1. 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)

2. 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)

3. 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.

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

5. 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

6. 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?

7. r-process
s-process

8. 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.

9. 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

10. 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

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

12. Robust r-process pattern
Sneden et al. 2003

13. Robust r-process pattern
Sneden et al. 2008

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

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

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

17. 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

18. 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

19. 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?

20. 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

21. 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.

22. 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.

23. 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?

24. 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.
M⊙ of proton ingestion.
Occurs for 20 M⊙≲ M ≲ 30 M⊙.

25. 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⊙

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

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

28. 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

29. Comparison with Observations
Mdil=700 M⊙
Mdil=100 M⊙ HE CS
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

30. 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

31. 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.

32. Muller et al 2016