Download The slow-neutron capture process at low metallicity

The slow-neutron capture
process at low metallicity
Amanda Karakas (Mt Stromlo, ANU)
Maria Lugaro (Monash), Richard Stancliffe (ANU)
& Carlos Rijs (Monash)
Overview of talk
1.  We present theoretical models for the slow neutron capture (s-process)
in asymptotic giant branch (AGB) stars of metallicity Z = 0.0001 or [Fe/H]
= -2.3
2.  Mass range from 0.9Msun to 6Msun
3.  AGB models from two stellar evolution codes: 1) Stromlo (Karakas
2010), and 2) STARS (Stancliffe 2010)
4.  We encountered 4 regimes of neutron-capture nucleosynthesis
5.  Models where 13C burns radiatively (≈2Msun) produce an overall good
match to the composition of carbon-enhanced metal-poor (CEMP)-s
stars
6.  None of the models produce a match to the composition of the CEMP-s/r
stars
Lugaro, Karakas, Stancliffe, & Rijs (2011, ApJ, submitted)
Carbon enhanced metal-poor stars
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Roughly 10-20% of old halo stars are C-rich ([C/Fe] > 1; Cohen et al.
2005; Carollo et al. 2011)
Of these ~2/3 show enrichments in heavier elements (e.g., Aoki et al.
2007)
CEMP-s/r
CEMP-s
Using the data and classification of Masseron et al. (2010)
The puzzle of the CEMP-s/r stars
•  About 50% of CEMP stars with an
s-process signature also show an
enrichment in r-process elements
•  It is puzzling how CEMP-s/r could
have formed in such large
numbers
•  Given that the s and r-processes
are thought to occur in
independent events
CEMP-s/r
–  s-process (AGB stars; M < 8Msun)
–  r-process (supernovae, M > 10Msun)
•  And CEMP-r stars are rarer in
nature (~10 known after targeted
surveys)
Definition of CEMP s/r:
•  [Eu/Fe] > 1
•  [Ba/Eu] > 0 but lower than for CEMP-s
(0.9 c.f. 0.6)
•  Appear distinct from CEMP-s
AGB stars and the s-process
The s process is responsible for
the production of about half the
abundances of elements heavier
than iron in the Galaxy
From low-mass stars (~1-3Msun)
[ls/Fe] = ([Sr/Fe]+[Y/Fe]+[Zr/Fe]/3)
[hs/Fe] = ([Ba/Fe]+[La/Fe]+[Ce/Fe])/3
s-process peaks
During the s process:
Time scale (n,g) << τβ
Light-s (ls)
heavy-s (hs)
Aims:
1.  Explain the light and heavy
element composition of CEMP-s
stars as a population
2.  Origin of CEMP-s/r stars
The s-process in AGB stars: 1) 22Ne(a,n)25Mg
The 22Ne(α,n)25Mg neutron source operates inside convective thermal pulses
when the temperature reaches ≥ 300 million K (Main regime for M > 3Msun)
tneutrons ~ few years
22Ne(α,n)25Mg
Interpulse phase (t ~ 103-4 years) for M ≥ 3Msun
Examples: 6Msun, [Fe/H] = -2.3
•  Dominated by production at the first s-process peak, notably Rb ([Rb/Fe]
= 2.1) with some Ba ([Ba/Fe] = 1.7) and Pb
•  Would produce a Nitrogen EMP, not a CEMP where [C/N] < 0
•  Could future surveys find the few objects that may have resulted from
mass transfer from such a low-metallicity, intermediate-mass AGB star?
2.5
6Msun, Z=0.0001
2
[X/Fe]
1.5
1
0.5
0
-0.5
30
40
50
60
Atomic Number, Z
70
80
2) Radiative 13C(α,n)16O
13C(α,n)16O
neutron source burns radiatively, with 13C produced by the
inclusion of a 13C pocket (Main regime for ~1.75Msun to 3Msun)
proton
diffusion
tneutrons ~ 103 years
13C(α,n)16O
Interpulse phase (t ~ 104-5 years) for M ≤ 3Msun
3) Convective 13C(α,n)16O
13C(α,n)16O
neutron source burns convectively during a thermal pulse, with
13C produced by the inclusion of a 13C pocket (first few TPs of ~1.25 and
1.5Msun)
proton
diffusion
13C
13C(α,n)16O
Interpulse phase (t ~ 105 years)
burnt as ingested
4) Proton ingestion and convective 13C(α,n)16O
13C(α,n)16O
neutron source burns convectively, with 13C produced by the
ingestion of a small number of protons in the TPs (Main regime for 0.9 and
1Msun models)
Proton
ingestion
13C(α,n)16O
Protons burnt as ingested
Interpulse phase (t ~ 105 years)
Examples: 0.9Msun, [Fe/H] = -2.3
•  No 13C pocket needed for production of s-process elements due to proton
ingestion during first few thermal pulses
•  Neutrons released in these conditions produce lower [Pb/ls] ratios than
when released in 13C pockets
2.5
•  Model becomes C rich, where
final [C/Fe] = 2.8
•  Proton ingestion leads to N
production, [N/Fe] = 1.9
•  Li, O, F, Na also produced
•  Very different to slightly more
massive 1Msun model, where
[C/Fe] = 1.9 and [N/Fe] = 0.9
•  See models of Suda &
Fujimoto (2010) and
Campbell & Lattanzio (2008) 0.9Msun, Z = 0.0001
2
[X/Fe]
1.5
1
0.5
0
-0.5
30
40
50
60
Atomic Mass, Z
70
80
Results: [Ba/Fe] versus [Eu/Fe]
Top panel: results of different masses,
scaled solar initial composition
Lower panel: results of variations in the
initial composition for the 2Msun
Stromlo model
Summary:
•  All models produce Ba and Eu
with the prediction lines following
the trend of the CEMP-s group
•  AGB models do not produce the
high [Eu/Fe] seen in the CEMP-s/
r stars
•  Increasing the initial [r/Fe]
produces same final [Ba/Fe]
•  Correlation between Ba and Eu
of CEMP-s/r group not
reproduced
Results: [ls/hs] versus [Mg/hs]
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Use “intrinsic” indicators,
elemental ratios that only
include elements produced in
AGB stars
Almost independent of model
uncertainties (third dredge-up,
mass loss, accretion, mixing
processes)
All our AGB models produce
[ls/hs] > -1, similar to CEMP-s
This is a basic fact about the sprocess and comes from
neutron-capture cross sections
CEMP-s/r have the lowest [ls/
CEMP data from Masseron et al. (2010). Data for ls is
hs] and [Mg/hs] values
taken from the SAGA database (Suda et al. 2008)
ls = light s-process elements (Sr, Y, Zr), hs = heavy s elements (Ba, La, Ce)
Results: Sodium and fluorine
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Models where 13C burns radiatively
provide a good match to the overall
composition of CEMP-s stars in
terms of their [Mg/hs], [ls/hs], and
[Pb/hs]
But produce too much Na and F
with respect to the heavy s-process
elements
Could be related to the formation of
the 13C pocket (and 14N pocket)
Leads to Na production via 14N(a,γ)
18O(a,γ)22Ne in intershell then 22Ne
(p,γ)23Na
CEMP data from Masseron et al. (2010)
Data for Na from Lucatello et al. (2011) Summary
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We present models for the slow neutron capture (s-process) in AGB stars
of metallicity, Z = 0.0001 [Fe/H] = -2.3
Mass range from 0.9Msun to 6Msun, where we have explored model
uncertainties by using two independent stellar evolution codes
We encountered 4 regimes of neutron-capture nucleosynthesis
Models where 13C burns radiatively (≈2Msun) produce an overall good
match to the composition of carbon-enhanced metal-poor (CEMP)-s stars
CEMP-s/r stars characterized by larger enhancements of elements at the
2nd s-process peak (e.g. Ba, La), of Eu and Pb, compared to elements the
1st s-process peak (e,g., Y, Zr)
Cannot be explained by AGB nucleosynthesis
Can these compositions be achieved by a “s/r” neutron-capture process?
Parametric models would be a first step establishing this possibility
CAIRNS CONVENTION CENTRE -­‐ AUSTRALIA Nuclei i n t he C osmos X II -­‐
Interna=onal S ymposium o n N uclear Astrophysics i n C airns, A ustralia
5th-­‐10th August 2012 http://www.nic2012.org/ The ANU NIC School will take place the week before in
Canberra, Australia (30th July – 3rd August 2012)
The s-process in AGB stars
Low mass AGBs
Lower temperature
~4 Msun
Intermediate mass AGBs
Higher temperature
proton
diffusion
13C(α,n)16O
22Ne(α,n)25Mg
Interpulse phase (t ~ 105 years)