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Stellar & Supernovae And Cosmic Chemical Evolution

Jim Truran

Astronomy and Astrophysics Enrico Fermi Institute University of Chicago and Argonne National Laboratory

“ The First and Evolution of the ” Institute for Nuclear Theory, Seattle July 5th, 2006 Constraining the First Stars?

 Critical questions concerning the first stars include:  What was the of the first stellar population?  What were its nucleosynthesis products?  Did the first stars differ from “normal stars” (stellar populations), and if so how so?  How are they related to the oldest stars (halo stars, globular clusters, dwarf spheroidals, …) observed in our and other today?

 How might we address these issues?  We note: (1) the first stars were born with a primordial (e.g. BBN) composition; (2) the synthesis of the heavy elements occurred in stars and supernovae; and (3) the compositions of stars as a function of time (increasing ) reflect the integrated nucleosynthesis enrichment history of the out of which they were formed.  Since distinctive abundance patterns can be identified with the nucleosynthesis products of stars of specific ranges of (and lifetime), we can use this to trace the chemical evolution of the oldest stars and stellar populations. Nucleosynthesis Sites and Production Timescales

 We need first to identify the most significant contributers to nucleosynthesis in galaxies. We consider three main nucleosynthesis sites and note the (production) timescales for the entry of the ejecta of each of these sites into the ISM of galaxies .

11 10 H,He  Massive stars (M > 10 M ) and SNe II: G  () 1010 abundances synthesis of most of the nuclear species 9 Carbon (AGB stars) (at the time of solar system formation) 10 8 α-elements from through zinc, and of the r- 10 (mostly Type II SN)

8 6 7 10 Fe peak process heavy elements (τ < 10 years) (mostly Type I SN) 106 105 N=82 s-process peak 4 Ba, La, Ce 10 (AGB stars) N=126  Stars (1 < M < 10 M): s-process peak 3 10 N=82 Pb,Bi synthesis of the heavy s-process elements r-process peak (AGB stars) 102 Te, Xe N-126 9 (Type II SN) r-process peak (τ > 10 years) 1 Os,Ir,Pt 10 (Type II SN) 0 U,Th 10 (TypeThU II SN)

Abundance relative to 10 10-1  SNe Ia: synthesis of the 1/2-2/3 of the 10-2 peak nuclei not produced by SNe II 10-3 0 50 100 150 200 (τ > 1.5-2 x109 years) Mass number Type Ia SNe Type II SNe

 “Standard model” (Hoyle & Fowler 1960):  “Standard model” (Hoyle & Fowler 1960):  SNe Ia are thermonuclear explosions of  SNe II are the product of the evolution C+O stars. of massive stars 10 < M < 100 M.  Evolution to criticality?  Evolution to criticality:  from a binary companion  A succession of nuclear burning stages (Whelan and Iben 1973) leads to growth of yield a layered compositional structure and a core dominated by 56Fe. the WD to the critical mass ( 1.4 M). 56  After ~1000 years of thermonuclear  Collapse of the Fe core yields a “cooking”, a violent explosion is triggered neutron or . at or near the center. Complete  The gravitational energy is released in incineration then ensues within two the form of neutrinos, which interact with seconds. the overlying matter and drive explosion.  Nucleosynthesis products include primarily  Nucleosynthesis contributions: elements from iron-peak nuclei; their contributions to oxygen to iron [(O-Ca)/Fe ~ 3 times Solar] and intermediate mass nuclei are at less significant neutron capture products from krypton through levels. (τ > 1.5 x 109 yrs) 8 nucleosynthesis uranium and thorium. (τnucleosynthesis < 10 yrs)  Light curve powered by radioactive decay of 56 56 56  Production of ≈ 0.1 M of Fe as Ni. Ni. (Nickel mass ≈ 0.6 M.) Peak ∝ M(56Ni). Nucleosynthesis: SNe II and SNe Ia

M = 15 M

Type Ia Nucleosynthesis

M = 25 M Type II Nucleosynthesis Synthesis of Nuclei Beyond Iron Nuclei heavier than iron (A > 60) are understood to be formed in neutron capture processes:  The shells of red giant stars (≈ 1-10 ) provide the s-process environment, with the 13C(α,n)16O reaction providing neutrons. (τ > 109 years)  Supernovae II provide the astronomical setting for the r-process. (τ < 108 years) Clues from Metal-Deficient Stars

Heavy Elements in CS 22892-052

Platinum

Germanium

Sneden et al. (2002) Halo Abundance Trends for -3 ≤ [Fe/H] ≤ -1 Oxygen and α-Elements R-Process Elements Calcium

Titanium

(Truran et al. 2002)

These behaviors are compatible with nucleosynthesis predictions for SNe II. s-Process/r-Process Chemical Evolution

Truran et al. (2002) Abundance Trends in Globular Clusters

[(α-elements)/Fe] (neutron-capture elements) Summary: Abundance Evolution Trends [Fe/H] > -3

 Extremely metal-deficient stars of [Fe/H] ~ -2 to –3 are characterized by both high O/Fe and (Ne-Ca)/Fe ratios and an r- process heavy element pattern  ⇒ SNe II production (τ ≤ 108 years)

 Signatures of an increasing s-process contamination first appear at [Fe/H] ≈ -2.5 to –2.0  ⇒ first input from AGB stars (τ ≈ 109 years)

 Evidence for entry of SNe Ia ejecta first appears at [Fe/H] ≈ -1.5 to –1.0, as evidenced in the [O/Fe] and [(Ne-Ca)/Fe] histories  ⇒ input from SNe Ia on timescales > 1.5-2 x 109 years Trends at the Lowest

[Cr/Fe] r-Process Scatter

[Mn/Fe]

[Zn/Fe]

Truran et al. (2002)

Cayrel et al. (2005) Summary: Abundance Evolution Trends -4 <[Fe/H]< -2.5

 Evidence for increasing scatter exists in the (r-process/Fe) ratio below metallicity [Fe/H] ~ -2.5, suggesting both that only a small fraction of massive stars form r-process nuclei - and that

 ⇒ the ISM was highly inhomogeneous at that epoch.

 In contrast, the scatter in abundance ratios of nuclei from Mg to Zn with respect to iron is remarkably small. Given the level of inhomogeneity reflected in the r-process/Fe ratio, this quite strongly implies

 ⇒ the massive stars responsible for these early products were extremely robust in their synthesis of nuclei through iron. (Keep in mind that the heavy elements introduced into stars formed at metallicities [Fe/H] ~ -4 are most likely to have come from a single progenitor.) Very Massive Primordial Stars

 Very massive stars (~ 100-300 M ) can be stable at low metallicities.  “Pair instability” supernovae eject nuclei from oxygen to iron.  Nucleosynthesis signatures include a pronounced odd-even variation.  These signatures are not observed in the most metal deficient stars.

Heger & Woosley (2000) Abundance Trends for [Fe/H] < -4 ?? Frebel et al. (2005)

 The abundances in the two most “iron-deficient” stars known do not trace the smooth trends found (Cayrel et al. 2004) above [Fe/H] = -4. The details of their evolutions remain uncertain. Understanding the Iron Peak Trends at Low Z

 The trends in iron peak nuclei identifiable in the Cayrel (2005) data include:  decreasing [Cr/Fe] with [Fe/H]  decreasing [Mn/Fe] with [Fe/H]  increasing [Co/Fe] with [Fe/H]  increasing [Zn/Fe] with [Fe/H]  constancy of [Ni/Fe] with [Fe/H]  Ge trends with [Fe/H] (figure)

 These suggest a high entropy (α- rich) freeze which tends to produce nuclei past iron at the expence of nuclei below iron. Note that if the trends in Ge arise in this manner it may likely implies buildup along the alpha chain

(at Ye ≈ 0.5 ) through the self-conjugate nucleus 72Se (yielding 72Ge). Note such conditions are achieved for hypernovae (Nomoto 2006). Explosive Nucleosynthesis of Fe-Peak Nuclei

68Se 72Kr

67As

64Ge 65Ge 66Ge 70Ge 72Ge

63Ga 69Ga 71Ga

60Zn 61Zn 62Zn 64Zn 66Zn 67Zn 68Zn 70Zn

59Cu 63Cu 65Cu ↑ 56Ni 57Ni 58Ni 60Ni 61Ni 62Ni 64Ni Z 55Co 59Co 52Fe 53Fe 54Fe 56Fe 57Fe 58Fe

51Mn 55Mn

48Cr 49Cr 50Cr 52Cr 53Cr 54Cr

47V 50V 51V

44Ti 45Ti 46Ti 47Ti 48Ti 49Ti 50Ti

43Sc 45Sc 40Ca 42Ca 43Ca 44Ca 46Ca 48Ca N → DLAs: Abundance Evolution with Red Shift

(Pettini 2003) (Lu et al. 1996)

 The paucity of DLAs with metallicities below [Fe/H] ≈ -3 is compatible with their having been enriched by only a very few stars - but in star 6 forming regions/clouds typically ~ 10 M.

 ⇒ (Note that the introduction of 10 M of metals from a single 6 ~ 20-30 M star is sufficient to enrich a ~ 10 M cloud to a metallicity -3 ≈ 10 Z.) Abundances in Dwarf Spheroidals and Globular Clusters

 Recent abundance determinations for dwarf spheroidal galaxies reveal distinctive trends relative to halo stars (e.g. Shetrone et al. 2003; Geisler et al. 2005).

 The presence of significant concentrations of s-process elements challenges the view (e.g. Gnedin and Kravtsov 2006) that most dSphs are “fossils” of recombination (see also Fenner et al. 2006).

 Outstanding issues include:

 How can we understand the distinctly non-halo-like behaviors of the [(α-element)/Fe] and [s-process/r-process] abundances of dSphs with current models for the hierarchical assembly history of our Galaxy.

 Why to the patterns for [(α-element)/Fe] and [s-process/r-process] as a function of metallicity generally parallel those of the field halo population while the dSphs seem not to do so? Abundances in Dwarf Spheroidal Galaxies [Ba/Eu]

[Alpha Elements/Fe]

Tolstoy et al. (2005) Geisler (2005) Summary

 Based upon existing observations of abundances in our Galaxy and QSO absorption line systems, we are led to conclude: Only “normal” stars in a Salpeter-like initial mass function seem required to produce the elements seen in the oldest stars.  While contributions from massive stars clearly dominate at early epochs, this may be simply a consequence of their shorter production time scales (lifetimes) rather than of an altered IMF.

 Very massive stars (100 < M < 300 M) do not appear to have made a significant contribution to early Galactic (or pre-galactic) nucleosynthesis.  Our present knowledge of the abundance history of our Galaxy provides no compelling evidence for a distinct earlier stellar Population (III?).  The absence of significant star-to-star scatter in the most metal deficient stars studied to date - given the fact that these stars likely boast in the mean only ~ one progenitor (e.g. Tumlinson 2005) - implies a robust nucleosynthesis mechanism and/or a narrow mass range of (massive) star zero-metallicity progenitors.  A significant degree of inhomogeneity - as reflected in (r-process/Fe) ratios - persists through metallicities [Fe/H] ~ -2.0.