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Metal-Poor Stars and the Chemical Enrichment of the Universe Observations, Stellar Abundances, and Chemical Evolution

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Metal-Poor Stars and the Chemical Enrichment of the Universe Observations, Stellar Abundances, and Chemical Evolution Metal-poor stars and the chemical enrichment of the universe Anna Frebel Observations, stellar abundances, and chemical evolution Prof. Anna Frebel Massachusetts Institute of Technology (MIT) & chemical enrichment Observations, abundances Observations, abundances 1 Bio sketch • 1999-2002: BSc equivalent in physics (Freiburg, Germany) Anna Frebel • 2002-2003: Work experience at Mt. Stromlo Observatory (Canberra, Australia) • 2003-2006: PhD at Australian National University (Canberra, Australia) • 2006-2008: McDonald Postdoctoral Fellow (Univ. of Texas, Austin) • 2009-2011: Clay Postdoctoral Fellow (Harvard-Smithsonian CfA) • 2012-now: Assistant Professor of Physics (MIT, USA) • Feel free to ask questions! • Available at NIC school: only Monday and Tuesday! & chemical enrichment at the 6.5m Magellan Telescope in Chile Observations, abundances Observations, abundances 2 Research interests Stellar archaeology: Near-field cosmology: Anna Frebel • The most metal-poor stars, • The first stars, • chemical evolution of the Milky • early star and galaxy Way and dwarf galaxies, formation, • stellar kinematics and galactic • galaxy assembly on structure, small scales, • supernova nucleosynthesis and • dwarf galaxies, neutron-capture processes, • the formation of the • nuclear astrophysics, Galactic halo • stellar evolution, (theoretically + observationally), • stellar abundances, • the age of the Universe. • spectroscopic observations and & chemical enrichment analyses. Observations, abundances Observations, abundances 3 Freely available at http://arxiv.org/abs/1102.1748 4 OutlineOutline Anna Frebel Demo! & chemical enrichment Observations, abundances Observations, abundances 5 Observations, abundances Anna Frebel & chemical enrichment 6 A long time ago... nd First stars 2 and later generations of stars (<1 M ) Anna Frebel (100 M) Big Bang today first galaxies today’s galaxies Larson & Bromm 2001 Cosmic time (not to scale) & chemical enrichment Observations, abundances Observations, abundances 7 chemical evolution All the atoms (except H, He & Li) Anna Frebel were created in stars! Pop III: zero-metallicity stars Pop II: old halo stars Pop I: young disk stars We are made of stardust! ⇒ Old stars contain fewer elements (e.g. iron) than younger stars & chemical enrichment Zentrum fuer Astronomie und Astrophysik, TU Berlin Astrophysik, Astronomieund Zentrum fuer Observations, abundances Observations, abundances 8 Stellar archaeology Through chemical abundance studies Anna Frebel Big Bang ~13 billion years in between early gas cloud: Metal-poor star formation stars today in the metal-poor stars Milky Way 9 About a star... Anna Frebel H He metals & chemical enrichment Observations, abundances Observations, abundances 10 Astronomer’s periodic table Anna Frebel Metals Z [ Fe often used to trace metallicity Z ] & chemical enrichment Observations, abundances Observations, abundances 11 What can we learn from old halo stars? Low-mass stars (M < 1 M) Hertzsprung-Russell-diagram Anna Frebel ⇒ lifetimes > 10 billion years ⇒ unevolved stars are still around! ________________________ Using “fossil” metal-poor stars to reconstruct... Origin and evolution of chemical elements Luminosity Relevant nucleosynthesis processes and sites Chemical and dynamical history of the Galaxy Lower limit to the age of the Universe APOD Temperature ... and to provide constraints Nature of the first stars & initial mass function Nucleosynthesis & chemical yields of first/early SNe Early star & early galaxy formation processes Hierarchical merging of galaxies (observed abundances are ‘end product’ that have to be reproduced by any comprehensive galaxy formation model) Formation of the galactic halo by detailed understanding of its stellar content & chemical enrichment Observations, abundances Observations, abundances 12 The Milky Way ✷ ✷ ✷ ✷ ✷ ✷ ✷ dwarf ✷ ✷ galaxies ✷ Halo Disk Bulge Metal-poor halo stars 13 The Role of Metal-poor stars The abundances of the elements in stars more metal-poor than the Sun have the potential to inform our understanding of conditions from the beginning of time through the formation of the first stars and Anna Frebel galaxies, and up to the relatively recent time when the Sun formed: • The most metal-poor stars ([Fe/H] ~ −4.0), with primitive abundances of the heavy elements (atomic number Z > 3), are most likely the oldest stars so far observed. • The lithium abundances of extremely metal-poor near main-sequence-turnoff stars have the potential to directly constrain conditions of the Big Bang. • The most metal-poor objects formed at epochs of redshifts z > 6, and probe conditions when the first heavy element producing objects formed. The study of objects with [Fe/H] < –3.5 permits insight into conditions at the earliest times that is not readily afforded by the study of objects at high redshift. • They constrain our understanding of the nature of the first stars, the initial mass function, the explosion of super- and hypernovae, and the manner in which their ejecta were incorporated into subsequent early generations of stars. • Comparison of detailed observed abundance patterns with the results of stellar evolution calculations and models of galactic chemical enrichment strongly constrains the physics of the formation and evolution of stars and their host galaxies. • In some stars w/ [Fe/H] ~ −3.0, overabundances of heavy-neutron-capture elements are so large that Th and U can be measured which leads to independent estimates of their ages and of the Galaxy. • Stars with [Fe/H] ~ –0.5 inform our understanding of the evolution of the Milky Way system. Relationships between abundance, kinematic, and age distributions – the defining characteristics of stellar & chemical enrichment populations – permit choices between the various paradigms of how the system formed and has evolved. Observations, abundances Observations, abundances 14 Solar abundance distribution Needs to be known in order to calculate other stellar abundances!! Anna Frebel (hence, if solar abundances change, everything else will change) & chemical enrichment Observations, abundances Observations, abundances 15 Solar abundances Photospheric (= “stellar” abundance) • Anders, Grevesse & Sauval ‘89 Anna Frebel • Grevesse & Sauval ‘98 • Asplund, Grevesse &Sauval ‘05 • Grevesse, Asplund & Sauval ‘07 • Asplund, Grevesse, Sauval & Scott ‘09 • Series of new papers ’14 • reference element: H • technique: calculation Meteoritic (= “star dust” grain analysis) • Lodders 03 • Lodders, Palme & Gail 09 • reference element: Si • technique: measurement • Volatile elements depleted, incl. the most abundant elements: H, He, C, N, O, Ne cannot rely on meteorites to determine the primordial Solar System abundances for such elements & chemical enrichment • For each application, the most similarly obtained solar abundances should be use to minimize systematic Observations, abundances Observations, abundances uncertainties! 16 Definitions: log ε(x) abundances Stellar ‘abundances’ are number density calculations with Anna Frebel respect to H and the solar value On a scale where H is 12.0: logε(X) = log10 (NX /N H ) +12 for element X This quantity is the output of all model atmospheres! i.e. MOOG code (of Chris Sneden, publicly available) + € Kurucz models (publicly available) For lithium, the abundance is mostly expressed as A(Li) = logε(Li); and for hydrogen, by definition, log10ε(H) = 12. & chemical enrichment For stellar abundances in the literature, results are generally presented relative to their values in the Sun, using the so-called “bracket notation”. Observations, abundances Observations, abundances 17 definitions: [fe/h] Anna Frebel where NFe and NH is the no. of iron and hydrogen atoms per unit of volume respectively. ⎡ ⎤ ⎡ ⎤ NO NO NFe NFe = ⎢log 10( )star − log10( )sun ⎥ −⎢log 10( )star − log10( )sun ⎥ ⎣ N H N H ⎦ ⎣ N H N H ⎦ € & chemical enrichment [A /H] − [B /H] = [A /B] for elements A and B Observations, abundances Observations, abundances 18 € spectroscopic comparison Anna Frebel “Look-back time” Galacticchemical evolution Abundances are derived from integrated absorption line strengths equals 1/250,000th of the solar Fe [Fe/H] = log(NFe/NH) − log(NFe/NH) & chemical enrichment * abundance Observations, abundances Observations, abundances 19 Observations, abundances Anna Frebel & chemical enrichment Success overdecades! 20 Observations, abundances Anna Frebel & chemical enrichment related definitions Metal-poor star 21 classification scheme Range Term Acronym # Anna Frebel [Fe/H] ≥ +0.5 Super metal-rich SMR some [Fe/H] = 0.0 Solar — a lot! [Fe/H] ≤ –1.0 Metal-poor MP very many [Fe/H] ≤ –2.0 Very metal-poor VMP many [Fe/H] ≤ –3.0 Extremely metal-poor EMP ~100 [Fe/H] ≤ –4.0 Ultra metal-poor UMP 1 [Fe/H] ≤ –5.0 Hyper metal-poor HMP 2 [Fe/H] ≤ –6.0 Mega metal-poor MMP -- & chemical enrichment Extreme Pop II stars! as suggested by Beers & Christlieb 2005 Observations, abundances Observations, abundances 22 What sort of stars are we looking for? Low-mass stars Anna Frebel with < 1 Msun ensures ⇒ Long lifetimes Unevolved nature ensures ⇒Unmixed surface layers This avoids surface abundances contamination with nuclear burning products from the stellar core and billion yr long preservation of & chemical enrichment abundances Observations, abundances Observations, abundances © B.J. Mochejska (APOD) 23 Three Observational Steps to Find Metal-Poor Stars Anna Frebel 1. Sample selection and visual inspection: Find appropriate candidates ! (Ca scales with Fe!) 2. Follow-up spectroscopy (medium
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