Christallo, S.

Delaude, D.

Gallino, R.

Husti, L.

Ryan, Sean G.

Straniero, O.

English

University of Hertfordshire Research Archive

Mem. S.A.It. Vol. 75, 706c© SAIt 2004 Memorie dellaLead and s-process elements in stars of variousmetallicities: AGB predictions compared withobservationD. Delaude1, R. Gallino1, S. Cristallo2, O. Straniero2, L. Husti3 and S. Ryan41 Dipartimento di Fisica dell’Universita´ di Torino, via P.Giuria 1, 10125 Torino, Italy2 Osservatorio Astronomico di Collurania, 64100 Teramo, Italy3 University of Bucharest, Romania4 Open University, Walton Hall, Milton Keynes, UKAbstract. We present AGB predictions for all heavy elements within a large range of13C-pocket efficiencies for stars of different metallicities, and compare them in detail with anumber of spectroscopic observations of s-rich and lead-rich in the Galaxy. The current con-cept of the s-process efficiency, specified by the [hs/ls] index, is shown to be inappropriatefor the metal poor AGB stars and a second independent index, [Pb/hs] or [Pb/ls], needs tobe introduced. The state-of-the-art concerning the interpretation of lead stars allows a verylarge spread of [Pb/hs] in metal poor stars, as typically observed. We discuss agreementsand discrepancies for a large range of elements.Key words. AGB stars – AGB nucleosynthesis – Lead stars – s-process1. IntroductionSpectroscopic detection of lead requires veryhigh-resolution spectroscopy. This is why lead-rich stars have only been observed in the lastfew years. As discussed in Travaglio et al.(2001), the s-process can bring forth a largeproduction of lead in AGB stars at low metal-licity. In fact lead and bismuth are at the termi-nation points of the s-fluence. Using a primary-like neutron source (like the 13C(α,n)16O reac-tion in interpulse phases) and starting with avery low initial metallicity, most iron seeds areconverted into 208Pb. So, when third dredge upepisodes mix the neutron capture products intoSend offprint requests to: R. GallinoCorrespondence to: gallino@ph.unito.itthe envelope, the star appears s-enhanced andlead-rich.2. The intrinsic index [Pb/hs]Commonly s-rich stars are classified as eitherintrinsic or extrinsic, with extrinsic being thosestars that become s-enhanced not because ofinternal nucleosynthesis but through receivings-rich material from a companion in a binarysystem by mass transfer. To characterize neu-tron capture process efficiencies, without dis-tinguishing between these two types of ob-jects, usually the intrinsic index [hs/ls] is used(where hs is the average abundance of theheavy s-elements Ba, La, Nd, Sm, and ls of thelight s-elements Y, Zr). However, at low metal-D. Delaude et al.: Lead and s-process elements in stars of various metallicities 707708 D. Delaude et al.: Lead and s-process elements in stars of various metallicitiesD. Delaude et al.: Lead and s-process elements in stars of various metallicities 709Fig. 4. Fits of a large sample of s-rich and lead-rich stars with AGB model predictions.licities, another intrinsic index is required.In fact at decreasing metallicities [hs/Fe] and[ls/Fe] converge within a small range, while[Pb/Fe] always increases (see the figures inGallino et al. 2004 - hereafter Paper I). Here,in Figure 1, [hs/ls] is plotted as a function ofmetallicity for various choices of 13C-pockets.The standard case (ST) is the one that for[Fe/H]=−0.3 best reproduces the main compo-nent of the solar system (Gallino et al. 1998).Different 13C-pockets can provide very simi-lar [hs/ls] indices, but still show a large rangeof [Pb/ls] and [Pb/hs] values. These are plottedrespectively in Figure 2 and Figure 3; for in-stance consider the cases ST*1.3 and ST/3 at[Fe/H]=−2.3. Comparison of models withobservationsUsing AGB nucleosynthesis models with dif-ferent 13C-pockets efficiencies, initial massesand metallicities, we tried to fit the spectro-scopic abundances of the s-rich and lead-richstars listed in the Table of Paper I. The fitis made by comparing the element distribu-tion observed in each star with the distribu-710 D. Delaude et al.: Lead and s-process elements in stars of various metallicitiesFig. 3. [Pb/hs] versus [Fe/H] for different 13C-pocket choices. Spectroscopic data of s-richand lead-rich stars are included for compari-son.Fig. 1. [hs/ls] versus [Fe/H] for different 13C-pocket choices.Fig. 2. [Pb/ls] versus [Fe/H] for different 13C-pocket choices.tion predicted by AGB models with different13C-pocket efficiencies. Such a large spread isjustified by observation of MS, S, C(N), Bastars in the disk (see Busso et. al 2001; Abiaet al. 2002). In Figure 4 we report the fits oflead stars not yet presented in Paper I. To com-pare our predicted abundances with observa-tions, in many cases we apply a dilution thatsimulates the effect of mixing of s-rich ma-terial in the envelope of extrinsic stars, us-ing the rule: dil=log(Minienv/Mtransf), where Minienvis the initial envelope mass and Mtransf is themass of transfered material. The choice of 13C-pocket efficiency, the initial mass, the initial[Eu/Fe] assumed (see below), the dilution fac-tor and the metallicity are indicated in eachplot. In AGB stars of low metallicity, the s-process feeds Eu at a consistent level, witha constant ratio [Ba/Eu]s ∼ 0.7. For severallead stars the spectroscopic observation indi-cates a lower [Ba/Eu] ratio than predicted,which may imply a different [Eu/Fe]ini in theparent cloud. Indeed, unevolved halo stars inthe same range of metallicity show an average[Eu/Fe] = 0.5, with a large spread ∆[Eu/Fe]= ± 0.5 dex (see Travaglio et al. 2004). Theadopted [Eu/Fe]ini is indicated in each panelof Figure 4. Notice that the rule adopted for[Eu/Fe]ini has been applied also to other el-ements of major r-process origin, e.g. for allthe elements from Eu to Tm. For the star CS31062-050 (Johnson et al. 2004) lines of Crand Mn have been detected, and somewhatnegative [Cr/Fe] and [Mn/Fe] values have beendeduced. The s-process in AGB stars producesvery little Cr and Mn, however unevolved halostars in the same range of metallicity show adepletion of both Cr and Mn, with an average[Cr/Fe]= −0.2 and [Mn/Fe]= −0.4 (see e.g.,Franc¸ois et al. 2004). As shown in Figure 4for CS 31062-050, adopting these initial val-ues a satisfatory agreement is reached. Forsome stars, the observed [Ba/Fe] appears sig-nificantly higher than [La/Fe], whereas AGBmodels predict [Ba/Fe] ' [La/Fe]. This mayindicate difficulties in the determination of theBa abundance. In general, lanthanum is a morerepresentative element of the second s-peak atneutron magic number, N = 82.In several cases we need to attain high[hs/ls] values without changing [Pb/hs]; ac-cording to our AGB models, those valuesD. Delaude et al.: Lead and s-process elements in stars of various metallicities 711can be reproduced using lower initial masseswith respect to the standard mass of 1.5M.Reducing the initial mass corresponds to a de-crease in the number of thermal pulses. Fromthe previous discussion it is clear that a gen-eral comparison of AGB predictions with allthe elements detected provides a better methodrather than being restricted to the average lsor hs values. Anyway, in Figure 5 the hs dataare compared with AGB predictions in the[Pb/hs] versus [Fe/H], showing that the largespread of spectroscopic data are well fittedwithin the large spread of 13C-pocket efficien-cies adopted.4. ConclusionA comparison is made of AGB model predic-tions of low metallicity with spectroscopic dataof a large sample of s-rich and lead-rich stars.Varying the initial mass and adopting a largespread of 13C-pocket efficiencies for a givenmetallicity a satisfactory reproduction of alldata is obtained.Acknowledgements. Part of this work wassupported by the Italian MIUR-FIRB Project“Astrophysical Origin of the Heavy Elementsbeyond Fe”.ReferencesAbia C., et al., 2002, ApJ, 579, 817Aoki W., et al., 2001, ApJ, 561, 346Aoki W., et al., 2002, ApJ, 580, 1149Busso M., et al., 2001, ApJ, 557, 802Cohen J. G., et al., 2003, ApJ, 588, 1082Franc¸ois P., et al., 2004, A&A, 421, 613Gallino R., et al., 1998, ApJ, 497, 388Gallino R., et al., 2004, Nucl. Phys. A, (inpress) (Paper I)Johnson J. A., Bolte M., 2002, ApJ, 579, L87Johnson J. A., Bolte M., 2004, ApJ, 605, 462Lucatello S., et al., 2003, AJ, 125, 875Sivarani T., et al., 2004, A&A, 413, 1073Travaglio C., et al., 2001, ApJ, 549, 346Travaglio C., et al., 2004, ApJ, 601, 864Van Eck S., et al., 2003, A&A, 404, 291

Lead and s-process elements in stars of various metallicities : AGB predictions and observations

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