Physics Of Massive Stars Research Articles

Direct Constraints on the Extremely Metal-poor Massive Stars Underlying Nebular C iv Emission from Ultra-deep HST/COS Ultraviolet Spectroscopy

Metal-poor nearby galaxies hosting massive stars have a fundamental role to play in our understanding of both high-redshift galaxies and low-metallicity stellar populations. But while much attention has been focused on their bright nebular gas emission, the massive stars that power it remain challenging to constrain. Here we present exceptionally deep Hubble Space Telescope ultraviolet spectra targeting six local (z < 0.02) galaxies that power strong nebular C iv emission approaching that encountered at z > 6. We find that the strength and spectral profile of the nebular C iv in these new spectra follow a sequence evocative of resonant scattering models, indicating that the hot circumgalactic medium likely plays a key role in regulating C iv escape locally. We constrain the metallicity of the massive stars in each galaxy by fitting the forest of photospheric absorption lines, reporting measurements driven by iron that lie uniformly below 10% solar. Comparison with the gas-phase oxygen abundances reveals evidence for enhancement in O/Fe 2–4 times above solar across the sample, robust to assumptions about the absolute gas-phase metallicity scale. This supports the idea that these local systems are more chemically similar to their primordial high-redshift counterparts than to the bulk of nearby galaxies. Finally, we find significant tension between the strong stellar wind profiles observed and our population synthesis models constrained by the photospheric forest in our highest-quality spectra. This reinforces the need for caution in interpreting wind lines in isolation at high redshift, but also suggests a unique path toward validating fundamental massive star physics at extremely low metallicity with integrated ultraviolet spectra.

Stroopwafel: simulating rare outcomes from astrophysical populations, with application to gravitational-wave sources

ABSTRACT Gravitational-wave observations of double compact object (DCO) mergers are providing new insights into the physics of massive stars and the evolution of binary systems. Making the most of expected near-future observations for understanding stellar physics will rely on comparisons with binary population synthesis models. However, the vast majority of simulated binaries never produce DCOs, which makes calculating such populations computationally inefficient. We present an importance sampling algorithm, stroopwafel, that improves the computational efficiency of population studies of rare events, by focusing the simulation around regions of the initial parameter space found to produce outputs of interest. We implement the algorithm in the binary population synthesis code compas, and compare the efficiency of our implementation to the standard method of Monte Carlo sampling from the birth probability distributions. stroopwafel finds ∼25–200 times more DCO mergers than the standard sampling method with the same simulation size, and so speeds up simulations by up to two orders of magnitude. Finding more DCO mergers automatically maps the parameter space with far higher resolution than when using the traditional sampling. This increase in efficiency also leads to a decrease of a factor of ∼3–10 in statistical sampling uncertainty for the predictions from the simulations. This is particularly notable for the distribution functions of observable quantities such as the black hole and neutron star chirp mass distribution, including in the tails of the distribution functions where predictions using standard sampling can be dominated by sampling noise.

Massive stars: stellar models and stellar yields, impact on Galactic Archaeology

AbstractThe physics of massive stars depends (at least) on convection, mass loss by stellar winds, rotation, magnetic fields and multiplicity. We briefly discuss the impact of the first three processes on the stellar yields trying to identify some guidelines for future works.

Low-metallicity (sub-SMC) massive stars

AbstractThe double distance and metallicity frontier marked by the SMC has been finally broken with the aid of powerful multi-object spectrographs installed at 8-10m class telescopes. VLT, GTC and Keck have enabled studies of massive stars in dwarf irregular galaxies of the Local Group with poorer metal-content than the SMC. The community is working to test the predictions of evolutionary models in the low-metallicity regime, set the new standard for the metal-poor high-redshift Universe, and test the extrapolation of the physics of massive stars to environments of decreasing metallicity. In this paper, we review current knowledge on this topic.

B-supergiants in IC1613: testing low-Z massive star physics and evolution

AbstractThe physical processes taking place in massive stars during their life and death are highly dependent on the metallicity (Z) of their parent cloud. Observations of these stars in low-Z nearby galaxies are crucial to understand these processes. IC1613 is the nearest Local Group galaxy with ongoing star formation and O-abundance lower than the SMC, although UV spectroscopy suggests it is not so metal poor. We performed a spectral analysis of early B-type stars in the galaxy, obtaining physical parameters and abundances. Our results confirm the low O-abundance of IC1613.

Recent advances in non-LTE stellar atmosphere models

AbstractIn the last decades, stellar atmosphere models have become a key tool in understanding massive stars. Applied for spectroscopic analysis, these models provide quantitative information on stellar wind properties as well as fundamental stellar parameters. The intricate non-LTE conditions in stellar winds dictate the development of adequate sophisticated model atmosphere codes. The increase in both, the computational power and our understanding of physical processes in stellar atmospheres, led to an increasing complexity in the models. As a result, codes emerged that can tackle a wide range of stellar and wind parameters.After a brief address of the fundamentals of stellar atmosphere modeling, the current stage of clumped and line-blanketed model atmospheres will be discussed. Finally, the path for the next generation of stellar atmosphere models will be outlined. Apart from discussing multi-dimensional approaches, I will emphasize on the coupling of hydrodynamics with a sophisticated treatment of the radiative transfer. This next generation of models will be able to predict wind parameters from first principles, which could open new doors for our understanding of the various facets of massive star physics, evolution, and death.

Spectropolarimetry of massive stars: Requirements and potential from today to 2030

AbstractWe develop the requirements and potential of spectropolarimetry as applied to understanding the physics of massive stars during the immediate, intermediate-term and long-term future.

Predicted gamma-ray line emission from the Cygnus complex

The Cygnus region harbours a huge complex of massive stars at a distance of 1.0-2.0kpc from us. About 170 O stars are distributed over several OB associations, among which the Cyg OB2 cluster is by far the most important with about 100-120 O stars. These massive stars inject large quantities of radioactive nuclei into the interstellar medium, such as 26Al and 60Fe, and their gamma-ray line decay signals can provide insight into the physics of massive stars and core-collapse supernovae. Past studies of the nucleosynthesis activity of Cygnus have concluded that the level of 26Al decay emission as deduced from CGRO/COMPTEL observations was a factor 2-3 above the predictions based on the theoretical yields available at that time and on the observed stellar content of the Cygnus region. We reevaluate the situation from new measurements of the gamma-ray decay fluxes with INTEGRAL/SPI and new predictions based on recently improved stellar models including some of the effects of stellar rotation for the higher mass stars and a coherent estimate of the contribution from SNIb/c. We developed a population synthesis code to predict the nucleosynthesis activity and corresponding decay fluxes of a given stellar population of massive stars. The observed decay fluxes from the Cygnus complex are found to be consistent with the values predicted by population synthesis at solar metallicity. The observed extent of the 1809keV emission from Cygnus is found to be consistent with the result of a numerical simulation of the diffusion of 26Al inside the superbubble blown by Cyg OB2. Our work indicates that the past dilemma regarding the gamma-ray line emission from Cygnus resulted from an overestimate of the 1809keV flux of the Cygnus complex, combined with an underestimate of the nucleosynthesis yields.

Developments in Physics of Massive Stars

AbstractNew constraints on stellar models are provided by large surveys of massive stars, interferometric observations and asteroseismology. After a review of the main results so far obtained, we present new results from rotating models and discuss comparisons with observed features. We conclude that rotation is a key feature of massive star physics.

Which massive stars are gamma-ray burst progenitors?

The collapsar model for gamma-ray bursts requires three essential ingredients: a massive core, removal of the hydrogen envelope, and enough angular momentum in the core. We study current massive star evolution models of solar metallicity to determine which massive star physics is capable of producing these ingredients. In particular, we investigate the role of hydrodynamic and magnetic internal angular momentum transport and binary mass and angular momentum transfer. We follow the evolution of rotating single stars and of binary systems that include rotational processes for both stars. Neglecting magnetic fields, we show that the cores of massive single stars can maintain a high specific angular momentum when evolved with the assumption that mean molecular weight gradients suppress rotational mixing processes. In binary systems that undergo mass transfer during core hydrogen burning the mass receiving star accretes large amounts of high angular momentum material, leading to a spin-up of the core. We find, however, that this merely compensates for the tidal angular momentum loss due to spin-orbit coupling, which leads to synchronous rotation before the mass transfer event. Therefore the resulting cores do not rotate faster than in single stars. We also present models that include magnetic fields generated by differential rotation and we consider the internal angular momentum transport by magnetic torques. We investigate the capability of magnetic torques to efficiently pump angular momentum into the cores of accreting stars. Despite our finding that this mechanism works, the magnetic coupling of core and envelope after the accreting star ends core hydrogen burning leads to slower rotation than in the non-magnetic case.

Evidence for a fundamental stellar upper mass limit from clustered star formation

The observed masses of the most massive stars do not surpass about 150 M ○. . This may either be a fundamental upper mass limit which is defined by the physics of massive stars and/or their formation, or it may simply reflect the increasing sparsity of such very massive stars, so that the observation of even higher mass stars becomes unlikely in the Galaxy and the Magellanic Clouds. It is shown here that if the stellar initial mass function (IMF) is a power law with a Salpeter exponent (a = 2. 35) for massive stars then the richest very young cluster R136 seen in the Large Magellanic Cloud (LMC) should contain stars with masses larger than 750 M ○. . If, however, the IMF is formulated by consistently incorporating a fundamental upper mass limit then the observed upper mass limit is arrived at readily even if the IMF is invariant. An explicit down-turn or cut-off of the IMF near 150 M ○. is not required: our formulation of the problem contains this implicitly. We are therefore led to conclude that a fundamental maximum stellar mass near 150 M ○. exists, unless the true IMF has a > 2.8.

Massive Stars in the Local Group: Implications for Stellar Evolution and Star Formation

▪ Abstract The galaxies of the Local Group serve as important laboratories for understanding the physics of massive stars. Here I discuss what is involved in identifying various kinds of massive stars in nearby galaxies: the hydrogen-burning O-type stars and their evolved He-burning evolutionary descendants, the luminous blue variables, red supergiants, and Wolf-Rayet stars. Primarily I review what our knowledge of the massive star population in nearby galaxies has taught us about stellar evolution and star formation. I show that the current generation of stellar evolutionary models do well at matching some of the observed features and provide a look at the sort of new observational data that will provide a benchmark against which new models can be evaluated.

Massive Star Evolution

The evolution of massive stars is far from being fully understood, as we outline by pointing to a number of open problems related to massive stars in the Magellanic Clouds. We argue that rotation may be a key ingredient in the physics of massive stars. We report on recent results obtained including rotation, and their relevance to these remaining questions.

Supernovae Types Ia/II and intracluster medium enrichment

We re-examine the respective roles played by supernovae (SNe) Types Ia and II in enriching the intracluster medium (ICM) of galaxy clusters, in light of the recent downward shift of the ASCA abundance ratios of alpha-elements to iron favoured by Ishimaru & Arimoto (1997, PASJ, 49, 1). Because of this shift, Ishimaru & Arimoto conclude that >50% of the ICM iron must have originated from within Type Ia SNe progenitors. A point not appreciated in their study, nor in most previous analyses, is the crucial dependence of such a conclusion upon the adopted massive star physics. Employing several alternative Type II SN yield compilations, we demonstrate how uncertainties in the treatment of convection and mass-loss can radically alter our perception of the relative importance of Type Ia and II SNe as ICM polluters. If mass-loss of the form favoured by Maeder (1992, A&A, 264, 105) or convection of the form favoured by Arnett (1996, Supernovae and Nucleosynthesis) is assumed, the effect upon the oxygen yields would lead us to conclude that Type Ia SNe play no part in polluting the ICM, in contradiction with Ishimaru & Arimoto. Apparent dichotomies still exist (e.g. the mean ICM neon-to-iron ratio implies a 100% Type II Fe origin, while the mean sulphur ratio indicates a 100% Type Ia origin) that cannot be reconciled with the currently available yield tables.

Ultraviolet Population Synthesis of Starburst Galaxies Observed with HST

Starburst galaxies are ideal laboratories to study both the physics of massive stars and processes important in galaxy formation and evolution. Observations of starburst galaxies at UV wavelengths are crucial because only in this spectral regime can we directly observe the spectroscopic signatures of the hot, massive stars that power the emission at other wavebands. UV signatures from massive stars are mainly broad spectral features (e.g. P Cygni profiles of SiIVλ1400, and CIVλ1550) formed in the stellar winds.