Primary neutron sources in AGB and massive stars

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Primary neutron sources in AGB and massive stars

• Roberto Gallino
• Universita di Torino
• A continuous collaboration with many friends.
• See the Posters
• BISTERZO
• CRISTALLO
• HUSTI
• PIGNATARI

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Outline

• The first observational constraint solar system
  isotopic composition of heavy elements
• Experimental constraints neutron capture cross
  sections
• AGB stars and the two sources of neutrons
• The choice of 13C-pocket guided by spectroscopic
  observations of s-process enriched stars
  intrinsic and extrinsic AGBs
• Ba stars and CEMP-s(r) stars
• The huge pulse and H remixing at very low
  metallicities
• The Sr, Y, Zr puzzle and the GCE connection
• The weak s component in massive stars

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Based on the neutron capture measuremens (Macklin
and Gibbons (60-70.ies) and solar system
isotopic abundances (Suess and Urey 1956), the
curve sigma(A)Ns(A)
appeared generally smooth, but interrupted by
steep decreases in correspondence of the magic
neutron numbers N 50, 82, 126, where the
neutron capture cross sections are very small and
the resulting s-process abundances are large.
(Clayton et al. 1961, Ann. of Phys. 12, 331
Seeger et al. 1965, Ap.J. Suppl. 11,121) see
Chap. 7 of D.D. Claytons book, 1968) . 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 208Pb (and 209Bi).
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Three s-process components were anticipated by
the classical analysis (Clayton and Rassbach
1974 Kaeppeler, Beer, Wisshak, Clayton,
Macklin, Ward 1982) the weak, the main, and the
strong s-component. The main and the strong
s-components are the outcome of many generations
of Asymptotic Giant Branch stars (AGB) polluting
the interstellar medium before the solar system
formed. Actually, even the main s-component is
far from being a unique process, depending on the
efficiency of the so-called C13-pocket, on the
initial mass, and on the metallicity.
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AGB thermally pulsing stars



thermally pulsing model star
(Straniero et al. 1997 Busso et al. 1999)
TP thermal pulses
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THE AGB ENGINE
( Busso, Gallino, Wasserburg 1999 ARAA, 37, 239 )
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The two neutron sources in AGB
stars 13C(a,n)16O
22Ne(a,n)25Mg
Needs 13C ! Major neutron source 13C-pocket Primar
y source! T8 0.9-1 Interpulse phase (1- 0.4)
105 yr Radiative conditions Nn 107 cm-3
Abundant 22Ne Minor neutron source Neutron
burst Secondary source T8 3 (low 22Ne
efficiency) Thermal pulse 6 yr Convective
conditions Nn (peak) 1010 cm-3
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Reproduction of the main component by an AGB
stellar model (Arlandini et al. 1990 updated
Bisterzo et al, 2006 NIC-IX)

• 13C-pocket choice
• ad hoc modulated
• constant Pulse by Pulse
• Fe/H -0.3
• artificially introduced in the third dredge up
  phases

208Pb
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For a given C-13 pocket efficiency, decreasing
the metallicity and starting from a solar
composition, the s-process fluence progressively
feeds the first s-peak at N50 with minor
contributions beyond A90, then the second s-peak
at N82 with minor contributions at the first
s-peak or at Pb-208, and eventually the third
s-peak at the termination point of the s-process
at N126, with the s-process distribution sharply
peaked at lead-208. As a matter of fact, below
Fe/H -1.5 AGB models offer the most likely
interpretation for the recent spectroscopically
discoveries of very-low metallicity C-rich and
s-process and lead-rich stars.
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• QUESTIONS
• 1 - IS THE s-PROCESS A PRIMARY PROCESS?
• ANSWER IS NO
• 2 - IS THE s-PROCESS A SECONDARY
• PROCESS ?
• ANSWER IS NO
• 3 - CAN ONE PREDICT THE r-PROCESS CONTRIBUTIONS
  TO SOLAR ABUNDANCES AS
• r 1 s ?
• ANSWER IS YES

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hs/ls vs Fe/H
First intrinsic indicator
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Pb/hs vs Fe/H Second intrinsic
indicator
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hs/ls Comparison with Observations.

Busso, Gallino, Lambert, Travaglio, Smith 2001,
ApJ 557, 802 (see also Abia et al. 2002)
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• BARIUM STARS. A recent sample of 26
• Barium giants, subgiants and Ba dwarfs
• at high resolution spectroscopy
• ( Allen and Barbuy 2006 AA) .
• Theoretical predictions versus observations.
• (See Poster HUSTI)

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• CEMP-s stars Extrinsic AGBs of very low
• metallicity
• The Lead stars
• Nb indicator of an extrinsic AGB in a binary
  system
• F, Na, Mg permit an estimate of the initial AGB
  stellar mass.
• (see Poster BISTERZO )

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Predictions updated NEW!
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The La to Eu ratio in metal-poor AGB stars
AGB of M 1.3 Msun, Fe/H -2.60
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CEMP sr. Vanhala and Cameron (1998) showed
through numerical simulations how the supernova
ejecta may interact with a nearby molecular
cloud, polluting it with fresh nucleosynthesized
material and triggering the condensation of
binary system of low mass. Note that according to
Lucatello et al. (2005) all C-rich and s-rich
metal-poor stars show binarity from their radial
velocity temporal variations. CEMP-sr is a
common phenomenon. The possibility of even a
strong pre-enrichment of r-process elements does
not affect the s-process nucleosynthesis
occurring later in the more massive AGB companion
followed by wind accretion on the observed star.
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CS 31062-050
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HE 0024-2523
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• IN LOW METALLICITY STARS
• 22Ne (primary) source of neutrons
• 22Ne major neutron poison
• 22Ne and primary Light Isotopes
• 22Ne source of s-process iron, bridge
• to s-process elements production

• (Gallino, Bisterzo, Husti, Kaeppeler, Cristallo,
  Straniero NiC-IX, CERN, Geneva 2006,
  isna.confE. PoS100)

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A. 22Ne (primary) source of neutrons

• 12C is mixed with the envelope by third dredge up
  episodes
• H-shell burning converts CNO into 14N, including
  primary 12C
• During the next thermal pulse 14N(a,g)18F(bn)18O
  (a,g)22Ne(a,n)25Mg
• A large abundance of primary 12C is produced by
  partial He burning during each thermal pulse (3-a
  reaction)
• A large fraction of 22Ne survives

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C. Primary 22Ne and Light Isotopes at very low
metallicity

• 19F, 20Ne, 21Ne, 22Ne, 23Na, (24Mg), 25Mg, 26Mg,
  27Al, 31P primarily produced by neutron capture
• Putting 22Ne(n,g)23Ne to zero, the abundances of
  23Na and its progenies decrease drastically

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22Ne as source of s-process iron

• 56Fe produced in this way is used to build up
  s-elements, up to lead
• At very low metallicities iron is produced by
  neutron captures starting on 22Ne

• TEST
• Initial abundance of 56Fe put to zero

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M1.5 M_sun Fe/H-2.6 Initial 56Fe
abundance 0
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• AGB MODELS
• AT VERY LOW METALLICITY
• THE HUGE-PULSE
• HUGE TDU
• HUGE C13 POCKET
• HUGE LEAD PRODUCTION
• HUGE 7Li, 13C, N, F, Na
• ( See POSTER CRISTALLO)

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Sergio Cristallo
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Cristallo, S. Straniero O. Dominguez, I. Gallino,
R.
M1.5MSUN
Fe/H-2.5
Maximum neutron density
Bibliography Hollowell, Iben, Fujimoto
1990 Fujimoto, Ikeda, Iben 2000 Iwamoto et al.
2004 Suda et al. 2004
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C N O FNeNa Mg
LIGHT ELEMENTS PRIMARY PRODUCTION AT VERY
LOW METALLICITY
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Effects of the Huge Pulse Surface C/O
Surface 12C/13C
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HOW MUCH LITHIUM?
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THE IMPACT OF THE GALACTIC CHEMICAL
EVOLUTION
(GCE) From the Galactic chemical evolution
model (Travaglio et al. 1999) we obtain that
AGB stars produced 94 of the solar 208Pb
(Note Solar Pb is given at 6 uncertainty) The
main component at Fe/H - 0.30 (Arlandini et
al. 1998) only accounts for 43 of the solar
208Pb
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Travaglio et al., ApJ 521, 691 1999
Ba/Fe, Eu/Fe and Ba/Eu vs Fe/H
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The Sr, Y and Zr puzzle how many n-capture
components?
Sr/Fe, Y/Fe and Zr/Fe vs Fe/H
Travaglio et al., ApJ 601, 864 (2004).
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GALACTIC CHEMICAL EVOLUTION AGB CONTRIBUTION AT
THE EPOCH OF SOLAR SYSTEM FORMATION
208Pb
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• The puzzle of Sr, Y, Zr in the Halo
• No classical-r no weak-s (nor main-s!)
• LEPP (Light Element Primary Production,
• John Cowan invenit)
• (Travaglio et al 2004 ApJ)
• LEPP in massive stars accounts for about 20 of
  the solar abundance of these elements.
  Relationship with the weak r-process?

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The weak s-process in Massive stars(see
Poster Pignatari)
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Meynet et al. 2006 AA 447 623 (and reference
therein) suggested a primary production of 14N
triggered by fast rotation effect, with X14
0.01 (!) for metallicities lower than 1/3
solar. This implies a huge primary 22Ne neutron
source at the end of core He-burning as well as
during the following convective shell
C-burning.
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COPPER and their twins Gallium and
Germanium.Synthesised in massive stars10 as
primary element inthe Silicon Burning region,
and90 by neutron captures in massive
starsduring core He burning and shell C
burning(light elements weak s-process)A best
example of a secondary-like process (Woosley and
Weaver 1995 ApJSTimmes, Woosley, Weaver 1995
ApJS)
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Post-Supernova composition (yields)
Cu
The secondary-like isotopes show an
overproduction of 100 respect to the initial
mass fraction in the region of C-shell and
He-shell
M25 Msun, ZZsun (Nucl. Data Page, A. Heger)
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New high-resolution spectroscopy results
r-rich stars BD173248 HD 155444 CS 22892-052
Primary contribution
Secondary-like contribution
SNIa contribution to Fe
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