Transcript jina2-talk

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 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
A detailed view of part of the
n-capture synthesis paths
La
Ba
138
139
p
s,r
130
132
134
135
136
137
138
P
P
s
s,r
s
s,r
s,r
133
Cs
s,r
Xe
128
129
130
131
132
134
136
s
s,r
s
s,r
s,r
r
r
s-process path
r-process path
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 e(A) = log10(NA/NH) + 12.0
Atmospheric parameters: Teff, log g, vt, [Fe/H]
Metallicity
[Fe/H]
Metal-poor halo star
Very metal-poor star
[Fe/H] < -1.5
[Fe/H] < -2.5
Sr II line strength variations at
lowest metallicities
All three stars
have similar
atmospheric
parameters and
[Fe/H] ~ -3.4
McWilliam et al. (1995)
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-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?
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 l’s
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
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
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)
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
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)
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 solarsystem r-process-only abundances
Heaviest stable elements: must use HST
Z < 56: need for another r-process?
A small spectral interval of a metalpoor 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
Li and Be
values are w.r.t.
to unevolved
stars of similar
metallicity
Sneden et al. (2002), in preparation
Terbium in the Sun and CS 22892-052
Relative Flux
1.1
1.0
0.9
0.80.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
Sun
CS22892-052
This is the
cleanest Tb II
feature in the
solar spectrum
n-capture-rich
metal-poor
stars are good
“laboratories”
for these lines
Summary of the latest n-capture
abundances for CS 22892-052
Sneden et al. (2003), in preparation
Z56 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
Perfect
agreement with
CS22892-052
would be a
horizontal line
Note difference
of HD 122563:
real or needing
better data?
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
35
50
65
100
%(153Eu) =
100 - %(151Eu)
Solar system:
%(151Eu) = 47.8
%(153Eu) = 52.2
Sneden et al. (2002)
Barium isotopic mixes
134
135
136
137
138
s only
s&r
s only
s&r
s&r
odd isotopes
18%
134
135
136
137
138
2.4%
6.6%
8.0%
11.2%
71.8%
odd isotopes
46%
134
135
136
137
138
0.0%
20.4%
53.9%
0.0%
25.7%
134
135
136
137
138
no
yes
no
yes
no
synthesis
cause
solar system
abundances
r-process
abundances
hyperfine
splitting?
odd isotopes are only11% of solar system s-process material
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)
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 ncapture synthesis
product
Rolfs & Rodney (1988)
Best Th II and U II lines
BD +17o3248
CS 31082-001
Cowan et al. (2002)
Cayrel et al. (2001)
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
<[Thobserved/Eu ]> = -0.58 +/- 0.02 (s = 0.07, # = 10)
<t> = 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
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
It is like a “blue straggler”
M68
It is a binary
(companion undetected)
Preston & Sneden ( 2000)
[M68 diagram from Walker 1994]
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
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 Z<56 n-capture elements
be understood?
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)
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
Results
confirmed
by Koch &
Evardsson
(2002)
Woolf et al. 1995
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
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)