chapter 16 – chemical analysis review of curves of growth –the linear part: the width is set by...
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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
• 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
Eu
BaSr, Y, ZrSc
Li, Be, B
CNO
Fe
-1
2
5
8
10 20 30 40 50 60 70 80
Atomic Number
Lo
g e
(H
=12
)
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
log A = log (gf/g’f) + log /’ – log /l – ex( – ’)
Using a good model
• The temperature distribution of the model - the 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
Spectrum Synthesis
II
(Jacobson)
• Oxygen abundance determinations• Matching the line profile for 3 different values of the
oxygen abundance, with [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?
[O I
]
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
Planets and
Metallicity
• What does this tell us about planet formation?
• What about 2nd order effects (O/Fe, Mg/Fe, Ca/Fe)???
Fisher & Valenti 2005
Iron in the Solar Neighborhood
[Fe/H] is not a good indicator of the age of the disk
Why Iron?
•Fe is abundant•Fe is easy•Fe is made in supernovae
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
Science Magazine
Alpha-process Elements:
Edvardsson et al.Pilachowski et al.McWilliam et al.
Excesses at low metallicity
/Fe ratio originally set by SN II production
Later, SN Ia produce a different Ca/Fe ratio
How to Make Heavy Metals:
neutron-capture processes
r-process– High neutron flux– Type II Supernovae (massive
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
Main s-process•Low mass stars•Double shell burning •Makes SrYZr, Ba, etc.
Weak s-process•Massive stars•He-core and shell Burning•Lower neutron flux makes SrYZr only
n-capture Synthesis Paths
Ba
La
Cs
Xe
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
r- and s-Process Elements
Zn
Ga
Ge
AsSe
Br
Kr
Rb
Sr
Y
Zr
NbMo
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
Xe
Cs
Ba
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
YbLu
Hf
Ta
W
ReOs
Ir
Pt
Au
Hg
TlPb
Bi
Th
U
0
1
Fra
ctio
n o
f r-
pro
cess
Zn As Kr Y Mo Pd In Te Cs Ce Sm Tb Er Lu W Ir Hg Bi
r-Process s-Process
Heavy Metal Abundances
Note: Scatter Deficiencies
at low metallicity
Excesses at intermediate metallicity
r-Process vs. s-Process
Transition from r-process onlyto r+s processat log(Ba)=+0.5
Corresponds to[Fe/H] ~ -2.5
S-process nucleosynthesis begins to
contribute to galactic chemical enrichment
At lower metallicities
only r-processcontributes
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?
SFH in Omega Centauri
• 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
Pancino et al. 2000
Lee et al 1999, Nature 402, 55
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