The s-process in low metallicity stars Roberto Gallino (1) Sara Bisterzo (1) Oscar Straniero (2) I. I. Ivans (3, 4) F. Kaeppeler (5) (1) Dipartimento di Fisica Generale, Università di Torino, Torino ( Italy) (2) Osservatorio Astronomico di Collurania – Teramo, (3) The Observatories of the Carnegie Institution of Washington, Pasadena, CA, (USA) (4) Princeton University Observatory, Princeton, NJ (USA) (5)Forschungszentrum Karlsruhe, Institut fuer Kernphysik, Karlsruhe (Germany) Winter School on Nuclear Astrophysics, Hirschegg January , 2006

The s-process is characterized by a generally smooth curve sigma(A)Ns(A) versus atomic mass number A, but interrupted by steep decreases in correspondence of magic neutron numbers N = 50, 82 or 126, where the neutron capture cross sections are very small and the resulting s-process abundances are large. This happens at the first s-peak at Sr, Y, Zr, at the second s-peak at Ba, La, Ce, Pr, Nd and eventually at the termination of the s-process involving Pb-208 (and Bi).

Three s-process components were anticipated by the classical analysis (Clayton and Rassbach 1974; Kaeppeler et al. 1982): the weak, the main, and the strong s-component. The main s-component is the outcome of many generations of Asymptotic Giant Branch stars (AGB) polluting the interstellar medium before the solar system formed. Actually, the main s-component is far from being a unique process, depending on the efficiency of the so-called C13-pocket, the initial mass, and metallicity.

Reproduction of the Solar Main Component (Gallino et al. 1998) 13 C-pocket choice: artificially introduced ad hoc modulated constant Pulse by Pulse AND METALLICITY [Fe/H] = -0.3

[ls/Fe] vs [Fe/H] ls =(Y, Zr) envelope last pulse condition

[hs/Fe] vs [Fe/H] hs =(Ba, La, Nd, Sm) envelope last pulse condition

[Pb/Fe] vs [Fe/H] envelope last pulse condition

[hs/ls] vs [Fe/H] First intrinsic indicator envelope last pulse condition

Second intrinsic indicator [Pb/hs] vs [Fe/H] envelope last pulse condition

Today, the typical mass of an intrinsic AGB HALO STAR is ~ 0.6 M sun (initial mass 0.8 – 0.9 Msun): NO TDU  No C or s-process enrichment observable. A. Intrinsic Halo AGBs B. Extrinsic Halo AGBs (Dwarfs - Giants) P ~ 2-3 yr (13 Gyr ago)

The s elements enhancement in low-metallicity stars interpreted by mass transfer in binary systems (extrinsic AGBs). For extrinsic AGBs [Zr/Nb] ~ 0. Instead, for intrinsic AGBs [Zr/Nb] ~ – 1. Zr over Nb: Intrinsic or Extrinsic AGBs Fig. 2 s-process path

Choice of initial abundances

UPDATED

Light elements C N O F Ne Na Mg Al