Massive Stars: Input Physics and Stellar Models

We present a general overview of the structure and evolution of massive stars of masses ≥12 M _⊙ during their pre-supernova stages. We think it is worth reviewing this topic owing to the crucial role of massive stars in astrophysics, especially in the evolution of galaxies and the universe. We have performed several test computations with the aim to analyze and discuss many physical uncertainties still encountered in massive-star evolution. In particular, we explore the effects of mass loss, convection, rotation, ¹²C(α,γ)¹⁶O reaction and initial metallicity. We also compare and analyze the similarities and differences among various works and ours. Finally, we present useful comments on the nucleosynthesis from massive stars concerning the s-process and the yields for ²⁶Al and ⁶⁰Fe.     

Presupernova evolution of the most massive stars

Taking as example a 60M _⊙ star of solar metallicity, the state of the art of model calculations for very massive, from the main sequence to the supernova stage, is reviewed. It is argued that — due to the simple internal structure of Wolf-Rayet stars — the post main sequence evolutionary phases are currently those which are better understood. A brief discussion of the supernova outcome from very massive stars is given. Then, the more uncertain main sequence evolution is discussed. A first attempt to incorporate results about pulsational instabilities of very massive stars in stellar evolutionary calculations is performed. On its basis, a new type of evolutionary sequence for very massive stars is obtained, namely O-star → Of-star → H-rich WNL → LBV → H-poor WNL → WNE → WC → SN. This scenario is shown to correspond better to many observed properties of very massive stars than the standard one. It includes a model for the prototype LBV P Cygni.     

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.     

Gamma-ray Lines From cr Source Regions

Gamma-ray lines arise from radioactivities produced in nucleosynthesis sites, and from deexcitation of nuclei which have been activated through energetic particle collisions. Since the bulk of nucleosynthesis activity relates to activities inside massive stars, both these processes are related to the likely sources of cosmic rays: Supernova remnants show radioactivity afterglows at time scales which bracket their likely phases of relevance as CR acceleration sites; ²⁶Al radioactivity may trace regions of intense wind interactions from groups of massive stars, and also encode information about the possible injection of matter into CR acceleration environments through interstellar dust grains. The status of -ray line measurements after the Compton Observatory mission is presented, with models and interpretations of current results, and the prospects of upcoming measurements.     

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.     

Evolution of massive close binaries

This paper briefly reviews the competition between massive single star and massive close binary evolution the last two decades. The status of the binary evolutionary model is summarized, the assumptions and simplifications are critically discussed. Using all computations performed since 1970, general conclusions are drawn and a comparison with massive single star evolution is presented. Special attention is given at the assumptions behind the commonly accepted model for the mass gainer and a new accretion model is proposed. The binary results in combinarion with single star evolution are compared with observations of massive stars with emphasis on the HR diagram, star number counts, WR stars, SN 1987A, OBN and OBC stars.     

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.     

The chemical composition of B-supergiant atmospheres

We present new estimates of He/H and CNO abundance ratios in the atmospheres of a selection of B2 supergiants which imply that the C/N ratio in the most luminous Ia stars is close to its equilibrium value. The is also some evidence for more moderate CN abundance anomalies in the B2Ib and B2II supergiants. These results, together with other recent work, imply that the effects of the CNO bi-cycle on the composition of B-supergiant atmospheres are most severe for the more luminous and massive stars. Furthermore, studies of LMC B-supergiants indicate that a small fraction of these very luminous stars are nitrogen weak. This picture is qualitatively consistent with theoretical predictions whenever massive stars perform blue loops in the HR diagram, returning from a red supergiant phase to become core helium burning blue supergiants with atmospheres contaminated by nuclear processed material.     

Cometary dynamics

The modern theory of cometary dynamics is based on Oort's hypothesis that the solar system is surrounded by a spherically symmetric cloud of 10¹¹ to 10¹² comets extending out to interstellar distances. Dynamical modeling and analysis of cometary motion have confirmed the ability of the Oort hypothesis to explain the observed distribution of energies for the long-period comet orbits. The motion of comets in the Oort cloud is controlled by perturbations from random passing stars, interstellar clouds, and the galactic gravitational field. Additionally, comets which enter the planetary region are perturbed by the major planets and by nongravitational forces resulting from jetting of volatiles on the surfaces of the cometary nuclei. The current Oort cloud is estimated to have a radius of 6 to 8 × 10⁴ AU, and to contain some 2 × 10¹² comets with a total mass of 7 to 8 Earth masses. Evidence has begun to accumulate for the existence of a massive inner Oort cloud extending from just beyond the orbit of Neptune to 10⁴ AU or more, with a population up to 100 times that of the outer Oort cloud. This inner cloud may serve as a reservoir to replenish the outer cloud as comets are stripped away by the various perturbers, and may also provide a more efficient source for the short-period comets. Recent suggestions of an unseen solar companion star or a tenth planet orbiting in the inner cloud and causing periodic comet showers on the Earth are likely unfounded. The formation site of the comets in the Oort cloud was likely the extended nebula accretion disc reaching from about 15 to 500 AU from the forming protosun. Comets which escape from the Oort cloud contribute to the flux of interstellar comets, though capture of interstellar comets by the solar system is extremely unlikely. The existence of Oort clouds around other main sequence stars has been suggested by the detection by the IRAS spacecraft of cool dust shells around about 10% of nearby stars.     

Massive stars: Setting the stage

The paper gives a summary of the situation mid-1993 of theory and observations regarding massive stars. I describe: stellar mass loss and its implications, pre-main-sequence evolution, the main sequence, problems of atmospheric instability, Luminous Blue Supergiants, Yellow Hypergiants, Wolf-Rayet stars and supernovae.     

Chemical and photometric evolution of elliptical galaxies

In this paper we present the new chemical-spectro-photometric models of population synthesis by Bressan, Chiosi & Fagotto (1993). The models are specifically designed for elliptical galaxies. They include the presence of dark matter and galactic winds triggered by the energy deposit from supernovae and winds of massive stars. The models are aimed at studying the UV-excess and its dependence on the metallicity, the color-magnitude relation, and the color evolution as a function of the redshift. It is shown that in order to explain the color-magnitude relation as a result of galactic winds, the energy input from massive stars is required. Supernovae alone cannot provide sufficient energy to start galactic wind before the metallicity and hence colors have got saturated. We show that the main source of the UV-excess are the old, hot HB and AGB manque stars of high metallicity present in varying percentages in the stellar content of a galaxy. Since in our model the mean and maximum metallicity are ultimately driven by the mass of the galaxy, this provides a natural explanation for the observed correlation between UV-excess and metallicity. Finally, looking at the color evolution as function of the redshift, we suggest that a sudden change occurring in the color (1550-V) at the onset of the old, hot HB and AGB manque stars of high metallicity, is a good age indicator. The detection of this feature at a certain redshift would impose firm constraints on the underlying cosmological model of the universe.     

The Be stars

Classical Be stars are defined and their relationship to normal B-type stars stated. Spectral classification of the underlying stars suggests that, on the average, Be stars are located 0.5–1.0 magnitude above the main sequence. Struve's rotational model for Be stars, and several tests which support the model, are reviewed. The best evidence at this time suggests that Be stars may not rotate with the critical velocity at which centrifugal force just balances the equatorial gravitational force, but a number of mechanisms for getting material out into the shell have been proposed and are discussed.The physical characteristics of Be shells were first derived from optical observations of shell stars, supplemented more recently by ultraviolet, infrared, radio, and polarization measurements. These data suggest that Be shells are probably lenticular with radii 3 to 20 times the radius of the underlying star, excitation temperatures lower than those of the reversing layers, and electron densities in the range 10¹⁰-10¹³ cm^-3.Variability of Be stars, from spectroscopic, photometric, and polarimetric observations, seems well established over time scales of years and months, but the evidence for night-to-night and hourly changes is somewhat conflicting. Of special interest are recent X-ray observations of several Be stars.Models for the envelopes of Be stars are reviewed, including state-state stellar wind models, time-dependent stellar wind models, the elliptical ring model, disk models, and binary models. Finally, the evolutionary status of Be stars is discussed, and some recommendations for future work made.     

A study of the SMC cluster NGC 330

We analyze the properties of the SMC cluster NGC 330 on the basis of a new CMD and of published effective temperatures and bolometric magnitudes for a smaller sample of stars (Caloi et al 1993, Stothers & Chin 1992a,b) to check for the kind of mixing taking place in massive stars. The existence of a few stars in the so-called HR gap led Stothers & Chin (1993) to claim that only models with semiconvective mixing and OPAL could match the HR diagram, whereas induced Caloi et al (1993) to conclude that neither semiconvective nor overshoot models are suited. The careful analysis of this cluster with new stellar models indicates that 1) models with convective overshoot and models with semiconvection can both explain the gross features of the HR diagram, 2) models with overshoot ought to be preferred.     

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.     

Development of a 1m-normal-incidence-EUV telescope

Astrophysical plasmas at temperatures in the range (0.5–5)×10⁵ K that occur e.g. in interstellar space, in the extended atmospheres around stars of essentially all spectral types, including the numerous late-type stars with low photospheric temperatures, and in the atmospheres of highly evolved stars, can best be studied at extreme ultraviolet wavelengths where they release the bulk of their energy. We report here the current development status of a 1m-normal-incidence-EUV-telescope that will be flown on an ARIES rocket to observe the spectra of nearby stars in the 350 – 700 mm range.     

The evolution of massive stars

The evolution of stars with masses between 15 M ₀ and 100M ₀ is considered. Stars in this mass range lose a considerable fraction of their matter during their evolution.The treatment of convection, semi-convection and the influence of mass loss by stellar winds at different evolutionary phases are analysed as well as the adopted opacities.Evolutionary sequences computed by various groups are examined and compared with observations, and the advanced evolution of a 15M ₀ and a 25M ₀ star from zero-age main sequence (ZAMS) through iron collapse is discussed.The effect of centrifugal forces on stellar wind mass loss and the influence of rotation on evolutionary models is examined. As a consequence of the outflow of matter deeper layers show up and when the mass loss rates are large enough layers with changed composition, due to interior nuclear reactions, appear on the surface.The evolution of massive close binaries as well during the phase of mass loss by stellar wind as during the mass exchange and mass loss phase due to Roche lobe overflow is treated in detail, and the value of the parameters governing mass and angular momentum losses are discussed.The problem of the Wolf-Rayet stars, their origin and the possibilities of their production either as single stars or as massive binaries is examined.Finally, the origin of X-ray binaries is discussed and the scenario for the formation of these objects (starting from massive ZAMS close binaries, through Wolf-Rayet binaries leading to OB-stars with a compact companion after a supernova explosion) is reviewed and completed, including stellar wind mass loss.     

The mass and helium discrepancy in massive stars: The case Vela X-1

A NLTE-analysis is presented of high S/N spectra of the optical component of the standard massive X-ray binary Vela X-1. In combination with the orbital parameters we conclude that the optical star is highly helium enriched and is significantly overluminous compared to standard evolutionary tracks of massive accretion stars. We then propose a new accretion model able to explain these features.     

Orion-2: First scientific results

The ultraviolet spectral images of thousands of faint stars, up to the 13th mag., in the wavelength region of 2000–5000 Å are obtained by means of the space astrophysical observatory Orion-2 aboard the spaceship Soyuz-13. These spectrograms were designed generally for an investigation of the continuous spectra of the stars in ultraviolet. The processing and measurement of part of the material available confirm the expectations for the solution of a large number of problems concerning the physics of stars and stellar atmospheres. Some of the results obtained are included in the present review. Particularly, the observed distribution of continuous energy in the ultraviolet of normal hot stars is in line, according to Orion-2 data, with theoretical prediction; the existence of a new type of high temperature (> 20000K) and low absolute luminosity stars is noticed; the blocking effect of the ultraviolet absorption lines expected for the A-type stars is confirmed; a number of empirical regularities concerning the behaviour of the ultraviolet doublet of ionized magnesium, 2800 Mg ii, in the stellar spectra are derived; the chromosphere in cold stars is detected; the role of a multiplet of ionized titanium, 3080 Ti ii, in stellar spectra is revealed; probably an abnormal silicon-rich stellar envelope around a Be-type star is discovered; a new method for the spectral classification of the stars by their ultraviolet spectral images is developed; a range of interesting facts relating to the structure of the ultraviolet spectra of middle type stars (F-K) come to the fore; an exceptional ultraviolet spectrogram for the planetary nebula II 2149 and its nuclei is obtained; the blocking effect of emission lines in the spectrum of the B-type emission and normal O-type stars has been detected; a remarkably faint (12^itm.6) and high temperature star (No. 1) of strange spectral structure has been discovered.