GRB progenitors at low metallicities

We calculated pre-supernova evolution models of single rotating massive stars. These models reproduce observations during the early stages of the evolution very well, in particular Wolf--Rayet (WR) populations and ratio between type II and type Ib,c supernovae at different metallicities (Z). Using these models we found the following results concerning long and soft gamma--ray burst (GRB) progenitors: - GRBs coming from WO--type (SNIc) WR stars are only produced at low Z (LMC or lower). - The upper metallicity limit for GRBs is reduced to Z ~ 0.004 (SMC) when the effects of magnetic fields are included. - GRBs are predicted from the second (and probably the first) stellar generation onwards.Comment: 5 pages, 1 figure, to appear in the proceedings of "Swift and GRBs: Unveiling the Relativistic Universe", San Servolo, Venice, 5-9 June 200

Stellar evolution with rotation XIII: Predicted GRB rates at various Z

We present the evolution of rotation in models of massive single stars covering a wide range of masses and metallicities. These models reproduce very well observations during the early stages of the evolution (in particular WR populations and ratio between type II and type Ib,c at different metallicities, see Meynet & Maeder 2005). Our models predict the production of fast rotating black holes. Models with large initial masses or high metallicity end their life with less angular momentum in their central remnant with respect to the break-up limit for the remnant. Many WR star models satisfy the three main criteria (black hole formation, loss of hydrogen-rich envelope and enough angular momentum to form an accretion disk around the black hole) for gamma-ray bursts (GRB) production via the collapsar model (Woosley 1993). Considering all types of WR stars as GRB progenitors, there would be too many GRBs compared to observations. If we consider only WO stars (type Ic supernovae as is the case for SN2003dh/GRB030329, see Matheson et al. 2003) as GRBs progenitors, the GRBs production rates are in much better agreement with observations. WO stars are produced only at low metallicities in the present grid of models. This prediction can be tested by future observations.Comment: ~16 pages, 14 figures, accepted by A&

Physics of rotation in stellar models

In these lecture notes, we present the equations presently used in stellar interior models in order to compute the effects of axial rotation. We discuss the hypotheses made. We suggest that the effects of rotation might play a key role at low metallicity.Comment: 32 pages, 7 figures, lectures, CNRS school, will be published by Springe

Very low metallicity massive star models: Pre-SN evolution and primary nitrogen production

Two series of models were computed. The first series consists of 20 solar mass models with varying initial metallicity (Z=0.02 down to Z=10^{-8}) and rotation (V_{ini}=0-600 km/s). The second one consists of models with an initial metallicity of Z=10^{-8}, masses between 9 and 85 solar masses and fast initial rotation velocities (V_{ini}=600-800 km/s). The most interesting models are the models with Z=10^{-8} ([Fe/H]~-6.6). In the course of helium burning, carbon and oxygen are mixed into the hydrogen burning shell. This boosts the importance of the shell and causes a reduction of the CO core mass. Later in the evolution, the hydrogen shell deepens and produces large amount of primary nitrogen. For the most massive models (M>~60 solar masses), significant mass loss occurs during the red supergiant stage. This mass loss is due to the surface enrichment in CNO elements via rotational and convective mixing. The 85 solar mass model ends up as a WO type Wolf-Rayet star. Therefore the models predict SNe of type Ic and possibly long and soft GRBs at very low metallicities. The rotating 20 solar mass models can best reproduce the observed CNO abundances at the surface of extremely metal poor (EMP) stars and the metallicity trends when their angular momentum content is the same as at solar metallicity (and therefore have an increasing surface velocity with decreasing metallicity). The wind of the massive star models can also reproduce the CNO abundances of the most metal-poor carbon-rich star known to date, HE1327-2326.Comment: A&A accepted, 18 pages, 13 figures WEBLINK: http://quasar.physik.unibas.ch/~hirschi/work/lowz.pd

Abundances of Baade's Window Giants from Keck/HIRES Spectra: II. The Alpha- and Light Odd Elements

We report detailed chemical abundance analysis of 27 RGB stars towards the Galactic bulge in Baade's Window for elements produced by massive stars: O, Na, Mg, Al, Si, Ca and Ti. All of these elements are overabundant in the bulge relative to the disk, especially Mg, indicating that the bulge is enhanced in Type~II supernova ejecta and most likely formed more rapidly than the disk. We attribute a rapid decline of [O/Fe] to metallicity-dependent yields of oxygen in massive stars, perhaps connected to the Wolf-Reyet phenomenon. he explosive nucleosynthesis alphas, Si, Ca and Ti, possess identical trends with [Fe/H], consistent with their putative common origin. We note that different behaviors of hydrostatic and explosive alpha elements can be seen in the stellar abundances of stars in Local Group dwarf galaxies. We also attribute the decline of Si,Ca and Ti relative to Mg, to metallicity- dependent yields for the explosive alpha elements from Type~II supernovae. The starkly smaller scatter of [/Fe] with [Fe/H] in the bulge, as compared to the halo, is consistent with expected efficient mixing for the bulge. The metal-poor bulge [/Fe] ratios are higher than ~80% of the halo. If the bulge formed from halo gas, the event occured before ~80% of the present-day halo was formed. The lack of overlap between the thick and thin disk composition with the bulge does not support the idea that the bulge was built by a thickening of the disk driven by the bar. The trend of [Al/Fe] is very sensitive to the chemical evolution environment. A comparison of the bulge, disk and Sgr dSph galaxy shows a range of ~0.7 dex in [Al/Fe] at a given [Fe/H], presumably due to a range of Type~II/Type~Ia supernova ratios in these systems.Comment: 51 pages, 6 tables, 27 figures, submitte

The impact of stellar rotation on the CNO abundance patterns in the Milky Way at low metallicities

We investigate the effect of new stellar models, which take rotation into account, computed for very low metallicities on the chemical evolution of the earliest phases of the Milky Way. We check the impact of these new stellar yields on a model for the halo of the Milky Way that can reproduce the observed halo metallicity distribution. In this way we try to better constrain the ISM enrichment timescale, which was not done in our previous work. The stellar models adopted in this work 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 computed for large rotational velocities have a tremendous impact on the interstellar medium nitrogen enrichment for log(O/H)+12 < 7 (or [Fe/H]< -3). We show that upon the inclusion of the new stellar calculations in a chemical evolution model for the galactic halo with infall and outflow, 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 initial rotational velocities of the order of 600-800kms-1. An important contribution to N from AGB stars is still needed in order to explain the observations at intermediate metallicities. One possibility is that AGB stars at very low metallicities also rotate fast. This could be tested in the future, once stellar evolution models for fast rotating AGB stars will be available.Comment: Contribution to Nuclei in the Cosmos IX (Proceedings of Science - 9 pages, 4 figs., accepted) - Version 2: one reference added in the caption of Fig.

SPINSTARS at low metallicities

The main effect of axial rotation on the evolution of massive PopIII stars is to trigger internal mixing processes which allow stars to produce significant amounts of primary nitrogen 14 and carbon 13. Very metal poor massive stars produce much more primary nitrogen than PopIII stars for a given initial mass and rotation velocity. The very metal poor stars undergo strong mass loss induced by rotation. One can distinguish two types of rotationnaly enhanced stellar winds: 1) Rotationally mechanical winds occurs when the surface velocity reaches the critical velocity at the equator, {\it i.e.} the velocity at which the centrifugal acceleration is equal to the gravity; 2) Rotationally radiatively line driven winds are a consequence of strong internal mixing which brings large amounts of CNO elements at the surface. This enhances the opacity and may trigger strong line driven winds. These effects are important for an initial value of $\upsilon/\upsilon_{\rm crit}$ of 0.54 for a 60 M$_\odot$ at $Z=10^{-8}$, {\it i.e.} for initial values of $\upsilon/\upsilon_{\rm crit}$ higher than the one ($\sim$0.4) corresponding to observations at solar $Z$. These two effects, strong internal mixing leading to the synthesis of large amounts of primary nitrogen and important mass losses induced by rotation, occur for $Z$ between about 10$^{-8}$ and 0.001. For metallicities above 0.001 and for reasonable choice of the rotation velocities, internal mixing is no longer efficient enough to trigger these effects.Comment: 5 pages, 4 figures, to be published in the conference proceedings of First Stars III, Santa Fe, 200