Download s-process

The Cecilia Payne-Gaposchkin
Lecture
Center for Astrophysics
May 9, 2002
Heavy Metal
from Ancient
Superstars
In collaboration with….
 Debra Burris (Oklahoma City CC)
 John Cowan (University of Oklahoma)
 Chris Sneden (University of Texas)
 Taft Armandroff (NOAO)
 Henry Roe (U.C. Berkeley)
Outline
 The high-redshift universe
 The Galaxy in time – a brief review of the
formation of the Milky Way and its structural
components
 The origin of the elements – back to B2FH!
 A stroll through the Periodic Table
 A timeline of Galactic chemical enrichment What does it all mean?
Metallicity at High Redshift
• Studies of the most metal-poor stars in the Galaxy
give us access to the state of the Universe at very
early times
Songaila & Cowie 2002
But - the most
metal-poor
stars in the
Galaxy have
[Fe/H]=-4
Metallicity Distribution Function
for Metal-Poor Stars
•
•
•
NO stars with
[Fe/H] < -4.0
Beers 1999
Have we found the
low metallicity end
of the MDF?
Did the first
generation(s) raise
the metallicity to
[Fe/H] = -4?
(Selection effects for [Fe/H] > -2)
The Milky Way….
Circa 1950
1990
Flattened Inner Halo
Halo
Thick Disk
Dwarf Spheroidal Companions
Dark Matter Corona
The Chemistry of Stellar
Populations……
The chemical compositions of stars
reflect the star formation histories of
stellar populations
The complexity of the Milky Way’s
history is reflected in the compositions
of its stars
Solar Abundances from Grevesse and Sauval
CNO
Log e (H=12)
8
Fe
5
Sr, Y, Zr
Sc
2
Ba
Li, Be, B
Eu
-1
10
20
30
40
50
Atomic Number
60
70
80
Jargon
 [m/H] = log N(m)/N(H)star – log N(m)/N(H)Sun
[Fe/H] = -1.0 is the same as 1/10 solar
[Fe/H] = -2.0 is the same as 1/100 solar
 [m/Fe] = log N(m)/N(Fe)star – log N(m)/N(Fe)Sun
[Ca/Fe] = +0.3 means twice the number of Ca
atoms per Fe atom
 Log e(metal) = log n(metal)/n(H) + 12
The Fe Chronometer
Why Iron?
•Fe is abundant
•Fe is easy
•Fe is made in
supernovae
But [Fe/H] is
not a good
In
the
halo,
[Fe/H]
is
a
function
indicator of
both
since star formation
the
age time
of
the disk
of
began and the star formation rate
• Nucleosynthesis in stars
leads to chemical
Chemical
enrichment of the Galaxy
• Rate of enrichment
Evolution
depends on sites and
• Primordial
mechanisms of
nucleosynthesis
nucleosynthesis
• Hydrogen burning
• The variables are:
–
–
–
–
Star formation rate
Initial mass function
Yields
Stellar evolution time
– Proton-capture chains
• Helium Burning
– C,Ne,O,Si burning
• Photodissociation
burning process
• Neutron-capture
processes
• Odd-ball stuff
Galactic Lithium Production: 10% of Big Bang origin
90% of Galactic origin
Figure from Con Deliyannis
“Alpha” Elements:
Excesses at low
metallicity from
C, Ne, O & Si
production in SN
II
Decline in
[alpha/Fe] due to
Fe production by
SN Ia
Edvardsson et al.; Pilachowski et al.; McWilliam et al.
How to Make Heavy Metals:
neutron-capture processes
Main s-process
•Low mass stars
r-process
•Double shell burning
– High neutron flux
– Type II Supernovae (massive •Makes SrYZr, Ba,
etc.
stars)
– No time for b-decay
– Eu, Gd, Dy, some Sr, Y, Zr, Ba, La…
s-process
– Low neutron flux
– Time for b-decay before next
neutron capture
– No Eu, Gd, Dy
Weak s-process
•Massive stars
•He-core and shell
Burning
•Lower neutron flux
makes SrYZr only
Solar System r- and s-Process Elements
Eu
1
Br
Fraction of r-process
As
Se
Rh
Te
Ag
Ru
Rb
Pd
Ge
Kr
Zn
Y
Mo
Nb
Tb
Cs
Sb
Sm
In
Xe
Pr
Cd
Ho
Tm
Lu
Yb Ta
Dy
Gd
Er
Os Au
Re Pt
Th
Bi
Ir
U
Hf
Hg
La Nd
Sn
Zr
Tl
Pb
Ce
I
Sr
W
Ba
Ga
0
Zn As Kr
Y
Mo Pd
In
Te
Cs Ce Sm Tb
r-Process
s-Process
Er
Lu
W
Ir
Hg Bi
Isotopes built by n-capture syntheses
The valley of b-stability
Rolfs & Rodney (1988)
n-capture Synthesis Paths
La
Ba
18% of solar system Ba is odd, but
48% of r-process Ba is odd
138
139
p
s,r
130
132
134
135
136
137
138
p
p
s
s,r
s
s,r
s,r
133
Cs
Xe
s,r
128
129
130
131
132
134
136
s
s,r
s
s,r
s,r
r
r
s-process path
r-process path
Spectrum of HD 126587
 Metal-poor giant
 [Fe/H] = -2.85
 Teff = 4910 K
 r-process rich
 Spectrum from
the Mayall 4-m
echelle
Star-to-Star [n-capture/Fe] variations
Stars of similar temperature and metallicity may have very different
neutron-capture element abundances
Burris et al. (2001)
The Scatter is in the Stars!
The r-process elements vary together
Burris et al. (2001)
Abundance Data Sources
-1.0 < [Fe/H] < 0.0
• Edvardsson et al. 1993
• Jehin et al. 1999
-3.0 < [Fe/H] < -1.0
• Burris et al. 2000
-4.0 < [Fe/H] < -2.0
• McWilliam et al. 1995, 1998
Heavy Metal
Abundances
Note:
Scatter
Deficiencies at
low metallicity
Excesses at
intermediate
metallicity
n-capture Abundances in BD+17o3248
Scaled solar-system r-process curve: Sneden 2002
Solar-System s-process Abundances DON’T Fit
Sneden (2002), Burris et al. (2000)
BD +17 3248 is typical of very metal poor stars
Sneden et al. (2000); Westin et al. (2000); Cowan et al. (2002)
r-Process vs. s-Process
Transition from
r-process only
to r+s process
at loge(Ba)=+0.5
and
[Fe/H] = -2.0
La/Eu at low metallicity
s-process
seen at
[Fe/H]=2.1
Simmerer et al. (2002)
When does the s-process start?
Main s-process occurs during thermal pulses in
AGB stars of 2-4 solar masses
H mixes inward, giving
12C(p,e+)13C
13C(a,n)16O
t ~ 108 years
s-process elements do not appear before this
r-process appears at [Fe/H]=-2.9
New r-process elements come from
deep in the Supernova
This may be
part of the
reason for the
n-capture
scatter.
Not all SN II
produce lots of
r-process
Rolfs & Rodney (1988)
The “light”
heavy metals
Production of Sr, Y,
and Zr requires an
additional neutron
capture process
Heavy
metals at
[Fe/H] = -4
At very low metallicity, the production of heavy
metals is dominated by an unknown process
What came before the r-process?
• Identify “weak
r-process stars”
to see yields of
very early
nucleosynthesis
The Earliest Star Formation
•
•
•
•
•
Formation of stars
as “pre-galactic”
objects from small
density fluctuations
H2 provides cooling
Masses from a few
tens to a few
hundred solar masses
Low mass star
formation is
suppressed by
reionization
Provides early
chemical enrichment
Abel, Bryan, & Norman 2002
Theoretical Framework
 Stochastic model for early chemical
evolution (Travaglio et al. 1999)
 Coalescing and fragmenting clouds
 Homogenization time scale ~ few x
108 years reduces scatter
 Suggests r-process from 8-10 MSun
 s-process elements from 1-3 MSun
 AGB stars after homogenization
Theoretical Models
of
Chemical Evolution
• Stochastic models
of Travaglio et al.
for r-process
production by 8-10
solar mass SN II
The scatter in the abundances of all of the n-capture elements
from star-to-star is of astrophysical origin, and the scatter
increases as metallicity decreases.
Conclusions
Significant production of r-process elements began when
the metallicity of the Galaxy reached [Fe/H] = -3.
The heavy n-capture elements were formed predominantly
by the r-process at metallicities below [Fe/H] = -2.1.
Elements from the s-process appear at a metallicity of [Fe/H] =
-2.1, when low-mass AGB stars begin to contribute from double
shell burning. The s-process then dominates Ba production.
The origin of heavy metals at the lowest Galactic metallicity
([Fe/H] = -4) is still not understood, but may be dominated by
the weak s-process, or by a separate r-process in massive stars.
The Epochs of Galactic Chemical Evolution
 Primordial Epoch -The Big Bang
 Epoch of Massive Stars @ [Fe/H] ~ -4
– Ca, O, Sr-Y-Zr + ?
 r-process Epoch - r-process elements from 8-10 MSun SNII
 The Double Shell Epoch yields s-process elements @ [Fe/H]=-2.1
(~ 109 years)
 The Iron Epoch – from SN Ia @ [Fe/H]=-2
 The Lithium Epoch @ [Fe/H]=-1.0 from ???
Key Concept – Stellar evolution timescales are important
The Oxygen
Abundance
Oxygen abundances are still uncertain, with
inconsistencies between the triplet and forbidden lines