Norbert Langer

How Massive Single Stars End Their Life

How massive stars die—what sort of explosion and remnant each produces—depends chiefly on the masses of their helium cores and hydrogen envelopes at death. For single stars, stellar winds are the only means of mass loss, and these are a function of the metallicity of the star. We discuss how metallicity, and a simplified prescription for its effect on mass loss, affects the evolution and final fate of massive stars. We map, as a function of mass and metallicity, where black holes and neutron stars are likely to form and where different types of supernovae are produced. Integrating over an initial mass function, we derive the relative populations as a function of metallicity. Provided that single stars rotate rapidly enough at death, we speculate on stellar populations that might produce gamma-ray bursts and jet-driven supernovae.

Presupernova Evolution of Rotating Massive Stars. I. Numerical Method and Evolution of the Internal Stellar Structure

The evolution of rotating stars with zero-age main-sequence (ZAMS) masses in the range 8-25 M☉ is followed through all stages of stable evolution. The initial angular momentum is chosen such that the stars equatorial rotational velocity on the ZAMS ranges from zero to ~ 70% of breakup. The stars rotate rigidly on the ZAMS as a consequence of angular momentum redistribution during the pre-main-sequence evolution. Redistribution of angular momentum and chemical species are then followed as a consequence of Eddington-Sweet circulation, Solberg-Hoiland instability, the Goldreich-Schubert-Fricke instability, and secular and dynamic shear instability. The effects of the centrifugal force on the stellar structure are included. Convectively unstable zones are assumed to tend toward rigid rotation, and uncertain mixing efficiencies are gauged by observations. We find, as noted in previous work, that rotation increases the helium core masses and enriches the stellar envelopes with products of hydrogen burning. We determine, for the first time, the angular momentum distribution in typical presupernova stars along with their detailed chemical structure. Angular momentum loss due to (nonmagnetic) stellar winds and the redistribution of angular momentum during core hydrogen burning are of crucial importance for the specific angular momentum of the core. Neglecting magnetic fields, we find angular momentum transport from the core to the envelope to be unimportant after core helium burning. We obtain specific angular momenta for the iron core and overlying material of 1016-1017 cm2 s-1. These values are insensitive to the initial angular momentum and to uncertainties in the efficiencies of rotational mixing. They are small enough to avoid triaxial deformations of the iron core before it collapses, but could lead to neutron stars which rotate close to breakup. They are also in the range required for the collapsar model of gamma-ray bursts. The apparent discrepancy with the measured rotation rates of young pulsars is discussed.

Formation of high mass x-ray black hole binaries

Abstract The discrepancy in the past years of many more black-hole soft X-ray transients (SXTs), of which a dozen have now been identified, had challenged accepted wisdom in black hole evolution. Reconstruction in the literature of high-mass X-ray binaries has required stars of up to ∼40 M ⊙ to evolve into low-mass compact objects, setting this mass as the limit often used for black hole formation in population syntheses. On the other hand, the sheer number of inferred SXTs requires that many, if not most, stars of ZAMS masses 20–35 M ⊙ end up as black holes ( Portegies Zwart et al., 1997 , Ergma and van den Heuvel, 1998 ). In this paper we show that this can be understood by challenging the accepted wisdom that the result of helium core burning in a massive star is independent of whether the core is covered by a hydrogen envelope, or ‘naked’ while it burns. The latter case occurs in binaries when the envelope of the more massive star is transferred to the companion by Roche Lobe overflow while in either main sequence or red giant stage. For solar metallicity, whereas the helium cores which burn while naked essentially never go into high-mass black holes, those that burn while clothed do so, beginning at ZAMS mass ∼20 M ⊙ , the precise mass depending on the 12C(α,γ)16O rate as we outline. In this way the SXTs can be evolved, provided that the H envelope of the massive star is removed only following the He core burning. Whereas this scenario was already outlined in 1998 by Brown et al. [NewA 4 (1999) 313], their work was based on evolutionary calculations of Woosley et al. [ApJ 448 (1995) 315] which employed wind loss rates which were too high. In this article we collect results for lower, more correct wind loss rates, finding that these change the results only little. We go into the details of carbon burning in order to reconstruct why the low Fe core masses from naked He stars are relatively insensitive to wind loss rate. The main reason is that without the helium produced by burning the hydrogen envelope, which is convected to the carbon in a clothed star, a central 12C abundance of ∼1/3 remains unburned in a naked star following He core burning. The later convective burning through 12C+12C reactions occurs at a temperature T∼80 keV. Finally, we show that in order to evolve a black hole of mass ≳10 M ⊙ such as observed in Cyg X-1 , even employing extremely massive progenitors of ZAMS mass ≳60 M ⊙ for the black hole, the core must be covered by hydrogen during a substantial fraction of the core burning. In other words, the progenitor must be a WNL star. We evolve Cyg X-1 in an analogous way to which the SXTs are evolved, the difference being that the companion in Cyg X-1 is more massive than those in the SXTs, so that Cyg X-1 shines continuously.

O Stars in Transition. II. Fundamental Properties and Evolutionary Status of Ofpe/WN9 Stars from HST Ultraviolet Observations*

We present new HST/FOS ultraviolet spectroscopic observations of seven LMC Ofpe/WN9 stars. We find that Ofpe/WN9 stars have slow winds with terminal velocities of about 400 km s-1 and high mass-loss rates of the order of 2-5 × 10-5 M☉ yr-1. Ofpe/WN9 stellar temperatures and radii are in the range 30,000-39,000 K, and 19-39 R☉, respectively. Stellar luminosities are between log (L/L☉) = 5.6 and 6.3. We study the Ofpe/WN9 stars winds and examine their evolutionary status. We find that Ofpe/WN9 stars are intermediate between O and W-R stars in terms of the wind momentum flux. We also find that the stellar properties and wind momentum of the Ofpe/WN9 sample place them in the evolutionary sequence: O → Of → H-rich WNL → Ofpe/WN9, for initial stellar masses less than ~100 M☉. In view of persisting discrepancies of standard massive star models with observations, we compute massive main-sequence models according to three different evolutionary scenarios. We find that both higher mass-loss rate and enhanced mixing between core and envelope are required in order to yield models compatible with the derived stellar and wind properties of Ofpe/WN9 stars. The emerging picture may be consistent with earlier evidence of Ofpe/WN9 stars being quiescent luminous blue variables (LBVs). This idea is further strengthened by the highly reduced surface H mass fractions of the Ofpe/WN9 stars. We derive Xs = 0.5 to 0.3, which still excludes Ofpe/WN9 stars from being core He-burning objects, but is almost identical to the Xs values recently measured in LBVs.

Nucleosynthesis in rotating massive stars

Abstract Observational evidence for rotationally induced mixing in massive stars is summarized. From these observations and the models required to explain them, we conclude that rotation will increase the primary metal yields of massive stars, enhance the production of H-burning secondary products (e.g. 14 N and 26 Al), and reduce the initial stellar mass limit for Type II supernova explosions. For the first time, these features are described quantitatively in the context of new evolutionary models for mass losing, rotating stars. These calculations include the effects of the centrifugal force on the structure as well as angular momentum transport and chemical element diffusion. The chemical yields of these models are presented and compared to those of other models evolved without rotation. Our models also indicate the presence of qualitatively new nucleosynthesis channels which may result in primary 14 N production in the H-burning shell and primary neutron processing in the He-burning shell of rotating stars. Implications for the supernova explosion and neutron star remnant are briefly described.

Progenitors of gravitational wave mergers: binary evolution with the stellar grid-based code ComBinE

The first gravitational wave detections of mergers between black holes and neutron stars represent a remarkable new regime of high-energy transient astrophysics. The signals observed with LIGO-Virgo detectors come from mergers of extreme physical objects which are the end products of stellar evolution in close binary systems. To better understand their origin and merger rates, we have performed binary population syntheses at different metallicities using the new grid-based binary population synthesis code ComBinE. Starting from newborn pairs of stars, we follow their evolution including mass loss, mass transfer and accretion, common envelopes and supernova explosions. We apply the binding energies of common envelopes based on dense grids of detailed stellar structure models, make use of improved investigations of the subsequent Case BB Roche-lobe overflow and scale supernova kicks according to the stripping of the exploding stars. We demonstrate that all the double black hole mergers, GW150914, LVT151012, GW151226, GW170104, GW170608 and GW170814, as well as the double neutron star merger GW170817, are accounted for in our models in the appropriate metallicity regime. Our binary interaction parameters are calibrated to match the accurately determined properties of Galactic double neutron star systems, and we discuss their masses and types of supernova origin. Using our default values for the input physics parameters, we find a double neutron star merger rate of about 3.0 Myr-1 for Milky-Way equivalent galaxies. Our upper limit to the merger-rate density of double neutron stars is R≃400 yr-1 Gpc-3 in the local Universe (z=0).

On Flamsteed's supernova Cas A

Abstract The Galactic supernova remnant Cas A is believed to be the result of a recent (∼ 300 yrs) and nearby (D ∼ 3 kpc) explosion of a massive star. The discovery of gamma-ray line emission due to radioactive 44 Ti with COMPTEL supports the massive star origin of Cas A. However, the ejection of 44 Ti should be accompanied by large amounts of 56 Ni and 57 Ni, which would have made this a very bright supernova. No 17th century sightings of this event exist, with the notable exception of the ∼ 6th mag star 3 Cas reported by Sir John Flamsteed. This mystery may be solved by assuming that the supernova was enshrouded by a dusty envelope, perhaps provided by a pre-supernova wind. X-ray observations with ROSAT provide evidence for this scenario through the simultaneous analysis of absorption along the line of sight and the extended X-ray scattering halo around Cas A. The supernova shock destroyed the dust, reducing the line-of-sight opacity to a value consistent with a 3 kpc path through an average ISM. ROSAT detects the evaporated dust as excess absorption.

The pre-supernova evolution of rotating massive stars

Massive stars are born rotating rigidly with a significant fraction of critical rotation at the surface. Consequently, rotationally-induced circulation and instabilities lead to chemical mixing in regions that would otherwise be stable, as well as a redistribution of angular momentum. Differential rotation also winds up magnetic fields, causing instabilities that can power a dynamo and magnetic stresses that lead to additional angular momentum transport. We follow the evolution of typical massive stars, their structure and angular momentum distribution, from the zero-age main sequence until iron core collapse. Without the action of magnetic fields, the resulting angular momentum is sufficiently large to significantly affect the explosion mechanism and neutron star formation. Sub-millisecond pulsars result that could encounter the r-mode instability. In helium cores massive enough, at least at low metalicity, the angular momentum is also sufficiently great to form a centrifugally supported accretion disk around a central black hole, powering the engine of the collapsar model for GRBs. Including current estimates of the effect of magnetic fields still allows the formation of rapidly rotating (fV 5 10 IDS) pulsars, but might leave too little angular momentum for collapsars.

Interferometric Studies of Late Phases of Stellar Evolution

Objects in late evolutionary phases are very interesting targets for VLTI observations. We discuss Mira stars, carbon stars, LBVs, and Proto-Planetary Nebulae. The resolution of VLTI observations is high enough to resolve the stellar disks of many Mira stars, to reveal surface inhomogeneities, to study the strong wavelength dependence of Mira star diameters, and to study circumstellar shells and bipolar outflows of stars in late evolutionary phases. For illustration of the feasibility of the above VLTI projects we show previous speckle masking observations with the 2.2 m ESO telescope and the 6 m SAO telescope. Fortunately, most of the objects mentioned above are much brighter than the limiting magnitude of VLTI observations (which is at least about 15th magnitude in the visible and 12th magnitude in the K-band). Multi-speckle long-baseline interferometry techniques (modified speckle masking combined with the building block method; Reinheimer & Weigelt 1987, Reinheimer et al. 1993, and in this proceedings volume) can be applied if adaptive optics wavefront compensation is not available (for example, in the visible) or if only partial compensation is obtained.

Interstellar CNO isotope Ratios

In an interpretation of interstellar, circumstellar, and solar system CNO isotope ratios, we find two scenarios which are free of internal inconsistencies. The first requires that the early solar system was enriched by material from massive stars, leading to enhanced 12C/13C and 18O/17O ratios and to a reduced 14N/15N ratio. The second involves infall of gas onto the galactic disk after the formation of the solar system. Both scenarios require that the bulk of the interstellar 16O, 18O and 15N originates from massive stars (>8M⊙), with 18O and perhaps 15N being destroyed in lower mass stars. 17O is mainly synthesized in stars of intermediate mass while 12C, 13C, and 14N are produced in stars of high and intermediate masses.