Metal dependence of light and heavy Metal dependence of light and heavy ss--process elements in AGB starsprocess elements in AGB stars

Roberto Gallino (1)

Sara Bisterzo (1)

Oscar Straniero (2)

I. I. Ivans (3, 4)

(1) Dipartimento di Fisica Generale , Università di Torino, 10125 (To) Italy

(2) Osservatorio Astronomico di Collurania – Teramo, 64100(3)The Observatories of the Carnegie Institution of Washington,

Pasadena, CA, (USA) (4)Princeton University Observatory, Princeton, NJ (USA)

IX Torino Workshop - Granada, February 5 - 10, 2006

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 generationsof 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-calledC13-pocket, the initial mass, and metallicity.

Reproduction of the Solar Main Component(Gallino et al. 1998)

13C-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

Zr over Nb: Intrinsic or Extrinsic AGBs

The s elements enhancement in low-metallicity stars interpreted by mass transferin binary systems (extrinsic AGBs). For extrinsic AGBs [Zr/Nb] ~ 0. Instead, for intrinsic AGBs [Zr/Nb] ~ – 1.

Fig. 2s-process path

Choice of initial abundances

UPDATED

Light elements

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5 [X

/Fe]

5 7 9 11 13 Z

CN

O

FNe

Na

Mg

Al

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

Second intrinsic indicator

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

C-rich and no s-rich AGB stars

ls hs Pb

ls hs Pb

Case of M = 1.5 Msun and ST/75

ls hs Pb

The The SrSr, Y and , Y and Zr Zr puzzle: how puzzle: how many nmany n--capture capture components?components?

[[SrSr/Fe], [Y/Fe] and /Fe], [Y/Fe] and [[ZrZr/Fe] /Fe] vsvs [Fe/H][Fe/H]

TravaglioTravaglio et al., et al., ApJApJ 601, 864 (2004).601, 864 (2004).

[[BaBa/Fe], [/Fe], [EuEu/Fe] and [/Fe] and [BaBa//EuEu]] vsvs [Fe/H][Fe/H]

Travaglio et al., ApJ 521, 691 1999

##Vanhala and Cameron (1998) showed through numerical simulationshow the supernova ejecta may interact with a nearbymolecular cloud, polluting it with fresh nucleosynthesizedmaterial and triggering the condensation ofbinary system of low mass. Note that according to Lucatello et al.(2005) all C-rich and s-rich metal-poor stars show binarityfrom their radial velocity temporal variations.The subsequent evolution of such close binary systems,and how the pre-enrichment of r-process elementsinfluences the s-process occurring in the more massiveAGB companion is discussed and compared with a dozenof very-low metallicity lead stars, s and r process rich.

##For a given C-13 pocket efficiency, by decreasing the metallicity and starting from a solar composition, the s-process fluence progressively feeds the first s-peak at N=50 with minor contributions beyond A=90,then the second s-peak at N=82 with minor contributionat the first s-peak or at Pb-208, and eventually the third s-peak at the termination pointof the s-process at N=126, with the s-process distributionsharply peaked at lead.As a matter of fact, for a given C13-pocket choice(Case ST of Gallino et al. 1998), below [Fe/H] = -1.5 AGB models offers the most likely interpretation forthe recent spectroscopically discoveries of very-low metallicity C-rich and s-process and lead-rich stars.

##For a given metallicity, with varying the C-13 pocket efficiency a large spread of[Pb/hs], by two orders of magnitudes, is predicted,in agreement with spectroscopic observations.In conclusion, the strong s-component in the solar systemis the result of all previous generations of haloAGB stars, which mostly produce Pb (50 percent of solar Pb-208) and 20 percent of solar Bi(Travaglio et al. 2001).Among the couple of dozen of very metal-poor starsC-rich and s-process rich discovered so farthere is a consistent number of objects showingthe unusual characteristic of being also r-process rich.

References1. J. A. Johnson, M. Bolte, ApJ 579, L87 (2002)2. W. Aoki, et al., ApJ 580, 1149 (2002)3. T. Sivarani, et al., A&A 413, 1073 (2004)4. J. A. Johnson, M. Bolte, ApJ 605, 462 (2004)5. W. Aoki, et al., ApJ 561, 346 (2001)6. S. Van Eck, S. Goriely, A. Jorissen, B. Plez, A&A 404, 291 (2003)7. S. Lucatello, et al., AJ 125, 875 (2003)

8. J. G. Cohen, N. Christlieb, Y. Z. Quian, G. J. Wasserburg, ApJ 588, 1082 (2003)9. B. Barbuy, et al., A&A 429, 1031 (2005)10. S. Lucatello, PhD Thesis (2003)11. A. V. Yushchenko, et al. A&A 413, 1105 (2004)12. I. Ivans et al., ApJ accepted (2005)

s-process peaks:

light-s-elements peak ( ls ): Sr, Y, Zr

heavy-s-elements peak ( hs ): Ba, La, Ce, Pr, Nd, Sm208Pb and Bi

are at the termination point

of the

s-fluence

(Ratzel et al. PRC 70 065803 2004)

With a primary-like 13C-pocket, lead becomes the major

s-process product at low metallicity

Spectroscopic Na (and Mg) Stellar Mass prediction

Case ST

[Fe/H] = -2.60

In Fig 4 we show the results of AGB models of M = 1.3, 1.5, 2, 3, and 5 Msun, for the same 13C-pocket, at [Fe/H] = – 2.60. A strong primaryproduction of 22Ne results in advanced pulses, by conversion of primary 12C to 14N in the H-burning ashes, followed by 2αcaptures on 14N in the thermal pulses and implies a primary production of 23Na via 22Ne(n,γ)23Na, (and 23Na(n,γ)24Na(β-)24Mg).

Fig. 4

M = 3 Msun

M = 1.3 Msun

M = 1.5 Msun M = 2 Msun

M = 5 Msun

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 eventuallyat the termination of the s-process involving Pb-208 (and Bi).