and chemical evolution 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 Sample selection and visual inspection: Find appropriate candidates (Ca scales with Fe!) 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

spectroscopic comparison “Look-back time” Galactic chemical evolution Abundances are derived from integrated absorption line strengths equals 1/250,000th of the solar Fe abundance [Fe/H] = log(NFe/NH)*  log(NFe/NH)

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): 5,000 (=> not so carbon-poor...) 5,000 and 12,000 times more carbon and nitrogen exist than iron! Synthetic spectrum: red lines Carbon (CH) band Huge nitrogen abundance ([N/Fe]= +4.1): (=> not so nitrogen-poor...) Reminder: Solar ratio [C,N/Fe] = 0 Synthetic spectrum: red lines Nitrogen (NH) band Frebel et al. 2008, ApJ subm.

Ca II K line HE 1327-2326 Calcium often used as proxy for the Fe abundance! (..and Fe for metallicity) Interstellar Ca (Ca scales with Fe!) –5.4 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 ‘Best fit’ synthetic spectrum Th Frebel et al. (2010), in prep. HE 1523-0901

Uranium in HE 1523-0901 ‘Best fit’ synthetic spectrum 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 Stronger line <=> lower excit <=> 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 vald@vald.astro.univie.ac.at Mon Nov 1 10:03:10 2010 Date: Tue, 31 Aug 2010 23:56:00 +0200 From: vald@vald.astro.univie.ac.at 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 1 1 1' 'Si 2', 4057.0090, 12.8390, -1.330, 0.000, 0.000, 0.000,99.000,' 2 2 2 2 2 2 2' 'F 3', 4057.0630, 54.8200, -0.340, 0.000, 0.000, 0.000,99.000,' 3 3 3 3 3 3 3' 'V 1', 4057.0650, 2.1220, -0.203, 8.158,-5.083,-7.799, 1.000,' 4 4 4 4 4 4 4' 'Cr 1', 4057.1370, 4.4460, -1.424, 8.330,-5.330,-7.720, 1.160,' 5 5 5 5 5 5 5' 'Co 1', 4057.1820, 0.2240, -3.249, 4.653,-6.374,-7.867, 1.230,' 6 6 6 6 6 6 6' Stronger line <=> lower excit <=> higher log gf

Definitions: log  Stellar ‘abundances’ are number density calculations with respect to H and the solar value On a scale where H is 12.0: This quantity is the output of all model atmospheres! i.e. MOOG code (of Chris Sneden, publicly available) + Kurucz models (=inhouse!) for element X

definitions: [fe/h] for elements A and B where NFe and NH is the no. of iron and hydrogen atoms per unit of volume respectively. 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

classification scheme Range Term Acronym # [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 -- 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 stellar orbit V1 Galactic disk V2 Three-component potential: disk, spheroid, halo (Johnston 1998) Accretion for 10 billions years?

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

most important reactions in stellar nucleosynthesis: * Hydrogen burning: - The proton-proton chain - The CNO cycle * Helium burning: - The triple-alpha process - The alpha process * Burning of heavier elements: - Carbon burning process - Neon burning process - Oxygen burning process - Silicon burning process * Production of elements heavier than iron: - Neutron capture: - The R-process - The S-process - Proton capture: - The Rp-process - Photo-disintegration: - The P-process All textbooks, wikipedia .... Timmes+ ~95 Woosely&Weaver 1995 Heger & Woosley 2008 Many details not known, but good models out there

neutron-capture processes -decay: n => p + e- v_e 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 s-process elements in the universe (through mass loss) produce s-rich companions: CH stars, Ba stars, s-rich metal-poor stars good knowledge of s-process theoretically; important for calculating the solar r-process component

The two neutron sources in AGB stars 13C(a,n)16O 22Ne(a,n)25Mg 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 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 At the He, 12C, 22Ne, s-process elements: Zr, Ba, ... stellar surface: C>O, s- process enhance ments He, 12C, 22Ne, s-process elements: Zr, Ba, ...

thermally pulsing AGB stars

r-process

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) [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] SN star -- decay -- today

Our Cosmic Lab

The r-Process Pattern Very good agreement with scaled solar r-process 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! Scaled solar r-process element pattern!! 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!

origin of the elements abundances trends chemical evolution origin of the elements abundances trends

Zentrum fuer Astronomie und Astrophysik, TU Berlin chemical evolution 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 Zentrum fuer Astronomie und Astrophysik, TU Berlin We are made of stardust! Old stars contain fewer elements (e.g. iron) than younger stars

How and when did these early stars form? Big Bang Primordial gas cloud First star exploding First chemical enrichment 2nd generation stars forming from enriched material e.g. HE 1327-2326 [X/Fe] Tominaga et al. 2007 Heger & Woosley 2008 Why important? Metal-poor stars provide the only available diagnosis for zero-metallicity Pop III nucleosynthesis and early chemical enrichment

Pre-enrichment by a “faint SN” “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 Iwamoto et al. 2005 Science 309 451 M=25M, Z=0, low E

… with some (new) upper limits Iwamoto et al. 2005 Science 309 451

Abundance trends Alpha-elements [Mg/Fe] Alpha elements multiple of He: (C,O), Ne, Mg, Si, S, Ar, Ca, Ti (not pure) Synthesis during stellar evolution and -capture in supernova explosion of massive stars (>8 M) [Si/Fe] [Ca/Fe] Pagel & Tautvaišiene (1995)) Fe and -elements produced in the explosions of massive stars (SN type II) Fe-rich ejecta from the SN of low-mass stars (SN type Ia)

What is so special About the most Fe-poor stars? Aoki, Frebel et al. 2006, ApJ hyper Fe-poor ultra Fe-poor extremely Fe-poor very hyper Fe-poor ultra Fe-poor extremely Fe-poor very A compilation of abundances of ~800 metal-poor stars with [Fe/H]~<-2.0 can be found at http://www.cfa.harvard.edu/~afrebel/abundances/abund.html (published in Frebel ‘10, review article on metal-poor stars) The very different chemical signature of the hyper iron-poor stars is crucial for understanding the formation of the elements!

plots with abundance trends https://www.cfa.harvard.edu/~afrebel/abundances/abund.html

Frebel et al. 06, ApJ submitted Lithium 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

the bigger picture using stars to study the hierarchical assembly of galaxy formation “near-field cosmology”

A long time ago... today 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 Ultra-faint dwarfs Classical dSphs Martin et al. (2007)

Metallicity distribution function of dSph galaxies More metal-poor stars in the ultra-faints than in halo!?! Kirby et al. (2008) (targets selected from Simon & Geha 2007) Ultra-faint dwarfs MW halo stars Classical dwarfs “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 metal-poor stars in dwarf galaxies should agree with those found in the MW halo

Mg, Ca, Ti (-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) (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. halo stars dSphs stars ultra-faints

ultra-faint dwarf galaxy abundances Excellent agreement with the MW chemical evolution Comparison with Cayrel+ 04 halo data (black open circles) Spread in some elements (C, n-capture 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