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.     

Radio and infrared observations of circumstellar envelopes

This review summarizes the observational achievements at radio and infrared wavelengths since 1984 concerning the circumstellar envelopes of evolved stars. The main emphasis is on our increased knowledge of late stellar evolution, in particular during the asymptotic giant branch phase and the transition to the planetary nebula stage, and on the properties of gas/dust envelopes formed by stellar mass loss. It is not a critical review, and it is essentially free from historical references.     

Stellar and solar abundances

Recent spectroscopic results on stellar and solar abundances are reviewed with special reference to (a) Standard abundance distribution (Sun, hot stars, diffuse nebulae); (b) Abundance peculiarities related to stellar evolution (red giants showing results of H-burning and s-process, peculiar and metallic-lined stars); and (c) Population effects that may be related to the evolution of the Galaxy (correlation between stellar age and metal abundance, differences in details of heavyelement mixture in atmospheric composition of normal stars that have not reached an advanced evolutionary stage).     

The Milky Way 3-Helium Abundance

We are making precise determinations of the abundance of the light isotope of helium, ³He. The ³He abundance in Milky Way sources impacts stellar evolution, chemical evolution, and cosmology. The abundance of ³He is derived from measurements of the hyperfine transition of ³He⁺ which has a rest wavelength of 3.46 cm (8.665 GHz). As with all the light elements, the present interstellar ³He abundance results from a combination of Big Bang Nucleosynthesis (BBNS) and stellar nucleosynthesis. We are measuring the ³He abundance in Milky Way H ii regions and planetary nebulae (PNe). The source sample is currently comprised of 60 H ii regions and 12 PNe. H ii regions are examples of zero-age objects that are young relative to the age of the Galaxy. Therefore their abundances chronicle the results of billions of years of Galactic chemical evolution. PNe probe material that has been ejected from low-mass (M≤ 2M _⊙) to intermediate-mass (M∼2–5M _⊙) stars to be further processed by future stellar generations. Because the Milky Way ISM is optically thin at centimeter wavelengths, our source sample probes a larger volume of the Galactic disk than does any other light element tracer of Galactic chemical evolution. The sources in our sample possess a wide range of physical properties (including object type, size, temperature, excitation, etc.). The ³He abundances we derive have led to what has been called “The ³He Problem”.     

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.     

Origin and Evolution of the Light Nuclides

After a short historical (and highly subjective) introduction to the field, I discuss our current understanding of the origin and evolution of the light nuclides D, ³He, ⁴He, ⁶Li, ⁷Li, ⁹Be, ¹⁰B and ¹¹B. Despite considerable observational and theoretical progress, important uncertainties still persist for each and every one of those nuclides. The present-day abundance of D in the local interstellar medium is currently uncertain, making it difficult to infer the recent chemical evolution of the solar neighborhood. To account for the observed quasi-constancy of ³He abundance from the Big Bang to our days, the stellar production of that nuclide must be negligible; however, the scarce observations of its abundance in planetary nebulae seem to contradict this idea. The observed Be and B evolution as primaries suggests that the source composition of cosmic rays has remained ∼constant since the early days of the Galaxy, a suggestion with far reaching implications for the origin of cosmic rays; however, the main idea proposed to account for that constancy, namely that superbubbles are at the source of cosmic rays, encounters some serious difficulties. The best explanation for the mismatch between primordial Li and the observed “Spite-plateau” in halo stars appears to be depletion of Li in stellar envelopes, by some yet poorly understood mechanism. But this explanation impacts on the level of the recently discovered early “⁶Li plateau”, which (if confirmed), seriously challenges current ideas of cosmic ray nucleosynthesis.     

Self-consistent Simulations of Alfv��n Wave Driven Winds from the Sun and Stars

We review our recent results of Alfvén wave-driven winds. First, we present the result of self-consistent 1D MHD simulations for solar winds from the photosphere to interplanetary region. Here, we emphasize the importance of the reflection of Alfvén waves in the density stratified corona and solar winds. We also introduce the recent Hinode observation that might detect the reflection signature of transverse (Alfvénic) waves by Fujimura and Tsuneta (Astrophys. J. 702:1443, 2009). Then, we show the results of Alfvén wave-driven winds from red giant stars. As a star evolves to the red giant branch, the properties of stellar winds drastically change from steady coronal winds to intermittent chromospheric winds. We also discuss how the stellar evolution affects the wave reflection in the stellar atmosphere and similarities and differences of accretion disk winds by MHD turbulence.     

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.     

Impact of Distance Determinations on Galactic Structure. II. Old Tracers

Here we review the efforts of a number of recent results that use old tracers to understand the build up of the Galaxy. Details that lead directly to using these old tracers to measure distances are discussed. We concentrate on the following: (1) the structure and evolution of the Galactic bulge and inner Galaxy constrained from the dynamics of individual stars residing therein; (2) the spatial structure of the old Galactic bulge through photometric observations of RR Lyrae-type stars; (3) the three-dimensional structure, stellar density, mass, chemical composition, and age of the Milky Way bulge as traced by its old stellar populations; (4) an overview of RR Lyrae stars known in the ultra-faint dwarfs and their relation to the Galactic halo; and (5) different approaches for estimating absolute and relative cluster ages.     

Blue and red supergiants and the age structure of the NGC 330 region

We analysed new UBVRI CCD photometry of the massive, blue SMC cluster NGC 330 and its surrounding field. The age structure and a new reddening value for the stellar population in this region of the SMC are derived and the implications for star formation in this part of the SMC and for stellar evolution are discussed.Based on observations obtained at ESO, La Silla     

The stellar content of the Orion OB1 association

We present a photometric investigation, using the VBLUW system, of the stellar content of Orion OB1. Physical parameters (logg, logT _eff) for the stars are derived with the aid of model atmospheres. From these, visual extinctions, absolute magnitudes and distance moduli are derived. The distance moduli are used to determine membership for the stars in each of the subgroups and distances to the subgroups are calculated. The ages of the subgroups are derived through isochrone fitting and the IMF is derived for each subgroup. The energy deposited into the ISM through stellar winds and supernovae is calculated and compared to observed large scale features in the ISM around Orion OB1.     

Element Abundances in X-ray Emitting Plasmas in Stars

Studies of element abundances in stars are of fundamental interest for their impact in a wide astrophysical context, from our understanding of galactic chemistry and its evolution, to their effect on models of stellar interiors, to the influence of the composition of material in young stellar environments on the planet formation process. We review recent results of studies of abundance properties of X-ray emitting plasmas in stars, ranging from the corona of the Sun and other solar-like stars, to pre-main sequence low-mass stars, and to early-type stars. We discuss the status of our understanding of abundance patterns in stellar X-ray plasmas, and recent advances made possible by accurate diagnostics now accessible thanks to the high resolution X-ray spectroscopy with Chandra and XMM-Newton.     

Big Bang Nucleosynthesis and the Density of Baryons in the Universe

Now that extragalactic deuterium observations are being made, Big Bang Nucleosynthesis (BBN) is on the verge of undergoing a transformation. Previously, the emphasis was on demonstrating the concordance of the Big Bang Nucleosynthesis model with the abundances of the light isotopes extrapolated back to their primordial values using stellar and Galactic evolution theories. Once the primordial deuterium abundance is converged upon, the nature of the field will shift to using the much more precise primordial D/H to constrain the more flexible stellar and Galactic evolution models (although the question of potential systematic error in 4He abundance determinations remains open). The remarkable success of the theory to date in establishing the concordance has led to the very robust conclusion of BBN regarding the baryon density. The BBN constraints on the cosmological baryon density are reviewed and demonstrate that the bulk of the baryons are dark and also that the bulk of the matter in the universe is non-baryonic. Comparison of baryonic density arguments from Lyman- clouds, x-ray gas in clusters, and the microwave anisotropy are made and shown to be consistent with the BBN value.     

Aeronomy of Extra-Solar Giant Planets

The intense stellar UV radiation field incident upon extra-solar giant planets causes profound changes to their upper atmospheres. Upper atmospheric temperatures can be tens of thousands of kelvins, causing thermal dissociation of H₂ to H. The stellar ionizing flux converts H to H⁺. The high temperatures also drive large escape rates of H, but for all but the planets with the smallest orbits, this flux is not large enough to affect planet evolution. The escape rate is large enough to drag off heavier atoms such as C and O. For very small orbits, when the hill sphere is inside the atmosphere, escape is unfettered and can affect planet evolution.     

Formation and evolution of low-mass Algols

We discuss the origin, evolution and fate of low-mass Algols (LMA) that have components with initial masses less than 2.5 M₀. The semi-major axes of orbits of pre-LMA do not exceed 20–25 R₀. The rate of formation of Algol-type stars is ～ 0.01/year. Magnetic stellar winds may be the factor that determines the evolution of LMA. Most LMA end their lives as double helium degenerate dwarfs with M₁/M₂ ～ 0.88 (like L870-2). Some of them even merge through angular momentum loss caused by gravitational waves.     

Reservoir for Comet Material: Circumstellar Grains

Primitive meteorites and interplanetary dust particles contain small quantities of dust grains with highly anomalous isotopic compositions. These grains formed in the winds of evolved stars and in the ejecta of stellar explosions, i.e., they represent a sample of circumstellar grains that can be analyzed with high precision in the laboratory. Such studies have provided a wealth of information on stellar evolution and nucleosynthesis, Galactic chemical evolution, grain growth in stellar environments, interstellar chemistry, and the inventory of stars that contributed dust to the Solar System. Among the identified circumstellar grains in primitive solar system matter are diamond, graphite, silicon carbide, silicon nitride, oxides, and silicates. Circumstellar grains have also been found in cometary matter. To date the available information on circumstellar grains in comets is limited, but extended studies of matter returned by the Stardust mission may help to overcome the existing gaps.     

Stellar coronae—Evidence for their existence from X- and UV observations

Stellar coronae were among the first predicted X-ray sources. Because of their relatively low X-ray luminosities, however, they have been discovered only during the last few years.In the present paper the current state of stellar coronal X- and UV observations has been reviewed, including some preliminary observational results from the HEAO-1 and IUE satellites, but still without any result from the recently launched X-ray satellite HEAO-2.Late 1978 about two dozens of stellar soft X-ray sources have been detected, e.g., normal stars like the Sun (e.g., Cen), very active stars (RS CVn systems), and possibly a corona around an intermediately hot white dwarf (Sirius B).The observational results of various objects have been discussed and compared with X-ray luminosity predictions based on minimum-flux coronal models.     

SN 1987A and SN 1993J: Testing stellar evolution theory?

Nearby supernovae like SN 1987A and SN 1993J provide valuable constraints on the late evolution of massive stars. For this purpose, we review evolutionary models for the progenitor of SN 1987A and confront them with five observational/theoretical tests we devised. We show that single-star models (with the possible exception of rapid-rotation models) fail at least two of these tests, while two binary models (accretion and merger models) are consistent with all available constraints. We conclude that it is most likely that the progenitor of SN 1987A had a binary companion, either at the time of the explosion or at least in the not-too-distant past, and that SN 1987A should therefore not be used to calibrate single stellar evolution theory. For SN 1993J, we infer from the presupernova photometry and the early light curve that its progenitor was a 15M star that lost almost all of its hydrogen-rich envelope prior to the supernova. This seems to require that the progenitor underwent stable case C mass transfer. We discuss future observational tests of binary models for both supernovae.     

Probing the era of galaxy formation via TeV gamma ray absorption by the near infrared extragalactic background

We present models of the extragalactic background light (EBL) based on several scenarios of galaxy formation and evolution. We have treated galaxy formation with the Press-Schecter approximation for both cold dark matter (CDM) and cold+hot dark matter (CHDM) models, representing a moderate (z _f 3) and a late (z _f 1) era of galaxy formation respectively. Galaxy evolution has been treated by considering a variety of stellar types, different initial mass functions and star formation histories, and with an accounting of dust absorption and emission. We find that the dominant factor influencing the EBL is the epoch of galaxy formation. A recently proposed method for observing the EBL utilizing the absorption of 0.1 to 10 TeV gamma-rays from active galactic nuclei (AGN) is shown to be capable of discriminating between different galaxy formation epochs. The one AGN viewed in TeV light, Mrk 421, does show some evidence for a cutoff above 3 TeV; based on the EBL models presented here, we suggest that this is due to extinction in the source. The large absorption predicted at energies > 200 GeV for sources at z > 0.5 indicates that observations of TeV gamma-ray bursts (GRB) would constrain or eliminate models in which the GRB sources lie at cosmological distances.Now at University of Chicago, Dept. of Astronomy & Astrophysics.