Sylvia Ekström

Grids of stellar models with rotation III. Models from 0.8 to 120 M at a metallicity Z = 0.002 ?

Aims. Many topical astrophysical research areas, such as the properties of planet host stars, the nature of the progenitors of different types of supernovae and gamma ray bursts, and the evolution of galaxies, require complete and homogeneous sets of stellar models at different metallicities in order to be studied during the whole of cosmic history. We present here a first set of models for solar metallicity, where the effects of rotation are accounted for in a homogeneous way. Methods. We computed a grid of 48 different stellar evolutionary tracks, both rotating and non-rotating, at Z = 0.014, spanning a wide mass range from 0.8 to 120 M� . For each of the stellar masses considered, electronic tables provide data for 400 stages along the evolutionary track and at each stage, a set of 43 physical data are given. These grids thus provide an extensive and detailed data basis for comparisons with the observations. The rotating models start on the zero-age main sequence (ZAMS) with a rotation rate υini/υcrit = 0.4. The evolution is computed until the end of the central carbon-burning phase, the early asymptotic giant branch (AGB) phase, or the core helium-flash for, respectively, the massive, intermediate, and both low and very low mass stars. The initial abundances are those deduced by Asplund and collaborators, which best fit the observed abundances of massive stars in the solar neighbourhood. We update both the opacities and nuclear reaction rates, and introduce new prescriptions for the mass-loss rates as stars approach the Eddington and/or the critical velocity. We account for both atomic diffusion and magnetic braking in our low-mass star models. Results. The present rotating models provide a good description of the average evolution of non-interacting stars. In particular, they reproduce the observed main-sequence width, the positions of the red giant and supergiant stars in the Hertzsprung-Russell (HR) diagram, the observed surface compositions and rotational velocities. Very interestingly, the enhancement of the mass loss during

Grids of stellar models with rotation - II. WR populations and supernovae/GRB progenitors at Z = 0.014

Context. In recent years, many very interesting observations have appeared concerning the positions of Wolf-Rayet (WR) stars in the Hertzsprung-Russell diagram (HRD), the number ratios of WR stars, the nature of Type Ibc supernova (SN) progenitors, long and soft gamma ray bursts (LGRB), and the frequency of these various types of explosive events. These observations represent key constraints on massive star evolution. Aims. We study, in the framework of the single-star evolutionary scenario, how rotation modifies the evolution of a given initial mass star towards the WR phase and how it impacts the rates of Type Ibc SNe. We also discuss the initial conditions required to obtain collapsars and LGRB. Methods. We used a recent grid of stellar models computed with and without rotation to make predictions concerning the WR populations and the frequency of different types of core-collapse SNe. Current rotating models were checked to provide good fits to the following features: solar luminosity and radius at the solar age, main-sequence width, red-giant and red-supergiant (RSG) positions in the HRD, surface abundances, and rotational velocities. Results. Rotating stellar models predict that about half of the observed WR stars and at least half of the Type Ibc SNe may be produced through the single-star evolution channel. Rotation increases the duration of the WNL and WNC phases, while reducing those of the WNE and WC phases, as was already shown in previous works. Rotation increases the frequency of Type Ic SNe. The upper –

Effects of rotation on the evolution of primordial stars

Context. Though still beyond our observational abilities, Populati on III stars are interesting objects in many perspectives. T hey are responsible for the re-ionisation of the inter-galactic me dium. They also left their chemical imprint in the early Universe, imprint which can be deciphered in the most metal-poor stars in the halo of our Galaxy. Aims. Rotation has been shown to play a determinant role at very low metallicity, bringing heavy mass loss where almost none was expected. Is this still true when the metallicity strictly e quals zero? The aim of our study is to get an answer to this question, and to determine how rotation changes the evolution and the chemical signature of the primordial stars. Methods. We have calculated seven differentially-rotating stellar models at zero metallicity, w ith masses between 9 and 200 M⊙. For each mass, we have also calculated a corresponding model without rotation. The evolution has been followed up to the pre-supernova stage. Results. We find that Z = 0 models rotate with an internal profile (r) close to local angular momentum conservation, because of a very weak core-envelope coupling. Rotational mixing drives a H-shell boost due to a sudden onset of CNO cycle in the shell. This boost leads to a high 14 N production, which can be as much as 10 6 times higher than the production of the non-rotating models. Generally, the rotating models produce much more metals than their non-rotating counterparts. The mass loss is very low, even for the models that reach the critical velocity during the main s equence. It may however have an impact on the chemical enrichment of the Universe, because some of the stars are supposed to collapse directly into black holes. They would contribute to the enrichment only through their winds. While in that case non-rotating st ars would not contribute at all, rotating stars may leave an i mprint in their surrounding. Due to the low mass loss and the weak coupling, the core retains a high angular momentum at the end of the evolution. The high rotation rate at death probably leads to a much stronger explosion than previously expected, changing the fate of the models. The inclusion of our yields in a chemical evolution model of the Galactic halo predicts log values of N/O, C/O and 12 C/ 13 C ratios of -2.2, -0.95 and 50 respectively at log O/H +12 = 4.2.

The Effects of Stellar Rotation. II. A Comprehensive Set of Starburst99 Models

We present a new set of synthesis models for stellar populations obtained with Starburst99 and based on new stellar evolutionary tracks with rotation. We discuss models with zero rotation velocity and with velocities of 40% of the break-up velocity on the zero-age main-sequence. These values are expected to bracket realistic rotation velocity distributions in stellar populations. The new rotating models for massive stars are more luminous and hotter due to a larger convective core and enhanced surface abundances. This results in pronounced changes in the integrated spectral energy distribution of a population containing massive stars. The changes are most significant at the shortest wavelengths where an increase of the ionizing luminosity by up to a factor of five is predicted. We also show that high equivalent widths of recombination lines may not necessarily indicate a very young age but can be achieved at ages as late as ~107 yr. Comparison of these two boundary cases (0% and 40% of the break-up velocity) will allow users to evaluate the effects of rotation and provide guidance for calibrating the stellar evolution models. We also introduce a new theoretical ultraviolet spectral library built from the Potsdam Wolf-Rayet atmospheres. Its purpose is to help identify signatures of Wolf-Rayet stars in the ultraviolet whose strength is sensitive to the particulars of the evolution models. The new models are available for solar and one-seventh solar metallicities. A complete suite of models can be generated on the Starburst99 Web site. The updated Starburst99 package can be retrieved from that Web site as well.

A strong case for fast stellar rotation at very low metallicities

We investigate the effect of new stellar models taking rotation into account and computing for a metallicity Z = 10 -8 on the chemical evolution of the earliest phases of the Milky Way. These models were computed under the assumption that the ratio of the initial rotation velocity to the critical velocity of stars is roughly constant with metallicity. This naturally leads to faster rotation at lower metallicity, as metal-poor stars are more compact than metal-rich ones. We find that the new Z = 10 -8 stellar yields have a tremendous impact on the nitrogen enrichment of the interstellar medium for log(O/H) + 12 < 7 (or [Fe/H] < -3). We show that including Z = 10 -8 stellar yields in chemical evolution models, both high N/O and C/O ratios are obtained in the very-metal poor metallicity range, in agreement with observations. Our results give further support to the idea that stars at very low metallicities could have rotational velocities of the order of 600-800 km s -1 .

Evolution towards the critical limit and the origin of Be stars

Context. More and more evidence leads to considering classical Be stars as rotating close to the critical velocity. If so, then the question that arises is the origin of this high surface velocity. Aims. We determine which mechanisms accelerate the surface of single stars during the main sequence evolution. We study their dependence on the metallicity and derive the frequency of stars with different surface velocities in clusters of various ages and metallicities. Methods. We have computed 112 stellar models of four different initial masses between 3 and 60 M� , at four different metallicities between 0 and 0.020, and with seven different values of the ratio Ω/Ωcrit between 0.1 and 0.99. For all the models, computations were performed until either the end of the main sequence evolution or until the critical limit was reached. Results. The evolution of surface velocities during the main sequence lifetime results from an interplay between meridional circulation (bringing angular momentum to the surface) and mass loss by stellar winds (removing it). The dependence on metallicity of these two mechanisms plays a key role in determining, for each metallicity, a limiting range of initial masses (spectral types) for stars able to reach or at least approach the critical limit. Present models predict a higher frequency of fast rotating stars in clusters with ages between 10 and 25 Myr. This is the range of ages where most of Be stars are observed. To reproduce the observed frequencies of Be stars, it is necessary to first assume that the Be star phenomenon already occurs for stars with υ/υcrit ≥ 0.7 and, second, that the fraction of fast rotators on the zero-age main sequence is higher at lower metallicities. Depending on the stage at which the star becomes a Be star, it may present either larger or less enrichments in nitrogen at the surface.

Evolution and fate of very massive stars

There is observational evidence that supports the existence of very massive stars (VMS) in the local universe. First, VMS (Mini ≲ 320 M⊙) have been observed in the Large Magellanic Clouds (LMC). Secondly, there are observed supernovae (SNe) that bear the characteristics of pair creation supernovae (PCSNe, also referred to as pair instability SN) which have VMS as progenitors. The most promising candidate to date is SN 2007bi. In order to investigate the evolution and fate of nearby VMS, we calculated a new grid of models for such objects, for solar, LMC and Small Magellanic Clouds (SMC) metallicities, which covers the initial mass range from 120 to 500 M⊙. Both rotating and non-rotating models were calculated using the GENEVA stellar evolution code and evolved until at least the end of helium burning and for most models until oxygen burning. Since VMS have very large convective cores during the main-sequence phase, their evolution is not so much affected by rotational mixing, but more by mass loss through stellar winds. Their evolution is never far from a homogeneous evolution even without rotational mixing. All the VMS, at all the metallicities studied here, end their life as WC(WO)-type Wolf-Rayet stars. Because of very important mass losses through stellar winds, these stars may have luminosities during the advanced phases of their evolution similar to stars with initial masses between 60 and 120 M⊙. A distinctive feature which may be used to disentangle Wolf-Rayet stars originating from VMS from those originating from lower initial masses would be the enhanced abundances of Ne and Mg at the surface of WC stars. This feature is however not always apparent depending on the history of mass loss. At solar metallicity, none of our models is expected to explode as a PCSN. At the metallicity of the LMC, only stars more massive than 300 M⊙ are expected to explode as PCSNe. At the SMC metallicity, the mass range for the PCSN progenitors is much larger and comprises stars with initial masses between about 100 and 290 M⊙. All VMS in the metallicity range studied here produce either a Type Ib SN or a Type Ic SN but not a Type II SN. We estimate that the progenitor of SN 2007bi, assuming a SMC metallicity, had an initial mass between 160 and 175 M⊙. None of models presented in this grid produces gamma-ray bursts or magnetars. They lose too much angular momentum by mass loss or avoid the formation of a black hole by producing a completely disruptive PCSN.

Populations of rotating stars - I. Models from 1.7 to 15 M⊙ at Z = 0.014, 0.006, and 0.002 with Ω/Ωcrit between 0 and 1

B-type stars are known to rotate at various velocities, including very fast rotators near the critical velocity as the Be stars. In this paper, we provide stellar models covering the mass range between 1.7 to 15 Msun, which includes the typical mass of known Be stars, at Z = 0.014, 0.006, and 0.002 and for an extended range of initial velocities on the zero-age main sequence. We used the Geneva stellar-evolution code, including the effects of shellular rotation, with a numerical treatment that has been improved so the code can precisely track the variation in the angular momentum content of the star as it changes under the influence of radiative winds and/or mechanical mass loss. We discuss the impact of the initial rotation rate on the tracks in the Hertzsprung-Russell diagram, the main-sequence (MS) lifetimes, the evolution of the surface rotation and abundances, as well as on the ejected masses of various isotopes. Among the new results obtained from the present grid we find that 1) fast-rotating stars with initial masses around 1.7 Msun present at the beginning of the core hydrogen-burning phase quite small convective cores with respect to their slowly rotating counterparts. This fact may be interesting to keep in mind in the framework of the asteroseismic studies of such stars. 2) The contrast between the core and surface angular velocity is higher in slower rotating stars. The values presently obtained are in agreement with the very few values obtained for B-type stars from asteroseismology. 3) At Z = 0.002, the stars in the mass range of 1.7 to 3 Msun with a mean velocity on the MS of the order of 150 km/s show N/H enhancement superior to 0.2 dex at mid-MS, and superior to 0.4 dex at the end of the MS phase. At solar metallicity the corresponding values are below 0.2 dex at any time in the MS.

Fundamental properties of core-collapse supernova and GRB progenitors: predicting the look of massive stars before death

We investigate the fundamental properties of core-collapse Supernova (SN) progenitors from single stars at solar meta llicity. For this purpose, we combine Geneva stellar evolutionary models with initial masses of Mini = 20− 120 M⊙ with atmospheric/wind models using the radiative transfer code CMFGEN. We provide synthetic photometry and high-resolution spectra of hot stars at t he pre-SN stage. For models with Mini = 9− 20 M⊙, we supplement our analysis using publicly available MARCS model atmospheres of RSGs to estimate their synthetic photometry. We employ well-established observational criteria of spectroscopic classifi cation and find that massive stars, depending on their initial mass and rotation , end their lives as red supergiants (RSG), yellow hypergian ts (YHG), luminous blue variables (LBV), and Wolf-Rayet (WR) stars of the WN and WO spectral types. For rotating models, we obtained the following types of SN progenitors: WO1‐3 (Mini ≥ 32 M⊙), WN10‐11 (25 40 M⊙), WN7‐8 (25< Mini ≤ 40 M⊙), WN11h/LBV (20< Mini ≤ 25 M⊙), and RSGs (9≤ Mini ≤ 20 M⊙). Our rotating models indicate that SN IIP progenitors are all RSG, SN IIL/b progenitors are 56% LBVs and 44% YHGs, SN Ib progenitors are 96% WN10-11 and 4% WOs, and SN Ic progenitors are all WO stars. We find that n ot necessarily the most massive and luminous SN progenitors are the brighter ones in a given filter, since this depends on t heir luminosity, temperature, wind density, and how the spectral energy distribution compares to a filter bandpass. We find that SN IIP progenitors (RSGs) are bright in the RI JHKS filters and faint in the U B filters. SN IIL /b progenitors (LBVs and YHGs), and SN Ib progenitors (WNs) are relatively bright in optical/infrared filters, while SN Ic progenitors (WOs) are faint in all optical filters. We argu e that SN Ib and Ic progenitors from single stars should be undetectable in the available pre-explosion images with the current magnitude limits, in agreement with observational results.

Stellar mass and age determinations - I. Grids of stellar models from Z = 0.006 to 0.04 and M = 0.5 to 3.5 M⊙

Aims. We present dense grids of stellar models suitable for comparison with observable quantities measured with great precision, such as those derived from binary systems or planet-hosting stars. Methods. We computed new Geneva models without rotation at metallicities Z = 0.006, 0.01, 0.014, 0.02, 0.03, and 0.04 (i.e. [Fe/H] from -0.33 to +0.54) and with mass in small steps from 0.5 to 3.5 M o . Great care was taken in the procedure for interpolating between tracks in order to compute isochrones. Results. Several properties of our grids are presented as a function of stellar mass and metallicity. Those include surface properties in the Hertzsprung-Russell diagram, internal properties including mean stellar density, sizes of the convective cores, and global asteroseismic properties. Conclusions. We checked our interpolation procedure and compared interpolated tracks with computed tracks. The deviations are less than 1% in radius and effective temperatures for most of the cases considered. We also checked that the present isochrones provide nice fits to four couples of observed detached binaries and to the observed sequences of the open clusters NGC 3532 and M67. Including atomic diffusion in our models with M < 1.1 Mo leads to variations in the surface abundances that should be taken into account when comparing with observational data of stars with measured metallicities. For that purpose, iso-Z surf lines are computed. These can be requested for download from a dedicated web page, together with tracks at masses and metallicities within the limits covered by the grids. The validity of the relations linking Z and [Fe/H] is also re-assessed in light of the surface abundance variations in low-mass stars.