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 ▲]Detailed reference viewed: 64 (10 ULg) NGC 1624-2: a slowly rotating, X-ray luminous Of?cp star with an extraordinarily strong magnetic fieldWade, G. A.; Maíz Apellániz, J.; Martins, F. et alin Monthly Notices of the Royal Astronomical Society (2012), 425This paper presents a first observational investigation of the faint Of?p star NGC 1624-2, yielding important new constraints on its spectral and physical characteristics, rotation, magnetic field ... [more ▼]This paper presents a first observational investigation of the faint Of?p star NGC 1624-2, yielding important new constraints on its spectral and physical characteristics, rotation, magnetic field strength, X-ray emission and magnetospheric properties. Modelling the spectrum and spectral energy distribution, we conclude that NGC 1624-2 is a main-sequence star of mass M ≃ 30 M[SUB]&sun;[/SUB], and infer an effective temperature of 35 ± 2 kK and log g = 4.0 ± 0.2. Based on an extensive time series of optical spectral observations we report significant variability of a large number of spectral lines, and infer a unique period of 157.99 ± 0.94 d which we interpret as the rotational period of the star. We report the detection of a very strong (5.35 ± 0.5 kG) longitudinal magnetic field , coupled with probable Zeeman splitting of the Stokes I profiles of metal lines confirming a surface field modulus of 14 ± 1 kG, consistent with a surface dipole of polar strength ≳20 kG. This is the largest magnetic field ever detected in an O-type star, and the first report of Zeeman splitting of Stokes I profiles in such an object. We also report the detection of reversed Stokes V profiles associated with weak, high-excitation emission lines of O III, which we propose may form in the close magnetosphere of the star. We analyse archival Chandra ACIS-I X-ray data, inferring a very hard spectrum with an X-ray efficiency of log L[SUB]x[/SUB]/L[SUB]bol[/SUB] = -6.4, a factor of 4 larger than the canonical value for O-type stars and comparable to that of the young magnetic O-type star θ[SUP]1[/SUP] Ori C and other Of?p stars. Finally, we examine the probable magnetospheric properties of the star, reporting in particular very strong magnetic confinement of the stellar wind, with η[SUB]*[/SUB] ≃ 1.5 × 10[SUP]4[/SUP], and a very large Alfvén radius, R[SUB]Alf[/SUB] = 11.4 R[SUB]*[/SUB]. Based on spectropolarimetric observations obtained at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers (INSU) of the Centre National de la Recherche Scientifique of France, and the University of Hawaii, as well as on observations obtained using the Narval spectropolarimeter at the Observatoire du Pic du Midi (France), which is operated by the INSU. The spectroscopic data were gathered with five facilities: the 9.2-m Hobby-Eberly Telescope at McDonald Observatory (MDO), the 3.5-m Telescope at Calar Alto Observatory (CAHA), the 1.5-m Telescope at the Observatorio de Sierra Nevada (OSN), the 4.2-m William Herschel Telescope at the Observatorio del Roque de los Muchachos (ORM) and the 2-m Himalayan Chandra Telescope at Indian Astronomical Observatory (IAO). [less ▲]Detailed reference viewed: 24 (2 ULg) Sur l'histoire de l'astronomie à LiègeGosset, Eric ; Nazé, Yaël Diverse speeche and writing (2012)Cette série de posters s'intéresse à l'astronomie à Liège : (1) l'astronomie avant l'institut (Y. Nazé), (2) l'institut et les premiers directeurs (E. Gosset), et (3) l'astronomie au milieu du 20e siècle ... [more ▼]Cette série de posters s'intéresse à l'astronomie à Liège : (1) l'astronomie avant l'institut (Y. Nazé), (2) l'institut et les premiers directeurs (E. Gosset), et (3) l'astronomie au milieu du 20e siècle (Y. Nazé). [less ▲]Detailed reference viewed: 70 (13 ULg) Magnetometry of a sample of massive stars in CarinaNazé, Yaël ; Bagnulo, Stefano; Petit, Véronique et alin Monthly Notices of the Royal Astronomical Society (2012), 423X-ray surveys of the Carina nebula have revealed a few hard and luminous sources associated with early-type stars. Such unusual characteristics for the high-energy emission may be related to magnetically ... [more ▼]X-ray surveys of the Carina nebula have revealed a few hard and luminous sources associated with early-type stars. Such unusual characteristics for the high-energy emission may be related to magnetically confined winds. To search for the presence of magnetic fields in these objects, we performed a limited spectropolarimetric survey using the Focal reducer and low dispersion spectrograph (FORS) instrument. The multi-object mode was used, so that a total of 21 OB stars could be investigated during a one-night-long run. A magnetic field was detected in two objects of the sample, with a 6σ significance: Tr16-22 and 13. Such a detection was expected for Tr16-22, as its X-ray emission is too bright, variable and hard, compared to other late-type O or O+OB systems. It is more surprising for Tr16-13, a poorly known star which so far has never shown any peculiar characteristics. Subsequent monitoring is now needed to ascertain the physical properties of these objects and enable a full modelling of their magnetic atmospheres and winds. Based on data collected at ESO under Program ID 386.D-0624A. [less ▲]Detailed reference viewed: 22 (1 ULg) HST/STIS spectroscopy of the magnetic Of?p star HD 108: the low state at ultraviolet wavelengthsMarcolino, W. L. F.; Bouret, J.-C.; Walborn, N. R. et alin Monthly Notices of the Royal Astronomical Society (2012), 422We present the first ultraviolet spectrum of the peculiar, magnetic Of?p star HD 108 obtained in its spectroscopic low state. The new data, obtained with the Space Telescope Imaging Spectrograph (STIS) on ... [more ▼]We present the first ultraviolet spectrum of the peculiar, magnetic Of?p star HD 108 obtained in its spectroscopic low state. The new data, obtained with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope, reveal significant changes compared to IUE spectra obtained in the high state: N Vλ1240, Si IVλ1400 and C IVλ1550 present weaker P Cygni profiles (less absorption) in the new data, while N IVλ1718 absorption is deeper, without the clear wind signature evident in the high state. Such changes contrast with those found in other magnetic massive stars, where more absorption is observed in the resonance doublets when the sightline is close to the plane of the magnetic equator. The new data show also that the photospheric Fe IV forest, at ˜1600-1700 Å, has strengthened compared to previous observations. The ultraviolet variability is large compared to that found in typical, non-magnetic O stars, but moderate when compared to the high-/low-state changes reported in the optical spectrum of HD 108 over several decades. We use non-local thermodynamic equilibrium (non-LTE) expanding-atmosphere models to analyse the new STIS observations. Overall, the results are in accord with a scenario in which the optical variability is mainly produced by magnetically constrained gas, close to the photosphere. The relatively modest changes found in the main ultraviolet wind lines suggest that the stellar wind is not substantially variable on a global scale. Nonetheless, multidimensional radiative-transfer models may be needed to understand some of the phenomena observed. [less ▲]Detailed reference viewed: 16 (0 ULg) Cabinet de curiositésNazé, Yaël Article for general public (2012)Detailed reference viewed: 19 (2 ULg) Global Modeling of X-Ray Spectra Produced in O-type Star WindsHervé, Anthony ; Rauw, Grégor ; Nazé, Yaël et alin Astrophysical Journal (2012), 748High-resolution X-ray spectra of O-type stars revealed less wind absorption than expected from smooth winds with conventional mass-loss rates. Various solutions have been proposed, including porous winds ... [more ▼]High-resolution X-ray spectra of O-type stars revealed less wind absorption than expected from smooth winds with conventional mass-loss rates. Various solutions have been proposed, including porous winds, optically thick clumps, or an overall reduction of the mass-loss rates. The latter has a strong impact on the evolution of the star. Our final goal is to analyze high-resolution X-ray spectra of O-type stars with a multi-temperature plasma model in order to determine crucial stellar and wind parameters such as the mass-loss rate, the CNO abundances, and the X-ray temperature plasma distribution in the wind. In this context we are developing a modeling tool to calculate synthetic X-ray spectra. We present here the main ingredients and physics necessary for such a work. Our code uses the most recent version of the AtomDB emissivities to compute the intrinsic emissivity of the hot plasma as well as the CMFGEN model atmosphere code to evaluate the opacity of the cool wind. Following the comparison between two formalisms of stellar wind fragmentation, we introduce, for the first time in X-rays, the effects of a tenuous inter-clump medium. We then explore the quantitative impact of different model parameters on the X-ray spectra such as the position in the wind of the X-ray emitting plasma. For the first time, we also show that the two formalisms of stellar wind fragmentation yield different results, although the differences for individual lines are small and can probably not be tested with the current generation of X-ray telescopes. As an illustration of our method, we compare various synthetic line profiles to the observed O VIII λ18.97 line in the spectrum of ζ Puppis. We illustrate how different combinations of parameters can actually lead to the same morphology of a single line, underlining the need to analyze the whole spectrum in a consistent way when attempting to constrain the parameters of the wind. [less ▲]Detailed reference viewed: 14 (2 ULg) Ejecta around evolved massive stars observed with HerschelVamvatira-Nakou, Chloi ; Hutsemekers, Damien ; Royer, P. et alConference (2012, March)Detailed reference viewed: 18 (8 ULg) Les secrets de l'astrolabe - suiteNazé, Yaël Article for general public (2012)Detailed reference viewed: 19 (1 ULg) High-resolution X-ray spectroscopy of the magnetic Of?p star HD148937Nazé, Yaël ; Zhekov, S. A.; Walborn, N. R.in Astrophysical Journal (2012), 746High-resolution data of the peculiar magnetic massive star HD148937 were obtained with Chandra-HETGS, and are presented here in combination with a re-analysis of the older XMM-RGS data. The lines of the ... [more ▼]High-resolution data of the peculiar magnetic massive star HD148937 were obtained with Chandra-HETGS, and are presented here in combination with a re-analysis of the older XMM-RGS data. The lines of the high-Z elements (Mg, Si, S) were found to be unshifted and relatively narrow (FWHM of about 800km/s), i.e. narrower than the O line recorded by RGS, which possibly indicates that the hot plasma is multi-thermal and has several origins. These data further indicate a main plasma temperature of about 0.6keV and a formation of the X-ray emission at about one stellar radius above the photosphere. From the spectral fits and the H-to-He line ratios, the presence of very hot plasma is however confirmed, though with a smaller relative strength than for the prototype magnetic oblique rotator $\theta^1$\,Ori\,C. Both stars thus share many similarities, but HD148937 appears less extreme than $\theta^1$\,Ori\,C despite having also a large magnetic confinement parameter. [less ▲]Detailed reference viewed: 184 (1 ULg) A detailed X-ray investigation of ζ Puppis. I. The dataset and some preliminary resultsNazé, Yaël ; Flores, C. A.; Rauw, Grégor in Astronomy and Astrophysics (2012), 538
Results: This first paper reports on the data reduction of this unique dataset and provides a few preliminary results. On the one hand, analysis of EPIC low-resolution spectra shows the star to have a remarkably stable X-ray emission from one observation to the next. On the other hand, fitting by a wind model of individual line profiles recorded by RGS confirms the wavelength dependence of the line morphology. Based on observations collected with XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member States and the USA (NASA).Table 1 is available in electronic form at http://www.aanda.org [less ▲]Detailed reference viewed: 21 (1 ULg) Le ciel pour tousNazé, Yaël Article for general public (2012)Detailed reference viewed: 30 (5 ULg) The first X-ray survey of Galactic Luminous Blue VariablesNazé, Yaël ; Rauw, Grégor ; Hutsemekers, Damien in Astronomy and Astrophysics (2012), 538Aims: The X-ray emission of massive stars has been studied when these objects are in their main-sequence phase, as well as in their Wolf-Rayet phase. However, the X-ray properties of the transitional ... [more ▼]Aims: The X-ray emission of massive stars has been studied when these objects are in their main-sequence phase, as well as in their Wolf-Rayet phase. However, the X-ray properties of the transitional Luminous Blue Variable (LBV) phase remain unknown. Methods: Using a dedicated but limited XMM survey as well as archival XMM and Chandra observations, we performed the first X-ray survey of LBVs: about half of the known LBVs or candidate LBVs are studied. Results: Apart from the well known X-ray sources eta Car and Cyg OB2 #12, four additional LBVs are detected in this survey, though some doubt remains on the association with the X-ray source for two of these. For the other LBVs, upper limits on the flux were derived, down to $\log[L_{\rm X}/L_{\rm BOL}]-9.4$ for PCyg. This variety in the strength of the X-ray emission is discussed, with particular emphasis on the potential influence of binarity. [less ▲]Detailed reference viewed: 25 (1 ULg) The spectral variability and magnetic field characteristics of the Of?p star HD 148937Wade, G. A.; Grunhut, J.; Gräfener, G. et alin Monthly Notices of the Royal Astronomical Society (2012), 419We report magnetic and spectroscopic observations and modelling of the Of?p star HD 148937 within the context of the Magnetism in Massive Stars (MiMeS) Large Program at the Canada-France-Hawaii Telescope ... [more ▼]We report magnetic and spectroscopic observations and modelling of the Of?p star HD 148937 within the context of the Magnetism in Massive Stars (MiMeS) Large Program at the Canada-France-Hawaii Telescope. 32 high signal-to-noise ratio circularly polarized (Stokes V) spectra and 13 unpolarized (Stokes I) spectra of HD 148937 were acquired in 2009 and 2010. A definite detection of a Stokes V Zeeman signature is obtained in the grand mean of all observations [in both least-squares deconvolved (LSD) mean profiles and individual spectral lines]. The longitudinal magnetic field inferred from the Stokes V LSD profiles is consistently negative, in contrast to the essentially zero field strength measured from the diagnostic null profiles. A period search of new and archival equivalent width measurements confirms the previously reported 7.03 d variability period. The variation of equivalent widths is not strictly periodic: we present evidence for evolution of the amount or distribution of circumstellar plasma. Interpreting the 7.03 d period as the stellar rotational period within the context of the oblique rotator paradigm, we have phased the equivalent widths and longitudinal field measurements. The longitudinal field measurements show a weak sinusoidal variation of constant sign, with extrema out of phase with the Hα variation by about 0.25 cycles. From our constraint on v sin i≤ 45 km s[SUP]-1[/SUP], we infer that the rotational axis inclination i≤ 30°. Modelling the longitudinal field phase variation directly, we obtain the magnetic obliquity β= 38[SUP]+17[/SUP][SUB]-28[/SUB]° and dipole polar intensity B[SUB]d[/SUB]= 1020[SUP]-380[/SUP][SUB]+310[/SUB] G. Simple modelling of the Hα equivalent width variation supports the derived geometry. The inferred magnetic configuration confirms the suggestion of Nazé et al., who proposed that the weaker variability of HD 148937 as compared to other members of this class is a consequence of the stellar geometry. Based on the derived magnetic properties and published wind characteristics, we find a wind magnetic confinement parameter η[SUB]*[/SUB]≃ 20 and rotation parameter W= 0.12, supporting a picture in which the Hα emission and other line variability have their origin in an oblique, rigidly rotating magnetospheric structure resulting from a magnetically channelled wind. [less ▲]Detailed reference viewed: 9 (0 ULg) (G)astronomie - Comète glacéeNazé, Yaël Article for general public (2012)Detailed reference viewed: 16 (2 ULg) Massive stars' nebulae, as seen through Herschel's eyesVamvatira-Nakou, Chloi ; Nazé, Yaël ; Hutsemekers, Damien et alPoster (2012)Detailed reference viewed: 51 (35 ULg) Cahier d'exploration du ciel II. Mesurer l'UniversNazé, Yaël Book published by Réjouisciences (2012)Detailed reference viewed: 18 (3 ULg) La cuisine du Cosmos. Traité de (G)astronomieNazé, Yaël Book published by Réjouisciences (2012)Detailed reference viewed: 35 (1 ULg) Cahier d'exploration du ciel I. découvrir l'Univers (réédition cahier 2009)Nazé, Yaël Book published by Réjouisciences (2012)Detailed reference viewed: 13 (1 ULg)