Ekström, S

Maeder, A

Meynet, G

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PoS(NIC-IX)091CNO production in the first generation starsSylvia Ekström∗, Georges Meynet & André MaederGeneva Observatory, SwitzerlandE-mails: sylvia.ekstrom@obs.unige.ch,georges.meynet@obs.unige.ch,andre.maeder@obs.unige.chBig Bang nucleosynthesis produces only light elements and the very first generation stars are thusformed from metal-free clouds. They start the production of heavy elements during their life,and enrich the interstellar medium through their explosive death. Stellar evolution models showthat the treatment of rotation has important effects on the evolution of those metal-free stars: forexample, rotating models produce up to five orders of magnitude more primary nitrogen thannon rotating models, due to internal mixing. This will have an impact in the composition of thesecond generation stars, some of which may now be observed in the Galactic halo. In the casePopulation III stars were very massive and would end up as direct black holes, rotation again havean interesting effect of enhancing mass loss through centrifugal force and surface enrichment.CNO composition patterns observed in ultra metal-poor halo stars may be explained by a ’windonly’ contribution.International Symposium on Nuclear Astrophysics — Nuclei in the Cosmos — IXJune 25-30 2006CERN, Geneva, Switzerland∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(NIC-IX)091CNO production in the first generation stars Sylvia Ekström1. Primordial stars1.1 BirthAfter the Big Bang, the early Universe is composed only from H and He, a little Be, B andLi, but no heavier elements. At the end of the so-called ’Dark Age’, halos of dark matter start tocollapse and form stars with the gas trapped within. The star-forming clouds are almost pure Hand He, so fragmentation cannot occur, leading to a massive or very massive star formation (seeOmukai & Palla 2003 and references therein). The mass domain covered by primordial stars isstill very much debated. Numerical simulations including known physics lead to the formationof very massive objects (see Bromm & Larson 2004), with masses up to 500-700 M. Stabilitystudies for such extreme stars show that they are stable enough against radial pulsations to evolveuntil advanced stages (see Baraffe & al. 2001). Observational constraints, though, favor a morestandard, slightly top-heavy Larson IMF in the range 1 - 100 M (see Schneider & al. 2006), whilethe mechanism for producing sufficient fragmentation in primordial clouds is not yet known.1.2 EvolutionThe new star is born metal-free. In its contracting core, the gas heats up until nuclear reac-tions start burning some H, but the only energy source is provided by pp-chains reactions, so thecollapse is only slowed down. The core reaches then a temperature that allows some He burningthrough 3α reaction, producing some C and allowing the CNO-cycle energy production to stop thecollapse. Primordial stars are thus born hotter and more compact, and their metal-free envelopelets them remaining so through the whole main sequence (MS). Since radiative winds scale withthe metallicity as M˙ ∼ Z0.65 (see Vink & al. 2001), the star experiences practically no mass lossduring its evolution, except if its luminosity is high enough to remove some mass just with H or Helines (see Kudritzki 2002).1.3 DeathAccording to Heger & al. (2003), the fate of a massive star depends on the mass of its He coreat the end of the evolution. We have just seen that primordial stars evolve at almost constant mass,so they end up their life with a heavy He core. Stars with initial mass ranging from 25 to 140 Mare supposed to collapse as direct black hole without ejecting any matter. Between 140 and 260M the star undergoes pair-creation instability and is supposed to be completely disrupted in theprocess. Heavier stars again form direct black hole.2. Rotation at (extremely) low metallicityA detailed description of the effects of rotation at low or very low metallicity is given inMeynet & al. (2006). We will highlight here two main effects: 1) because of the very low radiativewinds, the star cannot lose its angular momentum and is thus prone to reach the break-up velocityduring the MS; 2) because of a less efficient meridional circulation, the Ω-profiles are steeper, andthus rotational mixing is stronger (see also the contribution of G. Meynet in this conference).The first effect leads to enhanced mass loss through purely mechanical process, the uppermostlayers of the star being removed by the centrifugal force. The second effect takes place during core2PoS(NIC-IX)091CNO production in the first generation stars Sylvia EkströmHe-burning, enriching the surface with newly synthesized elements and allowing radiative windsto get stronger.3. ModelizationIn order to study the effects of rotation at Z = 0, we have computed, with the Geneva stellarevolution code, metal-free models ranging from 9 to 200 M (Fig. 2). For each mass, we havecomputed a non-rotating model and a differentially rotating one, where the equatorial velocityhas been chosen so that υ/υcrit ' 0.5 (depending on the mass, this gives υeq ' 500−800 km s−1),corresponding to the average value observed in massive stars. The initial composition of the modelsis X = 0.76, Y = 0.24 and Z = 0.Figure 1: Herztsprung-Russell diagram of the Z = 0 modelsThe input physics is the same as in Meynet & Maeder (2002), including rotation-inducedinstabilities as meridional circulation, secular and dynamical shear, and horizontal turbulence. Wehave used the Schwarzschild criterion for convection, and an overshooting parameter of 0.2HPduring core H- and He-burning and 0 otherwise. We have applied the mass loss rates proposed byKudritzki (2002) for the models reaching a luminosity L/L ≥ 6, with the same adaptation for thestrictly Z = 0 case as in Marigo & al. (2003).4. CNO productionThe models have been followed until the end of core O-burning, so the values presented hereare not the final yields of the model, but they give insights of the way differential rotation modifiesthe characteristics of a star. As expected, most of the models have reached the break-up velocityduring the MS, some of them very early in their evolution. The mass lost within this processhowever is not very important because only the uppermost layers of the star are removed, and those3PoS(NIC-IX)091CNO production in the first generation stars Sylvia Ekströmlayers have a low density. Rotation favors a redward evolution and, as seen above, an enhancementof the effective surface metallicity through mixing, so the radiative winds are stronger during He-burning. Again, in the Z = 0 case, this doesn’t lead to strong mass loss because the time involvedis short. Our heavier rotating model (200 M) loses just a little more than 10% of its initial mass(8.2% during MS, 2.2% after), but its non-rotating counterpart loses all added only 0.5%.Figure 2: CNO production of Z = 0 models. Left: assuming a SN explosion with a remnant mass deter-mined by the CO core mass as in Maeder (1992); right: following the supposed fate of the star according toHeger & al. (2003).At the end of the evolution, the CO cores of the rotating models are about 10-20% heavier thanthe non-rotating ones. Since the bulk of the CO produced comes from the core, the values obtainedfor both the rotating and non-rotating models are quite similar (see Fig. 2, left).The case of N is very different. Except for the 25 M model, the rotating values are threeto five orders of magnitude higher than the non-rotating ones. This is due to the fact that N ismainly produced in the H-burning shell, when some C and O from the He-burning shell is diffusedoutwards. Rotation-induced mixing favors this process. But for the 25 M without rotation, asLimongi & Chieffi (2005), we find a high primary N production about three orders of magnitudehigher than the value produced by the other non-rotating models. The value is even slightly largerthan the rotating model’s value. It can be explained by structure differences: in the non-rotatingcase, the convective zone associated with the He shell is more extended than in the rotating case,and reaches the H shell, leading to a strong production of primary N.The picture becomes very different if we take into account the supposed fate of the star ac-cording to Heger & al. (2003). In the non-rotating case, the models with masses between 25 and85 M do not contribute at all to the enrichment of the medium, having no winds and ending asdirect black holes. In the rotating case, the enrichment is made through a ’wind-only’ contribution,leading to low but non-zero contribution of the enrichment of the medium. An important thing tonote is that the mass lost through winds has very peculiar abundances compared to the compositionof SN-ejecta (see Fig. 2, right), with larger N/O, N/C ratios, and much lower 12C/13C (not shown).4PoS(NIC-IX)091CNO production in the first generation stars Sylvia EkströmTo widen the discussion, we have also computed fast rotating 60 M models at Z = 10−5and 10−8 (see Meynet & al. 2006). In those models, the effects described above are very strong,leading to an increase of the effective surface metallicity of 2 to 6 orders of magnitude, and a massloss of 40-60% of the initial mass. If we compare the wind-only ejecta and the abundances patternobserved in some of the most metal-poor stars of the halo of our Galaxy (see Frebel & al. 2006for HE 1327-2326 and Plez & Cohen 2005 for G77-61), we get an interesting matching able toreproduce the high C, N, and O at the same time (see Fig. 8 of Meynet & al. 2006).References[1] T. Abel, G.L. Bryan, & M.L. 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CNO production in the first generation stars

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