Nucleosynthesis in Early Massive Stars: Origin of Heavy Elements Projjwal Banerjee Shanghai Jiao Tong University With Yong-Zhong Qian (U. Minnesota, TDLI), Alexander Heger (Monash U., TDLI)

Low Mass VMP Stars => Window into the Early (high redshift) Galaxy Big Bang M ≳ 10 M⊙ M ≲ 0.8M⊙ Surface composition = snapshot of early gas composition Observed today Should have very little metals like Fe compared to the Sun 200 Myr 1 Gyr 13.7 Gyr Transition to formation of low mass stars Target: Low mass stars very metal poor stars ([Fe/H]<-2)

Low Mass VMP Stars => Window into the Early (high redshift) Galaxy Only the first and second generation massive stars contribute. Direct information about nature of first and very early stars. Very few nucleosynthetic events contribute. Can learn about individual events. Valuable information about early Galaxy formation, star formation, and chemical evolution.

Origin of Heavy Elements Problem: Coulomb repulsion too high for fusion beyond Fe Solution: Neutrons

Neutron Capture Processes Rapid neutron capture process Lot of neutrons for a short amount of time Slow neutron capture process Very few neutrons but for a very long amount of time

Neutron Capture Processes s-process r-process Capture one neutron beta decay Wait for next neutron Repeat Capture several neutrons beta decay Repeat i-process?

r-process s-process

Neutron Capture Processes s-process Main site: Late stages of stars of 1-3 M☉ during AGB phase. Require > 1 Gyr from birth. Should not contribute in the early Galaxy Nuclear physics inputs are well constrained by experiments.

Neutron Capture Processes r-process NS-NS and NS-BH mergers (GW170817!) CCSN: neutrino-wind Z≲50, Jets/MR CSSN? Associated with massive stars Problem: Considerable theoretical uncertainties Early Galaxy: Only r-process should operate in the early Galaxy All heavy element pattern should look like Solar-r

Neutron Capture Processes r-process NS-NS and NS-BH mergers (GW170817!) CCSN: neutrino-wind Z≲50, Jets/MR CSSN? Associated with massive stars Problem: Considerable theoretical uncertainties Solar heavy elements: s-process + r-process Accurately measured Can calculate accurately The rest

Solar s and r pattern [Ba/Eu]r = -0.8 [Ba/Eu]s = 1.6 Sneden et al 2008 Very different abundance patterns

Robust r-process pattern Sneden et al. 2003

Robust r-process pattern Sneden et al. 2008

Do all VMP stars have r-process pattern? Sneden et al. 2008

Heavy Elements in VMP Stars [Fe/H]≲-2 s star r star

Heavy Elements in VMP Stars [Fe/H]≲-2 r/s star r star

Puzzling Properties of s and r/s VMP Stars Almost all stars with s or r/s pattern are also enriched in C. New VMP Star Classification Carbon Enhanced Metal-Poor Stars ([C/Fe]>0.7) CEMP-s, CEMP-r/s Higher C, high enrichment of heavy elements CEMP-no Lower C, very low enrichment of heavy elements

CEMP-s Stars 1 M⊙ ≲ M ≲ 3 M⊙ LMS AGB WD LMS M ≲ 0.8M⊙ Transfer of C and s-process enriched material Binary configuration with WD remnant today But 10-30% of CEMP-s stars appear to be single (Hansen et al 2016) Requires s-process associated with massive stars

CEMP-r/s Stars Above simple picture does not work. 1 M⊙ ≲ M ≲ 3 M⊙ LMS AGB WD LMS M ≲ 0.8M⊙ Transfer of C and s-process enriched material Binary configuration with WD remnant today Initially enriched in r-process Above simple picture does not work. Required initial r-enrichment too high Solution: intermediate neutron capture process But where?

CEMP-no Stars Popular model: Weaker Core-collapse SN leading to preferential ejection of C relative to Fe (Nomoto & Umeda 2005, Heger & Woosley 2010, Tominaga 2014). They are likely the direct descendants of the first stars. CCSN from first stars can explain abundances of light elements reasonably well. Heger & Woosley 2010

CEMP-no Stars Popular model: Weaker Core-collapse SN leading to preferential ejection of C relative to Fe (Nomoto & Umeda 2005, Heger & Woosley 2010, Tominaga 2014). Can explain abundances of light elements reasonably well. Origin of heavy elements like Ba (Z=56) observed in many of the CEMP-no stars unknown.

Ubiquity of Heavy Element in VMP Stars Roederer 2013 Both Ba and Sr are almost always observed r-process in NSM/Jets is not frequent enough to explain this.

Unresolved Problems Origin of single CEMP-s stars? Origin of CEMP-r/s stars? Origin of heavy elements in CEMP-no stars? Ubiquity of Ba and heavier elements? Additional sites for neutron capture in early massive stars?

New Site for Neutron Capture in Massive Stars About 1 yr before collapse in a 25 M⊙ star H He C,O O,Ne Growth of convective He shell. Mixing can occur at the convective boundary. 10-3-10-5 M⊙ of proton ingestion. Occurs for 20 M⊙≲ M ≲ 30 M⊙.

Free Neutrons from Protons 13C depletion 17O(α,n)20Ne boost due to T>3x108 K 13C(α,n)16O Vs 16O(n,γ)17O 17O(α,n)20Ne Vs 16O(n,γ)17O p transport + 12C(p,γ)13N(e+ νe)13C t = 0, p ingestion i-process s-process Mp=10-4 M⊙

Heavy Element Synthesis Initial Fe in VMP star is converted into heavy elements

Heavy Element Synthesis ?? Initial Fe in VMP star is converted into heavy elements

Heavy Element Synthesis In first stars, minute amount of Ca from H burning ash is used! Can easily account of low amounts of heavy elements in CEMP-no stars

Comparison with Observations Mdil=700 M⊙ Mdil=100 M⊙ HE0338-3945 CS30301-015 Aoki et al 2002 Jonsell et al 2006 Single CEMP-s star CEMP-r/s star P ingestion ≳ 106 s before collapse P ingestion ≲106 s before collapse Low Dilution of ≲ 1000 M⊙ Higher Dilution: CEMP-no or regular VMP stars

Enrichment of ISM H Low Energy Explosion He Low Dilution CEMP-s, CEMP-r/s C,O O,Ne Si,S Medium Energy Explosion Medium Dilution CEMP-no Low-s, r/s enriched stars Fe High Energy Explosion High Dilution Other VMP stars Source of Sr and Ba ubiquity

Summary & Outlook Origin of heavy elements in the early Galaxy is still a puzzle. R, s, and i-process required. Need to be common in the early Galaxy to explain the ubiquity of Sr and Ba. New site in massive stars is a good candidate. Can constrain the masses of the early massive stars. More VMP stars with detailed abundance pattern required.

Muller et al 2016