arXiv subjects

Yaël Nazé

Publications and source records attributed to Yaël Nazé.

At least 19 recordsLinked to original sources

arXiv2026

A systematic assessment of the short-term X-ray behaviour of the $γ$ Cas analogues

Context. Overall, $γ$ Cas analogues represent a class of OBe stars displaying bright and hard X-ray emission. This emission was recently determined to be the result of accretion onto a companion white dwarf (WD), which might possibly be a magnetic WD. Aims. We present a systematic review of the properties of $γ$ Cas analogues on short-term (kilosecond, ks) scales to further investigate the nature of the X-ray source. In particular, the presence of periodicities could be associated with the rotation of a magnetic WD. Methods. We generated the power spectra and a generalised Fourier algorithm. We applied an epoch folding method, along with Z2n approaches, to 131 X-ray detections of 26 $γ$ Cas analogues and two candidate sources. Results. We identified five sources ($γ$ Cas, $ζ$ Tau, HD45314, V771 Sgr, and $π$ Aqr), with recurrent periodic variability (P $\sim$ 1.2-5.4 ks) across observations. Period values and pulsed fractions are consistent with those reported for intermediate polars. However, while similar periods are found in all exposures of an object, the exact period value or folded light curve shape can vary. We also identify another nine sources displaying periodicities that cannot be confirmed, as there was only one observation available for their analysis. In other cases, periodicities appear (at best) transient and/or marginally significant. Aperiodic variability could be fitted by a power law in 18 of the sources, while the power-law index always is $\sim$ 1, as observed in other accreting WDs. Conclusions. The global properties of the observed short-term X-ray variability of the $γ$ Cas analogues are consistent with those observed from other accreting WD systems.

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arXiv2026

HD3191, the high-mass X-ray binary that wasn't there

The rapidly rotating B1 IV:nn star HD3191 lies within the error box of a flaring Fermi gamma-ray source. Although the counterpart of the Fermi source is likely an active galaxy, HD3191 has nevertheless been suggested to be a high-mass X-ray binary, possibly hosting a black hole companion. The star displays roughly sinusoidal photometric variations with amplitudes of about 12 mmag for two frequencies $ν_1$ = 0.1235 d$^{-1}$ and $ν_2$ = 1.6038 d$^{-1}$. Half of the former frequency ($ν_1$/2) had previously been interpreted as the orbital frequency of a high-mass X-ray binary in which the B1 IV:nn primary undergoes ellipsoidal variations. We show that this scenario fails to account for the lack of significant radial velocity variations and for the overall properties of the star. Our spectroscopic observations instead unveil line profile variations, suggesting that the photometric and line profile variations arise from multi-mode pulsations rather than from orbital effects. Whilst we cannot rule out the possibility that HD3191 could be a nascent binary, consisting of a B1 IV:nn with a low-mass pre-main sequence companion, the most likely scenario is a single star displaying non-radial pulsations. The fast stellar rotation would thus be intrinsic to the B1 IV:nn star rather than being the result of a spin-up during a past mass-transfer episode.

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arXiv2026

Photometric variability of nitrogen-rich Wolf-Rayet stars in Magellanic Clouds with OGLE

We present a comprehensive analysis of the photometric variability of (presumably single) nitrogen-rich Wolf-Rayet (WN) stars in the Magellanic Clouds, using long-term observations from the OGLE survey. Our sample comprises 47 stars with no nearby Gaia counterparts. We characterize both overall and short-term variabilities, examining data dispersion and identifying periodicities. To validate our findings, we also compare the OGLE light curves with data from the MACHO and TESS missions. Variability is ubiquitous in our WR sample: about one third of stars display high variability, or four fifths if we include cases with moderate variations. The observed changes are found to be periodic in 11 cases, with timescales of 2-56 d. Such periodic variations originate in corotating wind structures, binary effects, or pulsations, thereby increasing the number of systems known to show these phenomena. Surprisingly, nine targets display (quasi-periodic) outbursts, making such changes a new type of WR variability. The variability shows a transient character, in about 30$\%$ of the sample, with changing amplitudes for periodic signals or for outbursts (they even sometimes completely disappear from view). Finally, we identified six long-period variables, four of which have been confirmed by at least two independent surveys.

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arXiv2025

Exploring the binary origin of B and Be rapid rotators

Observational evidence has continued to mount that a significant fraction of rapidly rotating early-B type stars are products of binary mass transfer. However, very few mid- and late-type B stars with rapid rotation have been demonstrated to be post-interaction products, despite a growing sample of SB1 binaries among stars within this range of spectral types. By considering the currently available information over the entire range of rapidly rotating B-type binaries, we argue that a significant fraction of the mid- and late-type rapid rotators found in binaries are also likely the result of past mass transfer episodes. The observed properties of this sample are compared to the predictions from the Binary Population and Spectral Synthesis code (BPASS), with attention given to the expected evolutionary pathways of stripped stars and the stellar and binary properties of both components of post-interaction systems across a range of initial conditions. Prospects for directly detecting and characterizing the stripped cores of the previous mass donors in such systems are described, and the implications for the role of binary interaction in causing rapid rotation are discussed. An accurate description of prevalence of binary interaction, the physics of mass transfer, and the post-interaction configuration of systems over a range of initial conditions has far-reaching implications including double-degenerate binaries and their eventual mergers, the output of ionizing UV flux of stellar populations, and the supernova explosions that can arise from stripped or rapidly-rotating progenitors.

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arXiv2025

The multi-faceted variability of HD 192639: Stochastic behaviour, regularities, and an exceptional mass-ejection event

Spectroscopic and photometric variability is widespread among O-type supergiants. It is linked to various phenomena affecting the star and its circumstellar environment, thereby providing direct information concerning them. To investigate such connections, we decided to revisit the prototypical O7.5 Iabf supergiant HD 192639. High-cadence spectroscopic monitoring was performed simultaneously with intensive space-borne photometric observations. The data were analysed with several methods to characterise the variability. Besides the usual stochastic, low-frequency photometric variability, our observations reveal the presence of recurrent - but transient - modulations on a timescale of about five days. The same signal is present in the spectroscopic data and was already seen two decades ago. This stability suggests that this timescale corresponds to the stellar rotation. Furthermore, our observations unveil, for the first time, an unusually strong dimming event in the light curve associated with absorption and emission changes in H I and He I lines. This unprecedented trough corresponds to an episodic ejection of a rather large amount of mass (its column density being comparable to that of the steady wind). While rare, such an event could hint at an overlooked aspect of mass loss in massive stars.

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arXiv2025

New Frontiers in the Study of Magnetic Massive Stars with the Habitable Worlds Observatory

High-mass stars are notable for several reasons: they are characterized by strong winds, which inject momentum and enriched material into their surroundings, and die spectacularly as supernovae, leaving behind compact remnants and heavy elements (such as those that make life on Earth possible). Despite their relative rarity, they play a disproportionate role in the evolution of the galaxies that host them, and likely also played a significant role in the early days of the Universe. A subset ($\sim$10\%) of these stars was also found to host magnetic fields on their surface. These fields impact their evolution, and may lead to exotic physics (e.g., heavy stellar-mass black holes, pair-instability supernovae, magnetars, etc.). However, the detection and measurement of magnetic fields is limited, due to current instrumentation, to nearby massive stars in the Milky Way. To truly understand how magnetism arises in massive stars, and what role it might have played in earlier stages of our Universe, we require next-generation hardware, such as the proposed near-infrared-to-ultraviolet spectropolarimeter Pollux, on the Habitable Worlds Observatory (HWO). In this contribution, we detail how Pollux @ HWO will enable new frontiers in the study of magnetic massive stars, delivering results that will profoundly impact the fields of stellar formation, stellar evolution, compact objects, and stellar feedback.

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arXiv2024

X-raying the wind-wind collisions in HD 168112 and HD 167971

The O-type long-period binary HD 168112 and triple HD 167971 star systems have been known for several decades for their non-thermal synchrotron radio emission. This emission arises from relativistic electrons accelerated in the hydrodynamic shocks of the wind collisions in these systems. Such wind collisions are expected to produce a strong X-ray emission that varies as a function of orbital phase. In wide eccentric binaries, such as our targets, the X-ray emission arises from an adiabatic plasma and its intensity should scale as the inverse of the orbital separation. We present a set of XMM-Newton observations of these systems which help us gain insight into the properties of their wind interactions.

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arXiv2023

Addressing Outstanding Problems in the Physics of Massive Stars with the Line Emission Mapper X-ray Probe

We present some of the salient aspects of the scientific motivation for high resolution soft X-ray spectroscopy of early-type stars with the Line Emission Mapper X-ray Probe. The major strength of {\it LEM} for hot star physics is its large effective area, aided by the inherent energy resolution of its microcalorimeter that readily achieves resolving powers of 1000 and obviates the need for relatively inefficient dispersive optical elements. This increased sensitivity enables much fainter and more distant high mass stars to be observed than are accessible with present-day facilities, greatly increasing the pool of potential targets. For brighter sources, the sensitivity opens up time domain studies, wherein sufficient signal can be garnered in short order and exposure times, probing source variations on ks timescales. We argue that these capabilities of {\it LEM} will yield breakthroughs in all types of hot star systems, from understanding single OB and WR star winds and how they vary with metallicity, to probing the shocks of colliding wind systems and the magnetically channeled winds of magnetic OB stars. {\it LEM} will also study the energetics of WR star bubbles and feedback from their powerful pre-SN stellar winds.

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arXiv2023

Revisiting the orbital motion of WR 138

The optical spectrum of WR 138 exhibits emission lines typical of a WN6o star and absorption lines from a rapidly-rotating OB star. Using a large set of spectroscopic data, we establish a new orbital solution of the WN6o star based on the radial velocities of highly-ionized nitrogen lines. We show that the WN6o star moves on a 4.3 yr orbit with a comparatively low eccentricity of 0.16. The radial velocities of the OB star display considerable scatter. Our best estimates of the velocities of He I absorption lines result in a mass-ratio of $m_{\rm WN6o}/m_{\rm OB} = 0.53 \pm 0.09$. We disentangle the spectra of the two stars and derive a projected rotational velocity of $v\,\sin{i} = 350 \pm 10$ km s$^{-1}$ for the OB star. Our best orbital parameters, combined with the Gaia parallax of WR 138, are at odds with a previous interferometric detection of the companion, suggesting that there is either a bias in this detection or that WR 138 is actually a triple system.

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arXiv2023

Diving into the magnetosphere of the Of?p star HD108

We analyse optical and X-ray spectroscopy of the Of?p star HD108, known for its strong dipolar magnetic field and its optical line profile variability with a timescale of $54 \pm 3$ yrs, interpreted as the stellar rotation period. Optical emission lines have now recovered from their minimum emission state reached in 2007 - 2008. The variations of the equivalent width of the H$α$ emission provide constraints on the inclination of the rotation axis ($i$) and the obliquity of the magnetic axis ($β$). The best agreement between model and observations is found for ($i$, $β$) pairs with $i + β\simeq 85^{\circ}$ and $i \in [30^{\circ},55^{\circ}]$. The Balmer emission lines display stochastic variability at the $\sim 5$ % level on timescales of a few days. TESS photometry unveils transient modulations on similar timescales in addition to prominent red noise variations. A Chandra X-ray observation of December 2021, when the star was at a higher emission level, indicates a slight increase of the flux and a spectral hardening compared to the August 2002 XMM-Newton observation, taken near minimum emission state. Magnetohydrodynamic simulations are used to compute synthetic X-ray spectra. With our current best estimate of the $\dot{M}_{B=0}$ mass-loss rate, the simulated X-ray luminosity and spectral energy distribution agree very well with the observations. Finally, the radial velocities vary on a period of 8.5 years with a peak-to-peak amplitude of 10 - 11 km s$^{-1}$, suggesting orbital motion with an unseen companion of at least 4 M$_{\odot}$.

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arXiv2022

The X-ray emission of gamma Cassiopeiae during the 2020-2021 disc eruption

gamma Cas is known for its hard and intense X-ray emission that could trace accretion by a compact companion, wind interaction with a hot sub-dwarf companion, or magnetic interaction between the star and its Be decretion disc. These scenarios should lead to diverse dependences of the hard X-ray emission on disc density. We collected X-ray observations of gamma Cas during an episode of enhanced disc activity around January 2021. We investigate the variations in the disc properties using time series of dedicated optical spectroscopy and existing broadband photometry. Epoch-dependent Doppler maps of the H-alpha, H-beta, and He I 5876 emission lines are built to characterise the emission regions in velocity space. We analyse 4 XMM-Newton observations taken at key phases of the enhanced disc activity episode. Archival data are used to study the long-term correlation between optical and X-ray emission. Optical spectroscopy unveils an increase in the radial extent of the emission regions during the episode of enhanced disc activity, whilst no increase in the V-band flux is recorded. Doppler maps do not reveal any stable feature in the disc resulting from the putative action of the companion on the outer parts of the Be disc. No increase in the hard emission is observed in relation to the enhanced disc activity. However, at two occasions, the soft X-ray emission of gamma Cas is strongly attenuated, suggesting more efficient obscuration by a large flaring Be disc. There is a strong correlation between the long-term variations in the X-ray flux and in the V-band photometry. The observed behaviour of gamma Cas suggests no direct link between the properties of the outer regions of the Be disc and the hard X-ray emission, but favours a link between the level of X-ray emission and the properties of the inner part of the Be disc. These results thus disfavour an accretion or colliding wind scenario.

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arXiv2022

Apsidal motion in massive eccentric binaries: The case of CPD-41$^\circ$7742, and HD 152218 revisited

This paper is part of a study of the apsidal motion in close eccentric massive binary systems, which aims to constrain the internal structure of the stars. We focus on the binary CPD-41°7742 and revisit HD152218. Independent studies of CPD-41°7742 in the past showed large discrepancies in the longitude of periastron of the orbit, hinting at the presence of apsidal motion. We perform a consistent analysis of all observational data, explicitly accounting for the apsidal motion. We use spectroscopic and photometric observations of CPD-41°7742 to infer values for the fundamental stellar and binary parameters. We apply a disentangling method to the spectra to simultaneously derive the RVs at the times of observation and reconstruct the individual stellar spectra. These are analysed by means of CMFGEN to determine the stellar properties. We determine the apsidal motion rate in two ways: We complement our RVs with those reported in the literature and we use the phase shifts between the primary and secondary eclipses. The light curves are analysed by means of Nightfall to constrain the orbital inclination and stellar masses. Stellar structure and evolution models are constructed with Cles. Different prescriptions for the mixing are adopted. Photometric data of HD152218 are analysed and stellar evolution models are built. The binary CPD-41°7742 displays apsidal motion at a rate of 15.38+0.42-0.51° yr-1. Our analysis of the observational data of CPD-41°7742 explicitly accounting for the apsidal motion allows us to explain the discrepancy in periastron longitudes. The age estimates are in good agreement with estimates obtained for other massive binaries in NGC 6231. This study confirms the need for enhanced mixing in the stellar evolution models to reproduce the observational stellar properties; this points towards larger convective cores than usually considered.

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arXiv2021

Ultraviolet Spectropolarimetry with Polstar: Clumping and Mass-loss Rate Corrections

The most massive stars are thought to lose a significant fraction of their mass in a steady wind during the main-sequence and blue supergiant phases. This in turn sets the stage for their further evolution and eventual supernova, with consequences for ISM energization and chemical enrichment. Understanding these processes requires accurate observational constraints on the mass-loss rates of the most luminous stars, which can also be used to test theories of stellar wind generation. In the past, mass-loss rates have been characterized via collisional emission processes such as H$α$ and free-free radio emission, but these so-called "density squared" diagnostics require correction in the presence of widespread clumping. Recent observational and theoretical evidence points to the likelihood of a ubiquitously high level of such clumping in hot-star winds, but quantifying its effects requires a deeper understanding of the complex dynamics of radiatively driven winds. Furthermore, large-scale structures arising from surface anisotropies and propagating throughout the wind can further complicate the picture by introducing further density enhancements, affecting mass-loss diagnostics. Time series spectroscopy of UV resonance lines with high resolution and high signal-to-noise are required to better understand this complex dynamics, and help correct "density squared" diagnostics of mass-loss rates. The proposed Polstar mission easily provides the necessary resolution at the sound-speed scale of 20 km s$^{-1}$, on three dozen bright targets with signal-to noise an order of magnitude above that of the celebrated IUE MEGA campaign, via continuous observations that track structures advecting through the wind in real time.

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arXiv2020

A Deep Exposure in High Resolution X-Rays Reveals the Hottest Plasma in the $ζ\,$Puppis Wind

We have obtained a very deep exposure (813 ks) of $ζ\,$Puppis (O4 supergiant) with the Chandra/HETG Spectrometer. Here we report on analysis of the 1-9 Å region, especially well suited for Chandra, which has a significant contribution from continuum emission between well separated emission lines from high-ionization species. These data allow us to study the hottest plasma present through the continuum shape and emission line strengths. Assuming a powerlaw emission measure distribution which has a high-temperature cut-off, we find that the emission is consistent with a thermal spectrum having a maximum temperature of 12 MK. This implies an effective wind shock velocity of $900\,\mathrm{km\,s^{-1}}$, well below the wind terminal speed of $2250\,\mathrm{km\,s^{-1}}$. For X-ray emission which forms close to the star, the speed and X-ray flux are larger than can be easily reconciled with strictly self-excited line-deshadowing-instability models, suggesting a need for a fraction of the wind to be accelerated extremely rapidly right from the base. This is not so much a dynamical instability as a nonlinear response to changing boundary conditions.

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arXiv2019

Quest for the tertiary component in Cyg OB2 #5

The Cyg OB2 #5 system is thought to consist of a short-period (6.6 d) eclipsing massive binary orbited by an OB-star orbiting with a period of ~6.7 yr; these stars in turn are orbited by a distant early B-star with a period of thousands of years. However, while the inner binary has been studied many times, information is missing on the other stars, in particular the third star whose presence was indirectly postulated from recurrent modulations in the radio domain. Besides, to this date, the X-ray light curve could not be fully interpreted, for example in the framework of colliding-wind emission linked to one of the systems. We obtained new optical and X-ray observations of Cyg OB2 #5, which we combined to archival data. We performed a thorough and homogeneous investigation of all available data, notably revisiting the times of primary minimum in photometry. In the X-ray domain, XMM-Newton provides scattered exposures over ~5000 d whilst Swift provides a nearly continuous monitoring for the last couple of years. Although the X-ray light curve reveals clear variability, no significant period can be found hence the high-energy emission cannot be explained solely in terms of colliding winds varying along either the short or intermediate orbits. The optical data reveal for the first time clear signs of reflex motion. The photometry indicates the presence of a 2366 d (i.e. 6.5 yr) period while the associated radial velocity changes are detected at the 3 sigma level in the systemic velocity of the He II 4686 emission line. With the revised period, the radio light curve is interpreted consistently in terms of a wind interaction between the inner binary and the tertiary star. From these optical and radio data, we derive constraints on the physical properties of the tertiary star and its orbit.

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arXiv2018

The Athena X-ray Integral Field Unit

The X-ray Integral Field Unit (X-IFU) is the high resolution X-ray spectrometer of the ESA Athena X-ray observatory. Over a field of view of 5' equivalent diameter, it will deliver X-ray spectra from 0.2 to 12 keV with a spectral resolution of 2.5 eV up to 7 keV on ~5 arcsecond pixels. The X-IFU is based on a large format array of super-conducting molybdenum-gold Transition Edge Sensors cooled at about 90 mK, each coupled with an absorber made of gold and bismuth with a pitch of 249 microns. A cryogenic anti-coincidence detector located underneath the prime TES array enables the non X-ray background to be reduced. A bath temperature of about 50 mK is obtained by a series of mechanical coolers combining 15K Pulse Tubes, 4K and 2K Joule-Thomson coolers which pre-cool a sub Kelvin cooler made of a 3He sorption cooler coupled with an Adiabatic Demagnetization Refrigerator. Frequency domain multiplexing enables to read out 40 pixels in one single channel. A photon interacting with an absorber leads to a current pulse, amplified by the readout electronics and whose shape is reconstructed on board to recover its energy with high accuracy. The defocusing capability offered by the Athena movable mirror assembly enables the X-IFU to observe the brightest X-ray sources of the sky (up to Crab-like intensities) by spreading the telescope point spread function over hundreds of pixels. Thus the X-IFU delivers low pile-up, high throughput (>50%), and typically 10 eV spectral resolution at 1 Crab intensities, i.e. a factor of 10 or more better than Silicon based X-ray detectors. In this paper, the current X-IFU baseline is presented, together with an assessment of its anticipated performance in terms of spectral resolution, background, and count rate capability. The X-IFU baseline configuration will be subject to a preliminary requirement review that is scheduled at the end of 2018.

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arXiv2018

The Of?p stars of the Magellanic Clouds: Are they strongly magnetic?

All known Galactic Of?p stars have been shown to host strong, organized, magnetic fields. Recently, five Of?p stars have been discovered in the Magellanic Clouds. They posses photometric \citep{Naze} and spectroscopic \citep{Walborn} variability compatible with the Oblique Rotator Model (ORM). However, their magnetic fields have yet to be directly detected. We have developed an algorithm allowing for the synthesis of photometric observables based on the Analytic Dynamical Magnetosphere (ADM) model of \citet{Owocki}. We apply our model to OGLE photometry in order to constrain their magnetic geometries and surface dipole strengths. We predict that the field strengths for some of these candidate extra-Galactic magnetic stars may be within the detection limits of the FORS2 instrument.

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arXiv2017

Chemical abundances of fast-rotating massive stars. II. Interpretation and comparison with evolutionary models

Aims: Past observations of fast-rotating massive stars exhibiting normal nitrogen abundances at their surface have raised questions about the rotational mixing paradigm. We revisit this question thanks to a spectroscopic analysis of a sample of bright fast-rotating OB stars, with the goal of quantifying the efficiency of rotational mixing at high rotation rates. Method: Our sample consists of 40 fast rotators on the main sequence, with spectral types comprised between B0.5 and O4. We compare the abundances of some key element indicators of mixing (He, CNO) with the predictions of evolutionary models for single objects and for stars in interacting binary systems. Results: The properties of half of the sample stars can be reproduced by single evolutionary models, even in the case of probable or confirmed binaries that can therefore be true single stars in a pre-interaction configuration. The main problem for the rest of the sample is a mismatch for the [N/O] abundance ratio (we confirm the existence of fast rotators with a lack of nitrogen enrichment) and/or a high helium abundance that cannot be accounted for by models. Modifying the diffusion coefficient implemented in single- star models does not solve the problem as it cannot simultaneously reproduce the helium abundances and [N/O] abundance ratios of our targets. Since part of them actually are binaries, we also compared their chemical properties with predictions for post-mass transfer systems. We found that these models can explain the abundances measured for a majority of our targets, including some of the most helium-enriched, but fail to reproduce them in other cases. Our study thus reveals that some physical ingredients are still missing in current models.

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