arXiv subjects

Yael Naze

Publications and source records attributed to Yael Naze.

At least 19 recordsLinked to original sources

arXiv2026

Long-term investigation of gamma Cas analogs

The subcategory of gamma Cas analogs gathers Be stars with bright and hard X-ray emission. Long-term variations are expected in such objects for two reasons: their Be disk builds and dissipates, and such stars are suspected long-period binaries. Seven targets are analysed in this paper: five of them benefit from a spectroscopic monitoring in the visible (ESO, TIGRE, and amateur data) and three of them have been repeatedly observed at X-ray wavelengths (using XMM-Newton, Chandra, and Swift). Broad-band photometric data are also examined. We confirm the binary status of five targets (HD44458, HD110432, HD119682, HD161103, and HD162718) and propose first orbital solutions for all of them (they remain preliminary for two cases). Their long periods (59-322 d) and small velocity amplitudes (K~5km/s) imply low-mass (~1 M_sol) companions, as in other Be binaries and in agreement with expectations from binary interaction models. In parallel, variations of the X-ray flux are detected in all three targets with a large dataset of X-ray observations. For NGC 6649 9 and HD162718, these changes are modest (a factor of three) and uncorrelated to simultaneous optical broad-band photometry (which remains rather stable). In contrast, SS397 varies by nearly one dex and the largest and best monitored X-ray changes correlate well with optical variations. At minimum flux, SS397 keeps a hard X-ray spectrum despite a nearly normal L_X/L_BOL ratio, which has not been seen yet among gamma Cas analogs. Finally, the photometric behaviours on short timescales of HD161103, SS397, and NGC 6649 9 appear linked to broad frequency groups, as typically found for Be stars. The frequency spectrum of HD162718 displays a complex mix of (isolated) periodicities with the main one at 6.658/d. This target is thus one of the rare gamma Cas analogs to display a strong high-frequency signal typical of beta Cep activity. [summarized]

astro-ph.SR↗

arXiv2026

Orbital motion detected in gamma Cas Fe K emission lines

A subset of Be stars, typified by the naked-eye star gamma Cas, exhibits unusually bright and hard X-ray emission, the origin of which has remained debated for five decades. We performed high-resolution X-ray spectroscopic monitoring of gamma Cas with the Resolve instrument aboard the X-Ray Imaging and Spectroscopy Mission (XRISM). X-ray lines from the ultra-hot plasma and fluorescence from cooler material exhibit Doppler shifts consistent with orbital motion, not of the Be star itself, but of its low-mass companion (previously shown to be a white dwarf). This first evidence of orbital motion for the hard X-ray emitting plasma uniquely links it to the scenario of accretion onto the white dwarf companion. The modest line broadening further indicates that fluorescence occurs on the white dwarf surface and excludes X-ray generation in the inner parts of an accretion disc. Our findings identify gamma Cas and its analogues as the previously elusive, but long predicted class of binaries composed of a Be and a white dwarf. Identifying the origin of the hard X-rays from gamma Cas and its analogues, which represent about 10% of early-type Be stars, provides a key input for population synthesis models of massive binary evolution.

astro-ph.SR↗

arXiv2025

A family of binaries with an extreme mass ratio

Multiplicity is ubiquitous among massive stars. While the stellar components usually display similar masses, some binaries with extremely low mass ratios were also observed. Some of them are primordial, while others arise from binary interactions. The identification of systems with extreme mass ratios brings valuable information, notably on the origin of fast rotation in massive stars. We identify new short-period systems with extreme mass ratios through the detection of eclipses and reflection effects. The physical properties of a dozen newly identified cases were precisely evaluated through high-quality photometry and spectroscopy. In addition to characterizing these binaries, we found a clear signature of apsidal motion in one system, and three other systems display long-term shifts in eclipse times. All systems we reported here are composed of a massive star and a cool low-mass companion. They are therefore primordial cases. This doubles the known number of these systems in the Galaxy. In this context, it is important to note that most massive stars in these systems, as well as in previous systems reported in the literature, rotate fast (supersynchronous compared to the orbital motion). The high incidence of fast rotation in these nascent binaries provides strong constraints for star formation models.

astro-ph.SR↗

arXiv2025

High energy emission powered by accreting companions of Be/gamma Cas stars

The origin of the hard, bright X-ray emission that defines the gamma Cas analog class of Be stars remains an outstanding question in Be star literature. This work explores the possibility that the X-ray flux is produced by accretion onto a white dwarf companion. We use three-dimensional smoothed particle hydrodynamics simulations to model the prototype gamma Cas system assuming a white dwarf companion and investigate the accretion of the circumstellar material by the secondary star. We contrast these results to a model for 59 Cyg, a non-gamma Cas Be star system with a stripped companion. We find that the secondary stars in both systems form disk-like accretion structures with Keplerian characteristics, similar to those seen in the Be decretion disks. We also find that white dwarf accretion can produce X-ray fluxes that are consistent with the observed values for gamma Cas, while the predicted X-ray luminosities are significantly lower for the non-degenerate companion in 59 Cyg. In addition, using the three-dimensional radiative transfer code, HDUST, we find that these models produce H-alpha emission consistent with the observations for both gamma Cas and 59 Cyg, and that the predicted polarization degrees across optical and UV wavelengths are at detectable levels. Finally, we discuss the impact that future UV spectropolarimetry missions could have on our understanding of these systems.

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arXiv2025

Apsidal motion and proximity effects in the massive binary BD+60 497

The eccentric short-period O-star binary BD+60 497 is an interesting laboratory in which to study tidal interactions in massive binary systems, notably via the detection and characterisation of apsidal motion. The rate of apsidal motion in such systems can help constrain their age and provide insight into the degree of mass concentration in the interior of massive stars. We used spectroscopic data collected over two decades to reconstruct the individual spectra of the stars and to establish their epoch-dependent radial velocities. An orbital solution, explicitly accounting for apsidal motion was adjusted to the data. Space-borne photometric time series were analysed with Fourier methods and with binary models. We derived a rate of apsidal motion of $6.15^{+1.05}_{-1.65}$ degree/yr, which suggests an age of $4.13^{+0.42}_{-1.37}$ Myr. The disentangled spectra unveiled a curious change in the spectral properties of the secondary star between the epochs 2002-2003 and 2018-2022 with the secondary spectrum appearing to be of an earlier spectral type over recent years. Photometric data show variability at the 6 mmag level on the period of the binary system, which is hard to explain in terms of proximity effects. Whilst the rate of apsidal motion agrees well with theoretical expectations, the changes in the reconstructed secondary spectrum hint at a highly non-uniform surface temperature distribution for this star. Different effects are discussed that could contribute to the photometric variations. The current most-likely explanation is a mix of proximity effects and tidally excited oscillations

astro-ph.SR↗

arXiv2025

X-ray Polarimetry of Accreting White Dwarfs: A Case Study of EX Hydrae

We present the first first X-ray polarization measurements of a white dwarf, the intermediate polar EX Hya. We measured significant polarization only in the 2 -- 3 keV energy band with a polarization degree of 8 percent at a $3\sigma$ significance. No significant polarization was detected above 3 keV, which we attribute to the higher energy bands having lower signal-to-noise. We found that the scattering surface detected by the IXPE is nearly perpendicular to the optical scattering plane, showing that the X-ray scattering surface is the WD and close to the base of the accretion column. Finally, we show how the polarization can be used to estimate the height of the accretion shock above the white dwarf's surface.

astro-ph.SR↗

arXiv2025

Another one (BH+OB pair) bites the dust

Most (or possibly all) massive stars reside in multiple systems. From stellar evolution models, numerous systems with an OB star coupled to a black hole would be expected to exist. There have been several claimed detections of such pairs in recent years and this is notably the case of HD96670. Using high-quality photometry and spectroscopy in the optical range, we revisited the HD96670 system. We also examined complementary X-ray observations to provide a broader view of the system properties. The TESS light curves of HD96670 clearly show eclipses, ruling out the black hole companion scenario. This does not mean that the system is not of interest. Indeed, the combined analysis of photometric and spectroscopic data indicates that the system most likely consists of a O8.5 giant star paired with a stripped-star companion with a mass of ~4.5Msol, a radius of ~1Rsol, and a surface temperature of ~50kK. While several B+sdOB systems have been reported in the literature, this would be the first case of a Galactic system composed of an O star and a faint stripped star. In addition, the system appears brighter and harder than normal OB stars in the X-ray range, albeit less so than for X-ray binaries. The high-energy observations provide hints of phase-locked variations, as typically seen in colliding wind systems. As a post-interaction system, HD96670 actually represents a key case for probing binary evolution, even if it is not ultimately found to host a black hole.

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arXiv2024

TESS observations of non-Be fast rotators

The variability of fast-rotating Oe/Be stars has been reported in detail in recent years. However, much less known about the behaviour of fast-rotating OB stars without known decretion disks, and hence it is difficult to identify the commonalities and differences in the photometric variability of these two populations, especially with regards to their pulsational properties and their link with the presence of circumstellar material. Via an in-depth literature search, we identified a set of fast-rotating (v sin(i)>200 km/s) early B-type stars not known to have disks. TESS and Kepler light curves were built for 58 stars that appear isolated (no bright neighbour within 1 arcmin and no known companion) to avoid contamination of the light curves. Frequency spectra were calculated and then analysed to determine the noise level and the presence of significant signals above the noise. Red noise is detected in all targets, without obvious correlations between noise and stellar parameters. Long-term changes are much less frequent than in Be stars, with only 12% of our targets having the variability below 0.5/d dominating their frequency spectrum. In contrast, strong frequency groups are detected in about a third of targets, as in Be stars. These groups generally occur in pairs with harmonic frequencies, as is usually seen in Be stars, but with the first group more often displaying larger amplitudes. Finally, the most frequent variability is due to isolated frequencies in the 0.5-6./d range (which is found in two-thirds of cases and dominates the spectra in 42% of the sample). Higher-frequency signals (up to 40/d) are sometimes also detected but rarely (only 12% of stars) appear as the strongest ones of the frequency spectra. Overall, fast-rotating B-type stars, with or without disks, display similar photometric properties, except as regards their longer-term behaviour.

astro-ph.SR↗

arXiv2024

X-raying the zeta Tau binary system

The Be star zeta Tau was recently reported to be a gamma Cas analog; that is, it displays an atypical (bright and hard) X-ray emission. The origin of these X-rays remains debated.The first X-ray observations indicated a very large absorption of the hot plasma component (N_H~ 10^{23}/cm^2). This is most probably related to the edge-on configuration of the zeta Tau disk. If the X-ray emission arises close to the companion, an orbital modulation of the absorption could be detected as the disk comes in and out of the line of sight. New XMM-Newton data were obtained to characterize the high-energy properties of zeta Tau in more detail. They are complemented by previous Chandra and SRG/eROSITA observations as well as by optical spectroscopy and TESS photometry. The high-quality XMM-Newton data reveal the presence of a faint soft X-ray emission, which appears in line with that recorded for non-gamma Cas Be stars. In addition, zeta Tau exhibits significant short-term variability at all energies, with larger amplitudes at lower frequencies (``red noise''), as is found in X-ray data of other gamma Cas stars. Transient variability (softness dip, low-frequency signal) may also be detected at some epochs. In addition, between X-ray exposures, large variations in the spectra are detected in the 1.5-4.keV energy band. They are due to large changes in absorption toward the hottest (9keV) plasma. These changes are not correlated with either the orbital phase or the depth of the shell absorption of the Halpha line. These observed properties are examined in the light of proposed gamma Cas models.

astro-ph.SR↗

arXiv2024

Ups and downs in the X-ray emission of the colliding wind binaries HD 168112 and HD 167971

The long-period O-star binary system HD 168112 and the triple O-star system HD 167971 are well-known sources of non-thermal radio emission that arises from a colliding wind interaction. The wind-wind collisions in these systems should result in phase-dependent X-ray emissions. The presence of a population of relativistic electrons in the wind interaction zone could affect the properties of the X-ray emission and make it deviate from the behaviour expected for adiabatic shocks. We investigate the X-ray emission of these systems with the goals of quantifying the fraction of the X-ray flux arising from wind interactions and determining whether these emissions follow the predictions for adiabatic wind-wind collisions. Six X-ray observations were collected with XMM-Newton. Three observations were scheduled around the most recent periastron passage of HD 168112. Spectra and light curves were analysed and compared with simple predictions of model calculations for X-ray emission from colliding wind systems. The X-ray emission of HD 168112 varies as the inverse of the orbital separation, as expected for an adiabatic wind interaction zone. The relative contribution of intrinsic X-ray emission from wind-embedded shocks varies between 38% at periastron to 81% at apastron. The wind-wind collision zone remains adiabatic even around periastron passage. The X-ray emission of HD 167971 displays variations on the orbital timescale of the inner eclipsing binary. The existing data of this system do not allow us to probe variations on the timescale of the outer orbit. Shock modification due to the action of relativistic electrons does not seem to be efficiently operating in the HD 168112 system. In the existing observations, a significant part of the emission of HD 167971 must arise in the inner eclipsing binary. The origin of this emission is as yet unclear.

astro-ph.SR↗

arXiv2024

Reysa Bernson, the unconventional head of the first French planetarium

The first modern planetarium was presented in 1923 in Jena, Germany. Very soon in the subsequent years, planetariums were installed in other parts of Europe as well as in America. France, however, got its first planetarium only in 1937, for the World Exhibition organized in Paris. The team that took care of that planetarium was headed by a female amateur astronomer named Reysa Bernson. This choice might seem surprising, but it was not made at random, thanks to her never-ending astronomical activities at that time. This paper aims to bring back memories of this very active amateur astronomer of the 1920s and 1930s, and show the many ways in which astronomy was disseminated a century ago.

physics.hist-ph↗

arXiv2023

A Morpho-Kinematic Study of the Enigmatic Emission Nebula NGC 6164/5 Surrounding the Magnetic O-type Star HD 148937

HD 148937 is a peculiar massive star (Of?p) with a strong magnetic field (1kG). The hourglass-shaped emission nebula NGC 6164/5 surrounds this star. This nebula is presumed to originate from episodic mass-loss events of the central O-type star, but the detailed formation mechanism is not yet well understood. Grasping its three-dimensional structure is essential to uncover the origin of this nebula. Here we report the high-resolution multi-object spectroscopic observation of NGC 6164/5 using the GIRAFFE on the 8.2m Very Large Telescope. Integrated intensity maps constructed from several spectral lines delineate well the overall shape of this nebula, such as the two bright lobes and the inner gas region. The position-velocity diagrams show that the two bright lobes are found to be redshifted and blueshifted, respectively, while the inner region has multiple layers. We consider a geometric model composed of a bilateral outflow harboring nitrogen-enriched knots and expanding inner shells. Its spectral features are then simulated by using a Monte-Carlo radiative transfer technique for different sets of velocities. Some position-velocity diagrams from simulations are very similar to the observed ones. According to the model that best reproduces the observational data, the two bright lobes and the nitrogen-enriched knots are moving away from HD 148937 at about 120 km s$^{-1}$. Their minimum kinematic age is estimated to be about 7,500 years. We discuss possible formation mechanisms of this nebula in the context of binary interaction.

astro-ph.SR↗

arXiv2023

Colliding winds in WR21 and WR31 -- I. The X-ray view

WR21 and WR31 are two WR+O binaries with short periods, quite similar to the case of V444 Cyg. The XMM-Newton observatory has monitored these two objects and clearly revealed phase-locked variations as expected from colliding winds. The changes are maximum in the soft band (0.5--2.keV, variations by a factor 3--4) where they are intrinsically linked to absorption effects. The increase in absorption due to the dense WR wind is confirmed by the spectral analysis. The flux maximum is however not detected exactly at conjunction with the O star in front but slightly afterwards, suggesting Coriolis deflection of the collision zone as in V444 Cyg. In the hard band (2.--10. keV), the variations (by a factor of 1.5--2.0) are much more limited. Because of the lower orbital inclinations, eclipses as observed for V444 Cyg are not detected in these systems.

astro-ph.SR↗

arXiv2023

SRG/eROSITA survey of Be stars

Massive stars are known X-ray emitters and those belonging to the Be category are no exception. One type of X-ray emission even appears specific to that category, the gamma Cas phenomenon. Its actual incidence has been particularly difficult to assess. Thanks to four semesters of sky survey data taken by SRG (Spectrum Roentgen Gamma)/eROSITA, we revisit the question of the X-ray properties of Be stars. Amongst a large catalog of Be stars, eROSITA achieved 170 detections (20% of sample), mostly corresponding to the earliest spectral types and/or close objects. While X-ray luminosities show an uninterrupted increasing trend with the X-ray-to-bolometric luminosity ratios, the X-ray hardness was split between a large group of soft (and fainter on average) sources and a smaller group of hard (and brighter on average) sources. The latter category gathers at least 34 sources, nearly all displaying early spectral types. Only a third of them were known before to display such X-ray properties. The actual incidence of hard and bright X-rays amongst early-type Be stars within 100--1000pc appears to be ~12%, which is far from negligible. At the other extreme, no bright supersoft X-ray emission seem to be associated to any of our targets.

astro-ph.SR↗

arXiv2023

Extreme mass ratios and fast rotation in three massive binaries

The origin of rapid rotation in massive stars remains debated, although binary interactions are now often advocated as a cause. However, the broad and shallow lines in the spectra of fast rotators make direct detection of binarity difficult. In this paper, we report on the discovery and analysis of multiplicity for three fast-rotating massive stars: HD25631 (B3V), HD191495 (B0V), and HD46485 (O7V). They display strikingly similar TESS light curves, with two narrow eclipses superimposed on a sinusoidal variation due to reflection effects. We complement these photometric data by spectroscopy from various instruments (X-Shooter, Espadons, FUSE...), to further constrain the nature of these systems. The detailed analyses of these data demonstrates that the companions of the massive OB stars have low masses (~1Msol) with rather large radii (2-4 Rsol) and low temperatures (<15 kK). These companions display no UV signature, which would exclude a hot subdwarf nature, but disentangling of the large set of X-Shooter spectra of HD25631 revealed the typical signature of chromospheric activity in the companion's spectrum. In addition, despite the short orbital periods (P=3-7d), the fast-rotating OB-stars still display non-synchronized rotation and all systems appear young (<20Myr). This suggests that, as in a few other cases, these massive stars are paired in those systems with non-degenerate, low-mass PMS companions, implying that fast rotation would not be a consequence of a past binary interactions in their case.

astro-ph.SR↗

arXiv2023

Science with the Ultraviolet Explorer (UVEX)

UVEX is a proposed medium class Explorer mission designed to provide crucial missing capabilities that will address objectives central to a broad range of modern astrophysics. The UVEX design has two co-aligned wide-field imagers operating in the FUV and NUV and a powerful broadband medium resolution spectrometer. In its two-year baseline mission, UVEX will perform a multi-cadence synoptic all-sky survey 50/100 times deeper than GALEX in the NUV/FUV, cadenced surveys of the Large and Small Magellanic Clouds, rapid target of opportunity followup, as well as spectroscopic followup of samples of stars and galaxies. The science program is built around three pillars. First, UVEX will explore the low-mass, low-metallicity galaxy frontier through imaging and spectroscopic surveys that will probe key aspects of the evolution of galaxies by understanding how star formation and stellar evolution at low metallicities affect the growth and evolution of low-metallicity, low-mass galaxies in the local universe. Such galaxies contain half the mass in the local universe, and are analogs for the first galaxies, but observed at distances that make them accessible to detailed study. Second, UVEX will explore the dynamic universe through time-domain surveys and prompt spectroscopic followup capability will probe the environments, energetics, and emission processes in the early aftermaths of gravitational wave-discovered compact object mergers, discover hot, fast UV transients, and diagnose the early stages of stellar explosions. Finally, UVEX will become a key community resource by leaving a large all-sky legacy data set, enabling a wide range of scientific studies and filling a gap in the new generation of wide-field, sensitive optical and infrared surveys provided by the Rubin, Euclid, and Roman observatories. This paper discusses the scientific potential of UVEX, and the broad scientific program.

astro-ph.GA↗

arXiv2022

Ultraviolet Spectropolarimetry:Conservative and Nonconservative Mass Transfer in OB Interacting Binaries

One objective of the Polstar spectropolarimetry mission is to characterize the degree of nonconservative mass transfer that occurs at various stages of binary evolution, from the initial mass reversal to the late Algol phase. The proposed instrument combines spectroscopic and polarimetric capabilities, where the spectroscopy can resolve Doppler shifts in UV resonance lines with 10 km/s precision, and polarimetry can resolve linear polarization with 1e-3 precision or better. The spectroscopy will identify absorption by mass streams seen in projection against the stellar disk as a function of orbital phase, hot accretion spots, as well as scattering from extended splash structures, circumbinary disks, and other flows in and above/below the orbital plane (e.g. jets) that fail to be transferred conservatively. The polarimetry affects more the light coming from material not seen against the stellar disk, allowing the geometry of the scattering to be tracked, resolving ambiguities left by the spectroscopy and light-curve information. For example, nonconservative mass streams ejected in the polar direction will produce polarization of the opposite sign from conservative transfer accreting in the orbital plane. Also, time domain coverage over a range of phases of the binary orbit are well supported by the Polstar observing strategy. Combining these elements will significantly improve our understanding of the mass transfer process and the amount of mass that can escape from the system, an important channel for changing the final mass, and ultimate supernova, of the large number of massive stars found in binaries at close enough separation to undergo interaction.

astro-ph.SR↗

arXiv2022

The X-ray emission of Be+stripped star binaries

Using observations from Chandra, Swift and XMM-Newton, we investigate the high-energy properties of all known (18) Be+sdO systems as well as 7 additional Be binaries suspected to harbour stripped stars. The observed X-ray properties are found to be similar to those observed for other Be samples. The vast majority of these systems (15 out of 25) display very faint (and soft) X-ray emission, and six others are certainly not bright X-ray sources. Only two systems display gamma-Cas characteristics (i.e. bright and hard X-rays), and one of them is a new detection: HD37202 (zeta Tau). It presents an extremely hard spectrum, due to a combination of high temperature and high absorption (possibly due to its high inclination). In parallel, it may be noted that the previously reported cyclic behaviour of this Be star has disappeared in recent years. Instead, shorter cycles and symmetric line profiles are observed for the Halpha line. It had been recently suggested that the peculiar X-ray emissions observed in gamma-Cas stars could arise from a collision between the disk of a Be star and the wind of its hot, stripped-star companion. The small fraction of gamma-Cas analogs in this sample, as well as the properties of the known companions of the gamma-Cas cases (low mass or not extremely hot, contrary to predictions), combined to the actual stripped-star and colliding-wind empirical knowledge, make the disk-wind collision an unlikely scenario to explain the gamma-Cas phenomenon.

astro-ph.SR↗