Inquiry into Low-Mass Star Survivors from the Early Universe and the Quest of First Generation Stars Masayuki Y. Fujimoto, Takuma Suda, Takanori Nishimura (Hokkaido University) Masayuki Aikawa (Universite Libre de Bruxelles) Nobuyuki Iwamoto (Japan Atomic Energy Research Institute) Icko Iben, Jr. (University of Illinois at Urbana- Champaign) Extremely Metal-Poor (EMP) Stars 1. Introduction 2. External pollution 3. Internal Pollution 4. Binary nature 5. Summary

1. Introduction ・ first stars formation  transition from the dark age to luminous world re-ionization (z ~ 15) from WMAP metal-production (z > 5) from quasers ・ low-mass stars, if born, survive today as nuclear burning, luminous stars ⇔ Top-heavy mass function, especially for the first stars ・ Extremely Metal-Poor (EMP) stars may serve as probe into the evolution of early universe

Recent Increase in number of known EMP stars Deep survey of metal-poor stars in the Galactic Halo : HK survey （ Beers et al ） + Hamburg/ESO (Christlieb et al. 2000) ・ more than 100 stars for – 3.5 < [Fe/H] < －３ ・ ～ 10 stars for – 4.0 < [Fe/H] < －３.5 ･ no stars for – 5 < [Fe/H] < －４ ・ ２ stars at [Fe/H] = －５. ３ and － 5.4 (2002 、 2005) Metallicity distribution from HK survey (1998) －５

History of search for Metal-Poor stars 1950’s: discovery of stars of [Fe/H] = -1~ -2 (Chamberlain & Aller 1951) 1970’s: survey up to B < 11.5 and no star found below [Fe/H]=-3 → idea of no low-mass stars formed with smaller metallicity (Bond 1980) 1980’s: a high-velocity dwarf G64-12 with [Fe/H]=-3.5 at B=11.8 (Carney & Peterson 1981) a blue subdwarf CD-38 ○ 245 with [Fe/H]=-4.5 at B=12.0 (Bessell & Norris1984; later revised as [Fe/H]=-3.9, Ryan et al. 1996) a high velocity carbon dwarf G77-61 with [Fe/H]=-5.6 at B=12.0 (V=13.9) (Gass et al. 1988; later revised as [Fe/H]=-4.0, Plez & Cohen 2005) 1990's: HK survey of B=15.5 (Beers et al. 1992) uncovered over 100 stars with [Fe/H]<-3, yet, no star with [Fe/H]< ’s: Hamburg/ESO survey of B=17.5 (Christlieb et al 1999) HE giant with [Fe/H]=-5.3 at B=15.89 (Christlieb et al. 2002) HE dwarf with [Fe/H]=-5.4 at V=13.5 (Frebel et al. 2005)

Probe into Early Universe with Low- Mass Survivors as Tool Numbers of EMP stars will increase and makes such study promising In actuality, EMP stars are known to display various peculiar abundance patterns, different from the stars of younger populations It is true, however, that during their long lives, these low- mass survivors may experience modifications of their surface characteristics, e.g., through accreting metal from interstellar gas and/or from evolved companion, and also, through internal nucleosynthesis and material mixing.

Our purpose: Based on the characteristics of evolutions of low- mass, extremely metal-poor stars, to separate these external and/or internal pollution and disclose their pristine nature, in particular, to identify the survivors of first stars, if exist, and to inquire into dawn of our Universe the first stars were born

Bondi Accretion First star survivors: [Fe/H] ~ - 3 for Dwarfs with Msurf,conv~0.001Mo [Fe/H] ~ - 5 for giants with Msurf,conv ~ 0.1Mo 2. External Pollution --- Surface Pollution through accreting metal-rich gas

Overpopulation of carbon stars ・ 25% of Extremely Metal Poor Stars [Fe/H] <-2.5 ・ A few % for Pop. I (N- R, J-type carbon stars, Ba Stars ) and Pop. II stars (CH stars) S-process element enhancement ・ large star-star variations in ・ A small Pb/Ba ratios 3. Internal Pollution - Nucleosynthesis and material mixing - Mass transfer from evolved companion

Extremely Metal-Poor Stars (EMPS) larger fraction of carbon stars （２５％） ⇔ a few ％ in Pop II. (Rossi et al. 1998) large variations in the abundances of s - process elements

3.1. The Origin of Carbon Stars Structure of AGB Stars TP-AGB phase during H and He double shell burning in stars of mass M > 1.5 M  for Pop. I ． II ⇒ large 12C/13C + s - process elements (a) Third dredge-up (Iben 1975) for Pop.I & II Helium flash convection

(b) H mixing into He convection below [Fe/H] ~ -2.5 Z=0 [Fe/H] ＝－ ４ [Fe/H] ＝－ ２ structure just before He core flash dredge-up by surface convection helium core flash helium shell flash Ｘ 水素 燃焼殻 s ＝ｃｏｎ ｓｔ larger entropy H mixing H-mixing lower metallicity  lower entropy in the hydrogen burning shell  lower barrier for mixing

Extremely Metal-Poor Carbon stars within the standard framework of stellar evolution Initial mass Initial metallicity Population I, II Population III ＥＭＰ Ｓ 0－10－1 (Fujimoto, Ikeda, Iben 2000) grow Carbon star earlier than Pop. I & II stars for M < 3 M . HE HE

EMP Stars: Earlier Evolution to Carbon Stars He-Flash Driven Deep Mixing for [Fe/H]<-2.5 Thermal Dredge-up for Pop. I and Pop. II Up to thermal pulse for Z=0.001 (Lattanzio 1986) Z=0 (Suda et al. 2004) Carbon star

3.2 s-Process Nucleosynthesis (Pop. I & II ) (a) radiative １３ C （ ,n) １６ O （ Straniero et al ） 1)The mixing mechanism of proton into the carbon-rich radiative shell is assumed. 2)The number of neutrons per seed nuclei increases for lower metallicity extra mixing - １３ C pocket- prediction: larger Pb/Ba ratio for metal poorer stars

Observed Pb/Ba from EMPSs and Pop. II EMPS (Aoki et al. 2001) small Pb/Ba ratio Large Pb/Ba ratio [Fe/H]=-2.71 [Fe/H]=-2.74 Pop.II ： CH stars ( Van EcK et al. 2001) [Fe/H]=-2.45 [Fe/H]=-1.67 =-1.7

(b) Convective １３ C （ ,n)16O below [Fe/H]~- 2.5 In EMP stars ， the engulfed H by the He-convection burns as: １２ Ｃ（ｐ，  ） １３ Ｎ （ｅ ＋  ） １３ Ｃ（  ， ｎ） １６ Ｏ Smaller neutrons/seed nuclei ratio. expected because of dilution mixed protons in the He flash convection. [Fe/H] ＝－２．７, 2M o Iwamoto et al ． 2004 H mixing He convection H convection.

But the radiative burning requires the reduction of mixed protons by large factors. Comparisons with EMPSs (Ryan et al.2001) The convective １３ C （ ,n) burning gives the neutron exposures consistent with the observations.

convectiveradiative Radiative vs. Convective 13 C( ,n) 16 O Burning Suda et al Why not the radiative 13 C burning works for [Fe/H] < -2.5? the inner edge of surface convection Helium Layer Surface Conv. He + C + Fe H + He + Fe Opacity

C+N and s-process elements evolution ／ mass transfer in close binary [C,N/H] ＝ 0 ~ -1 as a result of hydrogen mixing into the helium shell flash convection and the subsequent dredge-up by the surface convection. EMPS grows carbon star at an earlier evolutionary stage than the star of younger population. surface pollution with material transferred from the AGB companion in close binary. s-process may occur with mixed proton but can be dredged up for M > 1.5 M . (Suda et al ． 2004 ）

2 nd possibility: weak flashes of H-mixing without splitting the convection large 13 C/ 12 C ~ 0.01

4. Binary Nature of Carbon Stars --- over-populations of EMP carbon-rich stars --- 1au 100au 10au He Flash-Driven Deep Mixing Third Dredge-up Hot Bottom Burning of Primary Binary separation The mass of secondly 0.8 Mo [Fe/H] = -2 O/C = 2.7 Lower mass limit Surface C-abundance as a result of wind accretion for giants

Period distributions EMP Carbon stars vs. CH stars (Pop.II) log a (au) a: binary separation Numbers CH stars – concentrated at a = 1 ~ 10 au. EMP – smaller separation ~

Summary So far so good between the observed EMP stars and the theory of low mass star evolution Why so many carbon stars Why different trend in s-process elements Why so few evidence of binary nature By separating these effects, we may reveal the origin and pristine nature of EXP stars: EMP stars as the second stars, triggered by First Sne (Machida et al. 2005) No need for peculiar C-enhanced SNe proposed by Umeda & Nomoto (2003) Pair Instability SNe are incompatible with the abundance pattern of EMP stars in particular, the origin of HE , HE of [Fe/H] <-5

two stars of [Fe/H] < -5 as the candidates for the first star survivors (initially metal-free) → Key to understanding the nature of low mass star survivors from early Universe