L. M. Oskinova

Neglecting the porosity of hot-star winds can lead to underestimating mass-loss rates

Context. The mass-loss rate is a key parameter of massive stars. Adequate stellar atmosphere models are required for spectral analyses and mass-loss determinations. Present models can only account for the inhomogeneity of stellar winds in the approximation of small-scale structures that are optically thin. Compared to previous homogeneous models, this treatment of “microclumping” has led to reducing empirical mass-loss rates by factors of two to three. Further reductions are presently discussed in the literature, with far-reaching consequences e.g. for stellar evolution and stellar yields. Aims. Stellar wind clumps can be optically thick in spectral lines. We investigate how this “macroclumping” influences the radiative transfer and the emergent line spectra and discuss its impact on empirical mass-loss rates. Methods. The Potsdam Wolf-Rayet (PoWR) model atmosphere code is generalized in the “formal integral” to account for clumps that are not necessarily optically thin. The stellar wind is characterized by the filling factor of the dense clumps and by their average separation. An effective opacity is obtained by adopting a statistical distribution of clumps and applied in the radiative transfer. Results. Optically thick clumps reduce the effective opacity. This has a pronounced effect on the emergent spectrum. Our modeling for the O-type supergiant ζ Puppis reveals that the optically thin Hα line is not affected by wind porosity, but that the P v resonance doublet becomes significantly weaker when macroclumping is taken into account. The reported discrepancies between resonance-line and recombination-line diagnostics can be resolved entirely with the macroclumping modeling without downward revision of the mass-loss rate. In the case of Wolf-Rayet stars, we demonstrate for two representative models that stronger lines are typically reduced by a factor of two in intensity, while weak lines remain unchanged by porosity effects. Conclusions. Mass-loss rates inferred from optically thin emission, such as the Hα line in O stars, are not influenced by macroclumping. The strength of optically thick lines, however, is reduced because of the porosity effects. Therefore, neglecting the porosity in stellar wind modeling can lead to underestimating empirical mass-loss rates.

Evolution of X-ray emission from young massive star clusters

The evolution of X-ray emission from young massive star clusters is modelled, taking into account the emission from the stars as well as from the cluster wind. It is shown that the level and character of the soft (0.2‐10 keV) X-ray emission change drastically with cluster age and are tightly linked with stellar evolution. Using the modern X-ray observations of massive stars, we show that the correlation between bolometric and X-ray luminosity known for single O stars also holds for O + O and (Wolf‐Rayet) WR + O binaries. The diffuse emission originates from the cluster wind heated by the kinetic energy of stellar winds and supernova explosions. To model the evolution of the cluster wind, the mass and energy yields from a population synthesis are used as input to a hydrodynamic model. It is shown that in a very young cluster the emission from the cluster wind is low. When the cluster evolves, WR stars are formed. Their strong stellar winds power an increasing X-ray emission of the cluster wind. Subsequent supernova explosions pump the level of diffuse emission even higher. Clusters at this evolutionary stage may have no X-ray-bright stellar point sources, but a relatively high level of diffuse emission. A supernova remnant may become a dominant X-ray source, but only for a short time interval of a few thousand years. We retrieve and analyse Chandra and XMM‐Newton observations of six massive star clusters located in the Large Magellanic Cloud (LMC). Our model reproduces the observed diffuse and point-source emission from these LMC clusters, as well as from the Galactic clusters Arches, Quintuplet and NGC 3603. Ke yw ords: stars: winds, outflows ‐ stars: Wolf‐Rayet ‐ open clusters and associations: general ‐ Magellanic Clouds ‐ X-rays: stars.

AN INTRODUCTION TO THE CHANDRA CARINA COMPLEX PROJECT

The Great Nebula in Carina provides an exceptional view into the violent massive star formation and feedback that typifies giant H II regions and starburst galaxies. We have mapped the Carina star-forming complex in X-rays, using archival Chandra data and a mosaic of 20 new 60 ks pointings using the Chandra X-ray Observatorys Advanced CCD Imaging Spectrometer, as a testbed for understanding recent and ongoing star formation and to probe Carinas regions of bright diffuse X-ray emission. This study has yielded a catalog of properties of > 14,000 X-ray point sources;> 9800 of them have multiwavelength counterparts. Using Chandras unsurpassed X-ray spatial resolution, we have separated these point sources from the extensive, spatially-complex diffuse emission that pervades the region; X-ray properties of this diffuse emission suggest that it traces feedback from Carinas massive stars. In this introductory paper, we motivate the survey design, describe the Chandra observations, and present some simple results, providing a foundation for the 15 papers that follow in this special issue and that present detailed catalogs, methods, and science results.

High resolution X-ray spectroscopy of bright O type stars

Archival X-ray spectra of the four prominent single, non-magnetic O stars ζ Pup, ζ Ori, ξ Per, ζ Oph, obtained in high resolution with Chandra HETGS/MEG have been studied. The resolved X-ray emission line profiles prov ide information about the shocked, hot gas which emits the X-radiation, and about the bulk of comparably cool stellar wind material which partly absorbs this radiation. In the pr esent paper we synthesize X-ray line profiles with a model of a clumpy stellar wind. We find that the geometrical shape of the wind inhomogeneities is important: better agreement with the observations can be achieved with radially compressed clumps than with spherical clumps. The parameters of the model, i.e. chemical abundances, stellar radius, mass-loss rate a nd terminal wind velocity, are taken from existing analyses of UV and optical spectra of the program stars. On this basis we also calculate the continuum absorption coefficient of the cool- wind material, using the Potsdam Wolf-Rayet (POWR) model atmosphere code. The radial location of X-ray emitting gas is restricted from analyzing the fir line ratios of helium-like ions. The only remaining free parameter of our model is the typical distance between the clumps; here we assume that at any point in the wind there is one clump passing by per one dynamical timescale of the wind. The total emission in a model line is scaled to the observation. There is a good agreement between synthetic and observed line profiles. We conclude th at the X-ray emission line profiles in O stars can be explained by hot plasma embedded in a cool wind which is highly clumped in the form of radially compressed shell fragments.

X-ray line emission from a fragmented stellar wind

We discuss X-ray line formation in dense O star winds. A random distribution of wind shocks is assumed to emit X-rays that are partially absorbed by cooler wind gas. The cool gas resides in highly compressed fragments oriented perpendicular to the radial flow direction. For fully opaque fragments, we find that the blueshifted part of X-ray line profiles remains flat-topped even after severe wind attenuation, whereas the red part shows a steep decline. These box- type, blueshifted profiles resemble recent Chandra observations of the O3 star zeta Pup. For partially transparent fragments, the emission lines become similar to those from a homogeneous wind.

Early magnetic B-type stars: X-ray emission and wind properties

We present a comprehensive study of X-ray emission and wind properties of massive magnetic early B-type stars. Dedicated XMM-Newtonobservations were obtained for three early type B-type stars, ξ 1 CMa, V2052 Oph, and ζ Cas with recently discovered magnetic fields. We report the first detection of X-ray emission from V2 052 Oph and ζ Cas. The latter is one the softest X-ray sources among early type stars, while the former is one of the Xray faintest. The observations show that the X-ray spectra of our program stars are quite soft with the bulk of X-ray emitting material having a temperature of about 1 MK. We compile the complete sample of early B-type stars with detected magnetic fields to date and existing X-ray measurements, in order to study whether the X-ray emission can be used as a general proxy for stellar magnetism. We find that the X-ray properties of ea rly massive B-type magnetic stars are diverse, and that hard and strong X-ray emission does not necessarily correlate with the presence of a magnetic field corroborating similar conclusi ons reached earlier for the classical chemically peculiar magnetic Bp-Ap stars. We analyze the UV spectra of five non-supergiant B stars with m agnetic fields ( τ Sco, β Cep, ξ 1 CMa, V2052 Oph, and ζ Cas) by means of non-LTE iron-blanketed model atmospheres. The latter are calculated with the Potsdam Wolf-Rayet (PoWR) code, which treats the photosphere as well as the the wind, and also accounts for X-rays. With the exception of τ Sco, this is the first analysis of these stars by means of stell ar wind models. Our models accurately fit the stellar photospheric spectra in the optical and the UV. The parameters of X-ray emission, temperature and flux are included in the model in ac cordance with observations. We confirm the earlier findings that the filling factors of X-ray e mitting material are very high. Our analysis reveals that the magnetic early type B stars studied here have weak winds with velocities not significantly exceeding vesc. The mass-loss rates inferred from the analysis of UV lines are significantly lower than predicted by hydrody namically consistent models. We find that, although the X-rays strongly affect the ionisatio n structure of the wind, this effect is not sufficient in reducing the total radiative accelerati on. When the X-rays are accounted for at the intensity and temperatures observed, there is sti ll sufficient radiative acceleration to drive stronger mass-loss than we empirically infer from the UV spectral lines.

The Wolf-Rayet stars in the Large Magellanic Cloud - A comprehensive analysis of the WN class

Context. Massive stars, although being important building blocks of galaxies, are still not fully understood. This especially holds true for Wolf-Rayet (WR) stars with their strong mass loss, whose spectral analysis requires adequate model atmospheres. Aims. Following our comprehensive studies of the WR stars in the Milky Way, we now present spectroscopic analyses of almost all known WN stars in the LMC. Methods. For the quantitative analysis of the wind-dominated emission-line spectra, we employ the Potsdam Wolf-Rayet (PoWR) model atmosphere code. By fitting synthetic spectra to the observed spectral energy distribution and the available spectra (ultraviolet and optical), we obtain the physical properties of 107 stars. Results. We present the fundamental stellar and wind parameters for an almost complete sample of WN stars in the LMC. Among those stars that are putatively single, two different groups can be clearly distinguished. While 12% of our sample are more luminous than 10(6) L-circle dot and contain a significant amount of hydrogen, 88% of the WN stars, with little or no hydrogen, populate the luminosity range between log (L/L-circle dot) = 5.3 ... 5.8. Conclusions. While the few extremely luminous stars (log (L/L-circle dot) > 6), if indeed single stars, descended directly from the main sequence at very high initial masses, the bulk of WN stars have gone through the red-supergiant phase. According to their luminosities in the range of log (L/L-circle dot) = 5.3 ... 5.8, these stars originate from initial masses between 20 and 40 M-circle dot. This mass range is similar to the one found in the Galaxy, i.e. the expected metallicity dependence of the evolution is not seen. Current stellar evolution tracks, even when accounting for rotationally induced mixing, still partly fail to reproduce the observed ranges of luminosities and initial masses. Moreover, stellar radii are generally larger and effective temperatures correspondingly lower than predicted from stellar evolution models, probably due to subphotospheric inflation.

The conspicuous absence of X-ray emission from carbon-enriched Wolf-Rayet stars

Received idate?; Accepted idate? Abstract. The carbon-rich WC5star WR114 was not detected during a 15.9 ksec XMM-Newton observation, implying an upper limit to the X-ray luminosity of LX � 2.5 × 10 30 ergs −1 and to the X-ray to bolometric luminosity ratio of LX /Lbol � 4 × 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.

Clumped stellar winds in supergiant high‐mass X‐ray binaries: X‐ray variability and photoionization

ABSTRACT The clumping of massive star winds is an established paradigm confirmed by multiple lines ofevidence and supported by stellar wind theory. The purpose of this paper is to bridge the gapbetween detailed models of inhomogeneous stellar winds in single stars and the phenomeno-logical description of donor winds in supergiant high-massX-ray binaries (HMXBs). We useresults from time-dependent hydrodynamicalmodels of the instability in the line-driven windof a massive supergiant star to derive the time-dependentaccretion rate onto a compact objectin the Bondi-Hoyle-Lyttleton approximation. The strong density and velocity fluctuations inthe wind result in strong variability of the synthetic X-raylight curves. The model predicts alarge scale X-ray variability, up to eight orders of magnitude, on relatively short timescales.The apparent lack of evidence for such strong variability in the observed HMXBs indicatesthat the details of accretion process act to reducethe variability due to the stellar wind velocityand density jumps.We study the absorptionof X-rays in the clumpedstellar windby means of a 2-D stochas-tic wind model. The monochromatic absorption in cool stellar wind in dependence on orbitalphase is computed for realistic stellar wind opacity. We find that absorption of X-rays changesstrongly at different orbital phases. The degree of the variability due to the absorption in thewind depends on the shape of the wind clumps and is stronger in case of oblate clumps.We address the photoionization in the clumped wind, and show that the degree of ioniza-tion is affected by the wind clumping. A correction factor for the photoionization parameteris derived. It is shown that the photoionization parameter is reduced by a factor X comparedto the smooth wind models with the same mass-loss rate, whereXis the wind inhomogeneityparameter. We conclude that wind clumping must also be taken into account when comparingthe observed and model spectra of the photoionized stellar wind.Key words: accretion, X-rays: binaries, instabilities, stars: neutron, X-rays: stars

Macroclumping as solution of the discrepancy between Hα and P v mass loss diagnostics for O-type stars

Context. Recent studies of O-type stars have demonstrated that discrepant mass-loss rates are obtained when different diagnostic methods are employed. Fitting the unsaturated UV resonance lines (e.g., P v) gives drastically lower values than obtained from the Hα emission. Wind inhomogeneity (so-called “clumping”) may be the main cause of this discrepancy. Aims. In a previous paper, we presented 3D Monte-Carlo calculations for the formation of scattering lines in a clumped stellar wind. In the present paper we select five O-type supergiants (from O4 to O7) and test whether the reported discrepancies can be resolved this way. Methods. In the first step, the analyses started with simulating the observed spectra with Potsdam Wolf-Rayet (PoWR) non-LTE model atmospheres. The mass-loss rates are adjusted to fit to the observed Hα emission lines best. For the unsaturated UV resonance lines (i.e., P v) we then applied our 3D Monte-Carlo code, which can account for wind clumps of any optical depths (“macroclumping”), a non-void interclump medium, and a velocity dispersion inside the clumps. The ionization stratifications and underlying photospheric spectra were adopted from the PoWR models. The properties of the wind clumps were constrained by fitting the observed resonance line profiles. Results. Our results show that with the mass-loss rates that fit Hα (and other Balmer and He ii lines), the UV resonance lines (especially the unsaturated doublet of P v) can also be reproduced with no problem when macroclumping is taken into account. There is no need to artificially reduce the mass-loss rates or to assume a subsolar phosphorus abundance or an extremely high clumping factor, unlike what was claimed by other authors. These consistent mass-loss rates are lower by a factor of 1.3 to 2.6, compared to the mass-loss rate recipe from Vink et al. Conclusions. Macroclumping resolves the previously reported discrepancy between Hα and P v mass-loss diagnostics.