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Chapter 16 – Chemical Analysis
• Review of curves of growth
– The linear part:
• The width is set by the thermal width
• Eqw is proportional to abundance
– The “flat” part:
• The central depth approaches its maximum value
• Line strength grows asymptotically towards a constant value
– The “damping” part:
• Line width and strength depends on the damping constant
• The line opacity in the wings is significant compared to kn
• Line strength depends (approximately) on the square root of the abundance
• How does line strength depend on excitation
potential, ionization potential, atmospheric
parameters (temperature and gravity),
microturbulence
• Differential Analysis
• Fine Analysis
• Spectrum Synthesis
Determining Abundances
• Classical curve of growth analysis
• Fine analysis or detailed analysis
– computes a curve of growth for each individual
line using a model atmosphere
• Differential analysis
– Derive abundances from one star only relative
to another star
– Usually differential to the Sun
– gf values not needed – use solar equivalent
widths and a solar model to derive gf values
• Spectrum synthesis
– Uses model atmosphere, line data to compute
the spectrum
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
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
Basic Methodology for “Solar-Type” Stars
• Determine initial stellar parameters
–
–
–
–
Composition
Effective temperature
Surface gravity
Microturbulence
• Derive an abundance from each line
measured using fine analysis
• Determine the dependence of the derived
abundances on
– Excitation potential – adjust temperature
– Line strength – adjust microturbulence
– Ionization state – adjust surface gravity
Using stellar Fe I lines to determine
model atmosphere parameters
• derived abundance
should not depend
on line strength,
excitation potential,
or wavelength.
• If the model and
atomic data are
correct, all lines
should give the
same abundance
Adjusting for
Excitation
Potential
•
•
For weak lines on the linear part of the COG, curves of growth can
be shifted along the abcissa until they line up, using the difference
in excitation potential
If the temperature is right, all the curves will coincide
Dlog A = log (gf/g’f) + log l/l’ – log kn/kl – qex(c – c’)
Using a good
model
• The temperature distribution of the model - the T(t)
relation, can make a difference in the shape of the COG
• The differences depend on excitation potential because the
depth of formation depends on excitation potential
The COG for Fe II
lines depends on
gravity
• Fe II lines can be used to determine the
gravity
• The iron abundance from Fe II lines must
also match the iron abundance from Fe I
lines
Strong lines
• Strong lines are
sensitive to gravity and
to microturbulence
• The microturbulence in
the Sun is typically 0.5
km s-1 at the center of
the disk, and 1.0 km s-1
for the full disk
• For giants, the
microturbulence is
typically 2-3 km s-1
Spectrum Synthesis
• Compute the line profile to match the
observed spectrum
• Vary the abundance to get a good fit.
•Jacobson et al.
determination of the
sodium abundance
in an open cluster
giant
•Model profiles are
shown for 3 different
oxygen abundances
(Jacobson)
[O I]
Spectrum
Synthesis
II
• Oxygen abundance determinations
• Matching the line profile for 3 different values of the
oxygen abundance, with D[O/H] = 0.5 dex
• Note CN lines also present near the [O I] line. The
strength of CN also depends on the oxygen abundance
– When O is low, CN is stronger… Why?
Interesting Problems in Stellar Abundances
• Precision Abundances
– Solar iron abundance
– Effects of 3D hydro
– Solar analogs
• Stellar Populations
– SFH of the Galactic
thin/thick disk
– Population diagnostics
– Migrating stars
– Merger remnants
– Dwarf spheroidals
– Galactic Bulge
• Nucleosynthesis
– Abundance anomalies in
GC
– Extremely metal poor
stars
– Peculiar red giant stars
• Metallicity and Planets
• Evidence for mixing
and diffusion
Fisher & Valenti 2005
Planets
and
Metallicity
• What does this tell us about planet
formation?
• What about 2nd order effects (O/Fe,
Mg/Fe, Ca/Fe)???
Iron in the Solar Neighborhood
Why Iron?
•Fe is abundant
•Fe is easy
•Fe is made in
supernovae
[Fe/H] is not a good indicator of the age of the disk
Science Magazine
Ultra Metal-Poor
Stars
•Ultra metal-poor stars are
rare in the halo
•Most metal poor star
known is ~ [Fe/H] = -6
•Surveys use Ca II K line
Alpha-process
Elements:
Excesses at low metallicity
a/Fe ratio originally set by
SN II production
Later, SN Ia produce a
different Ca/Fe ratio
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
– B-decay before next n-capture
– No Eu, Gd, Dy
Weak s-process
•Massive stars
•He-core and shell
Burning
•Lower neutron flux
makes SrYZr only
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
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
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
Heavy Metal
Abundances
Note:
Scatter
Deficiencies
at low
metallicity
Excesses at
intermediate
metallicity
r-Process vs. s-Process
Transition from
r-process only
to r+s process
at loge(Ba)=+0.5
Corresponds to
[Fe/H] ~ -2.5
S-process
nucleosynthesis
begins to contribute
to galactic chemical
enrichment
At lower metallicities
only r-process
contributes
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)
Abundance Dispersions in Globular Clusters
Star Formation
History in DSps
• CMD for the
Carina dwarf
spheroidal
galaxy from
Smecker-Hane
• Note at least
two epochs of
star formation
• Abundance
differences?
Pancino et al. 2000
SFH in Omega Centauri
Lee et al 1999, Nature 402, 55
• The globular cluster Omega Cen also shows interesting
structure in its CMD indicating multiple epochs of star
formation
• Epochs of star formation reflected in metallicity
distribution function