Massive single star evolution: A comparison with observations

We firstly examine the critical model assumptions for massive stars, in particular regarding mixing, mass loss and metallicity. The comparisons of models and observations for main sequence stars reveal some interesting problems, such as the lack of O-stars close to the zero-age sequence, the so-called helium and mass discrepancies. We emphasize that this last discrepancy was probably due to the unsafe atmosphere modelling used by spectroscopists. The comparisons for supergiants enlighten a number of most interesting problems: the He and CNO abundances in blue supergiants, the distribution of supergiants in the HR diagram and above all the variations of the blue to red number ratios with metallicity. Then, we examine the properties and chemistry of WR stars and the observations and interpretations concerning the great changes of WR numbers in galaxies of different metallicites. Finally, we emphasize the main WR filiations.     

Observations of the atmospheres and winds of O-stars,LBVs and Wolf-Rayet stars

This review summarises recent studies of O-stars, Luminous Blue Variables (LBVs) and Wolf-Rayet (WR) stars, emphasising observations and analyses of their atmospheres and stellar winds yielding determinations of their physical and chemical properties. Studies of these stellar groups provide important tests of both stellar wind theory and stellar evolution models incorporating mass-loss effects. Quantitative analyses of O-star spectra reveal enhanced helium abundances in Of and many luminous O-supergiants, together with CNO anomalies in OBN and Ofpe/WN9 stars, indicative of evolved objects. Enhanced helium, and CNO-cycle products are observed in several LBVs, implying a highly evolved status, whilst for the WR stars there is strong evidence for the exposition of CNO-cycle products in WN stars, and helium-burning products in WC and WO stars. The observed wind properties and mass-loss rates derived for O-stars show, in general terms, good agreement with predictions from the latest radiation-driven wind models, although some discrepancies are apparent. Several LBVs show similar mass-loss rates at maximum and minimum states, contrary to previous expectations, with the mechanism responsible for the variability and outbursts remaining unclear. WR stars exhibit the most extreme levels of mass-loss and stellar wind momenta. Whilst alternative mass-loss mechanisms have been proposed, recent calculations indicate that radiation pressure alone may be sufficient, given the strong ionization stratification present in their winds.     

Nucleosynthesis in massive stars

We discuss three aspects of the nucleosynthesis in massive and intermediate-mass stars during their early evolutionary phases. These are related to the CNO abundances in giant or supergiant stars, to the²⁶Al yield from massive stars via stellar wind, and to the production of the s-process nuclei in massive stars.     

Wind asymmetries in massive stars

We are in the process of surveying the linear polarization in luminous, early-type stars. We here report on new observations of the B [e] stars S 18 and R 50, and of the Luminous Blue Variables HR Car, R 143, and HD 160529. Together with previously published data, these observations provide clear evidence for the presence of intrinsic polarization in 1 B[e] star (HD 34664) and in 5 LBVs ( Car, P Cyg, R 127, AG Car, and HR Car). The data indicate that anisotropic stellar winds are a common occurrence among massive stars in these particular evolutionary stages. For such stars, mass-loss rates estimated using the assumption of a spherical, homogeneous and stationary outflow may be in error.     

Massive star distribution in external galaxies and starburst regions

Counts of hot and luminous stars in a number of associations in the Galaxy and Magellanic Clouds enable one to directly investigate the numbers and types of massive stars. There seems to be little, if any, dependence of the slope of the Intial Mass Function, or theM _upper on the initial composition of the stars. Indirect estimates of numbers of massive stars in various more distant environments are reviewed and discussed within a framework of acalibration of the methods using the stellar census of 30 Doradus. Very young starbursts, containing large numbers of massive stars, seem to be composed of smaller sub-units similar or somewhat larger than that object. These units might be newly born globular clusters.     

A comparison between observed and predicted mass-loss rates and wind momentum of O stars

Empirical mass-loss rates were derived for 28 luminous O stars from radio fluxes and H equivalent widths. Comparison with theoretical values predicted by the theory of radiatively driven winds reveals a discrepancy of 0.30±0.05 dex, with the theoretical values being too low. We show that there is not only a mass-loss discrepancy but also a momentum flux discrepancy. The theoretically predicted momentum fluxes are too low by 0.17±0.04 dex. This discrepancy is independent of the adopted stellar mass. We demonstrate that the momentum discrepancy in the most luminous O stars is comparable to the one found in the least extreme Wolf-Rayet stars. We suggest that the physical reason for the break-down of the theory in Wolf-Rayet stars and the most luminous O stars may be related.     

A spectroscopic analysis of B stars in the SMC cluster NGC 330

Medium resolution (2A/px) but high s/n spectra of approximately twenty of the brightest blue stars in the young open cluster NGC 330 in the SMC have been analyzed in order to determine their atmospheric parameters and the evolutionary status. Stellar parameters are determined by comparison with LTE and NLTE model atmosphere calculations and an HR diagram constructed. Luminosities of the sample stars lie in the range 4.0L _*/L )<5.0 and spectral types between O9 and late-B. The stars in our sample appear to define 4 groups: main-sequence B-stars (B2-B4), B-supergiants (B4) in a blue-loop phase of evolution, a small number of blue stragglers (O9-B0 near main-sequence stars) and a group of luminous giants (B1-B2) which reside in the so-called post main-sequence gap of the HR diagram. Furthermore, we have confirmed spectroscopically the very high incidence of Be stars in this cluster. Finally the almost complete absence of metal lines (at this resolution) is in keeping with the expected very low metallicity of the SMC.     

Spectral analyses of Wolf-Rayet stars: Theory,results, conclusions

Stratified Non-LTE models for expanding atmospheres became available in the recent years. They are based on the idealizing assumptions of spherical symmetry, stationarity and radiative equilibrium. From a critical discussion we conclude that this standard model is basically adequate for describing real Wolf-Rayet atmospheres and hence can be applied for quantitative spectral analyses of their spectra.By means of these models, the fundamental parameters have been determined meanwhile for the majority of the known Galactic WR stars. Most of them populate a vertical strip in the Herzsprung-Russell diagram at effective temperatures of 35 kK, the luminosities ranging from 10^4.5 to 10^5.9 L . Only early-type WN stars with strong lines and WC stars are hotter. The chemical composition of WR atmospheres corresponds to nuclear-processed material (WN: hydrogen burning in the CNO cycle; WC: helium burning). Hydrogen is depleted but still detectable in the cooler part of the WN subclass.Different scenarios for the evolutionary formation of the Wolf-Rayet stars are discussed in the light of the empirical data provided from the spectral analyses. Post-red-supergiant evolution can principally explain the basic observational properties, except the rather low luminosities of a considerable fraction of WN stars. Among the alternative scenarios, close-binary evolution can theoretically produce the least-luminous WN stars. However, final conclusions about the evolutionary formation of the WR stars are not yet possible.     

The population of massive stars in R136 from HST/FOC UV observations

New ultraviolet (1300 A, 3400 A),HST FOC observations have been used to derive the UV color-magnitude diagram (CMD) of R136, with the main scientific goal of studying the upper end of the stellar mass function at ultraviolet wavelengths where the color degeneracy encountered in visual CMDs is less severe. The CMD has been compared to a set of theoretical isochrones, which have been computed using the latest generation of evolutionary models and model atmospheres for early type stars. Wolf-Rayet stars are included. Comparison of theTheoretical andobserved CMD suggests that there are no stars brighter than M₁₃₀–11. We use the observed main sequence turn-off and the known spectroscopic properties of the stellar population to derive constraints on the most probable age of R136. The presence of WNL stars and the lack of red supergiants suggests a most likely age of 3±1 Myr. A theoretical isochrone of 3±1 Myr is consistent with the observed stellar content of R136 if the most massive stars have initial masses around 50 M.Bases on Observations with the NASA/ESA Hubble Space Telescope, obtained at the STScI, which is operated by AURA, Inc., under NASA contract NAS5-26555.Astrophysics Division, Space Science Department, ESA     

A. M. Hubert and C. Jaschek (eds.), B[e] Stars

The B[e] stars are early type stars with hydrogen emission lines, forbidden [FeII] and [OI] emission lines, and with an IR excess due to circumstellar dust. These properties may occur in stars of quite different evolutionary stages. In fact, the group of B[e] stars is very inhomogeneous, and contains pre-main sequence stars, supergiants with disks, compact planetary nebulae, symbiotic stars, and a group of stars with unclear evolutionary phase. The book gives the proceedings of a workshop in Paris in 1997 in which the properties and evolutionary phases of the B[e] stars are discussed. It contains chapters on: (1) the definition of B[e] stars, (2) distances, kinematics and the distribution in our Galaxy, (3) spectroscopy, (4) infrared properties, (5) photometry, polarimetry and variability, (6) models for winds and disks, (7) evolutionary stages, (8) revised classification of B[e] stars. The book ends with an object list of all B[e] stars. The book is very useful for students and researchers of hot star winds and gives nice overviews of the observations and theories and remaining puzzles of these strange objects with winds and outflowing dust-forming disks. This revised version was published online in August 2006 with corrections to the Cover Date.     

The atmospheric composition,extinction and luminosity of the LBV R71

ESO 3.6m Caspec spectra of the LMC luminous blue variable (LBV) taken at minimum have been analysed using NLTE model atmospheres and line formation calculations to derive atmospheric parameters and chemical composition. Using the silicon ionization balance and the hydrogen Balmer lines we deriveT _eff =17250, log g=1.80 and a microturbulent velocity of 15–20 km/s. The analysis yields abundance ratios by number of approximately 0.43 for He/H, 0.03 for C/N and 0.14 for O/N, implying that enrichment of the atmosphere by processed material has taken place. We have re-evaluated the reddening of R71 using IUE low resolution data and published UBVRIJHKL photometry and derive a value for A _V of 0.63. We also construct an extinction curve using archive IUE data for mid-B LMC supergiants and show that the extinction is anomalous; the 2175A bump being almost absent and the far UV rise very pronounced. A comparison of our model flux in theV-band with the observed (dereddened)V magnitude and the D.M. of the LMC (18.45), implies that the bolometric magnitude or R71 is –9.9. This is significantly higher than the value of –9.0 usually adopted for R71 and suggests that this object may not in fact be a subluminous LBV.     

P Cygni: An Extraordinary Luminous Blue Variable

P Cygni is a prototype for understanding mass loss from massive stars. This textbook star is known first of all because of two great eruptions in the 17th century. In the first half of this century it has given its name to a class of stars which are characterized by spectral lines consisting of nearly undisplaced emissions accompanied by a blue-displaced absorption component. This characteristic P Cygni-type profile betrays the presence of a stellar wind, but P Cygni's wind is quite unlike that of other hot supergiants. P Cygni was the first star that showed the effects of stellar evoluton from a study of its photometric history. It shares some common properties with the so-called Luminous Blue Variables. However, P Cygni is a unique object. This review deals with P Cygni's photometric properties, its circumstellar environment - including infrared and radio observations - and its optical and ultraviolet spectrum. Smaller sections deal with P Cygni's wind structure and evolution. This revised version was published online in August 2006 with corrections to the Cover Date.     

Intrinsic parameters of massive OB stars

We present the results of our observations of stars of type O5 and earlier and show that inclusion of the line blocking between 228 and 912 A solves the problem found by Herrero et al. (1992) in the determination of their stellar parameters. We study the influence of the line blocking and other effects on the mass and helium discrepancies and show that the first one is reduced by the use of spherical, non hydrostatic model atmospheres and that the second one is probably due to exposure of CNO material.The INT is operated on the island of La Palma by the RGO in the Spanish Observatorio de El Roque de los Muchachos of the Instituto de Astrofsica de Canarias     

Double-zone model with diffusive mixing and the mass and helium discrepancies in OB-stars

A model for massive main sequence (MS) stars is proposed that quantitatively accounts for the mass and helium discrepancies in luminous OB stars. The radiative envelope of the model consists of two zones being mixed by rotationally induced turbulent diffusion during the star's evolution on the MS. The rate of the mixing in the outer zone is assumed to be substantially lower than that in the inner zone. Both, the mass and helium discrepancy, are shown to be due to helium enrichment in the envelope produced by turbulent diffusion. Some arguments to support this double-zone stellar model are given.Alexander von Humboldt Fellow     

Pulsations of the R Coronae Borealis stars

The radial pulsations of very luminous, low-mass models (L/M 10⁴, solar units), which are possible representatives of the R CrB stars, have been examined. These pulsations are extremely nonadiabatic. We find that there are in some cases at least one extra (strange) mode which makes interpretation difficult. The blue instability edges are also peculiar, in that there is an abrupt excursion of the blue edge to the blue for L/M sufficiently large. The range of periods of the model encompasses observed periods of the Cepheid-like pulsations of actual R CrB stars.     

Observational Appearance of Black Holes in X-Ray Binaries and AGN

Accretion onto black holes powers most luminous compact sources in the Universe. Black holes are found with masses extending over an extraordinary broad dynamic range, from several to a few billion times the mass of the Sun. Depending on their position on the mass scale, they may manifest themselves as X-ray binaries or active galactic nuclei. X-ray binaries harbor stellar mass black holes—endpoints of the evolution of massive stars. They have been studied by X-ray astronomy since its inception in the early 60-ies, however, the enigma of the most luminous of them—ultra-luminous X-ray sources, still remains unsolved. Supermassive black holes, lurking at the centers of galaxies, are up to hundreds of millions times more massive and give rise to the wide variety of different phenomena collectively termed “Active Galactic Nuclei”. The most luminous of them reach the Eddington luminosity limit for a few billions solar masses object and are found at redshifts as high as z≥5–7. Accretion onto supermassive black holes in AGN and stellar- and (possibly) intermediate mass black holes in X-ray binaries and ultra-luminous X-ray sources in star-forming galaxies can explain most, if not all, of the observed brightness of the cosmic X-ray background radiation. Despite the vast difference in the mass scale, accretion in X-ray binaries and AGN is governed by the same physical laws, so a degree of quantitative analogy among them is expected. Indeed, all luminous black holes are successfully described by the standard Shakura-Sunyaev theory of accretion disks, while the output of low-luminosity accreting black holes in the form of mechanical and radiative power of the associated jets obeys to a unified scaling relation, termed as the “fundamental plane of black holes”. From that standpoint, in this review we discuss formation of radiation in X-ray binaries and AGN, emphasizing their main similarities and differences, and examine our current knowledge of the demographics of stellar mass and supermassive black holes.     

Towards a Unified View of Inhomogeneous Stellar Winds in Isolated Supergiant Stars and Supergiant High Mass X-Ray Binaries

Massive stars, at least \(\sim10\) times more massive than the Sun, have two key properties that make them the main drivers of evolution of star clusters, galaxies, and the Universe as a whole. On the one hand, the outer layers of massive stars are so hot that they produce most of the ionizing ultraviolet radiation of galaxies; in fact, the first massive stars helped to re-ionize the Universe after its Dark Ages. Another important property of massive stars are the strong stellar winds and outflows they produce. This mass loss, and finally the explosion of a massive star as a supernova or a gamma-ray burst, provide a significant input of mechanical and radiative energy into the interstellar space. These two properties together make massive stars one of the most important cosmic engines: they trigger the star formation and enrich the interstellar medium with heavy elements, that ultimately leads to formation of Earth-like rocky planets and the development of complex life. The study of massive star winds is thus a truly multidisciplinary field and has a wide impact on different areas of astronomy.In recent years observational and theoretical evidences have been growing that these winds are not smooth and homogeneous as previously assumed, but rather populated by dense “clumps”. The presence of these structures dramatically affects the mass loss rates derived from the study of stellar winds. Clump properties in isolated stars are nowadays inferred mostly through indirect methods (i.e., spectroscopic observations of line profiles in various wavelength regimes, and their analysis based on tailored, inhomogeneous wind models). The limited characterization of the clump physical properties (mass, size) obtained so far have led to large uncertainties in the mass loss rates from massive stars. Such uncertainties limit our understanding of the role of massive star winds in galactic and cosmic evolution.Supergiant high mass X-ray binaries (SgXBs) are among the brightest X-ray sources in the sky. A large number of them consist of a neutron star accreting from the wind of a massive companion and producing a powerful X-ray source. The characteristics of the stellar wind together with the complex interactions between the compact object and the donor star determine the observed X-ray output from all these systems. Consequently, the use of SgXBs for studies of massive stars is only possible when the physics of the stellar winds, the compact objects, and accretion mechanisms are combined together and confronted with observations.This detailed review summarises the current knowledge on the theory and observations of winds from massive stars, as well as on observations and accretion processes in wind-fed high mass X-ray binaries. The aim is to combine in the near future all available theoretical diagnostics and observational measurements to achieve a unified picture of massive star winds in isolated objects and in binary systems.     

Magnetic Fields in Massive Stars, Their Winds, and Their Nebulae

Massive stars are crucial building blocks of galaxies and the universe, as production sites of heavy elements and as stirring agents and energy providers through stellar winds and supernovae. The field of magnetic massive stars has seen tremendous progress in recent years. Different perspectives—ranging from direct field measurements over dynamo theory and stellar evolution to colliding winds and the stellar environment—fruitfully combine into a most interesting and still evolving overall picture, which we attempt to review here. Zeeman signatures leave no doubt that at least some O- and early B-type stars have a surface magnetic field. Indirect evidence, especially non-thermal radio emission from colliding winds, suggests many more. The emerging picture for massive stars shows similarities with results from intermediate mass stars, for which much more data are available. Observations are often compatible with a dipole or low order multi-pole field of about 1 kG (O-stars) or 300 G to 30?kG (Ap/Bp stars). Weak and unordered fields have been detected in the O-star ζ Ori A and in Vega, the first normal A-type star with a magnetic field. Theory offers essentially two explanations for the origin of the observed surface fields: fossil fields, particularly for strong and ordered fields, or different dynamo mechanisms, preferentially for less ordered fields. Numerical simulations yield the first concrete stable (fossil) field configuration, but give contradictory results as to whether dynamo action in the radiative envelope of massive main sequence stars is possible. Internal magnetic fields, which may not even show up at the stellar surface, affect stellar evolution as they lead to a more uniform rotation, with more slowly rotating cores and faster surface rotation. Surface metallicities may become enhanced, thus affecting the mass-loss rates.     

Galactic Evolution of the Light Elements: A New Set of B Observations

The boron 2500 spectral region has been observed with the Goddard High Resolution Spectrograph (GHRS) of the Hubble Space Telescope (HST) in a new set of metal-poor stars and analyzed by spectrum synthesis technique, adopting the most recent model atmospheres. By taking into account the Li and Be abundances available from the literature for this same set of objects, the resulting patterns of their light elements abundances cannot be easily justified with the currently known stellar structure scenarios. The finding of real differences in the B content between stars with very similar stellar characteristics suggest that also production effects, rather than depletion and/or mixing only, should be taken into account as a possible and valuable explanation.