A rare and prolific r-process event in the ultra-faint dwarf galaxy Reticulum II Alex Ji MIT

Anna Frebel, MIT Ani Chiti, MIT Josh Simon, Carnegie

Nature, 531, 610 ApJ 830, 93 arxiv:1512.01558 arxiv:1607.07447 [email protected] Where does the r-process happen?

r-process

Alex Ji Two Main Candidates Core-collapse Supernovae Neutron Star Mergers

Continually synthesize Rare, prolific r-process; small amounts of r-process delay time for merger • Small r-process yield • Large r-process yield -7.5 (<10 Msun Eu per SN) (~10-4.5 Msun Eu per NSM) • Produced after any episode of • 1 NSM per ~1000-2000 SN star formation • Delay time for orbit decay - Most > 100 Myr

Alex Ji Images: Woosley and Janka 2005, Price and Rosswog 2006 Clues to the r-process site

• Stellar Archaeology: Using abundances of metal-poor stars to probe the chemical content of the early universe Low-mass Metals stars found in our Galaxy today!

Early 12-14 billion years Metal Star forming Source gas cloud

Observe chemical abundances of [Fe/H] ≲ -3 ➞ ~1 enriching supernova these -> chemical content of old gas clouds! Alex Ji Lessons from Stellar Archaeology

• universal halo r-process star r-process pattern [Fe/H] = -3 same in the Sun and metal-poor halo stars Good for Neutron Star Mergers • r-process happens = log N(X)/N(H) +12 early r-process stars are extremely metal poor ([Fe/H] ~ -3) Good for Core-Collapse Supernovae • r-process is stochastic large scatter in [ncap/Fe] Good for both

Alex Ji Sneden et al. 2008 The Stellar Halo is Complicated Dark Matter Halo

Pillepich et al. 2015

Stellar Halo 50 kpc (Where metal-poor halo stars are found) rings

Alex Ji Dwarf Galaxies are Simpler Classical dSphs Ultra-faint dwarfs SDSS DES

50 kpc rings

Alex Ji Galaxy Data from McConnachie 2012; Bechtol et al 2015; Drlica-Wagner 2015 Ultra-faint dwarf galaxies (UFDs) • Low luminosity (300 - 30,000 Lsun)

• Dark-matter-dominated (M/L > 100)

• Metal-poor (Mean [Fe/H] < -2)

• Old (Mean stellar age 13.3 +/- 1 Gyr)

• Solution to missing satellite problem?

• Coherent sample of metal-poor stars for abundance measurements Ultra-faint dwarfs Classical dSphs

Dwarf data from McConnachie 2012, Kirby et al. 2013, Alex Ji Simon et al. 2015, Walker et al. 2016 Stars in first nine UFDs have low neutron-capture element abundance

gray points: Milky Way halo stars

Alex Ji UFD star

r-process star

Alex Ji UFD star

r-process star

Alex Ji UFD star

r-process star

Alex Ji Ret II stars >100x higher neutron-capture element abundances than other UFDs

Alex Ji Ret II stars >100x higher neutron-capture element abundances than other UFDs

and it is r-process elements! Alex Ji Reticulum II was enriched by a rare and prolific r-process event A typical core-collapse supernova could not be responsible for the Reticulum II r-process elements! How Rare? How Prolific? 1 event out of 10 UFDs Need dilution mass: [X/H] → X

Stellar mass of all UFDs: Max dilution mass: ~107 Msun expected # of SN: ~2000 total gas mass in UFD galaxy

Consistent w/ expected Min dilution mass: ~105 Msun NSM rate of 1051erg explosion in typical ISM 1 per 1000-2000 SNe

Alex Ji Ret II abundance consistent with neutron star merger merger Neutron star Neutron Supernova

Alex Ji Where does the r-process happen?

r-process

Alex Ji