arXiv subjects

Yael Naze

Publications and source records attributed to Yael Naze.

88 records · Page 5Linked to original sources

arXiv2011

The Search for Low-mass Companions of B Stars in the Carina Nebula Cluster Trumpler 16

We have developed lists of likely B3--A0 stars (called "late B" stars) in the young cluster Trumpler 16. The following criteria were used: location within 3' of Eta Car, an appropriate V and B-V combination, and proper motion (where available). Color and magnitude cuts have been made assuming an E(B-V) =0.55 mag +/- 0.1, which is a good approximation close to the center of Trumpler 16. These lists have been cross-correlated with X-ray sources found in the Chandra Carina Complex Project (CCCP). Previous studies have shown that only very rarely (if at all) do late main sequence B stars produce X-rays. We present evidence that the X-ray detected sources are binaries with low-mass companions, since stars less massive than 1.4 Msun are strong X-ray sources at the age of the cluster. Both the median X-ray energies and X-ray luminosities of these sources are in good agreement with values for typical low-mass coronal X-ray sources. We find that 39% of the late B stars based on a list with proper motions have low-mass companions. Similarly, 32% of a sample without proper motions have low-mass companions. We discuss the X-ray detection completeness. These results on low-mass companions of intermediate mass stars are complementary to spectroscopic and interferometric results, and probe new parameter space of low mass companions at all separations. They do not support a steeply rising distribution of mass ratios to low masses for intermediate-mass (5 Msun) primaries, such as would be found by random pairing from the Initial Mass Function.

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arXiv2011

Carina OB Stars: X-ray Signatures of Wind Shocks and Magnetic Fields

The Chandra Carina Complex contains 200 known O- and B type stars. The Chandra survey detected 68 of the 70 O stars and 61 of 127 known B0-B3 stars. We have assembled a publicly available optical/X-ray database to identify OB stars that depart from the canonical Lx/Lbol relation, or whose average X-ray temperatures exceed 1 keV. Among the single O stars with high kT we identify two candidate magnetically confined wind shock sources: Tr16-22, O8.5 V, and LS 1865, O8.5 V((f)). The O4 III(fc) star HD 93250 exhibits strong, hard, variable X-rays, suggesting it may be a massive binary with a period of >30 days. The visual O2 If* binary HD 93129A shows soft 0.6 keV and hard 1.9 keV emission components, suggesting embedded wind shocks close to the O2 If* Aa primary, and colliding wind shocks between Aa and Ab. Of the 11 known O-type spectroscopic binaries, the long orbital-period systems HD 93343, HD 93403 and QZ Car have higher shock temperatures than short-period systems such as HD 93205 and FO 15. Although the X-rays from most B stars may be produced in the coronae of unseen, low-mass pre-main-sequence companions, a dozen B stars with high Lx cannot be explained by a distribution of unseen companions. One of these, SS73 24 in the Treasure Chest cluster, is a new candidate Herbig Be star.

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arXiv2010

Hot and cool: two emission-line stars with constrasting behaviours in the same XMM-Newton field

High-energy emissions are good indicators of peculiar behaviours in stars. We have therefore obtained an XMM-Newton observation of HD155806 and 1RXSJ171502.4-333344, and derived their spectral properties for the first time. The X-ray spectrum of HD155806 appears soft, even slightly softer than usual for O-type stars (as shown by a comparison with the O9 star HD155889 in the same XMM field). It is well-fitted with a two-component thermal model with low temperatures (0.2 and 0.6 keV), and it shows no overluminosity (log[LX/Lbol]=-6.75). The high-resolution spectrum, though noisy, reveals a few broad, symmetric X-ray lines (FWHM ~ 2500 km/s). The X-ray emission is compatible with the wind-shock model and therefore appears unaffected by the putative dense equatorial regions at the origin of the Oe classification. 1RXSJ171502.4-333344 is a nearby flaring source of moderate X-ray luminosity (log[LX/Lbol]=-3), with a soft thermal spectrum composed of narrow lines and presenting a larger abundance of elements (e.g. Ne) with a high first ionization potential (FIP) compared to lower-FIP elements. All the evidence indicates a coronal origin for the X-ray emission, in agreement with the dMe classification of this source.

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arXiv2009

Hot stars observed by XMM-Newton I. The catalog and the properties of OB stars

Aims : Following the advent of increasingly sensitive X-ray observatories, deep observations of early-type stars became possible. However, the results for only a few objects or clusters have until now been reported and there has been no large survey comparable to that based upon the ROSAT All-Sky Survey (RASS). Methods : A limited survey of X-ray sources, consisting of all public XMM observations (2XMMi) and slew survey data (XMMSL1), is now available. The X-ray counterparts to hot, massive stars have been searched for in these catalogs. Results : About 300 OB stars were detected with XMM. Half of them were bright enough for a spectral analysis to be possible, and we make available the detailed spectral properties that were derived. The X-ray spectra of O stars are represented well by low (<1keV) temperature components and seem to indicate that an absorption column is present in addition to the interstellar contribution. The X-ray fluxes are well correlated with the bolometric fluxes, with a scatter comparable to that of the RASS studies and thus larger than found previously with XMM for some individual clusters. These results contrast with those of B stars that exhibit a large scatter in the L_X-L_BOL relation, no additional absorption being found, and the fits indicate a plasma at higher temperatures. Variability (either within one exposure or between multiple exposures) was also investigated whenever possible: short-term variations are far more rare than long-term ones (the former affects a few percent of the sample, while the latter concerns between one third and two thirds of the sources). Conclusions : This paper presents the results of the first high-sensitivity investigation of the overall high-energy properties of a sizable sample of hot stars.

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arXiv2008

X-ray stellar population of the LMC

In the study of stars, the high energy domain occupies a place of choice, since it is the only one able to directly probe the most violent phenomena: indeed, young pre-main sequence objects, hot massive stars, or X-ray binaries are best revealed in X-rays. However, previously available X-ray observatories often provided only crude information on individual objects in the Magellanic Clouds. The advent of the highly efficient X-ray facilities XMM-Newton and Chandra has now dramatically increased the sensitivity and the spatial resolution available to X-ray astronomers, thus enabling a fairly easy determination of the properties of individual sources in the LMC.

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arXiv2008

High-resolution X-ray spectroscopy of Theta Car

Context : The peculiar hot star Theta Car in the open cluster IC2602 is a blue straggler as well as a single-line binary of short period (2.2d). Aims : Its high-energy properties are not well known, though X-rays can provide useful constraints on the energetic processes at work in binaries as well as in peculiar, single objects. Methods : We present the analysis of a 50ks exposure taken with the XMM-Newton observatory. It provides medium as well as high-resolution spectroscopy. Results : Our high-resolution spectroscopy analysis reveals a very soft spectrum with multiple temperature components (1--6MK) and an X-ray flux slightly below the `canonical' value (log[L_X(0.1-10.)/L_{BOL}] ~ -7). The X-ray lines appear surprisingly narrow and unshifted, reminiscent of those of beta Cru and tau Sco. Their relative intensities confirm the anomalous abundances detected in the optical domain (C strongly depleted, N strongly enriched, O slightly depleted). In addition, the X-ray data favor a slight depletion in neon and iron, but they are less conclusive for the magnesium abundance (solar-like?). While no significant changes occur during the XMM-Newton observation, variability in the X-ray domain is detected on the long-term range. The formation radius of the X-ray emission is loosely constrained to <5 R_sol, which allows for a range of models (wind-shock, corona, magnetic confinement,...) though not all of them can be reconciled with the softness of the spectrum and the narrowness of the lines.

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arXiv2008

The mysterious Of?p class and the magnetic O-star Theta Ori C

In recent years, the stars of the Of?p category have revealed a wealth of peculiar phenomena: varying line profiles, photometric changes, and X-ray overluminosity are only a few of their characteristics. Here we review their physical properties, to facilitate comparisons among the Galactic members of this class. As one of them has been proposed to resemble the magnetic oblique rotator Theta Ori C, though with a longer period, this latter object is also included in our study to illuminate its similarities and differences with the Of?p category.

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arXiv2008

Chandra monitoring of the very massive binary WR20a and the young massive cluster Westerlund2

Results: The two Wolf-Rayet stars WR20a (WN6ha+WN6ha) and WR20b (WN6ha) were analyzed in detail. They are both very luminous and display very hard spectra, but WR20b does not seem to vary. On the contrary, WR20a, a known eclipsing, colliding-wind binary, brightens in the X-ray domain during the eclipses, i.e. when the collision is seen face-on. This can be explained by the properties of the wind-wind collision zone, whose high density leads to a large absorbing column (2 10^24 cm^-2).All twelve O-type stars previously classified spectroscopically, two eclipsing binaries previously identified and nine newly identified O-type star candidates are detected in the high energy domain; ten of them could be analyzed spectroscopically. Four are overluminous, but the others present typical L_X/L_BOL ratios, suggesting that several O-type objects are actually binaries. Variability at the ~2sigma level was detected for a majority of the sources, of unknown origin for the putatively single objects. Faint, soft, diffuse emission pervades the entire field-of-view but no clear structure can be identified, even at the position of a blister proposed to be at the origin of the TeV source HESS J1023-575. Finally, the X-ray properties of PMS objects were also investigated, in particular the brightest flaring ones. They provided an additional argument in favor of a large distance (~8kpc) for the cluster.

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arXiv2007

Towards an understanding of the Of?p star HD 191612: optical spectroscopy

We present extensive optical spectroscopy of the early-type magnetic star HD 191612 (O6.5f?pe-O8fp). The Balmer and HeI lines show strongly variable emission which is highly reproducible on a well-determined 538-d period. Metal lines and HeII absorptions (including many selective emission lines but excluding He II 4686A emission) are essentially constant in line strength, but are variable in velocity, establishing a double-lined binary orbit with P(orb) = 1542d, e=0.45. We conduct a model-atmosphere analysis of the primary, and find that the system is consistent with a O8: giant with a B1: main-sequence secondary. Since the periodic 538-d changes are unrelated to orbital motion, rotational modulation of a magnetically constrained plasma is strongly favoured as the most likely underlying `clock'. An upper limit on the equatorial rotation is consistent with this hypothesis, but is too weak to provide a strong constraint.

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arXiv2007

First detection of phase-dependent colliding wind X-ray emission outside the Milky Way

After having reported the detection of X-rays emitted by the peculiar system HD5980, we assess here the origin of this high-energy emission from additional X-ray observations obtained with XMM-Newton. This research provides the first detection of apparently periodic X-ray emission from hot gas produced by the collision of winds in an evolved massive binary outside the Milky Way. It also provides the first X-ray monitoring of a Luminous Blue Variable only years after its eruption and shows that the dominant source of the X-rays is not associated with the ejecta.

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arXiv2006

Spectroscopy of Massive Stars

Although rare, massive stars, being the main sources of ionizing radiation, chemical enrichment and mechanical energy in the Galaxy, are the most important objects of the stellar population. This review presents the many different aspects of the main tool used to study these stars, i.e. spectroscopy. The first part consists in an introduction on these objects and their physical properties (mass, wind, evolution, relation with their environment). Next, the spectral behaviour of single massive stars is investigated, in the visible as well as in the X-ray domain. Finally, the last part of this paper deals with massive binaries, especially those exhibiting a colliding wind phenomenon.

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arXiv2006

Towards an understanding of the Of?p star HD191612: Phase-resolved multiwavelength observations

We present the analysis of phase-resolved X-ray and optical observations of the peculiar hot star HD191612 (Of?p). This star is known to display line-profile variations that are recurrent with a period of 538d and its spectrum was found to present the signature of a magnetic field. In the X-rays, it is slightly overluminous compared to the canonical L_X/L_BOL relation and appears brighter when the optical lines are strongest. Our XMM-Newton observations further reveal that the X-ray spectrum of \hd exhibits rather broad lines and is dominated by a `cool' (0.2-0.6 keV) thermal component, two characteristics at odds with the proposed magnetic rotator model. We also report for the first time the low-level variability of the metallic (abs/em) lines and HeII absorptions that appear to be associated with radial-velocity shifts. Finally, we compare our results with observations of the early-type stars and discuss several possible scenarios.

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arXiv2004

A Period and a Prediction for the Of?p Spectrum Alternator HD 191612

The observational picture of the enigmatic O-type spectrum variable HD191612 has been sharpened substantially. A symmetrical, low-amplitude light curve with a period near 540 d has recently been reported from Hipparcos photometry. This period satisfies all of the spectroscopy since at least 1982, including extensive new observations during 2003 and 2004, and it has predicted the next transition during September--October 2004. Measurements of the H alpha equivalent width reveal a sharp emission peak in the phase diagram, in contrast to the apparently sinusoidal light curve. The He II absorption-line strength is essentially constant, while He I varies strongly, possibly filled in by emission in the O6 state, thus producing the apparent spectral-type variations. The O8 state appears to be the "normal" one. Two intermediate O7 observations have been obtained, which fall at the expected phases, but these are the only modern observations of the transitions so far. The period is too long for rotation or pulsation; although there is no direct evidence as yet for a companion, a model in which tidally induced oscillations drive an enhanced wind near periastron of an eccentric orbit appears promising. Further observations during the now predictable transitions may provide a critical test. Ultraviolet and X-ray observations during both states will likely also prove illuminating.

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arXiv2002

Structure and Dynamics of Candidate O Star Bubbles in N44

Dynamical studies of superbubbles and Wolf-Rayet ring nebulae show discrepancies from the standard, adiabatic model for wind-blown bubbles. We therefore study the physical properties and kinematics of three candidate bubbles blown by single O stars, to evaluate whether these discrepancies are also found in these simpler objects. Our sample candidates are N44F, N44J, and N44M, in the outskirts of the HII complex N44 in the Large Magellanic Cloud. We have obtained ground-based and HST emission-line images and high dispersion echelle spectra for these objects. From the Halpha luminosities and the [OIII]/Halpha ratios of these nebulae, we estimate the spectral types of the ionizing stars to be O7V, O9.5V and O9.5V for N44F, N44J, and N44M, respectively. We find that the observed expansion velocity of 12 km/s for N44F is consistent with the stellar wind luminosity expected from the central ionizing star, as predicted by the standard bubble model. The observed upper limits for the expansion velocities of N44J and N44M are also compatible with the expected values, within the uncertainties. We also report the discovery in N44F of strongly-defined dust columns, similar to those seen in the Eagle Nebula. The photoevaporation of these dense dust features may be kinematically important and may actually govern the evolution of the shell. The inclusion of photoevaporation processes may thus undermine the apparent agreement between the observed bubble dynamics and the simple adiabatic models.

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arXiv2001

Interstellar Bubbles in Two Young HII Regions

Massive stars are expected to produce wind-blown bubbles in the interstellar medium; however, ring nebulae, suggesting the existence of bubbles, are rarely seen around main-sequence O stars. To search for wind-blown bubbles around main-sequence O stars, we have obtained high-resolution Hubble Space Telescope WFPC2 images and high-dispersion echelle spectra of two pristine HII regions, N11B and N180B, in the Large Magellanic Cloud. These HII regions are ionized by OB associations that still contain O3 stars, suggesting that the HII regions are young and have not hosted any supernova explosions. Our observations show that wind-blown bubbles in these HII regions can be detected kinematically but not morphologically because their expansion velocities are comparable to or only slightly higher than the isothermal sound velocity in the HII regions. Bubbles are detected around concentrations of massive stars, individual O stars, and even an evolved red supergiant (a fossil bubble). Comparisons between the observed bubble dynamics and model predictions show a large discrepancy (1--2 orders of magnitude) between the stellar wind luminosity derived from bubble observations and models and that derived from observations of stellar winds. The number and distribution of bubbles in N11B differ from those in N180B, which can be explained by the difference in the richness of stellar content between these two HII regions. Most of the bubbles observed in N11B and N180B show a blister-structure, indicating that the stars were formed on the surfaces of dense clouds. Numerous small dust clouds, similar to Bok globules or elephant trunks, are detected in these HII regions and at least one of them hosts on-going star formation.

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arXiv2001

What is the real nature of HD 108 ?

Since the beginning of the past century, the nature of HD 108 has been a subject of intense debate. One after another, astronomers explored its variability and attributed it either to binarity, or to changes in the stellar wind of a single star. In this article, we analyse a 30 years long campaign of spectroscopic observations of this star with special emphasis on the last 15 years during which photographic plates have been replaced by CCD detectors. Our investigation of the radial velocities of HD 108 yields no significant short- or long-term period and does not confirm the published periodicities either. Though the radial velocity of HD 108 appears clearly variable, the variations cannot be explained by the orbital motion in a spectroscopic binary. However, our data reveal spectacular changes in the H I Balmer lines and some He I profiles over the years. These lines continuously evolved from P Cygni profiles to `pure' absorption lines. A similar behaviour was already observed in the past, suggesting that these changes are recurrent. HD 108 seems to share several characteristics of Oe stars and we discuss different hypotheses for the origin of the observed long-term variations. As we are now in a transition period, a continuous monitoring of HD 108 should be considered for the next few years.

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