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Chemical evolution in the MW Observational evidence. Birgitta Nordström Niels Bohr Institute. Outline. First (oldest) stars in MW halo Long-lived stars in MW disk Age determination. Motivation for study.

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chemical evolution in the mw observational evidence

Chemical evolution in the MWObservational evidence

Birgitta Nordström

Niels Bohr Institute

  • First (oldest) stars in MW halo
  • Long-lived stars in MW disk
  • Age determination
motivation for study
Motivation for study

”The formation and evolution of galaxies is one of the great outstanding problems in astrophysics.”

(Freeman & Bland-Hawthorn, 2002)

We think we know the grand scheme of the story,

but how well do we understand the real physics ?

  • Nucleosynthesis and chemical enrichment history
  • Dynamics: Interplay of initial conditions & evolution

Progress reported in Copenhagen, June 2008 (Proceedings in 09).

evidence of evolution
Evidence of evolution

How do we know if/that the MW evolves?

Observational facts?

evidence of evolution for eksample
Evidence of evolution for eksample:
  • Z (metallicity) increases with time since BB
  • SN explode (yields – increase of Z)
  • Open Clusters form and disrupt
  • Globular Clusters are old (they do not form now)
  • We observe Giant stars (stellar evolution)
  • We observe gas infall and outflow (MW disk)
  • Velocity dispersion in (U,V, W) increases with time
  • Age-Metallicity relation – significant dispersion
  • Radial metallicity gradients in MW disk
  • . . . .

Big Bang

Primordial Nucleosynthesis

(hydrogen, helium, deuterium, lithium)




Protogalaxy Collapse

Halo+bulge formation (thick disk?)

Thin Disk formation

Elements “pollute” ISM

Interstellar Medium (ISM)


Stellar mass loss

Stellar death - (m)


Star Formation

Stellar Evolution

(elements cooking)

Stellar remnants

ism enrichment
ISM Enrichment

SN type II(core collapse)

Massive Stars

(M > 8 M)

Fe and r-process elements up to U


Low- and Interm.-mass Stars

(0.8 < M < 8 M )

Planetary Nebulae, ..

Single stars

He, C, N, heavy

s-process elements

Binary st. (WD)

SN type Ia(thermonuclear explosion)


from big bang to today origin of the elements
From Big Bang to todayOrigin of the elements
  • Big Bang (H, He, Li)
  • Fusion in stars (He to Fe)
  • Explosions (fission: Ni U)
overview of current theory of nucleosynthesis
Overview of Current Theory of Nucleosynthesis
  • Big Bang produces bland distribution of H, D, 3He, 4He, and traces of Li
  • First stars and galaxies form (poorly understood epoch)
  • Massive stars burn quickly and brightly. Becoming unstable, the most massive stars explode as SNe, ejecting newly synthesized elements from C to U
  • Ejected elements are incorporated into new generations of stars, planets, human beings, etc.
the first stars
The First Stars
  • Contain rests from Big Bang (H, He, Li)
  • Teach us about creation of chem. elements
    • C, N, O to Fe (iron)
    • Heavy elements ( Nickel – Uranium)
  • Tests of Supernova models
  • Radioactive age dating of Milky Way
  • Where are they and what do they look like?

Stellar spectrum


Na I


Mg I

Ca I

quantitative spectral analysis
Quantitative spectral analysis

Sun = 1



0 (hypot.)



elements after big bang
Elements after Big Bang

”Standard Big Bang”:

For each

1,000,000,000,000 1H

there was formed

70,000,000,000 4He

40,000,000 2H

7,000,000 3He

and 150 7Li

Average density /0 

(Baryon to photon ratio)

big bang nucleosynthesis tested
Cosmic Microwave Background (CMB)

Snapshot of universe T~eV

ionized neutral

opaque transparent

T Fluctuations (Anisotropy)

sensitive to baryon content of plasma

Indep. measure of

baryon density

Big Bang Nucleosynthesis Tested

Wilkinson Microwave Anisotropy Probe (WMAP)

Bennett et al 2003

lithium in early generations
Lithium in early generations




test lithium
Test: Lithium

New Li produced by cosmic radiation

and in stars


at the same time …

Li is destroyed

in stars

Li from

Big Bang


Masse kendt

Henfaldstid kendt

s proces

Intet kendt

p proces

r proces

rp proces



Kosmisk stråling

Big Bang

Videre opad i det



Pb (82)


Sn (50)


Fe (26)




a-kerner12C,16O,20Ne,24Mg, …. 40Ca

Gab v.B,Be,Li

Generel tendens: færre tunge kerner

r-proces toppe (lukkede kerneskaller)

s-proces toppe (lukkede kerneskaller)


Skarp top!




s process in real time
s-process in “real time”

”Katteøjet”  planetariske tåger  ”Håndvægten”

r process

”Crab nebula”: Supernova 1054 – 950 years after


Optical (ESO VLT)   X-rays (Chandra)

sn ia yields
SN Ia yields

Element M/Mo

Models by Nomoto et al.

Elemental abundances predicted for various progenitor mass and explosion energies.

th ii and u ii lines
Th II and U II lines

CS 31082-001

BD +17o3248

Cayrel et al. (2001)

age from uranium thorium
Age from uranium-thorium
  • Time = 46.67[log(Th/S)0 – log(Th/S)now]
  • Time = 14.84[log(U/S)0 – log(U/S)now]
  • Time = 21.76[log(U/Th)0 – log(U/Th)now]
  • Limit for age of Universe: ~14 Gyr

from star of [Fe/H] ~ -3

  • First discovery of Uranium in old star
  • First absolute and reliable age-dating of another star than the Sun
  • Age: 13-14 Gyr

? Is that the age of the MW or what is the origin of the star (and similar)?

he fusion in red giants 10 6 to 10 7 years
He Fusion in Red Giants (~ 106 to 107 years)

4He + 4He 8Be

8Be + 4He 12C + g

12C burning (<1000 years)

12C + 4He 16O

12C + 12C 20Ne + 4He + g

16O burning (<1 year)

16O + 16O 28Si + 4He + g

12C + 16O 24Mg + 4He + g

end of a red giant s life si combustion lasts about 1day
End of a Red Giant's life: Si combustion:lasts about 1day
  • 28Si + 4He 32S + γ
  • 32S + 4He 36Ar + γ
  • 36Ar + 4He 40Ca + γ
  • 40Ca + 4He 44Ti + γ 44Ca + 2b+
  • 44Ti + 4He 48Cr + γ 48Ti + 2b+
  • 48Cr + 4He 52Fe 52Cr + 2b+
  • 52Fe + 4He 56Ni + γ 56Fe + 2b+
  • 56Ni / 56Fe + 4He  impossible . . .

Results from the first supernovae

25M 20M 13M


incomplete explosive Si burning

Supernovae with

smaller masses

‘swallow’ more of the core (Fe, Co, Ni), when the star collapses to a black hole or a neutron star.



complete explosive Si burning



abundance ratios as cosmic clocks
Abundance Ratios as “Cosmic Clocks”
  • Different chemical elements are produced on different timescales by stars of different lifetimes.
  • ISM will be enriched:
  • - faster in elements produced by massive stars(α-elem.)

- slowly in those elements produced by SNIa and low- and intermediate mass stars (Fe, C, N)

origin of single chemical elements
Origin of single chemical elements

Detailed analysis of

189 nearby longlived

stars relative to the Sun

(Edvardsson et al.,1993)




[O/Fe] x [Fe/H]

([X/H] = log(X/H) –log(X/H)sun)

SN II enrichment

SN Ia + SNII +PN +..


Halo + thick disk

Thin disk

Solar level

[Fe/H] “time”


Globular clusters (GC)

Spherical dense clusters

of about 106 stars

Searle and Zinn (1978):

GC have different metallicities independent of distance from Galactic Centre.

Important for Galaxy formation


Messier 10 - a globular

cluster in the Galaxy

The Milky Way has about 150 of these clusters

example hr diagram of the star cluster m 55
Example: HR diagram of the star cluster M 55

High-mass stars evolved onto the giant branch

Turn-off point

Low-mass stars still on the main sequence


The 150 Milky Way clusters are old (~10-12 Gyr)

  • Deficient in most of the heavier chemical elements

relative to the sun.

  • The most metal-poor clusters have [Fe/H] = -2.5


  • But the most metal-poor halo stars have much lower

abundances, down to [Fe/H] < -5

observational diagnostics of galactic evolution
Observational Diagnostics of galactic evolution
  • Elemental abundances of stars / [Fe/H]
  • Metallicity distribution function
  • Age-Metallicity Relation
  • Radial metallicity gradients
  • Age-Velocity dispersion
  • Fe/H-Velocity dispersion

Study the disk as that is where most of the baryons are.

requirements of stellar test sample
Requirements of stellar test sample
  • Large sample, complete within some volume
  • Lifetimes large enough to cover history of disk
  • No selection biases in metallicity, kinematics, or age
  • Complete data on age, [Fe/H], UVW, galactic orbits, binary stars
  • Surface abundances representative of stellar interior

Complete, magnitude limited sample of FG dwarfs

hr diagram of rv sample
HR diagram of RV sample

Giants excluded

Mv from

Hipparcos &



Teff from uvby photometry

basic and derived data for the sample
Basic and derived data for the sample
  • Observations + calibrations:
  • Binaries identified; eliminate from study when needed
  • Calibration m1 [Fe/H] revisited (cool & hot stars!)
  • Distance, Mv, Teff, [M/H], , and RV for all stars
  • Quantities derived from these data:
  • Ages from Teff, Mv,[M/H], and isochrones
  • Space motions U, V, W from distance, , and RV
  • Orbital parameters Rm, e, zmax in symmetric potential
computing isochrone ages
Computing isochrone ages




Integrate probability for all points in diagram



= ’Well-defined’ age

Need to adjust Teff scale @ low [Fe/H] !

when isochrone ages are uncertain
When isochrone ages are uncertain :

±1 conf. level

Ages not well-defined outside favourable parts of the HR diagram!


Age determinations:

Takeda et al. 2007 – GCS new


2. Metallicity Distribution Function

(G-dwarf problem)

Volume complete sample(solid),

corrected (dashed), Casuso & Beckman 2004 (dotted)

metallicity distribution in solar vicinity conclusion
Metallicity distribution in Solar vicinity(conclusion)

The ‘G dwarf problem’ is real:

When analysed from a complete, unbiased sample of stars with complete kinematical data and binary detection as well as metal abundances, the deficit of metal-poor dwarfs is TWICE as large.

  • New challenge for model builders!

Explanation: Early preenrichment,infall of gas, . . . .

3 the age metallicity relation
3. ’The’ age-metallicity relation

Assumptions in classical chemical evolution models:

  • Prompt recycling of new metals (delay by SNe Ia)
  • Efficient azimuthal mixing in disk
  • Radial gradients in SFH and abundances not much
  • Radial mixing of stars
  • Inflow/outflows can be simply parametrized


  • [X/H] as function of time (X = O, Mg, Fe, …);   0
  • [X/Fe] as functions of radius; tight relations;   0
age metallicity diagram single stars
Age – metallicity diagram (single stars)

Reddened F giants (over-corrected?)

Full magnitude-limited sample : 7,566 stars with ’well-defined’ ages

Nordström et al. 2004

age metallicity relations now then single stars well defined ages
Age - metallicity relations: Now & then(single stars, well-defined ages)

462 single FGK dwarfs

189 F dwarfs

(no) GK dwarfs

(also found for open clusters)

Volume-limited sample (d < 40 pc)

Edvardsson et al. (1993)

conclusions from the new sample
Conclusions from the new sample
  • No net slope of [M/H] with age @ R = R0 thoughout life of disk
  • Earlier findings of significant slope based on biased samples
  • Large and highly significant scatter in [M/H] at all ages
  • Scatter more significant than any trend of the mean
  • Any theory that cannot explain why a large scatter exists
  • is missing a crucial piece of the physics!
  • Inefficient azimuthal mixing; different local enrichment rates

 Challenge for model builders!

4 radial metallicity gradients
4. Radial metallicity gradients

Nordström et al. local sampleShaver 1983; Fitch&Silkey 1991;

Vilchez & Esteban 1996

results on radial gradients
Results on radial gradients
  • Weak constraints from local sample (!)
  • Present gradients consistent with other studies
  • Some evolution with time seen; can guide models

New models by Schönrich Binney (2008)

age velocity dispersion relations
Age – velocity (-dispersion) relations

age < 25%

7,237 single stars with ”well-defined” ages and all data


Disk heating : Saturating or ongoing?

NO !

Freeman : Velocity dispersion of nearby F

stars saturates after 2 Gyr (data from E93)

saturation seen for ages > 2 Gyr !

(galaxy mergers traceable)

Our data

YoungOld Thick

age < 25%

conclusions on disk heating processes
Conclusions on disk heating processes
  • Disk heating continues thoughout the life of the disk
  • Insufficient to explain scatter in AMR by radial diffusion
  • Sufficient to erase traces of old mergers in disk stars
  • Stationary spiral arms, GMCs, or massive BH insufficient
  • But predictions for transient spiral arms match data
  • Only weak kinematical signature of thick disk 

Hard to distinguish between initial conditions & evolution

local kinematics vs galactic populations
Local kinematics vs. Galactic populations
  • Overlap in spatial distribution and kinematics
  • Kinematic population definitions often based on

biased samples & oversimplified assumptions

  • Kinematic selection criteria may not work !
known dynamical substructures
Known dynamical substructures

”Dynamical streams” identified by Famaey et al. (2005), large sample of K & M giants

How to distinguish these dynamical features from the substructure due to past mergers?

  • Velocity distribution of dynamical streams is different not-mixed in VR
  • The APL-space distribution is different…(Apocentric, Pericenter, Angular momentum)

Chemical tagging = elemental abundances

where are and how do we find the remains of accreted satellites
Where are and how do we find the remains of accreted satellites?
  • If stars conservememoryof theirorigin, it may be possible toreconstructthe merginghistoryof our Galaxy
  • Thismemorymay be in the form of SUBSTRUCTURES in phase-space, in age distribution, in chemical abundances
    • Expect substructures in phase-space because the volume occupied by a small satellite is much smaller than that available (i.e. that of the MW), and because volume in phase-space is conserved.
  • Need samples of stars with accurate positional & velocity information, abundances, ages… Natural place to look for debris: halo + thick disk
characterizing debris from past mergers
Characterizing debris from past mergers
  • N-body simulation of dwarf galaxy (5 £ 108 M¯ in stars) in Milky Way potential; orbit crossing Solar neighbourhood
  • Debris deposited in a “thick-disk” (inspired in Arcturus group: Eggen 1971; Navarro , Helmi & Freeman 2004)
  • No spatial correlations (after few Gyr; mixing timescales are short in the inner Galaxy)
  • Broad velocity distribution
space of conserved quantities
Space of conserved quantities
  • Look for a projection of phase-space such that a satellite galaxy defines a coherent “lump” at all times
  • Previously: E, L, Lz (Helmi & de Zeeuw 2000)
    • But E is not conserved if Galactic potential varies; L is not constant for an axisymmetric Galaxy
  • Apocentre, Pericentre and z-angular momentumLz (APL)-space shows very specific features
    • (Roughly) conserved quantities
conclusions from the new sample1
Conclusions from the new sample
  • No net slope of [M/H] with age @ R = R0 thoughout life of disk
  • Earlier findings of significant slope based on biased samples
  • Large and highly significant scatter in [M/H] at all ages
  • Scatter more significant than any trend of the mean
  • Any theory that cannot explain why a large scatter exists
  • is missing a crucial piece of the physics!
  • Inefficient azimuthal mixing; different local enrichment rates
  •  Challenge for model builders!
3 radial metallicity gradients
3. Radial metallicity gradients

Our local sampleShaver 1983; Fitch&Silkey 1991;

Vilchez & Esteban 1996

results on radial gradients1
Results on radial gradients
  • Weak constraints from local sample (!)
  • Present gradients consistent with other studies
  • Some evolution with time seen;can guide models
computing orbital trajectories
Computing orbital trajectories
  • Start from present positions & velocities
  • Assume smooth potential matching V(R) & 0
  • Errors of ~750 pc after 450 Myr (2 revolutions)

complete explosive Si burning

Zn: Trace element in Universe

25M 20M 13M



Zn does not bind to dust grains and is therefore a measure of the

metal content in interstellar gas. But the relative production of Zn

is largest in the earliest and/or most massive stars.

conclusion and prospects
Conclusion and prospects
  • Many new results (local surveys, SSDS,..)

- Spectroscopy (elemental abundances)

- Radial velocities, parallaxes, etc.

  • A lot of progress in models (galaxy evolution and galaxy formation)
  • Gaia (satellite) for larger samples 109 stars