arXiv subjects

Yael Naze

Publications and source records attributed to Yael Naze.

At least 55 records · Page 3Linked to original sources

arXiv2019

Variations on a theme -- the puzzling behaviour of Schulte 12

One of the first massive stars detected in X-rays, Schulte 12 has remained a puzzle in several aspects. In particular, its extreme brightness both in the visible and X-ray ranges is intriguing. Thanks to Swift and XMM-Newton observations covering ~5000d, we report the discovery of a regular 108d modulation in X-ray flux of unknown origin. The minimum in the high-energy flux appears due to a combination of increased absorption and decreased intrinsic emission. We examined in parallel the data from a dedicated spectroscopic and photometric monitoring in the visible and near-IR domains, complemented by archives. While a similar variation timescale is found in those data, they do not exhibit the strict regular clock found at high energies. Changes in line profiles cannot be related to binarity but rather correspond to non-radial pulsations. Considering the substantial revision of the distance of Schulte 12 from the second GAIA data release, the presence of such oscillations agrees well with the evolutionary status of Schulte 12, as it lies in an instability region of the HR diagram.

astro-ph.SR↗

arXiv2018

Extended main sequence turn-off originated from a broad range of stellar rotational velocities

Star clusters have long been considered to comprise a simple stellar population, but this paradigm is being challenged, since apart from multiple populations in Galactic globular clusters^{1,2}, a number of intermediate-age star clusters exhibit a significant colour spread at the main sequence turn-off (MSTO)^{3,4,5,6,7,8,9,10,11}. A sequential evolution of multiple generations of stars formed over 100-200 million years is a natural explanation of this colour spread^{12}. Another approach to explain this feature is to introduce the effect of stellar rotation^{13}. However, its effectiveness has not yet been proven due to the lack of direct measurements of rotational velocities. Here we report the distribution of projected rotational velocities (V sin i) of stars in the Galactic open cluster M11, measured through a Fourier transform analysis. Cluster members display a broad V sin i distribution, and fast rotators including Be stars have redder colours than slow rotators. Monte Carlo simulations infer that cluster members have highly aligned spin axes and a broad distribution of equatorial velocities biased towards high velocities. Our findings demonstrate how stellar rotation affects the colours of cluster members, suggesting that the colour spread observed in populous clusters can be understood in the context of stellar evolution even without introducing multiple stellar populations.

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arXiv2018

Hot stars observed by XMM-Newton II. A survey of Oe and Be stars

We perform a survey of Oe and Be stars in the X-ray range. To this aim, we cross-correlated XMM-Newton and Chandra catalogs of X-ray sources with a list of Be stars, finding 84 matches in total. Of these, 51 objects had enough counts for a spectral analysis. This paper provides the derived X-ray properties (X-ray luminosities, and whenever possible, hardness ratios, plasma temperatures, and variability assessment) of this largest ever sample of Oe and Be stars. The targets display a wide range in luminosity and hardness. In particular, the significant presence of very bright and hard sources is atypical for X-ray surveys of OB stars. Several types of sources are identified. A subset of stars display the typical characteristics of O-stars, magnetic OB stars, or pre-main-sequence (PMS) objects: their Be nature does not seem to play an important role. However, another subset comprises gamma Cas analogs, which are responsible for the luminous and hard detections. Our sample contains seven known gamma Cas analogs, but we also identify eight new gamma Cas analogs and one gamma Cas candidate. This nearly doubles the sample of such stars.

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arXiv2017

A changing wind collision

We report on the first detection of a global change in the X-ray emitting properties of a wind-wind collision, thanks to XMM-Newton observations of the massive SMC system HD5980. While its lightcurve had remained unchanged between 2000 and 2005, the X-ray flux has now increased by a factor of ~2.5, and slightly hardened. The new observations also extend the observational coverage over the entire orbit, pinpointing the lightcurve shape. It has not varied much despite the large overall brightening, and a tight correlation of fluxes with orbital separation is found, without any hysteresis effect. Moreover, the absence of eclipses and of absorption effects related to orientation suggests a large size for the X-ray emitting region. Simple analytical models of the wind-wind collision, considering the varying wind properties of the eruptive component in HD5980, are able to reproduce the recent hardening and the flux-separation relationship, at least qualitatively, but they predict a hardening at apastron and little change in mean flux, contrary to observations. The brightness change could then possibly be related to a recently theorized phenomenon linked to the varying strength of thin-shell instabilities in shocked wind regions.

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arXiv2017

A detailed X-ray investigation of zeta Puppis IV. Further characterization of the variability

Previously, the X-ray emission of zeta Puppis was found to be variable with light curves harbouring "trends" with a typical timescale longer than the exposure length. The origin of these changes was proposed to be linked to large-scale structures in the wind, but further characterization of the variability at high energies was needed. Since then, a number of new X-ray observations have become available. Furthermore, a cyclic behaviour with a 1.78d period was identified in long optical photometric runs, which is thought to be associated with the launching mechanism of large-scale wind structures. We analysed these new X-ray data, revisited the old data, and compared X-ray with optical data, including when simultaneous. We found that the behaviour in X-rays cannot be explained in terms of a perfect clock because the amplitude and shape of its variations change with time. For example, zeta Puppis was much more strongly variable between 2007 and 2011 than before and after this interval. Comparing the X-ray spectra of the star at maximum and minimum brightness yields no compelling difference beyond the overall flux change: the temperatures, absorptions, and line shapes seem to remain constant, well within errors. The only common feature between X-ray datasets is that the variation amplitudes appear maximum in the medium (0.6-1.2keV) energy band. Finally, no clear and coherent correlation can be found between simultaneous X-ray and optical data. Only a subgroup of observations may be combined coherently with the optical period of 1.78d, although the simultaneous optical behaviour is unknown. The currently available data do not reveal any obvious, permanent, and direct correlation between X-ray and optical variations. The origin of the X-ray variability therefore still needs to be ascertained, highlighting the need for long-term monitoring in multiwavelengths, i.e. X-ray, UV, and optical.

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arXiv2017

B stars seen at high resolution by XMM-Newton

We report on the properties of 11 early B stars observed with gratings on board XMM-Newton and Chandra, thereby doubling the number of B stars analysed at high resolution. The spectra typically appear soft, with temperatures of 0.2--0.6 keV, and moderately bright (log[L_{X}/L_{BOL}]~ -7) with lower values for later type stars. In line with previous studies, we also find an absence of circumstellar absorption, negligible line broadening, no line shift, and formation radii in the range 2 - 7 R*. From the X-ray brightnesses, we derived the hot mass-loss rate for each of our targets and compared these values to predictions or values derived in the optical domain: in some cases, the hot fraction of the wind can be non-negligible. The derived X-ray abundances were compared to values obtained from the optical data, with a fair agreement found between them. Finally, half of the sample presents temporal variations, either in the long-term, short-term, or both. In particular, HD44743 is found to be the second example of an X-ray pulsator, and we detect a flare-like activity in the binary HD79351, which also displays a high-energy tail and one of the brightest X-ray emissions in the sample.

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arXiv2017

An X-ray view of HD166734, a massive supergiant system

The X-ray emission of the O+O binary HD166734 was monitored using Swift and XMM-Newton observatories, leading to the discovery of phase-locked variations. The presence of an f line in the He-like triplets further supports a wind-wind collision as the main source of the X-rays in HD166734. While temperature and absorption do not vary significantly along the orbit, the X-ray emission strength varies by one order of magnitude, with a long minimum state (Delta(phi)~0.1) occurring after a steep decrease. The flux at minimum is compatible with the intrinsic emission of the O-stars in the system, suggesting a possible disappearance of colliding wind emission. While this minimum cannot be explained by eclipse or occultation effects, a shock collapse may occur at periastron in view of the wind properties. Afterwards, the recovery is long, with an X-ray flux proportional to the separation d (in hard band) or to d^2 (in soft band). This is incompatible with an adiabatic nature for the collision (which would instead lead to "F_X propto 1/d"), but could be reconciled with a radiative character of the collision, though predicted temperatures are lower and more variable than in observations. An increase in flux around phi~0.65 and the global asymmetry of the light curve remain unexplained, however.

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arXiv2017

pi Aqr is another gamma-Cas object

The gamma-Cas category is a subgroup of Be stars displaying a strong, hard, and variable thermal X-ray emission. An XMM-Newton observation of pi Aqr reveals spectral and temporal characteristics that clearly make this Be star another member of the gamma-Cas category. Furthermore, pi Aqr is a binary but, contrary to gamma-Cas, the nature of the companion to the Be star is known; it is a non-degenerate (stellar) object and its small separation from the Be star does not leave much room for a putative compact object close to the Be disk. This renders the accretion scenario difficult to apply in this system, and, hence, this discovery favors a disk-related origin for the gamma-Cas phenomenon.

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arXiv2017

Chemical abundances of fast-rotating massive stars. I. Description of the methods and individual results

Aims: Recent observations have challenged our understanding of rotational mixing in massive stars by revealing a population of fast-rotating objects with apparently normal surface nitrogen abundances. However, several questions have arisen because of a number of issues, which have rendered a reinvestigation necessary; these issues include the presence of numerous upper limits for the nitrogen abundance, unknown multiplicity status, and a mix of stars with different physical properties, such as their mass and evolutionary state, which are known to control the amount of rotational mixing. Methods: We have carefully selected a large sample of bright, fast-rotating early-type stars of our Galaxy (40 objects with spectral types between B0.5 and O4). Their high-quality, high-resolution optical spectra were then analysed with the stellar atmosphere modelling codes DETAIL/SURFACE or CMFGEN, depending on the temperature of the target. Several internal and external checks were performed to validate our methods; notably, we compared our results with literature data for some well-known objects, studied the effect of gravity darkening, or confronted the results provided by the two codes for stars amenable to both analyses. Furthermore, we studied the radial velocities of the stars to assess their binarity. Results: This first part of our study presents our methods and provides the derived stellar parameters, He, CNO abundances, and the multiplicity status of every star of the sample. It is the first time that He and CNO abundances of such a large number of Galactic massive fast rotators are determined in a homogeneous way.

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arXiv2016

The puzzling properties of the magnetic O star Tr16-22

(abridged version) Context : The detection of bright, hard, and variable X-ray emission in Tr16-22 prompted spectropolarimetric observations of this star, which in turn led to the discovery of a surface magnetic field. Aims : We want to further constrain the properties of this star, in particular to verify whether X-ray variations are correlated to changes in optical emission lines and magnetic field strength, as expected from the oblique rotator model that is widely accepted for magnetic O stars. Methods: We have obtained new low-resolution spectropolarimetric and long-term high-resolution spectroscopic monitoring of Tr16-22, and we also analyse new, serendipitous X-ray data. Results: The new X-ray observations are consistent with previous data, but their addition does not help to solve the ambiguity in the variation timescale because of numerous aliases. No obvious periodicity or any large variations are detected in the spectropolarimetric data of Tr16-22 obtained over three months. The derived field values appear to be in line with previous measurements, suggesting constancy of the field (though the possibility of small, short-term field variations cannot be excluded). Variations in the equivalent widths of Halpha are very small, and they do not appear to be related to the X-ray timescale; the overall lack of large variations in optical emission lines is consistent with the magnetic field constancy. In addition, variations of the radial velocities indicate that Tr16-22 is probably a SB1 binary with a very long period.

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arXiv2016

Chandra View of Magnetically Confined Wind in HD191612: Theory versus Observations

High-resolution spectra of the magnetic star HD191612 were acquired using the Chandra X-ray observatory at both maximum and minimum emission phases. We confirm the flux and hardness variations previously reported with XMM-Newton, demonstrating the great repeatability of the behavior of HD191612 over a decade. The line profiles appear typical for magnetic massive stars: no significant line shift, relatively narrow lines for high-Z elements, and formation radius at about 2R*. Line ratios confirm the softening of the X-ray spectrum at the minimum emission phase. Shift or width variations appear of limited amplitude at most (slightly lower velocity and slightly increased broadening at minimum emission phase, but within 1--2 sigma of values at maximum). In addition, a fully self-consistent 3D magnetohydrodynamic (MHD) simulation of the confined wind in HD191612 was performed. The simulation results were directly fitted to the data leading to a remarkable agreement overall between them.

astro-ph.SR↗

arXiv2016

X-ray and optical spectroscopy of the massive young open cluster IC1805

Very young open clusters are ideal places to study the X-ray properties of a homogeneous population of early-type stars. In this respect, the IC1805 open cluster is very interesting as it hosts the O4If$^+$ star HD15570 thought to be in an evolutionary stage intermediate between a normal O-star and a Wolf-Rayet star. Such a star could provide a test for theoretical models aiming at explaining the empirical scaling relation between the X-ray and bolometric luminosities of O-type stars. We have observed IC1805 with XMM-Newton and further collected optical spectroscopy of some of the O-star members of the cluster. The optical spectra allow us to revisit the orbital solutions of BD+60$^{\circ}$ 497 and HD15558, and provide the first evidence of binarity for BD+60$^{\circ}$ 498. X-ray emission from colliding winds does not appear to play an important role among the O-stars of IC1805. Notably, the X-ray fluxes do not vary significantly between archival X-ray observations and our XMM-Newton pointing. The very fast rotator BD+60$^{\circ}$ 513, and to a lesser extent the O4If$^+$ star HD15570 appear somewhat underluminous. Whilst the underluminosity of HD15570 is only marginally significant, its amplitude is found to be compatible with theoretical expectations based on its stellar and wind properties. A number of other X-ray sources are detected in the field, and the brightest objects, many of which are likely low-mass pre-main sequence stars, are analyzed in detail.

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arXiv2016

The X-ray light curve of the massive colliding wind Wolf-Rayet + O binary WR21a

Our dedicated XMM-Newton monitoring, as well as archival Chandra and Swift datasets, were used to examine the behaviour of the WN5h+O3V binary WR21a at high energies. For most of the orbit, the X-ray emission exhibits few variations. However, an increase in strength of the emission is seen before periastron, following a 1/D relative trend, where D is the separation between both components. This increase is rapidly followed by a decline due to strong absorption as the Wolf-Rayet (WR) comes in front. The fitted local absorption value appears to be coherent with a mass-loss rate of about 1x10^{-5} M_sol/yr for the WR component. However, absorption is not the only parameter affecting the X-ray emission at periastron as even the hard X-ray emission decreases, suggesting a possible collapse of the colliding wind region near to or onto the photosphere of the companion just before or at periastron. An eclipse may appear as another potential scenario, but it would be in apparent contradiction with several lines of evidence, notably the width of the dip in the X-ray light curve and the absence of variations in the UV light curve. Afterwards, the emission slowly recovers, with a strong hysteresis effect. The observed behaviour is compatible with predictions from general wind-wind collision models although the absorption increase is too shallow.

astro-ph.SR↗

arXiv2016

A test for the theory of colliding winds: the periastron passage of 9 Sagittarii I. X-ray and optical spectroscopy

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

astro-ph.SR↗

arXiv2016

X-ray emission from magnetic massive stars

Magnetically confined winds of early-type stars are expected to be sources of bright and hard X-rays. To clarify the systematics of the observed X-ray properties, we have analyzed a large series of Chandra and XMM observations, corresponding to all available exposures of known massive magnetic stars (over 100 exposures covering ~60% of stars compiled in the catalog of Petit et al. 2013). We show that the X-ray luminosity is strongly correlated with the stellar wind mass-loss-rate, with a power-law form that is slightly steeper than linear for the majority of the less luminous, lower-Mdot B stars and flattens for the more luminous, higher-Mdot O stars. As the winds are radiatively driven, these scalings can be equivalently written as relations with the bolometric luminosity. The observed X-ray luminosities, and their trend with mass-loss rates, are well reproduced by new MHD models, although a few overluminous stars (mostly rapidly rotating objects) exist. No relation is found between other X-ray properties (plasma temperature, absorption) and stellar or magnetic parameters, contrary to expectations (e.g. higher temperature for stronger mass-loss rate). This suggests that the main driver for the plasma properties is different from the main determinant of the X-ray luminosity. Finally, variations of the X-ray hardnesses and luminosities, in phase with the stellar rotation period, are detected for some objects and they suggest some temperature stratification to exist in massive stars' magnetospheres.

astro-ph.SR↗

arXiv2016

Tribute to an astronomer: the work of Max Ernst on Wilhelm Tempel

In 1964-1974, the German artist Max Ernst created, with the help of two friends, a series of works (books, movie, paintings) related to the astronomer Wilhelm Tempel. Mixing actual texts by Tempel and artistic features, this series pays homage to the astronomer by recalling his life and discoveries. Moreover, the core of the project, the book Maximiliana or the Illegal Practice of Astronomy, actually depicts the way science works, making this artwork a most original tribute to a scientist.

physics.hist-ph↗

arXiv2015

Magnetically Confined Wind Shocks in X-rays - a Review

A subset (~ 10%) of massive stars present strong, globally ordered (mostly dipolar) magnetic fields. The trapping and channeling of their stellar winds in closed magnetic loops leads to magnetically confined wind shocks (MCWS), with pre-shock flow speeds that are some fraction of the wind terminal speed. These shocks generate hot plasma, a source of X-rays. In the last decade, several developments took place, notably the determination of the hot plasma properties for a large sample of objects using XMM-Newton and Chandra, as well as fully self-consistent MHD modelling and the identification of shock retreat effects in weak winds. Despite a few exceptions, the combination of magnetic confinement, shock retreat and rotation effects seems to be able to account for X-ray emission in massive OB stars. Here we review these new observational and theoretical aspects of this X-ray emission and envisage some perspectives for the next generation of X-ray observatories.

astro-ph.SR↗

arXiv2015

X-ray emission from interacting massive binaries: a review of 15 years of progress

Previous generations of X-ray observatories revealed a group of massive binaries that were relatively bright X-ray emitters. This was attributed to emission of shock-heated plasma in the wind-wind interaction zone located between the stars. With the advent of the current generation of X-ray observatories, the phenomenon could be studied in much more detail. In this review, we highlight the progress that has been achieved in our understanding of the phenomenon over the last 15 years, both on theoretical and observational grounds. All these studies have paved the way for future investigations using the next generation of X-ray satellites that will provide crucial information on the X-ray emission formed in the innermost part of the wind-wind interaction.

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