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Chemical evolution in the MW Observational evidence

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

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  1. Chemical evolution in the MWObservational evidence Birgitta Nordström Niels Bohr Institute

  2. Outline • First (oldest) stars in MW halo • Long-lived stars in MW disk • Age determination

  3. 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).

  4. Any connection between the components?

  5. Evidence of evolution How do we know if/that the MW evolves? Observational facts?

  6. 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 • . . . .

  7. It is all very easy or…?

  8. Big Bang Primordial Nucleosynthesis (hydrogen, helium, deuterium, lithium) slow INFALL fast Protogalaxy Collapse Halo+bulge formation (thick disk?) Thin Disk formation Elements “pollute” ISM Interstellar Medium (ISM) IMF + SFR Stellar mass loss Stellar death - (m) YIELDS Star Formation Stellar Evolution (elements cooking) Stellar remnants

  9. 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) Fe

  10. Periodic Table (evolution)

  11. How it all started

  12. From Big Bang to todayOrigin of the elements • Big Bang (H, He, Li) • Fusion in stars (He to Fe) • Explosions (fission: Ni U)

  13. 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.

  14. 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?

  15. ESO Very Large Telescope 4x8m

  16. Stellar spectrum H Na I Fe Mg I Ca I

  17. Quantitative spectral analysis Sun = 1 1/10,000 1/200,000 0 (hypot.) [Fe/H]= log(Fe/H)star-log(Fe/H)sun

  18. 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)

  19. 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

  20. Lithium in early generations WMAP + SBBNS A(Li) [Fe/H]

  21. Test: Lithium New Li produced by cosmic radiation and in stars but, at the same time … Li is destroyed in stars Li from Big Bang

  22. Masse kendt Henfaldstid kendt s proces Intet kendt p proces r proces rp proces Supernovae H-Stjerner Kosmisk stråling Big Bang Videre opad i det periodiske system: Pb (82) protoner Sn (50) Neutronindfangning: Fe (26) H(1) neutroner

  23. 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) U,Th Skarp top! Fe Au Pb

  24. s-process in “real time” ”Katteøjet”  planetariske tåger  ”Håndvægten”

  25. r-process ”Crab nebula”: Supernova 1054 – 950 years after Pulsar Optical (ESO VLT)   X-rays (Chandra)

  26. SN Ia yields Element M/Mo Models by Nomoto et al. Elemental abundances predicted for various progenitor mass and explosion energies.

  27. Relation r- and s-processes (solar) Au Sr I

  28. Z56 stable n-capture elements: excellent match to solar r-process. • New star • •

  29. Th II and U II lines CS 31082-001 BD +17o3248 Cayrel et al. (2001)

  30. 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

  31. 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)?

  32. Age of the Universe

  33. 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

  34. Elements built up from He (alpha elem.)

  35. 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 . . .

  36. Results from the first supernovae 25M 20M 13M SN 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. sa=0.05 sb=0.05 complete explosive Si burning sa=0.13 sb=0.16

  37. 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)

  38. Origin of single chemical elements Detailed analysis of 189 nearby longlived stars relative to the Sun (Edvardsson et al.,1993) SN II SN Ia

  39. [O/Fe] x [Fe/H] ([X/H] = log(X/H) –log(X/H)sun) SN II enrichment SN Ia + SNII +PN +.. [O/Fe] Halo + thick disk Thin disk Solar level [Fe/H] “time”

  40. 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 I Messier 10 - a globular cluster in the Galaxy The Milky Way has about 150 of these clusters

  41. 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

  42. 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

  43. 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.

  44. 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

  45. HR diagram of RV sample Giants excluded Mv from Hipparcos & photometric distances Teff from uvby photometry

  46. 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

  47. Computing isochrone ages HR ’cube’ 1 Integrate probability for all points in diagram Age ±1 = ’Well-defined’ age Need to adjust Teff scale @ low [Fe/H] !

  48. When isochrone ages are uncertain : ±1 conf. level Ages not well-defined outside favourable parts of the HR diagram!

  49. Age determinations: Takeda et al. 2007 – GCS new

  50. Isochrones (8 Gyr, solar metallicity): Victoria-Regina, Padova, Geneva(thin lines)

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