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The carbon-rich WC5 star WR 114 was not detected during a 15.9 ksec XMM-Newton observation, implying an upper limit to the X-ray luminosity of Lx < 2.5 x 1030 ergs-1 and to the X-ray to bolometric luminosity ratio of Lx/Lbol < 4 x 10-9. This confirms indications from earlier less sensitive measurements that there has been no convincing X-ray detection of any single WC star. This lack of detections is reinforced by XMM-Newton and CHANDRA observations of WC stars. Thus the conclusion has to be drawn that the stars with radiatively-driven stellar winds of this particular class are insignificant X-ray sources. We attribute this to photoelectronic absorption by the stellar wind. The high opacity of the metal-rich and dense winds from WC stars puts the radius of optical depth unity at hundreds or thousands of stellar radii for much of the X-ray band. We believe that the essential absence of hot plasma so far out in the wind exacerbated by the large distances and correspondingly high ISM column densities makes the WC stars too faint to be detectable with current technology. The result also applies to many WC stars in binary systems, of which only about 20 % are identified X-ray sources, presumably due to colliding winds.
A considerable fraction of the central stars of planetary nebulae (CSPNe) are hydrogen-deficient. As a rule, these CSPNe exhibit a chemical composition of helium, carbon, and oxygen with the majority showing Wolf-Rayet-like emission line spectra. These stars are classified as CSPNe of a spectral type [WC]. We perform a spectral analysis of CSPN PB 8 with the Potsdam Wolf-Rayet (PoWR) models for expanding atmospheres. The source PB8 displays wind-broadened emission lines from strong mass loss. Most strikingly, we find that its surface composition is hydrogen-deficient, but not carbon-rich. With mass fractions of 55% helium, 40% hydrogen, 1.3% carbon, 2% nitrogen, and 1.3% oxygen, it differs greatly from the 30-50% of carbon which are typically seen in [WC]-type central stars. The atmospheric mixture in PB8 has an analogy in the WN/WC transition type among the massive Wolf-Rayet stars. Therefore we suggest to introduce a new spectral type [WN/WC] for CSPNe, with PB8 as its first member. The central star of PB8 has a relatively low temperature of T-* = 52 kK, as expected for central stars in their early evolutionary stages. Its surrounding nebula is less than 3000 years old, i.e. relatively young. Existing calculations for the post-AGB evolution can produce hydrogen-deficient stars of the [WC] type, but do not predict the composition found in PB8. We discuss various scenarios that might explain the origin of this unique object.
The central star of the PN LMC-N66 showed an impressive outburst in 1993 - 1994, returning to its initial conditions about 8 years later. Its spectrum resembles that of a WN4.5 star, being the only confirmed central star of planetary nebulae showing such a spectral type. Recent analysis for the central star parameters, performed by Hamann et al. (2003) is presented. They have found that the bolometric luminosity increased by a factor larger than 6, during the outburst. We discuss the possible scenarios which have been proposed to explain the exceptional stellar parameters and the outburst mechanism. The stellar characteristics and the morphology and kinematics of the planetary nebula suggest the presence of binary system (massive star with a less massive companion or, a white dwarf accreting matter in a close- binary system). These cases pose the least severe contradictions with observational constraints.
We present the XMM-Newton discovery of X-ray emission from the planetary nebula (PN) A78, the second born-again PN detected in X-rays apart from A30. These two PNe share similar spectral and morphological characteristics: they harbor diffuse soft X-ray emission associated with the interaction between the H-poor ejecta and the current fast stellar wind and a point-like source at the position of the central star (CSPN). We present the spectral analysis of the CSPN, using for the first time an NLTE code for expanding atmospheres that takes line blanketing into account for the UV and optical spectra. The wind abundances are used for the X-ray spectral analysis of the CSPN and the diffuse emission. The X-ray emission from the CSPN in A78 can be modeled by a single C VI emission line, while the X-ray emission from its diffuse component is better described by an optically thin plasma emission model with a temperature of kT = 0.088 keV (T approximate to 1.0 x 10(6) K). We estimate X-ray luminosities in the 0.2-2.0 keV energy band of L-X,L-CSPN =(1.2 +/- 0.3) x 10(31) erg s(-1) and L-X,L-DIFF =(9.2 +/- 2.3) x 10(30) erg s(-1) for the CSPN and diffuse components, respectively.
Context.
The supergiant ionized shell SMC-SGS 1 (DEM 167), which is located in the outer Wing of the Small Magellanic Cloud (SMC), resembles structures that originate from an energetic star-formation event and later stimulate star formation as they expand into the ambient medium. However, stellar populations within and surrounding SMC-SGS 1 tell a different story. Aims. We present a photometric study of the stellar population encompassed by SMC-SGS 1 in order to trace the history of such a large structure and its potential influence on star formation within the low-density, low-metallicity environment of the SMC.
Methods.
For a stellar population that is physically associated with SMC-SGS 1, we combined near-ultraviolet (NUV) photometry from the Galaxy Evolution Explorer with archival optical (V-band) photometry from the ESO Danish 1.54 m Telescope. Given their colors and luminosities, we estimated stellar ages and masses by matching observed photometry to theoretical stellar isochrone models. Results. We find that the investigated region supports an active, extended star-formation event spanning similar to 25-40 Myr ago, as well as continued star formation into the present. Using a standard initial mass function, we infer a lower bound on the stellar mass from this period of similar to 3 x 10(4) M-circle dot, corresponding to a star-formation intensity of similar to 6 x 10(-3) M-circle dot kpc(-2) yr(-1).
Conclusions.
The spatial and temporal distributions of young stars encompassed by SMC-SGS 1 imply a slow, consistent progression of star formation over millions of years. Ongoing star formation, both along the edge and interior to SMC-SGS 1, suggests a combined stimulated and stochastic mode of star formation within the SMC Wing. We note that a slow expansion of the shell within this low-density environment may preserve molecular clouds within the volume of the shell, leaving them to form stars even after nearby stellar feedback expels local gas and dust.
Stars that start their lives with spectral types O and early B are the progenitors of core-collapse supernovae, long gamma-ray bursts, neutron stars, and black holes. These massive stars are the primary sources of stellar feedback in star-forming galaxies. At low metallicities, the properties of massive stars and their evolution are not yet fully explored. Here we report a spectroscopic study of 320 massive stars of spectral types O (23 stars) and B (297 stars) in the Wing of the Small Magellanic Cloud (SMC). The spectra, which we obtained with the ESO Very Large Telescope, were analyzed using state-of-the-art stellar atmosphere models, and the stellar parameters were determined. We find that the stellar winds of our sample stars are generally much weaker than theoretically expected. The stellar rotation rates show broad, tentatively bimodal distributions. The upper Hertzsprung-Russell diagram (HRD) is well populated by the stars of our sample from a specific field in the SMC Wing. A few very luminous O stars are found close to the main sequence, while all other, slightly evolved stars obey a strict luminosity limit. Considering additional massive stars in evolved stages, with published parameters and located all over the SMC, essentially confirms this picture. The comparison with single-star evolutionary tracks suggests a dichotomy in the fate of massive stars in the SMC. Only stars with an initial mass below similar to 30 M-circle dot seem to evolve from the main sequence to the cool side of the HRD to become a red supergiant and to explode as type II-P supernova. In contrast, stars with initially more than similar to 30 M-circle dot appear to stay always hot and might evolve quasi chemically homogeneously, finally collapsing to relatively massive black holes. However, we find no indication that chemical mixing is correlated with rapid rotation. We measured the key parameters of stellar feedback and established the links between the rates of star formation and supernovae. Our study demonstrates that in metal-poor environments stellar feedback is dominated by core-collapse supernovae in combination with winds and ionizing radiation supplied by a few of the most massive stars. We found indications of the stochastic mode of massive star formation, where the resulting stellar population is fully capable of producing large-scale structures such as the supergiant shell SMC-SGS 1 in the Wing. The low level of feedback in metal-poor stellar populations allows star formation episodes to persist over long timescales.
Stellar winds appear as a persistent feature of hot stars, irrespective of their wide range of different luminosities, masses, and chemical composition. Among the massive stars, the Wolf-Rayet types show considerably stronger mass loss than the O stars. Among hot low-mass stars, stellar winds are seen at central stars of planetary nebulae, where again the hydrogen-deficient stars show much stronger winds than those central stars with "normal" composition. We also studied mass-loss from a few extreme helium stars and sdOs. Their mass-loss rate roughly follows the same proportionality with luminosity to the power 1.5 as the massive O stars. This relation roughly marks a lower limit for the mass loss from hot stars of all kinds, and provides evidence that radiation pressure on spectral lines is the basic mechanism at work. For certain classes of stars the mass-loss rates lie significantly above this relation, for reasons that are not yet fully understood. Mass loss from low-mass stars may affect their evolution, by reducing the envelope mass, and can easily prevent diffusion from establishing atmospheric abundance patterns. In close binary systems, their winds can feed the accretion onto a companion.
Context. Clusters or associations of early-type stars are often associated with a "superbubble" of hot gas. The formation of such superbubbles is caused by the feedback from massive stars. The complex N206 in the Large Magellanic Cloud (LMC) exhibits a superbubble and a rich massive star population. Aims. Our goal is to perform quantitative spectral analyses of all massive stars associated with the N206 superbubble in order to determine their stellar and wind parameters. We compare the superbubble energy budget to the stellar energy input and discuss the star formation history of the region. Results. We present the stellar and wind parameters of the OB stars and the two Wolf-Rayet (WR) binaries in the N206 complex. Twelve percent of the sample show Oe/Be type emission lines, although most of them appear to rotate far below critical. We found eight runaway stars based on their radial velocity. The wind-momentum luminosity relation of our OB sample is consistent with the expectations. The Hertzsprung-Russell diagram (HRD) of the OB stars reveals a large age spread (1-30 Myr), suggesting different episodes of star formation in the complex. The youngest stars are concentrated in the inner part of the complex, while the older OB stars are scattered over outer regions. We derived the present day mass function for the entire N206 complex as well as for the cluster NGC2018. The total ionizing photon flux produced by all massive stars in the N206 complex is Q(0) approximate to 5 x 10(50) s(-1), and the mechanical luminosity of their stellar winds amounts to L-mec = 1.7 x 10(38) erg s(-1). Three very massive Of stars are found to dominate the feedback among 164 OB stars in the sample. The two WR winds alone release about as much mechanical luminosity as the whole OB star sample. The cumulative mechanical feedback from all massive stellar winds is comparable to the combined mechanical energy of the supernova explosions that likely occurred in the complex. Accounting also for the WR wind and supernovae, the mechanical input over the last five Myr is approximate to 2.3 x 10(52) erg. Conclusions. The N206 complex in the LMC has undergone star formation episodes since more than 30 Myr ago. From the spectral analyses of its massive star population, we derive a current star formation rate of 2.2 x 10(-3) M-circle dot yr(-1). From the combined input of mechanical energy from all stellar winds, only a minor fraction is emitted in the form of X-rays. The corresponding input accumulated over a long time also exceeds the current energy content of the complex by more than a factor of five. The morphology of the complex suggests a leakage of hot gas from the superbubble.
Context. Massive stars severely influence their environment by their strong ionizing radiation and by the momentum and kinetic energy input provided by their stellar winds and supernovae. Quantitative analyses of massive stars are required to understand how their feedback creates and shapes large scale structures of the interstellar medium. The giant H II region N206 in the Large Magellanic Cloud contains an OB association that powers a superbubble filled with hot X-ray emitting gas, serving as an ideal laboratory in this context. Aims. We aim to estimate stellar and wind parameters of all OB stars in N206 by means of quantitative spectroscopic analyses. In this first paper, we focus on the nine Of-type stars located in this region. We determine their ionizing flux and wind mechanical energy. The analysis of nitrogen abundances in our sample probes rotational mixing. Methods. We obtained optical spectra with the multi-object spectrograph FLAMES at the ESO-VLT. When possible, the optical spectroscopy was complemented by UV spectra from the HST, IUE, and FUSE archives. Detailed spectral classifications are presented for our sample Of-type stars. For the quantitative spectroscopic analysis we used the Potsdam Wolf-Rayet model atmosphere code. We determined the physical parameters and nitrogen abundances of our sample stars by fitting synthetic spectra to the observations. Results. The stellar and wind parameters of nine Of-type stars, which are largely derived from spectral analysis are used to construct wind momentum luminosity relationship. We find that our sample follows a relation close to the theoretical prediction, assuming clumped winds. The most massive star in the N206 association is an Of supergiant that has a very high mass-loss rate. Two objects in our sample reveal composite spectra, showing that the Of primaries have companions of late O subtype. All stars in our sample have an evolutionary age of less than 4 million yr, with the O2-type star being the youngest. All these stars show a systematic discrepancy between evolutionary and spectroscopic masses. All stars in our sample are nitrogen enriched. Nitrogen enrichment shows a clear correlation with increasing projected rotational velocities. Conclusions. The mechanical energy input from the Of stars alone is comparable to the energy stored in the N206 superbubble as measured from the observed X-ray and H alpha emission.
Modeling expanding atmospheres is a difficult task because of the extreme non-LTE situation, the need to account for complex model atoms, especially for the iron-group elements with their millions of lines, and because of the supersonic expansion. Adequate codes have been developed e.g. by Hillier (CMFGEN), the Munich group (Puls, Pauldrach), and in Potsdam (PoWR code, Hamann et al.). While early work was based on the assumption of a smooth and homogeneous spherical stellar wind, the need to account for clumping became obvious about ten years ago. A relatively simple first-order clumping correction was readily implemented into the model codes. However, its simplifying assumptions are severe. Most importantly, the clumps are taken to be optically thin at all frequencies (”microclumping”). We discuss the consequences of this approximation and describe an approach to account for optically thick clumps (“macroclumping”). First results demonstrate that macroclumping can generally reduce the strength of spectral features, depending on their optical thickness. The recently reported discrepancy between the Hα diagnostic and the Pv resonance lines in O star spectra can be resolved without decreasing the mass-loss rates, when macroclumping is taken into account.
Spectrum formation in clumped stellar winds : consequences for the analyses of Wolf-Rayet spectra
(1998)
HST UV and optical spectra of the early-type [WC] star SMP 61 in the LMC are analyzed by means of line blanketed non-LTE models for expanding atmospheres. The known distance to the LMC allows a reliable determination of the stellar parameters. The low iron surface abundance of the object possibly indicates a preceding evolution through a very late thermal pulse (VLTP).
The analysis of Wolf-Rayet spectra requires adequate model atmospheres which treat the non-LTE radiation transfer in a spherically expanding medium. Present state-of-the-art calculations account for complex model atoms with, typically, a few hundred energy levels and a few thousand spectral lines of He and CNO elements. In the most recent version of our model code, blanketing by millions of lines from iron-group elements is also included. These models have been widely applied for the spectral analysis of WN stars in the Galaxy and LMC. WN spectra can be well reproduced in most cases. WC stars have not yet been analyzed comprehensively, because the agreement with observations becomes satisfactory only when line-blanketed models are applied. The introduction of inhomogeneities (clumping), although treated in a rough approximation, has significantly improved the fit between synthetic and observed spectra with respect to the electron-scattering wings of strong lines. The mass-loss rates obtained from spectral analyses become smaller by a factor 2-3 if clumping is accounted for. A pre-specified velocity law is adopted for our models, but the radiation pressure can be evaluated from our detailed calculation and can be compared a posteriori with the required wind acceleration. Surprisingly we find that the line-blanketed models are not far from being hydrodynamically consistent, thus indicating that radiation pressure is probably the main driving force for the mass-loss from WR stars.
Spectral analyses of central of planetary nebulae of early WC-type / NGC 6751 and Sanduleak 3
(1997)
To investigate if magnetic fields are present in Wolf-Rayet stars, we selected a few stars in the Galaxy and one in the Large Magellanic Cloud (LMC). We acquired low-resolution spectropolarimetric observations with the European Southern Observatory FORS 2 (FOcal Reducer low dispersion Spectrograph) instrument during two different observing runs. During the first run in visitor mode, we observed the LMC Wolf-Rayet star BAT99 7 and the stars WR 6, WR 7, WR 18, and WR 23 in our Galaxy. The second run in service mode was focused on monitoring the star WR 6. Linear polarization was recorded immediately after the observations of circular polarization. During our visitor observing run, the magnetic field for the cyclically variable star WR 6 was measured at a significance level of 3.3 sigma (< B-z > = 258 +/- 78 G). Among the other targets, the highest value for the longitudinal magnetic field, < B-z > = 327 +/- 141 G, was measured in the LMC star BAT99 7. Spectropolarimetric monitoring of the star WR 6 revealed a sinusoidal nature of the < B-z > variations with the known rotation period of 3.77 d, significantly adding to the confidence in the detection. The presence of the rotation-modulated magnetic variability is also indicated in our frequency periodogram. The reported field magnitude suffers from significant systematic uncertainties at the factor of 2 level, in addition to the quoted statistical uncertainties, owing to the theoretical approach used to characterize it. Linear polarization measurements showed no line effect in the stars, apart from WR 6. BAT99 7, WR 7, and WR 23 do not show variability of the linear polarization over two nights.
Observations and theory suggest that line driven winds from hot stars and luminous accretion disks adopt a unique, critical solution which corresponds to maximum mass loss rate. We analyze the numerical stability of the infinite family of shallow wind solutions, which resemble solar wind breezes, and their transition to the critical wind. Shallow solutions are sub-critical with respect to radiative (or Abbott) waves. These waves can propagate upstream through shallow winds at high speeds. If the waves are not accounted for in the Courant time step, numerical runaway results. The outer boundary condition is equally important for wind stability. Assuming pure outflow conditions, as is done in the literature, triggers runaway of shallow winds to the critical solution or to accretion flow.
The planetary nebula A30 is believed to have undergone a very late thermal pulse resulting in the ejection of knots of hydrogen-poor material. Using multi-epoch Hubble Space Telescope images, we have detected the angular expansion of these knots and derived an age of 850(-150)(+280) yr. To investigate the spectral and spatial properties of the soft X-ray emission detected by ROSAT, we have obtained Chandra and XMM-Newton deep observations of A30. The X-ray emission from A30 can be separated into two components: a point source at the central star and diffuse emission associated with the hydrogen-poor knots and the cloverleaf structure inside the nebular shell. To help us assess the role of the current stellar wind in powering this X-ray emission, we have determined the stellar parameters and wind properties of the central star of A30 using a non-LTE model fit to its optical and UV spectra. The spatial distribution and spectral properties of the diffuse X-ray emission are highly suggestive that it is generated by the post-born-again and present fast stellar winds interacting with the hydrogen-poor ejecta of the born-again event. This emission can be attributed to shock-heated plasma, as the hydrogen-poor knots are ablated by the stellar winds, under which circumstances the efficient mass loading of the present fast stellar wind raises its density and damps its velocity to produce the observed diffuse soft X-rays. Charge transfer reactions between the ions of the stellar winds and material of the born-again ejecta have also been considered as a possible mechanism for the production of diffuse X-ray emission, and upper limits on the expected X-ray production by this mechanism have been derived. The origin of the X-ray emission from the central star of A30 is puzzling: shocks in the present fast stellar wind and photospheric emission can be ruled out, while the development of a new, compact hot bubble confining the fast stellar wind seems implausible.
With a deep Chandra/HETGS exposure of WR 6, we have resolved emission lines whose profiles show that the X-rays originate from a uniformly expanding spherical wind of high X-ray-continuum optical depth. The presence of strong helium-like forbidden lines places the source of X-ray emission at tens to hundreds of stellar radii from the photosphere. Variability was present in X-rays and simultaneous optical photometry, but neither were correlated with the known period of the system or with each other. An enhanced abundance of sodium revealed nuclear-processed material, a quantity related to the evolutionary state of the star. The characterization of the extent and nature of the hot plasma in WR 6 will help to pave the way to a more fundamental theoretical understanding of the winds and evolution of massive stars.
The study of massive stars in different metallicity environments is a central topic of current stellar research. The spectral analysis of massive stars requires adequate model atmospheres. The computation of such models is difficult and time-consuming. Therefore, spectral analyses are greatly facilitated if they can refer to existing grids of models. Here we provide grids of model atmospheres for OB-type stars at metallicities corresponding to the Small and Large Magellanic Clouds, as well as to solar metallicity. In total, the grids comprise 785 individual models. The models were calculated using the state-of-the-art Potsdam Wolf-Rayet (PoWR) model atmosphere code. The parameter domain of the grids was set up using stellar evolution tracks. For all these models, we provide normalized and flux-calibrated spectra, spectral energy distributions, feedback parameters such as ionizing photons, Zanstra temperatures, and photometric magnitudes. The atmospheric structures (the density and temperature stratification) are available as well. All these data are publicly accessible through the PoWR website.
We present new grids of Potsdam Wolf-Rayet (PoWR) model atmospheres for Wolf-Rayet stars of the nitrogen sequence (WN stars). The models have been calculated with the latest version of the PoWR stellar atmosphere code for spherical stellar winds. The WN model atmospheres include the non-LTE solutions of the statistical equations for complex model atoms, as well as the radiative transfer equation in the co-moving frame. Iron-line blanketing is treated with the help of the superlevel approach, while wind inhomogeneities are taken into account via optically thin clumps. Three of our model grids are appropriate for Galactic metallicity. The hydrogen mass fraction of these grids is 50%, 20%, and 0%, thus also covering the hydrogen-rich late-type WR stars that have been discovered in recent years. Three grids are adequate for LMC WN stars and have hydrogen fractions of 40%, 20%, and 0%. Recently, additional grids with SMC metallicity and with 60%, 40%, 20%, and 0% hydrogen have been added. We provide contour plots of the equivalent widths of spectral lines that are usually used for classification and diagnostics.
We discuss the connection between wind overloading and discrete absorption components in P Cygni line profiles from O stars. Overloading can create horizontal plateaus in the radial wind speed that cause the extra absorption in the line profile. The upstream propagation speed of these velocity plateaus is analyzed. The second part of the paper deals with X-ray emission from O stars. X-ray line profiles observed with Chandra and XMM are often symmetric, contrary to what is expected for lines from a homogeneous wind. We discuss the influence on line symmetry of photon escape channels in a strongly clumped wind.
Very massive stars, 100 times heavier than the sun, are rare. It is not yet known whether such stars can form in isolation or only in star clusters. The answer to this question is of fundamental importance. The central region of our Galaxy is ideal for investigating very massive stars and clusters located in the same environment. We used archival infrared images to investigate the surroundings of apparently isolated massive stars presently known in the Galactic Centre (GC). We find that two such isolated massive stars display bow shocks and hence may be 'runaways' from their birthplace. Thus, some isolated massive stars in the GC region might have been born in star clusters known in this region. However, no bow shock is detected around the isolated star WR 102ka (Peony nebula star), which is one of the most massive and luminous stars in the Galaxy. This star is located at the centre of an associated circumstellar nebula. To study whether a star cluster may be 'hidden' in the surroundings of WR 102ka, to obtain new and better spectra of this star, and to measure its radial velocity, we obtained observations with the integral-field spectrograph SINFONI at the ESO's Very Large Telescope. Our observations confirm that WR 102ka is one of the most massive stars in the Galaxy and reveal that this star is not associated with a star cluster. We suggest that WR 102ka has been born in relative isolation, outside of any massive star cluster.
Context. beta Cep-type variables are early B-type stars that are characterized by oscillations observable in their optical light curves. At least one beta Cep-variable also shows periodic variability in X-rays.
Aims. Here we study the X-ray light curves in a sample of beta Cep-variables to investigate how common X-ray pulsations are for this type of stars.
Methods. We searched the Chandra and XMM-Newton X-ray archives and selected stars that were observed by these telescopes for at least three optical pulsational periods. We retrieved and analyzed the X-ray data for kappa Sco, beta Cru, and alpha Vir. The X-ray light curves of these objects were studied to test for their variability and periodicity.
Results. While there is a weak indication for X-ray variability in beta Cru, we find no statistically significant evidence of X-ray pulsations in any of our sample stars. This might be due either to the insufficient data quality or to the physical lack of modulations. New, more sensitive observations should settle this question.
Spectral lines formed in stellar winds from OB stars are observed to exhibit profile variations. Discrete Absorption Components (DACs) show a remarkably slow wavelength drift with time. In a straightforward interpretation, this is in sharp contradiction to the steep velocity law predicted by the radiation-driven wind theory, and by semi- empirical profile fitting. In the present paper we re-discuss the interpretation of the drift rate. We show that the Co- rotating Interaction Region (CIR) model for the formation of DACs does not explain their slow drift rate as a consequence of rotation. On the contrary, the apparent acceleration of a spectral CIR feature is even higher than for the corresponding kinematical model without rotation. However, the observations can be understood by distinguishing between the velocity field of the matter flow, and the velocity law for the motion of the patterns in which the DAC features are formed. If the latter propagate upstream against the matter flow, the resulting wavelength drift mimics a much slower acceleration although the matter is moving fast. Additional to the DACs, a second type of recurrent structures is present in observed OB star spectra, the so-called modulations. In contrast to the DACs, these structures show a steep acceleration compatible with the theoretically predicted velocity law. We see only two possible consistent scenarios. Either, the wind is accelerated fast, and the modulations are formed in advected structures, while the DACs come from structures which are propagating upstream. Or, alternatively, steep and shallow velocity laws may co-exist at the same time in different spatial regions or directions of the wind.
Context. Spectroscopic analysis remains the most common method to derive masses of massive stars, the most fundamental stellar parameter. While binary orbits and stellar pulsations can provide much sharper constraints on the stellar mass, these methods are only rarely applicable to massive stars. Unfortunately, spectroscopic masses of massive stars heavily depend on the detailed physics of model atmospheres.
Aims. We demonstrate the impact of a consistent treatment of the radiative pressure on inferred gravities and spectroscopic masses of massive stars. Specifically, we investigate the contribution of line and continuum transitions to the photospheric radiative pressure. We further explore the effect of model parameters, e.g., abundances, on the deduced spectroscopic mass. Lastly, we compare our results with the plane-parallel TLUSTY code, commonly used for the analysis of massive stars with photospheric spectra.
Methods. We calculate a small set of O-star models with the Potsdam Wolf-Rayet (PoWR) code using different approaches for the quasi-hydrostatic part. These models allow us to quantify the effect of accounting for the radiative pressure consistently. We further use PoWR models to show how the Doppler widths of line profiles and abundances of elements such as iron affect the radiative pressure, and, as a consequence, the derived spectroscopic masses.
Results. Our study implies that errors on the order of a factor of two in the inferred spectroscopic mass are to be expected when neglecting the contribution of line and continuum transitions to the radiative acceleration in the photosphere. Usage of implausible microturbulent velocities, or the neglect of important opacity sources such as Fe, may result in errors of approximately 50% in the spectroscopic mass. A comparison with TLUSTY model atmospheres reveals a very good agreement with PoWR at the limit of low mass-loss rates.
The blue hypergiant Cyg OB2 12 (B3Ia(+)) is a representative member of the class of very massive stars in a poorly understood evolutionary stage. We obtained its high-resolution X-ray spectrum using the Chandra observatory. PoWR model atmospheres were calculated to provide realistic wind opacities and to establish the wind density structure. We find that collisional de-excitation is the dominant mechanism depopulating the metastable upper levels of the forbidden lines of the He-like ions Si XIV and Mg XII. Comparison between the model and observations reveals that X-ray emission is produced in a dense plasma, which could reside only at the photosphere or in a colliding wind zone between binary components. The observed X-ray spectra are well-fitted by thermal plasma models, with average temperatures in excess of 10 MK. The wind speed in Cyg OB2 12 is not high enough to power such high temperatures, but the collision of two winds in a binary system can be sufficient. We used archival data to investigate the X-ray properties of other blue hypergiants. In general, stars of this class are not detected as X-ray sources. We suggest that our new Chandra observations of Cyg OB2 12 can be best explained if Cyg OB2 12 is a colliding wind binary possessing a late O-type companion. This makes Cyg OB2 12 only the second binary system among the 16 known Galactic hypergiants. This low binary fraction indicates that the blue hypergiants are likely products of massive binary evolution during which they either accreted a significant amount of mass or already merged with their companions.
Among the different types of massive stars in advanced evolutionary stages is the enigmatic WN8h type. There are only a few Wolf-Rayet (WR) stars with this spectral type in our Galaxy. It has long been suggested that WN8h-type stars are the products of binary evolution that may harbor neutron stars (NS). One of the most intriguing WN8h stars is the runaway WR 124 surrounded by its magnificent nebula M1-67. We test the presence of an accreting NS companion in WR 124 using similar to 100 ks long observations by the Chandra X-ray observatory. The hard X-ray emission from WR 124 with a luminosity of L-X similar to 10(31) erg s(-1) is marginally detected. We use the non-local thermodynamic equilibrium stellar atmosphere code PoWR to estimate the WR wind opacity to the X-rays. The wind of a WN8-type star is effectively opaque for X-rays, hence the low X-ray luminosity of WR 124 does not rule out the presence of an embedded compact object. We suggest that, in general, high-opacity WR winds could prevent X-ray detections of embedded NS, and be an explanation for the apparent lack of WR+NS systems.
Context:
The first directly detected gravitational waves (GW 150914) were emitted by two coalescing black holes (BHs) with masses of ≈ 36 M⊙ and ≈ 29 M⊙. Several scenarios have been proposed to put this detection into an astrophysical context. The evolution of an isolated massive binary system is among commonly considered models.
Aims:
Various groups have performed detailed binary-evolution calculations that lead to BH merger events. However, the question remains open as to whether binary systems with the predicted properties really exist. The aim of this paper is to help observers to close this gap by providing spectral characteristics of massive binary BH progenitors during a phase where at least one of the companions is still non-degenerate.
Methods:
Stellar evolution models predict fundamental stellar parameters. Using these as input for our stellar atmosphere code (Potsdam Wolf-Rayet), we compute a set of models for selected evolutionary stages of massive merging BH progenitors at different metallicities.
Results:
The synthetic spectra obtained from our atmosphere calculations reveal that progenitors of massive BH merger events start their lives as O2-3V stars that evolve to early-type blue supergiants before they undergo core-collapse during the Wolf-Rayet phase. When the primary has collapsed, the remaining system will appear as a wind-fed high-mass X-ray binary. Based on our atmosphere models, we provide feedback parameters, broad band magnitudes, and spectral templates that should help to identify such binaries in the future.
Conclusions:
While the predicted parameter space for massive BH binary progenitors is partly realized in nature, none of the known massive binaries match our synthetic spectra of massive BH binary progenitors exactly. Comparisons of empirically determined mass-loss rates with those assumed by evolution calculations reveal significant differences. The consideration of the empirical mass-loss rates in evolution calculations will possibly entail a shift of the maximum in the predicted binary-BH merger rate to higher metallicities, that is, more candidates should be expected in our cosmic neighborhood than previously assumed.
Non-LTE models of WR winds
(2000)
We present the analysis of the optical variability of the early, nitrogen-rich Wolf-Rayet (WR) star WR 7. The analysis of multisector Transiting Exoplanet Survey Satellite (TESS) light curves and high-resolution spectroscopic observations confirm multiperiodic variability that is modulated on time-scales of years. We detect a dominant period of 2.6433 +/- 0.0005 d in the TESS sectors 33 and 34 light curves in addition to the previously reported high-frequency features from sector 7. We discuss the plausible mechanisms that may be responsible for such variability in WR 7, including pulsations, binarity, co-rotating interaction regions (CIRs), and clumpy winds. Given the lack of strong evidence for the presence of a stellar or compact companion, we suggest that WR 7 may pulsate in quasi-coherent modes in addition to wind variability likely caused by CIRs on top of stochastic low-frequency variability. WR 7 is certainly a worthy target for future monitoring in both spectroscopy and photometry to sample both the short (less than or similar to 1 d) and long (greater than or similar to 1000 d) variability time-scales.
Massive stars are extremely luminous and drive strong winds, blowing a large part of their matter into the galactic environment before they finally explode as a supernova. Quantitative knowledge of massive star feedback is required to understand our Universe as we see it. Traditionally, massive stars have been studied under the assumption that their winds are homogeneous and stationary, largely relying on the Sobolev approximation. However, Observations with the newest instruments, together with progress in model calculations, ultimately dictate a cardinal change of this paradigm: stellar winds are highly inhomogeneous. Hence, we are now advancing to a new stage in our understanding of stellar winds. Using the foundations laid by V.V. Sobolev and his school, we now update and further develop the stellar spectral analysis techniques. New sophisticated 3-D models of radiation transfer in inhomogeneous expanding media elucidate the physics of stellar winds and improve classical empiric mass-loss rate diagnostics. Applications of these new techniques to multiwavelength observations of massive stars yield consistent and robust stellar wind parameters. (C) 2016 Elsevier Ltd. All rights reserved.
Aims. To close this gap, we perform a comparative analysis of the optical companion in two important systems: IGR J175442619 (SFXT) and Vela X-1 (SGXB). We analyze the spectra of each star in detail and derive their stellar and wind properties. As a next step, we compare the wind parameters, giving us an excellent chance of recognizing key differences between donor winds in SFXTs and SGXBs. Methods. We use archival infrared, optical and ultraviolet observations, and analyze them with the non-local thermodynamic equilibrium (NLTE) Potsdam Wolf-Rayet model atmosphere code. We derive the physical properties of the stars and their stellar winds, accounting for the influence of X-rays on the stellar winds. Results. We find that the stellar parameters derived from the analysis generally agree well with the spectral types of the two donors: O9I (IGR J17544-2619) and B0.5Iae (Vela X-1). The distance to the sources have been revised and also agree well with the estimations already available in the literature. In IGR J17544-2619 we are able to narrow the uncertainty to d = 3.0 +/- 0.2 kpc. From the stellar radius of the donor and its X-ray behavior, the eccentricity of IGR J17544-2619 is constrained to e < 0.25. The derived chemical abundances point to certain mixing during the lifetime of the donors. An important difference between the stellar winds of the two stars is their terminal velocities (v(infinity) = 1500 km s(-1) in IGR J17544-2619 and v(infinity) = 700 km s(-1) in Vela X-1), which have important consequences on the X-ray luminosity of these sources. Conclusions. The donors of IGR J17544-2619 and Vela X-1 have similar spectral types as well as similar parameters that physically characterize them and their spectra. In addition, the orbital parameters of the systems are similar too, with a nearly circular orbit and short orbital period. However, they show moderate differences in their stellar wind velocity and the spin period of their neutron star which has a strong impact on the X-ray luminosity of the sources. This specific combination of wind speed and pulsar spin favors an accretion regime with a persistently high luminosity in Vela X-1, while it favors an inhibiting accretion mechanism in IGR J17544-2619. Our study demonstrates that the relative wind velocity is critical in class determination for the HMXBs hosting a supergiant donor, given that it may shift the accretion mechanism from direct accretion to propeller regimes when combined with other parameters.
Macroclumping in WR 136
(2015)
Macroclumping proved to resolve the discordance between different mass-loss rate diagnostics for O-type stars, in particular between Hα and the P v resonance lines. In this paper, we report first results from a corresponding investigation for WR stars. We apply our detailed 3-D Monte Carlo (MC) line formation code to the P v resonance doublet and show, for the Galactic WNL star WR136, that macroclumping is require to bring this line in accordance with the mass-loss rate derived from the emission-line spectrum.
Context. Metal-poor massive stars are assumed to be progenitors of certain supernovae, gamma-ray bursts, and compact object mergers that might contribute to the early epochs of the Universe with their strong ionizing radiation. However, this assumption remains mainly theoretical because individual spectroscopic observations of such objects have rarely been carried out below the metallicity of the Small Magellanic Cloud. Aims. Here we explore the predictions of the state-of-the-art theories of stellar evolution combined with those of stellar atmospheres about a certain type of metal-poor (0.02 Z(circle dot)) hot massive stars, the chemically homogeneously evolving stars that we call Transparent Wind Ultraviolet INtense (TWUIN) stars. Methods. We computed synthetic spectra corresponding to a broad range in masses (20 130 M-circle dot) and covering several evolutionary phases from the zero-age main-sequence up to the core helium-burning stage. We investigated the influence of mass loss and wind clumping on spectral appearance and classified the spectra according to the Morgan-Keenan (MK) system. Results. We find that TWUIN stars show almost no emission lines during most of their core hydrogen-burning lifetimes. Most metal lines are completely absent, including nitrogen. During their core helium-burning stage, lines switch to emission, and even some metal lines (oxygen and carbon, but still almost no nitrogen) are detected. Mass loss and clumping play a significant role in line formation in later evolutionary phases, particularly during core helium-burning. Most of our spectra are classified as an early-O type giant or supergiant, and we find Wolf-Rayet stars of type WO in the core helium-burning phase. Conclusions. An extremely hot, early-O type star observed in a low-metallicity galaxy could be the result of chemically homogeneous evolution and might therefore be the progenitor of a long-duration gamma-ray burst or a type Ic supernova. TWUIN stars may play an important role in reionizing the Universe because they are hot without showing prominent emission lines during most of their lifetime.
We have obtained time-resolved observations of line-profile variations of the two Wolf-Rayet stars WR 135 and WR 111. The spectra, taken during two consecutive nights, cover a broad range from 4470 to 6590 Ang. The profile variability of the C iii emission line at 5696 Ang in WR 135 is shown in detail. The principal difficulties to constrain the velocity law from the frequency drift of discrete spectral features is discussed, emphasizing the crucial dependence on the adopted location of the line-emission region, and the possible necessity to distinguish between the motion of structures and the flow of the matter. - Full access to the observational data is provided via anonymous file transfer.