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