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化学組成で探る天の川銀河の進化

辻本拓司 ( 国立天文台 ). ４つのテーマに焦点. 化学組成で探る天の川銀河の進化.  prompt Type Ia supernovae (2006~)  the IMF variation (classical issue)  stellar migration (2008~)  He-enriched stars in globular clusters (2004~) .  Galactic thick/thin disk  Galactic bulge  Galactic halo  the Fornax dSph galaxy.

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化学組成で探る天の川銀河の進化

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  1. 辻本拓司 (国立天文台) ４つのテーマに焦点 化学組成で探る天の川銀河の進化 prompt Type Ia supernovae(2006~) the IMF variation (classical issue) stellar migration (2008~) He-enriched stars in globular clusters (2004~) Galactic thick/thin disk Galactic bulge Galactic halo the FornaxdSph galaxy 天の川銀河研究会２０１２, ９月６−８日 in 鹿児島

  2. Delay Time Distribution (DTD) of SNeIa tIa~tGW a4 fsep a-1 Ia型超新星の寿命は短かった observed results from SN Ia surveys for extragalaxies theoretical models double-degenerate scenario Young progenitors for SNeIa are dominant (Mannucci et al. 2006; Sullivan et al. 2006) Totani et al. 2008 Kirby et al. 2011 single-degenerate scenario (Totani et al. 2008; Maoz et al. 2010) Hachisu et al. 2008 about 70% of SNeIa explodes with a time delay within 1 Gyr a significant impact on Galactic chemical evolution

  3. Drastic change in typical timescale of SN Ia progenitors ~1 Gyr ~ 0.1Gyr imply Toomre diagram T=(U2+V2)0.5 break in [a/Fe] among solar neighborhood stars apply Venn et al. 2004 Galactic stars are now well kinematically separated. -1 [Fe/H] Pagel & Tautvaisiene 1995 thin disk no break no high a/Fe stars no low Fe/H stars Yoshii et al. 1996 thick disk the presence of break! DTD discussion on a/Fe break should be assessed by comparing the modeled chemical feature of the thick disk with the corresponding observed one.

  4. Updates of s-process yield in AGB stars s-process元素はもう一つの元素(宇宙)時計 Mainly due to large uncertainties in convective mixing and 13C-pocket efficiencies, the s-process nucleosynthesis allows a wide range for the level of a possible production. The renewed picture of a SNIa clocking should be tied up with another nucleosynthesis clock, the s-process operating in an AGB star. Abundances of the surface of AGB stars can be directly compared with the nucleosynthesis results. the best empirical Ba yield as a function of stellar [Fe/H] so as to reproduce the Ba evolution of thin disk stars Theoretically allowable range TT & Bekki 2011 fails to reproduce the chemical evolution of the thin disk Busso et al. 2001

  5. Chemical Evolution of Disks ✓Thick and thin disks are separately modeled. ✓First, thick disk is rapidly formed, and subsequently thin disk is gradually formed. see next slide ✓Formation of two disks are connected in a sense that thin disk stars start forming from a remaining gas of thick disk. ✓We examine the evolution of [Mg/Fe] and [Ba/Mg]. not [Ba/Fe] so as to prevent the effect of s-processing from being hidden by SN Ia contamination three nucleosynthesis clocks: SNe II, SNeIa, AGB stars ✓We focus on the chemical evolution for [Fe/H] ≤ 0. since the origin of metal-rich disk stars should be assessed with an extra evolution factor such as stellar migration

  6. Formation of the thick disk through minor merging between the first generation of the Galactic thin disk and a dwarf galaxy about ~10 Gyr ago (Bekki &TT 2011, ApJ, 738, 4) The thick disk can be regarded as a first disk which is heated up by an ancient minor merger, that is subsequently followed by the gradual formation of a secondary disk, i.e., the thin disk. no metallicity gradient in the thick disk obs. flattening of metallicity gradient resulting from radial mixing induced by minor merging Kinematic properties can be also reproduced. AllendePrieto et al. 2006 model (but not a positive correlation.. : Spagna et al. 2010; Lee et al. 2011)

  7. Chemical evolution of the thick disk • model parameters • = 2 Gyr-1: SFR coefficient • DSF= 1.5 Gyr: SF duration • tin = 0.5 Gyr: timescale of an infall A successful reproduction of the [Mg/Fe] feature suggests that a new SN Ia DTD revealed by extragalaxy studies is compatible with the Milky Way case. data from Bensby et al. 2005 Ruchti et al. 2011 Venn et al. 2004 an indication of pre-enrichment including the s-processing due to enriched bulge winds ? or data from Bensby et al. 2005 Venn et al. 2004 by s-process elements from fast-rotating massive stars ? TT & Bekki 2012 (Pignatari et al. 2008; Chiappini et al. 2011)

  8. Chemical evolution of the thin disk • model parameters • = 0.4 Gyr-1: SFR coefficient • DSF= 12 Gyr: SF duration • tin = 5 Gyr: timescale of an infall The thin disk stars start forming from the thick disk's remaining gas (corresponding to ~10 % of the original gas) mixed with the infall gas accreted onto the disk. [Fe/H] and [Mg/Fe] decreases and increases, respectively, owing to dilution by metal-poor infalling gas from the halo. data from Bensby et al. 2005 Venn et al. 2004 This reverse evolution comes to an end when the chemical enrichment by star formation exceeds the effect of gas dilution, and subsequently an usual evolutionary path appears. data from Bensby et al. 2005 Venn et al. 2004 the absence of metal-poor thin disk stars as observed TT & Bekki 2012 the low Ba yield case

  9. The Galactic Bulge the presence of two populations two-peaked MDF Babusiaux et al. (2010) studied the correlation between kinematics and metallicities in Baade’s Window. red clump stars in Baade’s window two distinct populations Hill et al. 2011 vertex deviation microlensed dwarf and subgiant stars an old spheroid or a thick disc a bar-like kinematics 0 [Fe/H] Bensby et al. 2011

  10. Formation of the Galactic bulge from a two-component stellar disk (Bekki &TT 2011, MNRAS, 416, L60) Two-component scenario The first disk is disturbed by an ancient minor merger, which induces a vertical growth of the disk and transforms it into a thick disk, and subsequently the thin disk starts to form with an accompanying bar formation in the central region. simultaneous reproduction cylindrical rotation vertical metallicity gradient A vertical mixing induced by a bar buckling functions incompletely in a sense that the high latitude region in the thick disk is not well mixed. Note that in general, it is expected that a disk instability forming the bulge induces a vertical mixing, which leads to erasing a metallicity gradient along a minor axis.

  11. 銀河系バルジの起源は？ bar-induced? merger-built? YES YES metallicity gradient cylindrical rotation [Fe/H]=-0.03 Zoccali et al. 2008 では、混在？ [Fe/H]=-0.17 NO [Fe/H]=-0.28 Howard et al. 2009 single population Minitti et al. 2005

  12. two-component bulge model data from Hill et al. 2011 Bensby et al. 2011 metal-poor component n= 4 Gyr-1 DSF = 1 Gyr tin = 0.3 Gyr IMF: x= -1.35 metal-rich component n= 3 Gyr-1 DSF = 4 Gyr tin = 1.5 Gyr IMF: x= -1.05 ✓A top-heavy IMF is indispensable to make a metal-rich MDF as observed. ✓A large age span of bulge stars is predicted. data from Bensby et al. 2011 Gonzalez et al. 2011 Bensby et al. 2011 but, the color-magnitude diagram is… ✓the enriched gas an end result of chemical processing associated with the halo formation or the s-processing in massive stars ?? data from Bensby et al. 2011 TT & Bekki 2012

  13. one-component bulge model red giants in Baade window (Fulbright et al. 2006; Zoccali et al. 2008) x=-1.35 • = 2 Gyr-1 • DSF = 2 Gyr • tin = 0.3 Gyr • IMF: x= -1.05 A top-heavy IMF is suggested. The MDF with a Salptere IMF is entirely skewed to a low metallicity. In addition, the predicted [Mg/Fe] is lower than the observed data in a metal-rich regime. The predicted [Ba/Mg] exhibits a sharper rise from a much lower metallicity than is expected from the observation. This incon- sistency is resolved by the model with a flatter IMF. TT & Bekki 2012

  14. Halo vs. short-delayed SNeIa no indication of SNeIa for the elemental abundance of halo stars, exhibiting a plateau of [a/Fe] ratio over a whole metallicity range implies Halo stars must be rapidly formed in the Galactic building blocks with a short timescale (~108yr), while an assembly of them finally makes the stellar halo which exhibits an age span of a few Gyr. to check if it is likely or not…. The s-process elements from AGB stars starting to release with a timescale of a few 108yr are imprinted in the abundances of halo stars?? But,….. The compatibility of the presence of s-process among halo stars with the short-delayed DTD can be also understood consistently if s-process elements are produced in fast- rotating massive stars (Pignatari et al. 2008; Chiappini et al. 2011). s-process: no [La/Eu] continues to make a plateau s-process: yes [Pb/Eu] shows an upward trend with an increasing [Fe/H] Roederer et al. 2010

  15. Unusual elemental feature of dwarf spheroidals high s abundance low a/Fe ratio in the FornaxdSph Letarte et al. 2010 previous results from six dSphs [Fe/H] Venn et al. 2004 the LMC, too! also seen in the SagitarriusdSph(Sbordone et al. 2007) [Fe/H] Pompeia et al. 2008

  16. previous study strong galactic wind model occurrence of winds stop the SF no more r-process Eu from SNe II delayed s-process Ba from AGB stars [Fe/H] [Fe/H] Lanfranchi et al. 2008 proposed idea GC’s data (Letarte et al. 2006) Fe & aelements s-process r-process Ba([Ba/Eu]~-0.7) 2 8 10 50 [Fe/H] stellar mass (M) Letarte et al. 2010 ~1.5-3 M r-process cut-off ~25-30 M

  17. A truncated IMF in dwarf galaxies observationally Low-surface brightness galaxies have massive O-type stars A low ratio Meurer et al. 2009 less massive stars a smaller number of very massive stars theoretically A high mass end of the IMF depends on the mass of the star clusters. In the low density environment, the formation of massive star clusters is suppressed. Kroupa & Weidner 2003, Pflamm-Altenburg & Kroupa 2008 Their model predictions have been shown to be consistent with the observed trend for the Ha-to-FUV flux ratio (Lee et al. 2009).

  18. Model result TT 2011 no metallicity dependence for low metallicity Bas-process yield FnxdSph Busso’s results the Galaxy Cescutti et al. 2006 FnxdSph case: Mu=25 MwithΔSF=1.5 Gyr stars with age> 10 Gyr A rapid enrichment is implied by 3 Mo Coleman & de Jong 2008 1.5Mo

  19. これまで化学進化の分野では、ほとんど無視されていたが…….これまで化学進化の分野では、ほとんど無視されていたが……. 惑星を持つ太陽近傍星はmetal-rich 惑星を持つ星の ほとんどが[Fe/H]>0 のmetal-richな星。 Santos et al. 2003

  20. 重元素量 時間 化学進化の描像 およそ２０％の星が太陽よりmetal-rich ([Fe/H]>0) 基本的に重元素は時間と ともに増えていくもの 0 metal-richな星は太陽近傍の化学進化の終着点 Nordstrom et al. 2004 重元素量(metallicity) ではなさそう。。 Cepheids OB stars have [Fe/H]~ 0 H II regions ? 時間 時間

  21. Metal-richな星は簡単には作れない 重元素量頻度分布(ADF)からわかること the key features of the ADF ・the deficiency of metal-poor stars 　　　ー　the G-dwarf problem ー ・[Fe/H]peak= -0.2 ~ -0.1 & 　・[Fe/H]present > +0.2 [Fe/H]present=+0.4 [Fe/H]present=+0.03 +0.4まではありそう The predicted ADFs are biased to metal-rich as compared with the observations. ([Fe/H]~+0.3-0.4) Metal-richな星は進化の終着点 とは考えにくい。 Tsujimoto 2007

  22. Stellar migration (Radial mixing)I stellar migration due to resonant scattering with transient spiral arms Sellwood & Binney 2002 Roskar et al. 2008 explains a large scatter in age-metallicity relation final initial Two-dimensional histogram of final particle radii vs. particle formation radii Roskar et al. 2008

  23. Stellar migration (Radial mixing)II stellar migration due to resonant scattering with transient spiral arms Sellwood & Binney 2002 Roskar et al. 2008 predicts a steepening of abundance gradient reproduce metal-rich stars present past Roskar et al. 2008 w/migration w/o migration

  24. 金属量勾配の時間変化 I flattening flattening Maciel et al. 2006 Daflon & Cunha 2004 現在 現在 GCE models obs. predict a steepening(Chiappini et al. 2001) or a flattening (Hou et al. 2001) The predicted change in abundance gradient in the last several Gyr is <0.02 dex kpc-1 - too small - abundance gradient from R~4 kpc to R~14 kpc -0.1 dex kpc-1 -0.04 dex kpc-1 in the last severalGyr

  25. 金属量勾配の時間変化 II old stars old open clusters time young stars Cepheids TT, Bland-Hawthorn, & Freeman 2010 Yong et al. 2006 銀河系を見る限り、steepeningの証拠はなさそう (過去ほど、勾配が緩やか)

  26. double main-sequence in wCen VLT reveals HST reveals spectroscopic result photometric result Bedin et al. 2004 blue MS red MS fbMS~ 0.2-0.3 Piotto et al. 2005 the origin of bMS 1. Y~0.4 2. very low metallicity ([Fe/H]<-2) bMS stars are more metal-rich than rMS stars. D[Fe/H]~0.3

  27. super helium-rich stars in GCs 47 Tucanae NGC 2808 Milone et al. 2012 Milone et al. 2012 Y=025/0.32/0.38 ✓GC is not a single population! (note: a spread in [Fe/H]:wCen, M22, Terzan 5, NGC 2419 a subgiant-branch split: NGC 1851, M22, 47 Tuc,…..) ✓the presence of super He-rich stars only in massive GCs? >17% halo stars from disintegrated GCs (Martell et al. 2011) seen for metal-rich stars in the bulge (Nataf & Gould 2012) the origin of He-rich stars likely, the formation from AGB ejecta NGC 6566

  28. まとめ 大きく認識を変えたいこと ✓Ia型超新星は結構早く爆発する ~1億年 ✓球状星団（の一部）は単一の星の種族ではない ヘリウム過剰星の存在 今後注視していきたいディスク銀河進化のドライバー ✓stellar migration (星の動径方向の大移動) 　但し、過大評価の可能性もある IMFの普遍性問題 ✓バルジ、近傍矮小銀河にIMF variationの証拠 top-heavy top-light

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