Chap.2 Stellar populations and chemical evolution • Stars in a color-magnitude diagram – nearby stars, globular clusters • Stellar evolution and population synthesis – evolutionary tracks, metallicity vs. age – star formation, single starburst model • Origin of elements and yields – Supernovae and hypernovae • Extremely metal-poor stars – Neutron capture elements, CEMP stars • Chemical evolution – IMF, SFR, Simple model, G-dwarf problem 1
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Chap.2 Stellar populations and chemical evolutionchiba/lecture/Nagoya2017/...Chap.2 Stellar populations and chemical evolution • Stars in a color-magnitude diagram – nearby stars,
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Chap.2 Stellar populations and chemical evolution
• Stars in a color-magnitude diagram– nearby stars, globular clusters
• Stellar evolution and population synthesis– evolutionary tracks, metallicity vs. age– star formation, single starburst model
• Origin of elements and yields– Supernovae and hypernovae
1) Light odd-Z elements (Na and Al): Mainly made in the hydrostatic burning shells of massive stars. Their yields arerelated to the mass of the shell, which is related to the initial mass of the star
2) Magnesium: Made in the hydrostatic burning shells of massive stars (specificallythe C-burning shell), and the yield is related to the initial mass of the star.
3) The other alpha elements (O, Si, Ca, and Ti):O is formed in a hydrostatic burning shell (the He-burning shell). The heavieralpha-elements Si, Ca and Ti are formed deep within massive stars duringthe explosive burning phase of a supernova (SN).
4) Fe-peak elements (Sc, V, Cr, Mn, Fe, Co, Ni, Cu and Zn): With the exception of Cu and maybe Zn, these elements are made in bothType Ia and Type II SNe during the explosive phases. Co and possibly Zn aremade almost exclusively in Type II SNe. Hypernovae is required for Zn.
5) Light s-process elements (Sr, Y, and Zr):(Nearly all the elements heavier than Zn are made by neutron-capture processes.)Made in metal-rich AGB stars. The peak of the s-process production moves tolighter elements as metallicity increases because there are more Fe-group“seed” nuclei at higher metallicity,
6) Heavy s-process elements (Ba and La):Made in metal-poor AGB stars, although some of the inventory of both elementsin the Sun came form the r-process.
7) r-process element (Eu): Provided by the explosive phase of Type II SNe or merging of neutron stars.
Families of elements
•Lithium (Z=3): Produced in Big Bang nucleosynthesis and cosmic ray spallation.•Carbon (Z=6): Results from the triple-alpha He-burning process. Isotope ratiosbetween 12C and 13C are affected by hydrogen burning on the CNO cycle.
•Oxygen (Z=8): Results from hydrostatic He-burning burning in massive stars, yield related to the mass of the He-burning shell, which is a function of the star’s initial mass.
•Sodium (Z=11): Results mostly from carbon-burning. Production depends on the n/p ratio, so there is a predicted metallicity dependence of the yield from SN II.Can also be affected by H-burning in intermediate-mass stars, as seen in the so-called “Na-O anti-correlation” often seen in globular cluster stars.
•Magnesium (Z=12): Results from carbon-burning. Effectively 12C → 24Mgvia 20Ne + 4He. Released from SN II.
•Aluminum (Z=13): Carbon-burning; closely tied to the production of the minor Mgisotopes 25,26Mg. Production depends on the n/p ratio, so there is a predictedmetallicity dependence of the yield from SN II. Can also be affected by H-burning inintermediate-mass stars, as seen in “Na-O anti-correlation” in globular cluster stars.
•Silicon (Z=14): Explosive oxygen burning via 2O→Si + He, with Mg + He→Si. SN II+SN Ia.
•Calcium (Z=20): Oxygen and silicon burning, both hydrostatic and explosive. SN II.•Scandium (Z=21): SN II from oxygen burning + the alpha-rich freezeout.•Titanium (Z=22): Explosive Si burning, + alpha-rich freezeout, including white dwarfs(SN Ia). Appears to be mostly SN II.
List of elements and their production sites
•Vanadium (Z=23): Explosive oxygen burning + silicon burning. SN Ia probablydominate production. The [V/Fe] value is very sensitive to the value of Teff.
•Chromium (Z=24): Equilibrium process in explosive Si burning. SN II + SN Ia, butdominated by SN II.
•Manganese (Z=25): Explosive Si burning + alpha-rich freezeout. SN II. Metallicity dep.•Iron (Z=26): Equilibrium process. SN II + SN Ia, with a large yield from SN Ia.•Cobalt (Z=27): Explosive Si burning + alpha-rich freezeout (which producesa large Co/Fe yield). Possibly metallicity-dependent yields in Type II SN.
•Nickel (Z=28):. Explosive Si burning + alpha-rich freezeout. SN II + SN Ia•Copper (Z=29): Possibly from SN II “only” with metallicity-dependent yields. Minorcontributions from the s-process and SN Ia.
•Zinc (Z=30): Explosive Si burning + alpha-rich freezeout + s-process. Zn does notform on dust grains, so it is used in the study of damped Lyman-alpha systemsas metallicity indicator.
•Barium (Z=56): Heavy s-process. AGB stars. [heavy s/light s]= f(Z). •Lanthanum (Z=57): Heavy s-process. AGB stars. [heavy s/light s]= f(Z).•Europium (Z=63): Bypassed by s-process (mostly), best r-process “only” element inthe optical. The r-processes is believed to occur in a sub-class of SN II, most likelythe lower-mass SN II.
Where elements came from
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2.4 Extremely metal-poor stars
[Fe/H] ≤ -2.5
These stars were enrichedby just one supernova.
Their abundance patternsreflect the mass ofa progenitor star (first star).
α-elements
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Atomic Number
r-process elementsfor a star with [Fe/H]=-3.1
s-process elementsfor a star with [Fe/H]=-2.7
Universal mechanism (by SNe IIor merging of neutron stars)is at work for r-process.
Carbon-enhanced extremely metal-poor star(CEMP)
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Nucleosynthesis from Hypernovae(Tominaga et al. 2007)
Z
[X/Fe]
Mixing-fallback model
M=25Msun, E=1052 erg
Observed large [Zn/Fe] & [Co/Fe] ratios are reproduced.