Signatures of Peculiar Supernova Nucleosynthesis in Extremely α-enhanced Metal-poor Stars

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1 Signatures of Peculiar Supernova Nucleosynthesis in Extremely α-enhanced Metal-poor Stars Hye-Eun Jang 1, Sung-Chul Yoon 1, Young Sun Lee 2, Ho-Gyu Lee 3, Wonseok Kang 4 and Sang-Gak Lee 1 1 Seoul National University, 2 Chungnam National University, 3 Korea Astronomy and Space Science Institute, 4 National Youth Space Center

2 Introduction Galactic metal-poor stars with [Fe/H] < -1 Old stars that were formed 10 Gyrs ago Have signatures of nucleosynthesis of the early universe Core-collapse and Type Ia supernova nucleosynthesis Core-collapse supernovae (CCSNe) produce large amounts of α-elements (Mg, Si, Ca ), and they played the dominant role in the chemical evolution of the early universe Type Ia supernovae produce large amounts of iron group elements (Fe, Ni ), and they started to make contribution to the chemical evolution from about t ~ 1Gyr.

3 [Mg/Fe] α abundances [Mg/Fe] vs metallicity [Fe/H] Plateau at [Fe/H] < -1 : CCSN enrichment in the early universe & delayed iron contribution by Type Ia SN α-enhanced stars : Strong enhancement above the plateau May imply exotic chemical environment in the early universe (e.g : Pair-instability supernova, Fall-back supernova etc) α-enhanced stars [Fe/H]

4 Original motivation Exotic nucleosynthesis: Pair-instability supernovae (PISNe) Thermonuclear explosion of very massive stars ( M) Produce large amount of α-elements (esp. silicon), unique nucleosynthesis pattern (odd-even effect etc) Observation : No PISN signature was found Z ~0.001 Kozyreva [α/fe] ~ 2 150M PISN Z= M CCSN Z=0.002

5 [Mg/Fe] Target selection Select stars with high [Mg/Fe] from the SEGUE database. (abundance & stellar parameters derived by Lee+ 2008) + FGK type, main-sequence (log g > 4) and g mag < 16 : Targets : SDSS data Seven north targets -1.5 < [Fe/H] < -1 [Mg/Fe] > K < T < 6000K log g > 4 g mag < 16 [Fe/H]

6 Observation & Data Reduction Observation GRACES : Gemini North 8.2m telescope + CHFT ESPaDOnS Echelle Spectrograph Wavelength coverage: 4,000-10,000A Resolution: 40,000 (star+sky mode) S/N ratio: 50 (4500A) to 100 (7000A) CFHT Gemini Data reduction DRAGRACES + some IDL Procedures written by authors

7 log eps log eps Normalized flux Abundance Analysis Equivalent width estimation Using IDL-based code TAME (Kang & Lee, 2012) # of absorption lines for each star : ~400 Abundance analysis Fe I A Wavelength (A) Kurucz (α-enhanced) atmosphere model (Castelli & Kurucz, 2004) + MOOG (Sneden, 1973) Atmosphere parameter estimation using ~100 Fe I & II lines (E.g.) N1 Fe I line abundance vs excitation potential and equivalent width Excitation Potential Equivalent Width

8 Results & Discussion Stellar parameters & element abundance ratios [X/Fe] of 7 program stars (part) [Fe/H] T_eff Log g V_r O Mg Si Ca Sc Ti V Mn N N N N N N N VMP stars (Cayrel+, 2004) N7 and N8 have high oxygen abundance [O/Fe] > 0.7 Large [Mg/Fe] & [Ca/Fe] but relatively small [Si/Fe] ( Not compatible with PISN nucleosynthesis) N8 has very high [Ca/Fe], [Ti/Fe] and [V/Fe] ratio along with relatively strong [Mn/Fe] N8: Evidence for peculiar SN nuclesynthesis?

9 Abundance pattern of our program stars SAGA (-1.5 < [Fe] < -1) Caryel+2004 ([Fe] < -2) N8 ( ) Overabundance of Ca, Ti and V & relatively low abundance of Si are clearly shown! Abundance pattern of N1 ( ) also deviates from normal galactic stars

10 Element abundance of N8 and Implication on SN nucleosynthesis Possible scenarios to explain abundance pattern of N8 Should produce large amounts of Mg, Ca, Ti, V, Mn rather than Si 1) Helium detonation in accreting white dwarfs (Waldman+ 2011) 2) Contaminated by nearby Ca-rich core-collapse supernova (Gvaramadze+ 2017) White dwarf Dwarf (N8) SN progenitor CO He M i = 8-10 M sun (Low mass) Helium accretion from companion Ejecta accretion on the surface Ejecta spread into ISM Supernova explosion! Disappear Strong Ca emission lines Supernova explosion! (Ca-rich) Helium detonation process in the close binary system Schematic view of Ca-rich CCSN enrichment

11 Comparison of He detonation model and N : ㅇ SAGA: avg. [X/Fe] of stars with -1.5 < [Fe] < -1 + He detonation: CO.45+HE.2 yields from Waldman+ (2011)

12 Future works Comparison of Ca-rich supernova model and N8 Low-mass progenitor model (8-10M sun ) + average abundance at [Fe/H] = -1.5 Additional abundance measure of N8 Iron-group elements (Preliminary: [Ni/Fe] = and [Zn/Fe] = ) s-process/r-process elements (s-process: [Ba/Fe] ~ -0.1 and [Y/Fe] ~ +0.1) Physical parameter estimation Derive mass (< 1M sun?) and distance (~ 1kpc) of N8 from stellar parameters (temperature, gravity, magnitude etc.) Galactic location and radial velocity Nearby halo or thick disk star? Sun GC +66º N8-334 km/s Sun Galactic location of N8 N8 +35º Galactic plane

13 Summary We took spectroscopic observation of 7 magnesium-enhanced galactic metalpoor stars selected from SEGUE database. We find our program star N8 shows strong Ca, Ti enhancement compared to Si. These abundance pattern cannot be explained by the standard chemical evolution model from CCSN and Type-Ia SN nucleosynthesis. Helium detonation or Ca-rich CCSN model could be a possible explanation of our result. Helium detonation model may explain enhancement of Ca, Ti and V, while oxygen and silicon abundance is not reproduced by helium detonation model.