Amanda KarakasSchool of Physics & Astronomy, Monash University, Australia

Collaborators

I would like to acknowledge my three main collaborators in this work: Maria Lugaro, Chiaki Kobayashi and John Lattanzio

How to disentangle the vast quantity of information from current and future stellar abundance surveys? (e.g., HERMES, LAMOST, GAIA-ESO…)

Introduction

Chemical or chemo-dynamical models require accurate stellar yields from stars of all mass ranges

One substantial uncertainty is the lack of yields, particularly for

heavy elements (e.g., talks by Hampel, Cescutti)

The cycle of matter

How are elements produced by stars and recycled through galaxies?

And on what timescale did this happen?

Which stars? • Stars that have ages less than the age of the Universe• This means single stars more massive than ≈ 1Msun • Or binaries that will merge/interact in a Hubble time

Credit: HST public archive

Galactic Chemical Evolution

Q [Fe/H] and [X/Fe] evolve in a galaxy: fossils that retain the evolution history of the galaxy → Galactic Archaeology

Slide from Chiaki Kobayashi

SNII = core collapse supernovaHN = hypernova

SNIa = Type Ia supernova

How long do stars live?

Initial mass (Msun) Total lifetime1

(Z ≈ Zsolar)Total lifetime2

(Z ≈ 0.1 Zsolar)

25 7.5 Myr 7.8 Myr

15 13 Myr 15 Myr

5 116 Myr 90 Myr

2 1.5 Gyr 813 Myr

1 12 Gyr 6.8 Gyr

0.8 28 Gyr > 15 Gyr

Age of the galaxy ≈ 12 x 109 years; Universe ≈ 13.7 x 109 years

1) Ages from Karakas & Lugaro (2016); Woosley et al. (2002)2) Ages from Fishlock et al. (2014); Schaller et al. (2002)

The origin of the elements

The aim of nucleosynthesis is to explain this graph!From Pagel (1997): Abundances normalised to 106 Si atoms

The origin of the elements: massive stars

Core collapse supernova produce the majority of the elements from C to Fe (Nomoto, Kobayashi & Tominaga 2013)

Timescale? Quick! ≲ 30 Myr

The origin of the elements: massive stars

Additionally, high energy supernova (hypernova) produce a few key iron-peak elements including Co and Zn at low metallicities(Nomoto, Kobayashi & Tominaga 2013)

Timescale? Quick! ≲ 10 Myr

The origin of the elements: Type Ia SNe

The majority of iron-peak elements including Mn are synthesized in Type Ia supernova (Nomoto, Kobayashi & Tominaga 2013)

Timescale? Generally slow, > 1 Gyr

The origin of the elements: AGB stars

AGB stars produce copious amounts of a few key light elements Li, C, N, F as well as neutron-rich isotopes of O, Ne, Mg, Si (Karakas & Lattanzio 2014)

Timescale? Generally slow, > 1 Gyr

SN+HN (CK+ 06); SN+HN+AGB (Kobayashi, Karakas, Umeda 2011)

Chemical evolution of elements

time[A/B] = log10(A/B)star – log10(A/B)sun

Production of heavy elements

• By heavy elements we mean heavier than iron (Fe)

• Lots of protons à the electrostatic repulsion inhibits fusion reactions

• Most heavy nuclei are formed by neutron addition onto Fe-peak elements

• Two processes:– r-process (rapid neutron

capture)– s-process (slow neutron

capture)

AGB Stars

Asymptotic Giant Branch stars:

• After core He-burning, the C-O core contracts and star becomes a giant again

• Double-shell configuration• He-burning shell is thermally

unstable and flashes every ~105

years• Rapid, episodic mass loss

erodes the envelopeà Major source of the s-process

See review by Karakas & Lattanzio (2014)

AGB surface abundancesFRUITY database: From Cristallo et al. (2015)

Rb (Z= 37)

Rb

Ba (Z= 56)

Ba

Pb (Z= 82)

Pb

Z = proton number

AGB chemical yieldsExample: [Fe/H] = 0 (solar) from Karakas & Lugaro (2016)

From Karakas (2010)Yields and surface abundances for hydrogen to sulphur

Yield = amount of an isotope ejected into the ISM over the star’s lifetime

Black dots = weighted by an IMF

12C 14N

17O

19F

Yields of s-process elements

Light s-process (Y, Sr, Zr, Rb) versus heavy s-process elements (Ba, La, Nd)• Light s-process – particularly Rb – is typically produced in massive (4-

8Msun) AGB stars with short lifetimes (< 100Myr) • Heavy s-process produced in lower mass AGB stars with longer lifetimesà Elements trace different star formation histories and processes in galaxies

Black points: Fishlock+2014Red stars: FRUITY/Cristallo

The origin of s-process elements

• AGB stars produce the majority of the elements between Sr and Pb at the solar composition (Karakas & Lattanzio 2014; Sneden et al. 2008)

• With rotating massive stars adding to the inventory of Sr, Y, Zr, especially at low metallicities (e.g., Frischknect et al. 2016; Pignatari et al. 2010)

Timescale? Generally slow, > 1 Gyr for AGBTimescale? ≲ 30 Myr for massive stars

The rapid neutron capture process

• Ji et al. (2016) showed that the ultra-faint dwarf galaxy Reticulum II experienced a single, unusual r-process event

• Merged neutron stars or magneto-rotationally induced supernova?

A rare source for the r-process?

• Wallner et al. (2016) find that the rarity of the heavy 244Pu (half-life = 81 Myr) in the deep sea floor also points towards a rare source for the heaviest radioactive elements in the Galaxy

à Production lower than expected if core collapse supernovae made 244Pu

Merging neutron stars

• There are questions about the rate of neutron-star neutron-star mergers • Can enough occur early enough in the Galaxy to make the r-process? • Advanced LIGO is already helping to constrain these numbers (The

LIGO Scientific Collaboration; Abbot et al. 2016, arXiv: 1607.07456)

Figure from Hotokezaka et al. (2013, Phys. Rev. D): simulations of two neutron stars

How many nucleosynthesis processes exist in the early Galaxy?

• Hansen, Montes & Arcones (2014): How many neutron-capture processes existed in the early Galaxy?

• L component: elements from Sr (Z = 38) to about Ag (Z = 47)

• H component: elements from Ba (Z = 56) higher (r-process)

• After Qian & Wasserburg (2007)Sources?• Neutrino-driven winds of core collapse

supernovae make the light component à unlikely to make Ba and heavier

• Merging neutron stars or massive star precursor for long Gamma Ray Bursts (Nakamura et al. 2013) for heavy component

• Multiple sites for the r-process?

Evolution of elements in the Universe

From Chiaki Kobayashi (preliminary results)

-1

+1

-1

+1 -6 -4 -2 0 -6 -4 -2 0[Fe/H]

For some elements, the s-process alone can produce the solar compositionThis is not true for low metallicities (i.e., the early Universe)

Beyond the standard model of nucleosynthesis

• Proton ingestion episodes into a carbon and helium-rich region will produce neutrons

• This produces 13C pockets when the rate of proton ingestion is slow à What if it’s fast? e.g., into a convective region

àBurst of neutron production above what we find in s-process models

àThe intermediate or “i-process” (Cowan & Rose 1977)

The i-process

• Do proton ingestion episodes and the i-process cause the low Pb abundance in post-AGB stars?

(Herwig et al. 2011; De Smedt et al. 2012, 2014; Lugaro et al. 2015)

• What about the origin of the CEMP s/r stars?

(Dardelet et al. 2015; Jones et al. 2016)

à Ubiquitous in metal-poor stars throughout the Galaxy?

à Roederer, Karakas et al. (2016)

HD 94028 from Roederer et al. (2016)

What are the site(s) of the i-process?

• There are quantitative problems fitting s-process predictions to observations in low-mass, low-metallicity post-AGB stars (e.g., de Smedt et al. 2012, 2014)

• It has been suggested that these stars experience an i-process instead (Lugaro et al. 2015)

• Perhaps super-AGB stars > 6-8Msun, also of low metallicity (Jones et al. 2016)

• New predictions suggest i-process a better fit to CEMP s/r stars (Hampel et al. 2016) than an s-process (Abate et al. 2015a,b, 2016; Lugaro et al. 2012)How will the i-process – if it’s really a thing – affect the

(early) chemical evolution of the Galaxy?

• AGB stars and supernovae are important contributors to the chemical enrichment of galaxies

• We are now finally in a position to start making quantitative chemical evolution predictions including heavy elements

• Which is timely, given the release of stellar abundance data from surveys for 100,000+ stars (e.g., GAIA-ESO survey; Galah in Australia, De Silva et al. 2015; K2 mission, e.g. Huber et al. 2016)

• The question of the origin of the r-process is however a persistent problem

• First Galah and GAIA data releases this year! Yay!!! This is an exciting time!

Summary

Thanks for listening!

Credit: S Harris’s astronomy cartoonshttp://www.sciencecartoonsplus.com/gallery/astronomy/

Super-AGB stars

• M > 8Msun (for Z = Zsolar) up to 10Msun

à Core carbon burning before ascending the AGB

à Super-AGB stars à ONe cores

• Maybe they produce a new class of Type Ia supernovae? Type Iax(Denissenkov et al. 2015; Kobayashi et al. 2015)

• Electron capture supernova? If so do they make r-process elements? (Wanajo et al. 2015)

From Doherty et al. (2015)See also Siess (2010), Doherty et al. (2014a,b), Jones et al. (2013)

Z= m

etal

licity