Charbonnel, C

Decressin, T

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PoS(NIC-IX)086Nucleosynthesis and mixing in rotating AGB stars atlow metallicityT. Decressin∗a, C. CharbonnelabaGeneva Observatory, 51, chemin des Maillettes, 1290 Sauverny, SwitzerlandbLA2T, OMP, CNRS UMR 5572, 14, Av. E. Belin, 31400 Toulouse, FranceE-mail: Thibaut.Decressin@obs.unige.ch,Corinne.Charbonnel@obs.unige.chModels of rotating stars at Z = 5×10−4 and 10−5 with masses between 2.5 and 7 M⊙ are com-puted from the pre-main sequence up to the end of the asymptotic giant branch phase (AGB).We discuss the impact of rotation on nucleosynthesis. We show that it plays a major role duringthe phase of central He-burning. Rotational-induced mixing transports indeed freshly producedcarbon and oxygen into the H-burning shell where some new primary elements are synthesized.In the most massive early-AGB stars, those elements are revealed at the surface during the seconddredge-up event. We also follow the changes in surface and wind composition due to the interplaybetween third dredge-up and hot bottom burning during the TP-AGB phase. We present the finalstellar yields and discuss our predictions in the context of the chemical anomalies seen in galacticglobular clusters.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)086Rotating AGB stars T. Decressin1. Physical inputs of the stellar modelsBoth rotating and standard models for stars with initial masses between 2.5 to 7 M⊙ are com-puted at Z = 5×10−4 ([Fe/H] = −1.6). The metallicity dependence of our theoretical predictionsis investigated for 7 M⊙ for which we compute a model at Z = 10−5. The initial rotation velocityof our models is taken equal to 300 km.s−1.During main sequence, we follow in a self-consistent way the transport of angular momentumand of chemicals through meridional circulation and turbulence using the prescriptions by [1] and[2] (see also [3]). During He-burning, only local angular momentum conservation is used as struc-tural changes now dominate the momentum evolution. Rotational mixing is stopped during AGBphase.During the AGB phase mass loss is switched from Reimers prescription [4] to Blöcker one[5] (with η = 0.05) with an explicit √Z dependence. Diffusive overshooting [6] ( fover = 0.016) isadded at the bottom of the convective envelope during the TP-AGB phase.2. Pre-AGB phasesFigure 1: Chemical profiles of selected nuclei for rotating (left) and standard (right) 5 M⊙ stars at the endof central He-burning phase. The arrows indicate the deepest penetration of the convective envelope duringthe second dredge-up phase.During the pre-AGB phases, rotation-induced mixing modifies the chemical structure of thestar with respect to the standard predictions. One striking effect can be seen in Fig. 1 where thechemicals profiles of some key elements at the end of the central He-burning phase are displayedfor the 5 M⊙ for rotating and standard cases. The non rotating models present a sharp CO coresurrounded by a He-rich layer where the signatures of the CNO cycle are well visible. On the otherhand, the rotating models show the signatures of an important diffusion of C and O from the core2PoS(NIC-IX)086Rotating AGB stars T. DecressinRotating models Standard modelsFigure 2: Ratio of surface abundances after 2DUP completion to initial ones for the rotating (left) andstandard model (right).to the more external layers. Part of this C and N is converted into N in the He-rich layers wheresome protons are also mixed.For these patterns to be revealed at the surface requires the occurrence of the second dredge-up(2DUP) event. This happens only in the higher mass stars (M ≥ 4 M⊙; in less massive stars theconvective envelope sinks only into the H/He discontinuity during the 2DUP). In these objects theconvective envelope then sinks into the He-rich layers as shown in Fig. 1 where the arrows indicatethe deepest penetration of the convective envelope during the 2DUP phase.Fig. 2 illustrates the surface abundance variations after the 2DUP for all the stellar massesconsidered, in both the standard and rotating cases. Only the elements lighter than Mg are affected.For the low mass stars, only the layers where the CNO cycle operates outside of the He-rich layersare reached by the deepening convective envelope at that phase. More massive rotating stars show astrong increase of primary C, O, N and F abundance at the surface after the 2DUP as those elementsare synthesized in the He-rich layers before being dredged by the convective envelope. The 7 M⊙rotating model at Z = 10−5 shows stronger abundance variations.3. Winds on the TPAGB phase and comparison with globular cluster starsTable 1 presents some structural parameters of AGB models. Core is more massive in rotatingmodels at the beginning of the AGB phase as some mixing have extended this core during centralHe-burning phase. Recurring third dredge-up (3DUP) prevent strong increase during TPAGB asthe final mass of the remnant is only a few 0.1 M⊙ heavier.Now we turn our attention to the chemical changes for low mass stars (M ≤ 3 M⊙). Thesurface abundance patterns obtained during the 2DUP is only modified by recurring 3DUP events(see Fig. 3). It mainly increases the carbon abundance. Fluorine abundance raises after each 3DUPand a positive production if found in the mean composition of winds.3PoS(NIC-IX)086Rotating AGB stars T. DecressinRotating models Standard modelsM (M⊙) M2DUP (M⊙) NTP λmax Mf (M⊙) M2DUP (M⊙) NTP λmax Mf2.5 0.72 24 0.73 0.78 0.67 30 0.68 0.743 0.84 24 0.82 0.86 0.77 30 0.77 0.834 0.86 33 0.80 0.89 0.86 27 0.85 0.885 0.89 49 0.79 0.91 0.90 30 0.88 0.927 1.07 >28 0.86 – 1.01 >15 1.00 –Table 1: Structural parameter of TPAGB phase for rotating and standard models at Z = 5×10−4: core massafter 2DUP completion, number of thermal pulses, maximum dredge-up efficiency and final core massRotating models Standard modelsFigure 3: Ratio of mean abundances in stellar winds to initial composition for rotating (left) and standard(right) models. The typical abundance variations seen in globular cluster stars are indicated by grey bars.In massive TP-AGB stars (M ≥ 4 M⊙) the bottom of the convective envelope reaches temper-atures between 50 and 70× 106 K, and hot bottom burning (HBB) occurs. It converts part of thefresh primary carbon and neon produced within the convective thermal pulses into nitrogen andsodium respectively. However the Na abundance stays close to the level obtained after 2DUP. Mgand Al surface abundance have a similar evolution in the rotating and standard cases because theywere not modified in the pre TP-AGB phases. 25Mg and 26Mg increase due to their synthesis withinthe thermal pulses followed by 3DUP. As 24Mg is not consumed the total Mg abundance increases.AGB are often thought to be responsible of abundance anomalies seen exhibited by globularcluster (GC) stars (see [7] in these proceedings). In Fig. 3 we thus compare our predictions for thewind composition with the GC data. The C-N anticorrelation [8] is obtained in none of our modelsas AGB winds display strong carbon and nitrogen enrichment due respectively to the 3DUP andHBB. Low-mass rotating AGBs slightly deplete O as the results of rotational mixing during thepre-AGB phase. However this O depletion is much too small to explain the oxygen data shown inGCs. In all the models the raise in the Na abundance well matches the observational trends. Last4PoS(NIC-IX)086Rotating AGB stars T. Decressinbut not least the theoretical wind composition is at odd with the Mg-Al anticorrelation seen in GCsas all our AGB stars produce magnesium together with little quantities of aluminum. In additionthe theoretical magnesium isotopic composition that does not recover the one found by [10] (24Mgappears to be depleted in observed GC stars).4. ConclusionRotation has several effects on the chemical patterns of low- and intermediate-mass stars.During the central He-burning phase in particular it leads to the mixing of the He-burning productsinto the He-rich layers where some protons are also transported from the radiative envelope. ThereN is synthesized. This effect is dominant in high-mass AGB stars whose convective envelopepenetrates into those deep layers during the 2DUP. Very low metallicity stars are very sensitive tothis mechanism: due to their low initial abundance the dilution is stronger.The 3DUP and HBB modify the patterns resulting from the 2DUP. In low mass AGBs theevolution of the surface abundances of nitrogen, oxygen is dominated by the 2DUP whereas theircarbon abundance raises mainly thank to the 3DUP on the TP-AGB. The same is true for massiveAGB except that nitrogen increases also thanks to HBB on the TP-AGB.When compared to GC data one finds that only the Na increase can be reproduced by allthe models; however only the low-mass rotating AGB stars slightly deplete O. None of the otherobserved patterns is reproduced by the AGB models. We must thus conclude that these stars areprobably not responsible for the GC abundance anomalies.References[1] Zahn J.-P., 1992, A&A, 265, 115[2] Maeder A., Zahn J.-P., 1998, A&A, 334, 1000[3] Palacios, A., Charbonnel, C., Talon, C., Siess, L., 2005, A&A, 453, 261[4] Reimers D., 1975, Mem. Soc. Roy. Sci. Liège, 6th Ser., 8, 369[5] Blöcker T., 1995, A&A, 297, 727[6] Herwig F., 2000, A&A, 360, 952[7] Charbonnel C., 2006, in proceedings of Nuclei in the Cosmos - IX[8] Carretta E., Gratton R.G. Lucatello S. et al., 2005, A&A, 433, 597[9] Gratton R.G, Bonifacio P. & Bragaglia A., 2001 A&A, 369, 87[10] Yong D., Lambert D. L., Ivans I. I., 2003, ApJ, 599, 1357[11] Pasquini L., Bonifacio P., Molaro P. et al., 2005, A&A, 441, 549[12] Smith V.V., Cunha K., Ivans I.I. et al., 2005 ApJ, 633, 3925

Nucleosynthesis and mixing in rotating AGB stars at low metallicity

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