Observations of NeutronCapture Elements in the Early Galaxy

1
Observations of Neutron-Capture Elements in the
Early Galaxy

• Chris Sneden
• University of Texas at Austin

2
Involving the Efforts of Many People, Including

• John Cowan
• Jim Truran
• Scott Burles
• Tim Beers
• Jim Lawler
• Inese Ivans
• Jennifer Simmerer
• Caty Pilachowski

• Andy McWilliam
• George Preston
• Debra Burris
• Bernd Pfeiffer
• Karl-Ludwig Kratz
• Francesca Primas
• Rica French
• Taft Armandroff

3
Talk outline

• Reminder of solar r- and s-process breakdown
• General n-capture trends in the Galactic halo
• Star-to-star scatter
• Shift to r-process dominance
• Detailed abundance distributions in a few stars
• Elemental
• Isotopic
• Radioactive element observations
• There is more to halo star life than the
  r-process
• Summary, future questions

4
A detailed view of part of the n-capture
synthesis paths
139
138
La
s,r
p
130
132
134
135
136
137
138
Ba
P
P
s,r
s,r
s,r
s
s
133
Cs
s,r
132
131
130
129
128
134
136
Xe
s,r
r
s,r
r
s,r
s
s
r-process path
s-process path
5
ELEMENTAL r- and s-process solar-system abundances
Data from Burris et al. (2000)
6
General halo n-capture bulk abundance trends
LARGE scatter

• Large-sample surveys are needed to show this
• Gilroy et al. (1988), McWilliam et al. (1995)
  Ryan et al. (1996) Burris et al. (2000) Johnson
  Bolte (2001)
• Obvious from simple spectrum comparisons
• sn-capture/Fe 1 dex
• local nucleosynthesis events occurring
  in a poorly mixed early Galactic halo

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Stellar Spectroscopic Definitions

• A/B log10(NA/NB)star log10(NA/NB)Sun
• log e(A) log10(NA/NH) 12.0
• Atmospheric parameters Teff, log g, vt, Fe/H
• Metallicity Fe/H
• Metal-poor halo star Fe/H
• Very metal-poor star Fe/H

8
Sr II line strength variations at lowest
metallicities
All three stars have similar atmospheric
parameters and Fe/H -3.4
McWilliam et al. (1995)
9
Strontium abundance scatter at lowest
metallicities
McWilliam et al. (1995) filled circles
Gratton Sneden (1994) open squares
10
n-capture/Fe variations are obvious even in
spectra of higher metallicity stars
These two metal-poor (Fe/H-2.3) giants have
similar atmospheric parameters
Burris et al. (2000)
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n-capture abundance variations do not occur at
random
Comparison with an ordinary metal
Comparison with nearby n-capture element Dy
Burris et al. (2000)
12
General halo n-capture abundance ratios trend
toward pure r-process

• Not considered here carbon-rich stars
  with/without s-process overabundances
• Usual comparison Ba/Eu
• Basolar-system 90 s-process
• Eusolar-system 90 r-process
• Ba/Eu -0.9 pure r-process value
• at Fe/H -3.0
• Scatter is higher than desirable blame
• the Ba abundances?

13
The decline of Ba/Eu at lowest metallicities
The solar-system r-process-only ratio
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An alternative La/Eu

• La also sensitive to s-process (70 s-process in
  solar system)
• Both elements have several useful lines at
  accessible ls
• Atomic parameters of Eu, La lines very well known
• Can determine La/Eu with higher accuracy than
  Ba/Eu
• Can use same transitions over 3 dex metallicity
  range

15
Previous lanthanum work
The La/Eu (e.g, the s-/r-) ratio is constant???
Burris et al. (2000) ,magenta points Johnson
Bolte (2001), black points
16
La II lines in the solar spectrum synthetic
spectra fits with new atomic data
hyperfine structure pattern
Green line is the solar observed spectrum
Lawler et al. (2001)
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La/Eu at low metallicity
The Ba/Eu (e.g, the s-/r-) ratio is NOT constant
Simmerer et al. (2002)
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A better idea employ abundances of more elements
than just Ba and Eu
Four stars, with mean abundance levels scaled
to the solar-system curves by their average Ba,
La, Ce, Sm, and Eu abundances
Johnson Bolte (2001)
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Detailed elemental abundance distributions in a
few very low metallicity stars

• Stars with of n-capture abundances 15
• CS 22892-052 (Sneden et al. 2000) HD 115444
  (Westin et al. 2000) BD17o3248 (Cowan et al.
  2002) CS 31082-001 (Hill et al. 2002)
• Rare earths perfect agreement with
  solar-system r-process-only abundances
• Heaviest stable elements must use HST
• Z

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A small spectral interval of a metal-poor but
n-capture-rich star
Sneden et al. (2000)
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First example BD17o3248

• Most metal-rich of n-capture-enhanced stars
• Fe/H -2.1
• A warmer giant by about 500K than other examples
• Extensive high res, high S/N HST data in hand
• First metal-poor star with gold detection
• Takes advantage of large sets of new atomic data
• La II (Lawler et al. 2001) Ce II (Palmeri et al.
  2000)
• Pr II (Ivarsson et al. 2001) Tb II (Lawler et
  al. 2001)
• Eu II (Lawler et al. 2002)

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Detection of n-capture elements in HST STIS
spectra
HD 122563 is n-capture-poor BD17o3248 is
n-capture-rich
Cowan et al. (2002)
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Discovery of gold in a metal-poor star
Cowan et al. (2002)
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n-capture abundances in BD17o3248 1st
solar-system comparison
Scaled solar-system r-process curve Burris et
al. (2000)
Cowan et al. (2002)
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The BD17o3248 abundances are not compatible with
s-process synthesis
Scaled solar-system s-process curve Burris et
al. (2000)
Cowan et al. (2002)
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Second example CS 22892-052

• First metal-poor star discovered with extreme
  r-process
• Fe/H -3.1 Eu/Fe 1.6
• One puzzle also carbon-rich C/Fe 1.0
• Good high res, high S/N ground-based spectra and
  lower quality HST data in hand
• Even more exploration of atomic data (Mo, Yb, Lu,
  Ga, Ge, Sn, etc.)
• Abundances or significant upper limits for 57
  elements

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Abundance Summary
Colors identify different element groups
Li and Be values are w.r.t. to unevolved stars of
similar metallicity
Sneden et al. (2002), in preparation
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Terbium in the Sun and CS 22892-052
0.80.8 0.8 0.8 0.8 0.8 0.8 0.8
1.1
This is the cleanest Tb II feature in the solar
spectrum
1.0
Relative Flux
0.9
Sun
0.8
n-capture-rich metal-poor stars are good
laboratories for these lines
CS22892-052
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Summary of the latest n-capture abundances for CS
22892-052
Sneden et al. (2003), in preparation
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Z?56 stable n-capture elements excellent match
to solar r-process
Sneden et al. (2003), in preparation
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Zquestions
The upper limits for Sn and especially for Ga, Ge
are significant
Ga and Ge share the metal poverty of Fe-peak and
lighter elements
Sneden et al. (2003), in preparation
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Comparison with CS 22892-052 abundances
Perfect agreement with CS22892-052 would be a
horizontal line
Note difference of HD 122563 real or needing
better data?
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Some attempts to get isotopic abundances

• Need large hyperfine and/or isotopic splitting
• Rare-earth lines provide best opportunity
• Some elements have only one stable isotope
• Barium and now europium have been studied in
  metal-poor stars
• See Ivans et al. poster at this meeting

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An example of Eu II syntheses the 4205.05A line
The Eu abundance is altered by 0.2 dex for each
synthesis
35
Eu isotopic fractions are very similar to
solar-system values
(151Eu)
0
35
50
65
100
(153Eu) 100 - (151Eu)
Solar system (151Eu) 47.8 (153Eu) 52.2
Sneden et al. (2002)
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Barium isotopic mixes
134
135
137
136
138
synthesis cause
s r
s only
s only
s r
s r
134
135
137
136
138
solar system abundances
odd isotopes 18
6.6
2.4
8.0
11.2
71.8
135
137
134
138
136
r-process abundances
odd isotopes 46
25.7
0.0
0.0
20.4
53.9
134
135
137
136
138
hyperfine splitting?
yes
no
no
yes
no
odd isotopes are only11 of solar system
s-process material
37
Barium Isotopic Abundances in HD 140283
odd isotopes
10
31
52
31 is best fit
Solar system total 18 r-only 46 s-only
11
Lambert Allende Prieto (2002)
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Radioactive cosmochronometry for metal-poor stars
Galactic chemical evolution effects do not matter
for radioactive elements Th and U frozen into
metal-poor stars born near the start of the
Galaxy.
?
Daughter product Pb is also a direct n-capture
synthesis product
Rolfs Rodney (1988)
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Best Th II and U II lines
CS 31082-001
BD 17o3248
Cowan et al. (2002)
Cayrel et al. (2001)
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Age computations for halo stars

• t1/2(Th) 14.0 Gyr t1/2(U) 4.5 Gyr
• So for thorium
  
• NTh,now/NTh,start exp(-t/tmean)
  exp(-t/20.3Gyr)
• Cannot know NTh,start assume
  NTh,start/NEu and compare that to N
  Th,observed/NEu
• IF solar-system r-process abundances can be
  assumed to extend to U, then can use
  Thobserved/Eu as a measure of Th decay
• -0.58 /- 0.02 (s 0.07,
  10)
• 13.6
  /-1.0 Gyr (s 3.6 Gyr)
• But in CS 31082-001 the Th/Eu ratio is much
  larger
• Th/U t 12.5 Gyr
• Th/Eu t 4 to 5 Gyr

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Thorium-to-europium ratios in some halo stars
Open circles new data
Filled squares Johnson Bolte (2001)
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The curious chemical composition of CS 29497-030
It is like a blue straggler
It is a binary (companion undetected)
M68
Preston Sneden ( 2000)
M68 diagram from Walker 1994
43
CS 22947-030 is another example of lead-enriched
metal-poor stars
These are s-process enrichments!
All data for CS 29497-030 point to mass transfer
from former AGB companion
Log e(Pb)solar system 1.9
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Summary, future work

• Large star-to-star scatter in n-capture levels
  below Fe/H -2 established but not well
  interpreted
• Switch from r,s-process contributions to r-only
  abundances is seen in many low metallicity stars
• Th, U radioactive element chronometry is in its
  nfancy, but is a promising technique
• Extreme s-process stars may be understood?
• Do Th/Eu ratios always yield same ages?
• Are there more U detections be had?
• Can the abundances of Zunderstood?

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(No Transcript)
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Total r- and s-process synthesis paths
Bi is the end of the s-process
The r-process alone makes radioactive
chronometer elements Th and U
Rolfs Rodney (1988)
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What are s-/r- trends in the Galactic disk?

• Woolf et al. (1995) derived Eu/Fe in disk dwarf
  stars with Fe/H -1
• Woolf spectra also contain 4123Å La II line
• One La II and one Eu II line used to derive La/Eu
  for disk metallicity stars
• Complements Mashonkina Gehren study of Ba/Eu

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Europium in Galactic disk stars
Results confirmed by Koch Evardsson (2002)
Woolf et al. 1995
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La/Eu at high metallicity
Does La/Eu have a break at Fe/H -0.4 ?
Simmerer et al. (2002)
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La/Eu and space velocity
s.s. total
The s-/r- process abundance ratio correlates with
space velocity as much as (more than?) Fe/H
s.s. r-process
Simmerer et al. (2002)