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