Andre Maeder

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

Stellar evolution with rotation - X. Wolf-Rayet star populations at solar metallicity

We examine the properties of Wolf-Rayet (WR) stars predicted by models of rotating stars taking account of the new mass loss rates for O-type stars and WR stars (Vink et al. 2000. 2001; Nugis & Lamers 2000) and of the wind anisotropies induced by rotation. We find that the rotation velocities v of WR stars are modest, i.e. about 50 km s - 1 , not very dependent on the initial ν and masses. For the most massive stars, the evolution of ν is very strongly influenced by the values of the mass loss rates; below 12 M O . the evolution of rotation during the MS phase and later phases is dominated by the internal coupling. Massive stars with extreme rotation may skip the LBV phase. Models having a typical v for the O-type stars have WR lifetimes on the average two times longer than for non-rotating models. The increase of the WR lifetimes is mainly due to that of the H-rich eWNL phase. Rotation allows a transition WN/WC phase to be present for initial masses lower than 60 M O .. The durations of the other WR subphases are less affected by rotation. The mass threshold for forming WR stars is lowered from 37 to 22 M O . for typical rotation. The comparisons of the predicted number ratios WR/O, WN/WC and of the number of transition WN/WC stars show very good agreement with models with rotation, while this is not the case for models with the present-day mass loss rates and no rotation. As to the chemical abundances in WR stars, rotation brings only very small changes for WN stars, since they have equilibrium CNO values. However, WC stars with rotation have on average lower C/He and O/He ratios. The luminosity distribution of WC stars is also influenced by rotation.

Stellar Evolution with Rotation

The existing grids of star models without rotation allow us to already explain a number of properties of massive and WR stars (cf. Maeder and Conti,1994). In particular, the sequences of star clusters are well reproduced, as well as the relations of filiation between the various kinds of massive stars: O-type stars, blue-and red-supergiants, Wolf-Rayet (WR) stars. The observed abundances of CNO elements in WN stars and of He, C, O and Ne in WC stars are nicely accounted for, as well as the luminosities of WR stars, the number ratios WR/O and WC/WN in galaxies of various metallicities. The consequences of mass loss for the chemical yields are quite important and allow us in particular to understand the evolution of the CNO elements during the galactic history.

The Effective Temperature Scale of Galactic Red Supergiants: Cool, but Not as Cool as We Thought

We use moderate-resolution optical spectrophotometry and the new MARCS stellar atmosphere models to determine the effective temperatures of 74 Galactic red supergiants (RSGs). The stars are mostly members of OB associations or clusters with known distances, allowing a critical comparison with modern stellar evolutionary tracks. We find we can achieve excellent matches between the observations and the reddened model fluxes and molecular transitions, although the atomic lines Ca I ?4226 and Ca II H and K are found to be unrealistically strong in the models. Our new effective temperature scale is significantly warmer than those in the literature, with the differences amounting to 400 K for the latest type M supergiants (i.e., M5 I). We show that the newly derived temperatures and bolometric corrections give much better agreement with stellar evolutionary tracks. This agreement provides a completely independent verification of our new temperature scale. The combination of effective temperature and bolometric luminosities allows us to calculate stellar radii; the coolest and most luminous stars (KW Sgr, Case 75, KY Cyg, HD 206936=? Cep) have radii of roughly 1500 Rsolar (7 AU), in excellent accordance with the largest stellar radii predicted from current evolutionary theory, although smaller than that found by others for the binary VV Cep and for the peculiar star VY CMa. We find that similar results are obtained for the effective temperatures and bolometric luminosities using only the dereddened V-K colors, providing a powerful demonstration of the self-consistency of the MARCS models.

Stellar evolution with rotation VII: Low metallicity models and the blue to red supergiant ratio in the SMC

We calculate a grid of models with and without the eects of axial rotation for massive stars in the range of 9t o 60M and metallicity Z =0 :004 appropriate for the SMC. Remarkably, the ratios =crit of the angular velocity to the break{up angular velocity grow strongly during the evolution of high mass stars, contrary to the situation at Z =0 :020. The reason is that at low Z, mass loss is smaller and the removal of angular momentum during evolution much weaker, also there is an ecient outward transport of angular momentum by meridional circulation. Thus, a much larger fraction of the stars at lower Z reach break{up velocities and rotation may thus be a dominant eect at low Z. The models with rotation well account for the long standing problem of the large numbers of red supergiants observed in low Z galaxies, while current models with mass loss were predicting no red supergiants. We discuss in detail the physical eects of rotation which favour a redwards evolution in the HR diagram. The models also predict large N enrichments during the evolution of high mass stars. The predicted relative N{enrichments are larger at Z lower than solar and this is in very good agreement with the observations for A{type supergiants in the SMC.

Stellar evolution with rotation - VIII. Models at

We calculate a grid of star models with and without the eects of axial rotation for stars in the mass range between 2 and 60 M for the metallicity Z= 10 5 . Star models with initial masses superior or equal to 9 M were computed up to the end of the carbon-burning phase. Star models with masses between 2 and 7 M were evolved beyond the end of the He-burning phase through a few thermal pulses during the AGB phase. Compared to models at Z = 0:02, the low Z models show faster rotating cores and stronger internal-gradients, which favour an important mixing of the chemical elements. The enhancement of N/C at the surface may reach 2 to 3 orders of magnitude for fast rotating stars. Surface enrichments may make the evolved stars less metal poor than they were initially. In very low Z models, primary nitrogen is produced during the He-burning phase by rotational diusion of 12 C into the H-burning shell. A large fraction of the primary 14 N escapes further destruction and enters the envelope of AGB stars, being ejected during the TP-AGB phase and the formation of a planetary nebula. The intermediate mass stars of very lowZ are the main producers of primary 14 N, but massive stars also contribute to this production; no significant primary nitrogen is made in models at metallicity Z= 0:004 or above. We calculate the chemical yields in He, C, N, O and heavy elements and discuss the chemical evolution of the CNO elements at very low Z. Remarkably, the C/ Ov s. O/H diagram is mainly sensitive to the interval of stellar masses, while the N/ Ov s. O/H diagram is mainly sensitive to the average rotation of the stars contributing to the element synthesis. The presently available observations in these diagrams seem to favour contributions either from stars down to about 2 M with normal rotation velocities or from stars above 8 M but with very fast rotation.

Z

We describe the latest developments of the Geneva stellar evolution code in order to model the pre-supernova evolution of rotating massive stars. Rotating and non-rotating stellar models at solar metallicity with masses equal to 12, 15, 20, 25, 40 and 60 Mwere computed from the ZAMS until the end of the core silicon burning phase. We took into account meridional circulation, secular shear instabilities, horizontal turbulence and dynamical shear instabilities. We find that dynamical shear instabilities mainly smoothen the sharp angular velocity gradients but do not transport angular momentum or chemical species over long distances. Most of the differences between the pre-supernova structures obtained from rotating and non-rotating stellar models have their origin in the effects of rotation during the core hydrogen and helium burning phases. The advanced stellar evolutionary stages appear too short in time to allow the rotational instabilities considered in this work to have a significant impact during the late stages. In particular, the internal angular momentum does not change significantly during the advanced stages of the evolution. We can therefore have a good estimate of the final angular momentum at the end of the core helium burning phase. The effects of rotation on pre-supernova models are significant between 15 and 30 M� . Indeed, rotation increases the core sizes (and the yields) by a factor ∼1.5. Above 20 M� , rotation may change the radius or colour of the supernova progenitors (blue instead of red supergiant) and the supernova type (IIb or Ib instead of II). Rotation affects the lower mass limits for radiative core carbon burning, for iron core collapse and for black hole formation. For Wolf-Rayet stars (M > 30 M� ), the pre-supernova structures are mostly affected by the intensities of the stellar winds and less by rotational mixing.

= 10

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 –

^\mathsf{-5}

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.

and CNO yields for early galactic evolution

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 .