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Nucleosynthesis, stellar abundances, and chemical evolution Anna Frebel P-329 Guest lecture “Stars and planets” class by Dimitar Sasselov, Fall 2010 Ulitmate question How did the solar abundances come about? Stellar spectra What sort of stars are we looking for? unevolved, low-mass stars; <1 Msun to ensure long lifetimes =>unmixed, too, to avoid surface abundances contamination with nuclear burning products © B.J. Mochejska (APOD) Three Observational Steps to Find Metal-Poor Stars 1. Sample selection and visual inspection: Find appropriate candidates (Ca scales with Fe!) 2. Follow-up spectroscopy (medium resolution): Derive estimate for [Fe/H] from the Ca II K line 3. High-resolution spectroscopy: Detailed abundances analysis Frebel et al. 2005b “Look-back time” spectroscopic comparison Abundances are derived from integrated absorption line strengths [Fe/H] = log(NFe/NH) log(NFe/NH) * equals 1/250,000th of the solar Fe abundance important spectral absorption lines in stars • H lines 6562Å, 4860Å, 4340Å, 4101Å • CH g-band @ 4313Å and others • Li @ 6707Å • Mg b lines @ ~5170Å • Ca K line @ 3933Å • Na D lines @ ~5880Å • Eu @ 4129Å • Sr @ 4077Å, 4215Å • Ba @ 4554Å Fe lines are everywhere in the spectrum -- always easily accessible Carbon & nitrogen Huge carbon abundance ([C/Fe]= +3.7): (=> not so carbon-poor...) Synthetic spectrum: red lines Carbon (CH) band 5,000 and 12,000 times more carbon and nitrogen exist than iron! Huge nitrogen abundance ([N/Fe]= +4.1): (=> not so nitrogen-poor...) Synthetic spectrum: red lines Reminder: Solar ratio [C,N/Fe] = 0 Nitrogen (NH) band Frebel et al. 2008, ApJ subm. HE 1327-2326 Ca II K line Calcium often used as proxy for the Fe abundance! (..and Fe for metallicity) –5.4 Interstellar Ca (Ca scales with Fe!) Frebel et al. (2005), Nature Mg b lines Eu Thorium II Line 4019Å Abundance: Synthetic spectrum (based on atomic data and model atmosphere) to match observed spectrum Synthetic spectrum that includes NO thorium Th HE 1523-0901 Frebel et al. (2010), in prep. ‘Best fit’ synthetic spectrum Uranium in HE 1523-0901 Synthetic spectrum that includes NO uranium Synthetic spectrum with U abundance if it had NOT decayed Frebel et al. (2007) ‘Best fit’ synthetic spectrum How do we interpret stellar spectra? need to know: model atmosphere analysis techniques knowledge about nucleosynthesis, stellar evolution, chemical evolution, cosmological understanding of galaxy formation Model atmosphere analysis techniques Stellar parameters fully characterize a star: effective temperature Teff surface gravity log g metallicity [Fe/H] (microturbulent velocity vmic) ATomic data • every absorption line is an atomic transition • determined by atomic physics parameters • Vienna Atomic Line Database (VALD) http://vald.astro.univie.ac.at/~vald/php/vald.php • National Institute for Standards and Technology (NIST) http://www.nist.gov/pml/data/asd.cfm From [email protected] Mon Nov 1 10:03:10 2010 Date: Tue, 31 Aug 2010 23:56:00 +0200 From: [email protected] Subject: Re: ============= job.012302 ============= # begin request # extract all # default configuration # short format # # 4057.0, 4058.5 # end request Damping parameters Lande Elm Ion WL(A) Excit(eV) log(gf) Rad. Stark Waals factor References 'Ti 1', 4057.0060, 2.3340, -4.645, 7.735,-5.924,-7.491, 0.230,' 1 1 1 1 'Si 2', 4057.0090, 12.8390, -1.330, 0.000, 0.000, 0.000,99.000,' 2 2 2 2 'F 3', 4057.0630, 54.8200, -0.340, 0.000, 0.000, 0.000,99.000,' 3 3 3 3 'V 1', 4057.0650, 2.1220, -0.203, 8.158,-5.083,-7.799, 1.000,' 4 4 4 4 'Cr 1', 4057.1370, 4.4460, -1.424, 8.330,-5.330,-7.720, 1.160,' 5 5 5 5 'Co 1', 4057.1820, 0.2240, -3.249, 4.653,-6.374,-7.867, 1.230,' 6 6 6 6 Stronger line <=> lower excit <=> higher log gf 1 2 3 4 5 6 1 2 3 4 5 6 1' 2' 3' 4' 5' 6' Definitions: log Stellar ‘abundances’ are number density calculations with respect to H and the solar value On a scale where H is 12.0: log (X) log 10 N X /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 (=inhouse!) definitions: [fe/h] QuickTime™ and a decompressor are needed to see this picture. where NFe and NH is the no. of iron and hydrogen atoms per unit of volume respectively. QuickTime™ and a decompressor are needed to see this picture. NO NO N Fe N Fe log 10 ( ) star log 10 ( ) sun log 10 ( ) star log 10 ( ) sun N N N N H H H H A /H B /H A /B for elements A and B Solar abundances Photospheric (=‘stellar’ abundance) • • • • • Anders, Grevesse & Sauval ‘89 Grevesse & Sauval ‘98 Asplund, Grevesse &Sauval ‘05 Grevesse, Asplund & Sauval ‘07 Asplund, Grevesse, Sauval & Scott ‘09 • • reference element: H calculation Meteoritic (=‘star dust’ grain analysis) • • Lodders 03 Lodders, Palme & Gail 09 • • reference element: Si 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 For each application, the most similarly obtained solar abundances should be use to minimize systematic uncertainties! how to calculate chemical abundances • Need a spectrum => measure equivalent width of absorption lines (=integrated line strength) • Need atomic data (excit. potential+log gf values) => feed both into “model atmosphere” • Get: calculated abundance (number density) log (X) • Calculate [Fe/H] with solar abundances • • • • • Example: log (Mg)star = 5.96; log (Fe)star = 5.50 log (Mg)sun = 7.60; log (Fe)sun = 7.50 [Mg/H] = log (Mg)star - log (Mg)sun = -1.64 [Mg/Fe] = [Mg/H] - [Fe/H] = -1.64 - (-2.0) = 0.36 How metal-poor? classical example: early universe: primordial gas how metal-poor is the next-generation star? canonical SN Fe yield: 0.1 Msun available gas mass: 106 Msun M tot 10 6 M sun NH mH mH QuickTime™ and a decompressor are needed to see this picture. N Fe M tot 0.1M sun mFe 56m H log (Fe) sun log( N Fe /N H ) sun 12 7.50 log( N Fe /N H ) sun 7.50 12 4.50 N Fe 0.1M sun mH 107 6 NH 56m H 10 M sun 56 107 [Fe /H] log( ) (4.50) 4.2 56 classification scheme Range [Fe/H] ≥ +0.5 [Fe/H] = 0.0 [Fe/H] ≤ –1.0 [Fe/H] ≤ –2.0 [Fe/H] ≤ –3.0 [Fe/H] ≤ –4.0 [Fe/H] ≤ –5.0 [Fe/H] ≤ –6.0 Term Acronym # Super metal-rich SMR some Solar — a lot! Metal-poor MP very many Very metal-poor VMP many Extremely metal-poor EMP ~100 Ultra metal-poor UMP 1 Hyper metal-poor HMP 2 Mega metal-poor MMP -- Extreme Pop II stars! as suggested by Beers & Christlieb 2005 Halo Metallicity distribution function (MDF) Previous ‘as observed’, raw MDF is not a realistic presentation! (but shows that we have been doing a good job in finding these stars..) Non-zero tail!!! Schoerck et al. 2008 The most metal-poor stars are extremely rare but extremely important! “Surface Pollution” through accretion Bondi-Hoyle-Lyttleton accretion: dM/dt = 4 G2M2 / v3 V1 Accretion for 10 billions years? stellar orbit Galactic disk V2 Three-component potential: disk, spheroid, halo (Johnston 1998) blue, bluer, the bluest Lower metallicity leads to decreased opacity stars are hotter than solar equivalents look bluer (bluer colors) needs to be taken into account! for temperature measurements, abundance analyses, stellar populations studies Nucleosynthesis All elements heavier than Li, Be, B are made during stellar evolution and supernova explosions Stellar nucleosynthesis QuickTime™ and a decompressor are needed to see this picture. most important reactions in stellar nucleosynthesis: * Hydrogen burning: - The proton-proton chain All textbooks, wikipedia - The CNO cycle .... * Helium burning: - The triple-alpha process - The alpha process * Burning of heavier elements: Timmes+ ~95 - Carbon burning process Woosely&Weaver 1995 - Neon burning process Heger & Woosley 2008 - Oxygen burning process - Silicon burning process * Production of elements heavier than iron: - Neutron capture: - The R-process Many details not - The S-process known, but good models - Proton capture: out there - The Rp-process - Photo-disintegration: - The P-process neutron-capture processes _ -decay: n => p + e- v e QuickTime™ and a decompressor are needed to see this picture. • s-process: neutron-capture longer than beta-decay timescale • r-process: neutron-capture shorter than beta-decay timescale slow n-cap process • in 1-8Msun AGB stars; AGB stars are major providers of C and sprocess elements in the universe (through mass loss) • produce s-rich companions: CH stars, Ba stars, s-rich metal-poor stars good knowledge of sprocess theoretically; QuickTime™ and a decompressor important for calculating are needed to see this picture. the solar r-process component The two neutron sources in AGB stars 13C(a,n)16O Needs 13C ! Major neutron source 13C-pocket Primary source! T8 = 0.9-1 Interpulse phase (1- 0.4) 105 yr Radiative conditions Nn = 107 cm-3 lower mass AGBs 22Ne(a,n)25Mg Abundant 22Ne Minor neutron source Neutron burst Secondary source T8 = 3 (low 22Ne efficiency) Thermal pulse 6 yr Convective conditions Nn (peak) = 1010 cm-3 higher mass AGBs the AGB engine He, 12C, 22Ne, s-process elements: Zr, Ba, ... At the stellar surface: C>O, sprocess enhance ments thermally pulsing AGB stars r-process QuickTime™ and a None decompressor are needed to see this picture. r-Process Enhanced Stars (rapid neutron-capture process) Responsible for the production of the heaviest elements Most likely production site: SNe II => pre-enrichment Chemical “fingerprint” of previous nucleosynthesis event (only “visible” in the oldest stars because of low metallicity) ~5% of metal-poor stars with [Fe/H] < 2.5 (Barklem et al. 05) Only 15-20 stars known so far with [Eu/Fe] > 1.0 Nucleo-chronometry: obtain stellar ages from decaying Th, U and stable r-process elements (e.g. Eu, Os) SN star -- decay -- today [Th and U can also be measured in the Sun, but the chemical evolution has progressed too far; required are old, metal-poor stars from times when only very few SNe had exploded in the universe] Our Cosmic Lab The r-Process Pattern Very good agreement with scaled solar rprocess pattern for Z>56 scaled solar r-process pattern decayed Th,U HE 1523-0901 According to metal-poor star abundances, the r-process is universal! Frebel et al. (2007) Precision at work! CS 22892-052 HD 115444 Scaled solar r-process element pattern!! BD +17 3248 CS 31082-001 HD 221170 HE 1523-0901 Cowan 2007, priv. comm They all have the same abundance pattern, particularly among heavy neutron-capture elements! r-process must be a universal process! chemical evolution origin of the elements abundances trends chemical evolution Zentrum fuer Astronomie und Astrophysik, TU Berlin All the atoms (except H, He & Li) 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 How and when did these early stars form? e.g. HE 1327-2326 Big Bang First star exploding First chemical enrichment Heger & Woosley 2008 Primordial gas cloud 2nd generation stars forming from enriched material Why important? Metal-poor stars provide the only available diagnosis for zerometallicity Pop III nucleosynthesis and early chemical enrichment Pre-enrichment by a “faint SN” Iwamoto et al. 2005 Science 309 451 • “Faint” SN with mixing and fallback: Post-explosion abundance distribution –Explains high C, N, O, Mg (Smaller mass cut for HE1327-2326 to account for high [Mg/Fe]) –Explain other metal-poor stars with [Fe/H]<3.5 –Neutron-capture elements not included M=25M, Z=0, low E Iwamoto et al. 2005 Science 309 451 … with some (new) upper limits Abundance trends [Mg/Fe] Alpha-elements [Si/Fe] Alpha elements multiple of He: (C,O), Ne, Mg, Si, S, Ar, Ca, Ti (not pure) [Ca/Fe] Synthesis during stellar evolution and a-capture in supernova explosion of massive stars (>8 M) Fe and a-elements produced in the explosions of massive stars (SN type II) Fe-rich ejecta from the SN of low-mass stars (SN type Ia) Aoki, Frebel et al. 2006, ApJ What is so special About the most Fe-poor stars? hyper Fe-poor ultra Fe-poor extremely Fe-poor very Fe-poor hyper Fe-poor ultra Fe-poor extremely Fe-poor very Fe-poor A compilation of abundances of ~800 metal-poor stars with The very different chemical signature of the hyper iron-poor stars [Fe/H]~<-2.0 can be found at is crucial for understanding the formation of the elements! http://www.cfa.harvard.edu/~afrebel/abundances/abund.html (published in Frebel ‘10, review article on metal-poor stars) plots with abundance trends https://www.cfa.harvard.edu/~afrebel/abundances/abund.html • Li in HE 1300 depleted in accordance with the star’s evolutionary status (subgiant) • Majority of depletion seems to be taking place in the range 5500-5600 K • Li depletion does not significantly depend on metallicity Frebel et al. 06, ApJ submitted Lithium the bigger picture using stars to study the hierarchical assembly of galaxy formation “near-field cosmology” A long time ago... 2nd and later generations of stars (<1 M) First stars (100 M) Big Bang today first galaxies today’s galaxies Larson & Bromm 2001 Cosmic time (not to scale) metallicity-luminosity relation Ultra-faint dwarfs Stellar archaeology with the most metal-poor stars in MW satellite galaxies Classical dSphs Ultra-faint dwarfs Martin et al. (2007) Metallicity distribution function of dSph galaxies More metal-poor stars in the ultra-faints than in halo!?! Ultra-faint dwarfs MW halo stars Classical dwarfs Kirby et al. (2008) (targets selected from Simon & Geha 2007) “classical” dSph have no extremely metal-poor stars?!? (Helmi et al. 2006) => yes, they do! What can we learn from the existing dwarf galaxies? Stellar archaeology: examine the chemical history in search for their oldest population to learn about - early chemical evolution in small systems - chemical signatures that relate dwarf galaxies to MW If surviving dwarfs are analogs of early MW building blocks then we should find chemical evidence of it! Stellar metallicities & abundances of metalpoor stars in dwarf galaxies should agree with those found in the MW halo Mg, Ca, Ti (a-elements) No discrepancy of ultra-faint dwarf galaxy stars with those of MW halo (at low metallicities)! • Stars in ultra-faint dwarfs studied by AF and colleagues (Ursa MajorII, Coma Berenices, LeoIV) (Frebel+2010, Simon+2010) • Stars in ultra-faint dwarfs studied by others (Hercules, Bootes) (Koch+2008, Norris+2009) • Stars in classical dSphs (Sculptor, Carinae, Draco, Sextans, Ursa Minor, Fornax, Leo) halo stars ultra-faints dSphs stars (Shetrone+2001,2003, Venn+2004, Sadakane+04, Aoki+2009) • Halo stars (e.g. Cayrel+2004, Barklem+2005, Aoki+2005, Lai+2008 plus many others!) See Frebel (2010) review for a complete list of abundances and references. ultra-faint dwarf galaxy abundances Excellent agreement with the MW chemical Comparison with evolution Cayrel+ 04 halo data (black open circles) Spread in some elements (C, ncapture elements) red squares: Ursa Major II blue dots: Coma Berenices black diamond: MW halo giant Frebel et al. (2010a) Summary • • • • • • • • • spectroscopy abundance analyses stellar atmospheres stellar evolution nucleosynthesis SN physics and explosions nuclear+atomic physics chemical evolution (near-field) cosmology