Isotopic constraints on nucleosynthesis, Solar System composition & accretion Nikitha Susan Saji Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen

Outline of the talk Stellar nucleosynthesis Nucleosynthetic heterogeneity Cases of Cr, Ti, Ba, Nd, Sm etc.. Accretionary relationships Chondrites and Earth..

Origin of elements - Nucleosynthesis  Primordial Big Bang nucleosynthesis created H, He & Li  All heavier nuclei (except Be and B) up to Uranium created in stars by nuclear fusion or supernova explosion

Formation of light elements  H-burning to form He  He-burning to from 12 C  Alpha-process to form 16 O, 20 Ne from 12 C  C, O and Ne burning to form Mg, Na, Al, Si, S  28 Si burning and photodisintegration grow elements upto Ni; 56 Ni decays to 56 Fe at which stage fusion ceases due to nuclear statistical equilibrium The onion-like layers of a massive, evolved star just prior to core collapse (Not to scale)

Creating the remainder of the periodic table: Elements beyond Fe  Neutron-capture No coulomb barrier but a free supply of neutrons required. However, a nucleus of fixed Z cannot absorb arbitrarily many neutrons without becoming unstable and initiating β-decay When neutron-flux is low, each capture which results in an unstable nuclei is followed by a β- decay before the next capture  s-process (“slow”) When neutron-flux is high, multiple neutron-captures before β- decay  r- process (“rapid”)  Proton-capture Requires the coulomb-barrier to be overcome creating proton-rich nuclei  p-process

Process and setting s-process – mainly operates in the red-giant (AGB) phase; creates isotopes that lie in the “valley of stability” (Z≈N) r-process – occurs during supernova-explosion of more massive stars; creates the heavier nuclei of any element p-process - possibly also in supernovae; creates the lightest nuclei of any element Now to our own Solar system… Sun, being a low-mass star, likely formed in a cluster within a Giant Molecular Cloud. It is possible to infer this astrophysical setting of Solar System formation from the relative abundances of r-, s- and p-isotopes in primitive Solar System objects.

Where to look for clues?

Composition the Solar protoplanetary disk Solar system elemental abundances that match galactic nucleosynthetic models suggest that bulk of Solar system is a molecular cloud-inherited old galactic component However, presence of short-lived radionuclides ( 26 Al, 41 Ca, 60 Fe, 53 Mn, 182 Hf etc) inferred from their daughter isotope excesses in primitive meteorites requires the solar nebula to be polluted by freshly synthesized stellar matter at the end stages of its formation Pre-solar grains with both old AGB-type signatures and young SN-type signature present in primitive meteorites Therefore, SS = old dust + new dust in the most simplistic model  The question then is to what extent these different components were mixed or not mixed..  The present-day bulk planetary signatures (Earth, Mars) need to be understood in the context of these components..

Isotopic heterogeneity in the Solar system Widespread heterogeneity amongst solar system planets and meteorites in the stable isotopes of iron-group elements – 50 Ti, 54 Cr, 64 Ni Initially interpreted to be disk heterogeneity from incomplete mixing but correlation between isotopes from distinct nucleosynthetic pathways rules out this possibility Initially well-mixed but later unmixing due to thermal processing of dust Similar argument holds good for Ca and Mo stable isotope signatures Some other elements on the other hand show no evidence of any heterogeneity (Os, Hf ) Tranquier et al (2009)

Emerging picture of accretion in the Solar nebula.. Inner Solar System objects (Rocky planets + Enstatite & Ordinary chondrites) accreted from material that is distinctly different in isotopic signature from that of Outer Solar System objects (Gas giants + Carbonaceous chondrites + Comets)

The curious case of Nd An excess of 142 Nd (s-dominant with small r-contribution) on modern terrestrial mantle relative to ordinary and carbonaceous chondrites that has been attributed to 146 Sm (a p-process isotope) decay from an early silicate differentiation or a non-chondritic Sm/Nd on Earth But, chondrites have significant variability in their 142 Nd composition Former scenarios does not hold good if there is a component of nucleosynthetic heterogeneity present in the parent material that formed Earth

Ba isotope signatures suggest an s- deficiency and r- excess in C- chondrites relative to terrestrial value All C-chondrites have a low 144 Sm/ 152 Sm value suggesting p- deficiency Drawing conclusions on Nd from Ba and Sm could be inaccurate: need to better constrain the stable isotopic composition of Nd in chondrites to say more about the 142 Nd signature of Earth with respect to chondrites

Summary Evidences for the astrophysical setting of Solar system formation can be discerned from the isotopic signatures in primitive meteorites In the simplest model, Solar system composition is a mixture of an old molecular cloud-inherited galactic component and younger supernovae-derived component Initial heterogeneity erased by mixing in the nebula; what is observed now is likely from a secondary processing Inner solar system bodies accreted from material distinctly different in isotopic composition from outer solar system bodies The stable isotope composition of chondrites with respect to Nd need to be understood better to ascertain the chondritic origin of Earth and its early global silicate differentiation

Evolution of a high-mass star

Evolution of a low-mass star

Elemental Abundances in the Solar System

Schiller et al (2015)