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Observations of Neutron-Capture Elements in the Early Galaxy
Chris Sneden
University of Texas at Austin
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
Talk outline
Reminder of solar r- and s-process breakdownGeneral 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 observationsThere is more to halo star life than the r-processSummary, future questions
A detailed view of part of the n-capture synthesis paths
Ba
La
Cs
139
132131130129128
130 132
133
134 136
134 135 136 137 138
138
PP s,rs,r s,r
s,r
s,r
s,r
s
rs,r r
p
s
s,r ss
r-process paths-process path
Xe
ELEMENTAL r- and s-process solar-system abundances
Data from Burris et al. (2000)
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
σ[n-capture/Fe] > 1 dex
local nucleosynthesis events occurring in a poorly mixed early Galactic halo
Stellar Spectroscopic Definitions
[A/B] = log10(NA/NB)star – log10(NA/NB)Sun
log (A) = log10(NA/NH) + 12.0
Atmospheric parameters: Teff, log g, vt, [Fe/H]
Metallicity [Fe/H]
Metal-poor halo star [Fe/H] < -1.5
Very metal-poor star [Fe/H] < -2.5
Sr II line strength variations at lowest metallicities
McWilliam et al. (1995)
All three stars have similar atmospheric parameters and[Fe/H] ~ -3.4
Strontium abundance scatter at lowest metallicities
McWilliam et al. (1995): filled circles
Gratton & Sneden (1994): open squares
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)
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)
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-processEusolar-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?
The decline of Ba/Eu at lowest metallicities
The solar-system r-process-only ratio
An alternative: La/Eu
La also sensitive to s-process (70% s-process in solar system)Both elements have several useful lines at accessible ’sAtomic parameters of Eu, La lines very well knownCan determine La/Eu with higher accuracy than Ba/EuCan use same transitions over 3 dex metallicity range
Previous lanthanum work
Burris et al. (2000) ,magenta points; Johnson & Bolte (2001), black points
The La/Eu (e.g, the s-/r-) ratio is constant???
La II lines in the solar spectrum: synthetic spectra fits with new atomic data
hyperfinestructurepattern
Green lineis the solarobserved spectrum
Lawler et al. (2001)
La/Eu at low metallicity
The Ba/Eu (e.g, the s-/r-) ratio is NOT constant
Simmerer et al. (2002)
A better idea: employ abundances of more elements than just Ba and Eu
Johnson & Bolte (2001)
Four stars, withmean abundance levels scaled tothe solar-systemcurves by their average Ba, La,Ce, Sm, and Euabundances
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); BD+17o3248 (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 < 56: need for another r-process?
A small spectral interval of a metal-poor but n-capture-rich star
Sneden et al. (2000)
First example: BD+17o3248
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)
Detection of n-capture elements in HST STIS spectra
HD 122563 is n-capture-poor; BD+17o3248 is n-capture-rich
Cowan et al. (2002)
Discovery of gold in a metal-poor star
Cowan et al. (2002)
n-capture abundances in BD+17o3248: 1st solar-system comparison
Scaled solar-system r-process curve: Burris et al. (2000)
Cowan et al. (2002)
The BD+17o3248 abundances are not compatible with s-process synthesis
Scaled solar-system s-process curve: Burris et al. (2000)
Cowan et al. (2002)
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
Abundance Summary
Colors identify different element groups
Sneden et al. (2002), in preparation
Li and Be values are w.r.t. to unevolved stars of similar metallicity
Terbium in the Sun and CS 22892-052
0.80.80.80.80.80.80.80.8
0.8
0.9
1.0
1.1
Rel
ativ
e F
lux
Sun
This is the cleanest Tb II feature in the solar spectrum
n-capture-rich metal-poor stars are good “laboratories” for these lines
CS22892-052
Summary of the latest n-capture abundances for CS 22892-052
Sneden et al. (2003), in preparation
Z56 stable n-capture elements: excellent match to solar r-process
Sneden et al. (2003), in preparation
Z<56 n-capture elements: some deviations, some questions
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
Comparison with CS 22892-052 abundances
Note differenceof HD 122563: real or needing better data?
Perfect agreement with CS22892-052 would be a horizontal line
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
An example of Eu II syntheses: the 4205.05A line
The Eu abundance is altered by 0.2 dex for each synthesis
Eu isotopic fractions are very similar to solar-system values
%(151Eu):0
355065
100
%(153Eu) = 100 - %(151Eu)
Solar system:%(151Eu) = 47.8%(153Eu) = 52.2
Sneden et al. (2002)
Barium isotopic mixes
134 135
s & rs only
137136 138
s only s & r s & r
135
25.7%0.0%
137134 138
0.0% 20.4% 53.9%
134 135
yesno
137136 138
no yes no
134 135
6.6%2.4%
137136 138
8.0% 11.2% 71.8%
136
synthesiscause
solar systemabundances
r-process abundances
hyperfinesplitting?
odd isotopes18%
odd isotopes 46%
odd isotopes are only11% of solar system s-process material
Barium Isotopic Abundances in HD 140283
Lambert & Allende Prieto (2002)
odd isotopes:10%
31%
52%
Solar system:total = 18%
r-only = 46%s-only = 11%
31% is best fit
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)
Best Th II and U II lines
Cowan et al. (2002) Cayrel et al. (2001)
BD +17o3248 CS 31082-001
Age computations for halo stars
1/2(Th) = 14.0 Gyr; 1/2(U) = 4.5 GyrSo for thorium:
NTh,now/NTh,start = exp(-t/mean)= 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<[Thobserved/Eu ]> = -0.58 +/- 0.02 ( = 0.07, # = 10)
<t> = 13.6 +/-1.0 Gyr ( ~ 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
Thorium-to-europium ratios in some halo stars
Open circles:new data
Filled squares:Johnson &Bolte (2001)
The curious chemical composition of CS 29497-030
M68
[M68 diagram from Walker 1994]
It is like a “blue straggler”
It is a binary (companion undetected)
Preston & Sneden ( 2000)
CS 22947-030 is another example of lead-enriched metal-poor stars
These are s-process enrichments!
Log (Pb)solar system = 1.9
All data for CS 29497-030 point to mass transfer from former AGB companion
Summary, future work
Large star-to-star scatter in n-capture levels below [Fe/H] ~ -2: established but not well interpretedSwitch from r,s-process contributions to r-only abundances is seen in many low metallicity starsTh, U radioactive element chronometry is in its nfancy, but is a promising techniqueExtreme 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 Z<56 n-capture elements be understood?
Total r- and s-process synthesis paths
The r-process alone makes radioactive chronometer elements Thand U
Bi is the end of the s-process
Rolfs & Rodney (1988)
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
Europium in Galactic disk stars
Woolf et al. 1995
Results confirmed by Koch & Evardsson (2002)
La/Eu at high metallicity
Does La/Eu have a break at [Fe/H] -0.4 ?
Simmerer et al. (2002)
La/Eu and space velocity
The s-/r- process abundance ratio correlates with space velocity as much as (more than?) [Fe/H]
Simmerer et al. (2002)
s.s. r-process
s.s. total