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DARS, October 9, 2008
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
”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). Progress reported in Copenhagen, June 2008 (Proceedings in 09).
Any connection between the components?
Evidence of evolution
How do we know if/that the MW evolves?
Observational facts?
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• . . . .
It is all very easy or…?
Big Bang
Primordial Nucleosynthesis
(hydrogen, helium, deuterium, lithium)
Protogalaxy CollapseHalo+bulge formation (thick disk?)
Thin Disk formation
Interstellar Medium (ISM)
Stellar Evolution(elements cooking)
INFALLslowfast
Star FormationStellar mass lossStellar death - (m)
Stellar remnants
IMF + SFR
Elements “pollute” ISM
YIELDS
ISM Enrichment
Massive Stars SN type II (core collapse)
(M > 8 M ) elements Fe and r-process elements up to ULow- and Interm.-mass
StarsHe, C, N, heavy s-process elements
Planetary Nebulae, ..
(0.8 < M < 8 M )
+
Single stars
Binary st. (WD)
Fe
SN type Ia (thermonuclear explosion)
Periodic Table (evolution)
How it all started
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
• 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• 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?
ESO Very Large Telescope 4x8m
Stellar spectrum
H
Na I
Mg I
Ca I
Fe
Quantitative spectral analysis
Sun = 1
1/10,000
1/200,000
0 (hypot.)
[Fe/H]=log(Fe/H)star-log (Fe/H)sun
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
T Wilkinson Microwave Anisotropy Probe (WMAP)
Bennett et al 2003
Lithium in early generations
[Fe/H]
A(L
i)
WMAP + SBBNS
Test: Lithium
Li fromBig Bang
New Li produced by cosmic radiation and in stars
BBe,, Li ON,C,p
but,at the same time …
Li is destroyed in stars
neutroner
protoner
rp procesrp proces
r procesr proces
Masse kendtHenfaldstid kendtIntet kendt
s process proces
H-StjernerH-Stjerner
Big BangBig Bang
p procesp proces
SupernovaeSupernovae
Kosmisk strålingKosmisk strålingH(1)
Fe (26)
Sn (50)
Pb (82)
Neutronindfangning:
Videre opad i det periodiskesystem:
Hydrogen i brøkdel af massen X = 0.71
Heliums I brøkdel af massen Y = 0.28
“Metalindhold” (brøkdelen af alle andre grundstoffer) Z = 0.019
Brøkdel af tunge grundstoffer (fra Ni og opefter) 0.000004
0 50 100 150 200 250m a ss num b e r
10-13
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
100
num
be
r fra
ctio
n
Gab v.B,Be,Li
-kerner12C,16O,20Ne,24Mg, …. 40Ca
Skarp top!
s-proces toppe (lukkede kerneskaller)
r-proces toppe (lukkede kerneskaller)
AuFe Pb
U,Th
Generel tendens: færre tunge kerner
s-process in “real time”
”Katteøjet” planetariske tåger ”Håndvægten”
r-process
Pulsar
Optical (ESO VLT) X-rays (Chandra)
”Crab nebula”: Supernova 1054 – 950 years after
SN Ia yieldsElement M/Mo
Models by Nomoto et al.
Elemental abundances predicted for various progenitor mass and explosion energies.
Relation r- and s-processes (solar)
Sr IAu
Z56 stable n-capture elements: excellent match
to solar r-process. • New star
••
Th II and U II lines
Cayrel et al. (2001)
BD +17o3248 CS 31082-001
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
Result
• 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)?
Radio-activity Th/Eu
14-17 Gyr ± 10 Gyr
Radio-activity Th/U
13.5 Gyr ± 1.5 Gyr
Microwave-
Background
13.7 Gyr ± 0.2 Gyr
Universe
expansion
13-14 Gyr ± 2 Gyr
White dwarfs11- 12 Gyr ± 1 Gyr
Age of the Universe
He Fusion in Red Giants (~ 106 to 107 years)4He + 4He 8Be8Be + 4He 12C +
12C burning (<1000 years)12C + 4He 16O12C + 12C 20Ne + 4He +
16O burning (<1 year)16O + 16O 28Si + 4He + 12C + 16O 24Mg + 4He +
Elements built up from He (alpha elem.)
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 . . .
a=0.13b=0.16
a=0.05b=0.05
incomplete explosive Si burning
complete 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.
25M 20M 13MSN
Results from the first supernovae
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 SN Ia and low- and intermediate mass stars (Fe, C, N)
Origin of single chemical elements
Detailed analysis of 189 nearby longlivedstars relative to the Sun
(Edvardsson et al.,1993)
SN II
SN Ia
[O/Fe] x [Fe/H] ([X/H] = log(X/H)
–log(X/H)sun)
[O/Fe]
[Fe/H] “time”
SN II enrichment SN Ia + SNII +PN +..
Solar level
Thin disk
Halo + thick disk
Globular clusters (GC)Globular clusters (GC)
Messier 10 - a globular cluster in the Galaxy
The Milky Way has about 150 of these clusters
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
I
Example: HR diagram of the star cluster M 55
High-mass stars evolved onto the
giant branch
Low-mass stars still on the main
sequence
Turn-off point
The 150 Milky Way clusters are old (~10-12 Gyr)
•Deficient in most of the heavier chemical elementsrelative 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
1. Elemental abundances of stars / [Fe/H]
2. Metallicity distribution function
3. Age-Metallicity Relation
4. Radial metallicity gradients
5. Age-Velocity dispersion
6. Fe/H-Velocity dispersionStudy the disk as that is where most of the baryons are.
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
Giants excluded
Mv from
Hipparcos &photometricdistances
Teff from uvby photometry
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
Need to adjust Teff scale @ low [Fe/H] !
±1
HR’cube’
= ’Well-defined’ age
Age
Integrate probability for all points in diagram
1
When isochrone ages are uncertain :
Ages not well-defined outside favourable parts of the HR diagram!
±1 conf. level
Age determinations:Age determinations:
Takeda et al. 2007 – GCS newTakeda et al. 2007 – GCS new
Isochrones (8 Gyr, solar metallicity):
Victoria-Regina, Padova, Geneva (thin lines)
2. Metallicity Distribution Function2. Metallicity Distribution Function(G-dwarf problem)(G-dwarf problem)
Volume complete sample(solid), Volume complete sample(solid),
corrected (dashed), Casuso & Beckman 2004 (dotted)corrected (dashed), Casuso & Beckman 2004 (dotted)
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
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
Predictions:• [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)
Full magnitude-limited sample : 7,566 stars with ’well-defined’ ages
Reddened F giants (over-corrected?)
Nordström et al. 2004
Age - metallicity relations: Now & then(single stars, well-defined ages)
Volume-limited sample (d < 40 pc)
189 F dwarfs (no) GK dwarfs
Edvardsson et al. (1993)
462 single FGK dwarfs
(also found for open clusters)
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
Nordström et al. local sample Shaver 1983; Fitch&Silkey 1991; Vilchez & Esteban 1996
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)
5. UVW velocities – Age
Age – velocity (-dispersion) relations
7,237 single stars with ”well-defined” ages and all data
age < 25%
Freeman : Velocity dispersion of nearby Fstars saturates after 2 Gyr (data from E93)
Disk heating : Saturating or ongoing?Disk heating : Saturating or ongoing?
Young Old Thick
saturation seen for ages > 2 Gyr !(galaxy mergers traceable)
NO !
Our data
age < 25%
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
• 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”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?
• If stars conserve memorymemory of their originorigin, it may be possible to reconstructreconstruct the merging historyhistory of our Galaxy
• This memorymemory may be in the form of SUBSTRUCTURES 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
• 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
Colour Magnitude Diagrams of the groups
Isochrones from Yonsei-Yale
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 momentum Lz (APL)-space shows very specific features– (Roughly) conserved quantities
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
Our local sample Shaver 1983; Fitch&Silkey 1991; Vilchez & Esteban 1996
Results on radial gradients
• Weak constraints from local sample (!)
• Present gradients consistent with other studies
• Some evolution with time seen; can guide models
• Start from present positions & velocities
• Assume smooth potential matching V(R) & 0
• Errors of ~750 pc after 450 Myr (2 revolutions)
Computing orbital trajectories
The Geneva-Copenhagen Survey: “The Movie”
Long-lived stars near the sun
H I circulation of gas disk-halo (Fraternali et al. 2005)
complete explosive Si burning
25M 20M 13M
a=0.13b=0.08
Zn: Trace element in Universe
Zn does not bind to dust grains and is therefore a measure of themetal content in interstellar gas. But the relative production of Zn is largest in the earliest and/or most massive stars.
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