XMM-Newton campaign towards the young open cluster NGC 6231 and we focus on its rich OB star population. We present a literature-based census of the OB stars in the field of view with more than one hundred objects, among which 30 per cent can be associated with an X-ray source. All the O-type stars are detected in the X-ray domain as soft and reasonably strong emitters. In the 0.5-10.0 keV band, their X-ray luminosities scale with their bolometric luminosities as log L-X - log L-bol = -6.912 +/- 0.153. Such a scaling law holds in the soft (0.5-1.0 keV) and intermediate (1.0-2.5 keV) bands but breaks down in the hard band. While the two colliding wind binaries in our sample clearly deviate from this scheme, the remaining O-type objects show a very limited dispersion (40 or 20 per cent according to whether 'cool' dwarfs are included or not), much smaller than that obtained from previous studies. At our detection threshold and with our sample, the sole identified mechanism that produces significant modulations in the O star X-ray emission is related to wind interaction. We thus propose that the intrinsic X-ray emission of non-peculiar O-type stars can be considered as constant for a given star. In addition, the level of X-ray emission is accurately related to the star luminosity or, equivalently, to its wind properties. Among B-type stars, the detection rate is only about 25 per cent in the subtype range B0-B4 and remains mostly uniform throughout the different subpopulations while it drops significantly at later subtypes. The associated X-ray spectra are harder than those of O-type stars. Our analysis points towards the detected emission being associated with a physical (in a multiple system) PMS companion. However, we still observe a correlation between the bolometric luminosity of the B stars and the measured X-ray luminosity. The best-fitting power law in the 0.5-10.0 keV band yields log L-X = 0.22(+/- 0.06) log L-bol + 22.8(+/- 2.4). [less ▲]

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See detailGCIRS 16SW: A massive eclipsing binary in the Galactic center
Martins, F.; Trippe, S.; Paumard, T. et al

in Astrophysical Journal (2006), 649(2, Part 2), 103-106

We report on the spectroscopic monitoring of GCIRS 16SW, an Ofpe/WN9 star and LBV candidate in the central parsec of the Galaxy. SINFONI observations show strong daily spectroscopic changes in the K band ... [more ▼]

We report on the spectroscopic monitoring of GCIRS 16SW, an Ofpe/WN9 star and LBV candidate in the central parsec of the Galaxy. SINFONI observations show strong daily spectroscopic changes in the K band. Radial velocities are derived from the He I 2.112 mu m line complex and vary regularly with a period of 19.45 days, indicating that the star is most likely an eclipsing binary. Under various assumptions, we are able to derive a mass of similar to 50 M circle dot for each component. [less ▲]

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See detailThe atypical emission-line star Hen 3-209
Nazé, Yaël ULg; Rauw, Grégor ULg; Hutsemekers, Damien ULg et al

in Monthly Notices of the Royal Astronomical Society (2006), 371(4), 1594-1600

We analyse observations, spanning 15 yr, dedicated to the extreme emission-line object Hen 3-209. Our photometric data indicate that the luminosity of the star undergoes marked variations with a peak-to ... [more ▼]

We analyse observations, spanning 15 yr, dedicated to the extreme emission-line object Hen 3-209. Our photometric data indicate that the luminosity of the star undergoes marked variations with a peak-to-peak amplitude of 0.65 mag. These variations are recurrent, with a period of 16.093 +/- 0.005 d. The spectrum of Hen 3-209 is peculiar with many different lines (H I, He I, Fe II,. showing P Cygni profiles. The line profiles are apparently changing in harmony with the photometry. The spectrum also contains [O III] lines that display a saddle profile topped by three peaks, with a maximum separation of about 600 km s(-1). Hen 3-209 is most likely an evolved luminous object suffering from mass ejection events and maybe belonging to a binary system. [less ▲]

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See detailXMM-Newton observations of the massive colliding wind binary and non-thermal radio emitter Cyg OB2#8A [O6If+O5.5III(f)]
De Becker, Michaël ULg; Rauw, Grégor ULg; Sana, H. et al

in Monthly Notices of the Royal Astronomical Society (2006), 371(3), 1280-1294

We report on the results of four XMM-Newton observations separated by about ten days from each other of Cyg OB2 #8A [O6If + O5.5III(f)]. This massive colliding wind binary is a very bright X-ray emitter ... [more ▼]

We report on the results of four XMM-Newton observations separated by about ten days from each other of Cyg OB2 #8A [O6If + O5.5III(f)]. This massive colliding wind binary is a very bright X-ray emitter-one of the first X-ray emitting O-stars discovered by the Einstein satellite-as well as a confirmed non-thermal radio emitter whose binarity was discovered quite recently. The X-ray spectrum between 0.5 and 10.0 keV is essentially thermal, and is best fitted with a three-component model with temperatures of about 3, 9 and 20 MK. The X-ray luminosity corrected for the interstellar absorption is rather large, i.e. about 1034 erg s(-1). Compared to the 'canonical' L-X/L-bol ratio of O-type stars, Cyg OB2 # 8A was a factor of 19-28 overluminous in X-rays during our observations. The EPIC spectra did not reveal any evidence for the presence of a non-thermal contribution in X-rays. This is not unexpected considering that the simultaneous detections of non-thermal radiation in the radio and soft X-ray (below 10.0 keV) domains is unlikely. Our data reveal a significant decrease in the X-ray flux from apastron to periastron with an amplitude of about 20 per cent. Combining our XMM Newton results with those from previous ROSAT-PSPC and ASCA-SIS observations, we obtain a light curve suggesting a phase-locked X-ray variability. The maximum emission level occurs around phase 0.75, and the minimum is probably seen shortly after the periastron passage. Using hydrodynamic simulations of the wind-wind collision, we find a high X-ray emission level close to phase 0.75, and a minimum at periastron as well. The high X-ray luminosity, the strong phase-locked variability and the spectral shape of the X-ray emission of Cyg OB2 # 8A revealed by our investigation point undoubtedly to X-ray emission dominated by colliding winds. [less ▲]

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See detailOn the nature of the hard X-ray source IGR J2018+4043
Bykov, Andrei Mikhailovich; Krassilchtchikov, Alexandre Mikhailovich; Uvarov, Yuri Alexandrovich et al

in Astrophysical Journal (2006), 649(1, Part 2), 21-24

We found a very likely counterpart to the recently discovered hard X-ray source IGR J2018 + 4043 in the multiwavelength observations of the source field. The source, originally discovered in the 20-40 keV ... [more ▼]

We found a very likely counterpart to the recently discovered hard X-ray source IGR J2018 + 4043 in the multiwavelength observations of the source field. The source, originally discovered in the 20-40 keV band, is now confidently detected also in the 40-80 keV band, with a flux of (1.4 +/- 0.4) x 10(-11) ergs cm(-2) s(-1). A 5 ks Swift observation of the IGR J2018 + 4043 field revealed a hard pointlike source with an observed 0.5-10 keV flux of 3.4(-0.8)(+0.7) x 10(-12) ergs cm(-2) s(-1) (90% confidence level) at alpha = 20(h)18(m)38(s).55, delta = +40 degrees 41'00.4" (with a 4".2 uncertainty). The combined Swift-INTEGRAL spectrum can be described by an absorbed power-law model with photon index Gamma = 1.3 +/- 0.2 and N-H = 6.1(-2.2)(+3.3) x 10(22) cm(-2). In archival optical and infrared data we found a slightly extended and highly absorbed object at the Swift source position. There is also an extended VLA 1.4 GHz source peaked at a beamwidth distance from the optical and X-ray positions. The observed morphology and multiwavelength spectra of IGR J2018 + 4043 are consistent with those expected for an obscured accreting object, i.e., an AGN or a Galactic X-ray binary. The identification suggests possible connection of IGR J2018 + 4043 to the bright gamma-ray source GEV J2020 + 4023 detected by COS B and CGRO EGRET. [less ▲]

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See detailL'univers des Hautes Energies dévoilé par Integral
Nazé, Yaël ULg; Rauw, Grégor ULg

Article for general public (2006)

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See detailThe OB binary HD 152219: a detached, double-lined, eclipsing system
Sana, H.; Gosset, Eric ULg; Rauw, Grégor ULg

in Monthly Notices of the Royal Astronomical Society (2006), 371(1), 67-80

We present the results of an optical spectroscopic campaign on the massive binary HD 152219 located near the core of the NGC 6231 cluster. Though the primary to secondary optical brightness ratio is ... [more ▼]

We present the results of an optical spectroscopic campaign on the massive binary HD 152219 located near the core of the NGC 6231 cluster. Though the primary to secondary optical brightness ratio is probably about 10, we clearly detect the secondary spectral signature and we derive the first reliable SB2 orbital solution for the system. The orbital period is close to 4.2403 d and the orbit is slightly eccentric (e = 0.08 +/- 0.01). The system is most probably formed by an O9.5 giant and a B1-2 V-III star. We derive minimal masses of 18.6 +/- 0.3 and 7.3 +/- 0.1 M-circle dot for the primary and secondary, respectively, and we constrain the stellar radius at values about 11 and 5 R-circle dot. INTEGRAL-Optical Monitoring Camera (OMC) data reveal that HD 152219 is the third O-type eclipsing binary known in NGC 6231. In the Hertzsprung-Russell (HR) diagram, the primary component lies on the blue edge of the beta Cep-type instability strip and its spectral lines display clear profile variations that are reminiscent of those expected from non-radial pulsations. Finally, we report the analysis of XMM-Newton observations of the system. The X-ray spectrum is relatively soft and is well reproduced by a two-temperature mekal model with kT(1) = 0.26 keV and kT(2) = 0.67 keV. The X-ray flux is most probably variable on a time-scale of days. The average X-ray luminosity during our campaign is log(L-X) approximate to 31.8 (erg s(-1)), but shows fluctuations of about 10 per cent around this value. [less ▲]

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See detailEarly-type stars in the young open cluster IC 1805. II. The probably single stars HD 15570 and HD 15629, and the massive binary/triple system HD 15558
De Becker, Michaël ULg; Rauw, Grégor ULg; Manfroid, Jean ULg et al

in Astronomy and Astrophysics (2006), 456

Aims.We address the issue of the multiplicity of the three brightest early-type stars of the young open cluster IC 1805, namely HD 15570, HD 15629 and HD 15558. Methods: .For the three stars, we measured ... [more ▼]

Aims.We address the issue of the multiplicity of the three brightest early-type stars of the young open cluster IC 1805, namely HD 15570, HD 15629 and HD 15558. Methods: .For the three stars, we measured the radial velocity by fitting Gaussian curves to line profiles in the optical domain. In the case of the massive binary HD 15558, we also used a spectral disentangling method to separate the spectra of the primary and of the secondary in order to derive the radial velocities of the two components. These measurements were used to compute orbital solutions for HD 15558. Results: .For HD 15570 and HD 15629, the radial velocities do not present any significant trend attributable to a binary motion on time scales of a few days, nor from one year to the next. In the case of HD 15558 we obtained an improved SB1 orbital solution with a period of about 442 days, and we report for the first time on the detection of the spectral signature of its secondary star. We derive spectral types O5.5III(f) and O7V for the primary and the secondary of HD 15558. We tentatively compute a first SB2 orbital solution although the radial velocities from the secondary star should be considered with caution. The mass ratio is rather high, i.e. about 3, and leads to very extreme minimum masses, in particular for the primary object. Minimum masses of the order of 150 ± 50 and 50 ± 15 M_ȯ are found respectively for the primary and the secondary. Conclusions: .We propose that HD 15558 could be a triple system. This scenario could help to reconcile the very large minimum mass derived for the primary object with its spectral type. In addition, considering new and previously published results, we find that the binary frequency among O-stars in IC 1805 has a lower limit of 20%, and that previously published values (80%) are probably overestimated. [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 detailHigh Resolution X-ray Spectroscopy of Early-Type Stars
Rauw, Grégor ULg; Pollock, A. M. T.; Nazé, Yaël ULg

in High Resolution X-ray Spectroscopy: towards XEUS and Con-X (2006, August 01)

Although X-ray emission from massive, early-type stars has been known for almost thirty years, it is only with the advent of the high-resolution spectrographs aboard Chandra and XMM-Newton that a detailed ... [more ▼]

Although X-ray emission from massive, early-type stars has been known for almost thirty years, it is only with the advent of the high-resolution spectrographs aboard Chandra and XMM-Newton that a detailed investigation of the X-ray spectra of these stars has become possible. The existing observations of both single and binary early-type stars have been used to study the properties of their stellar winds. While some results agree with the expectations from theoretical models, other spectra show unexpected features. In this contribution, we review our current knowledge on the X-ray emission of stars of spectral type O and Wolf-Rayet and discuss the possibilities to test some new ideas with future high-resolution X-ray observations. [less ▲]

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See detailAn XMM-Newton view of the young open cluster NGC 6231 - I. The catalogue
Sana, H.; Gosset, Eric ULg; Rauw, Grégor ULg et al

in Astronomy and Astrophysics (2006), 454(3), 1047-115

This paper is the first of a series dedicated to the X-ray properties of the young open cluster NGC 6231. Our data set relies on an XMM-Newton campaign of a nominal duration of 180 ks and reveals that NGC ... [more ▼]

This paper is the first of a series dedicated to the X-ray properties of the young open cluster NGC 6231. Our data set relies on an XMM-Newton campaign of a nominal duration of 180 ks and reveals that NGC 6231 is very rich in the X-ray domain too. Indeed, 610 X-ray sources are detected in the present field of view, centered on the cluster core. The limiting sensitivity of our survey is approximately 6 x 10(-15) erg cm(-2) s(-1) but clearly depends on the location in the field of view and on the source spectrum. Using different existing catalogues, over 85% of the X-ray sources could be associated with at least one optical and/or infrared counterpart within a limited cross-correlation radius of 3" at maximum. The surface density distribution of the X-ray sources presents a slight N-S elongation. Once corrected for the spatial sensitivity variation of the EPIC instruments, the radial profile of the source surface density is well described by a King profile with a central density of about 8 sources per arcmin(2) and a core radius close to 3.1 arcmin. The distribution of the X-ray sources seems closely related to the optical source distribution. The expected number of foreground and background sources should represent about 9% of the detected sources, thus strongly suggesting that most of the observed X-ray emitters are physically belonging to NGC 6231. Finally, beside a few bright but soft objects - corresponding to the early-type stars of the cluster - most of the sources are relatively faint (similar to 5 x 10(-15) erg cm(-2) s(-1)) with an energy distribution peaked around 1.0-2.0 keV. [less ▲]

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