Stellar Spectroscopy and Elemental Abundances

1. Stellar Spectroscopy and Elemental Abundances Definitions Solar Abundances Relative Abundances Origin of Elements

2. Definitions • X, Y, Z: amounts of H, He, and rest (metals) by mass (total of 1; log Z/Zsun in Kurucz models) • Solar: X=0.735, Y=0.248, Z=0.017 • Abundance as number density relative to HA=N(element)/N(H)Usually given as log A or log A +12 • [Fe/H]=log[N(Fe)/N(H)]star – log[N(Fe)/N(H)]sun • [M/H] sometimes reported as mean metallicity

3. Solar Abundances • From spectroscopy and meteorites • Gray Table 16.3 • Scott, Grevesse et al.2015, A&A, 573, A252014arXiv1405.0279S

4. Relative Abundances

5. Origin of the Elementshttp://ned.ipac.caltech.edu/level5/Pagel/Pagel_contents.html • Hydrogen is most abundant element, followed fairly closely by helium. • He formed in the Big Bang, with some increase from the primordial He abundance (Yp =0.24) by subsequent H-burning in stars (Y = 0.248 here and now).

6. Light Elements: Li, Be, B • Li, Be and B are very scarce, mostly destroyed in the harsh environment of stellar interiors • Li abundance comes from measurements in meteorites; it is still lower in the solar photosphere because of destruction by mixing with hotter layers below. • Abundant in primary cosmic rays as a result of fusion and spallation reactions between p and (mainly) CNO nuclei at high energies. • Deuterium and some Li formed in Big Bang.

7. Carbon (6) to Calcium (20) • Downward progression modulated by odd:even and shell effects in nuclei which affect their binding energy. • From successive stages in stellar evolution: exhaustion of one fuel is followed by contraction, heating, alpha=He capture fusion. • Onset of Ca burning leads to Mg and nearby elements; accompanied by neutrino emission(ever faster evolution).

8. Iron Group • Fe-group elements represent approximate nuclear statistical equilibrium at T ≈109 K • Result of shock that emerges from the core of a massive star that has collapsed into a neutron star (SN II) OR sudden ignition of C in a white dwarf that has accreted enough material from a companion to bring it over the Chandrasekhar mass limit (SN Ia). • Dominant product is 56Ni, most stable nucleus with equal numbers of protons and neutrons, which later decays into 56Fe.

9. s-process: slow addition of neutrons • Nucleosynthesis beyond the Fe group occurs by neutron capture. Captures on a seed nucleus (mostly 56Fe) lead to the production of a β-unstable nucleus (e.g. 59Fe). • Outcome depends on relative time-scales for neutron addition and decay. • s-process: slow addition, so that unstable nuclei have time to undergo decay • Nuclei form along the stability valley to 209Bi.

10. s- and r-process decaysin neighborhood of Tin (Sn) β decay

11. r-process: rapid addition of neutrons • Many neutrons are added under conditions of very high T, neutron density; build unstable nuclei up to the point where (n, γ) captures are balanced by (γ, n) photodisintegrations • After neutron supply is switched off, products undergo further decay ending at the nearest stable isobar (neutron-rich side of the stability valley). • Some elements from both r- and s-processes.

12. Stability Valley Abundance peaks occur corresponding to closed shells with 50, 82 or 126 neutrons

13. Summary of Relative Abundances

14. Metal rich vs. metal poor stars:Frebel et al.2005, Nature, 434, 871[Fe/H]=-5.4 • SMSS0313-6708[Fe/H]<-6.5Nordlander et al.2017, A&A, 597, A6

15. Abundance Trends • Metals higher in Pop I stars (younger, disk) than in Pop II stars (older, halo);Galactic enrichment with time • Metals higher closer to Galactic center • Evolutionary changes:Li decrease with ageCNO-processed gas in stars with mixingC enhancement in older stars with He-burning • Magnetic fields can create patches with unusual abundance patterns: Ap stars