How To Understand The Evolution Of The Galaxy

Transcription

1 Galactic Chemical Evolution Ena Choi

2 Contents Introduction Formation and evolution of the Milky Way Galactic Halo Galactic Disk Cosmic Chemical Evolution References

3 Introduction Simple chemical evolution model Star Formation Stellar Evolution IMF Remnant Mass Life time of the star Stellar Yields Scalo (1986) Remnant Mass Maeder (1992) Yields Yields Tinsley(1980) Maeder(1992)+Herwig(2004) & Schaller Maeder(1992)+Herwig(2004)

4 1) Galactic Halo A. α-capture abundance ratios Overall trend O, Mg, Si, Ca and Ti are overabundant in metal-poor halo stars ([X/Fe]~ ) -> short-lived high mass stars dominated early galactic halo necleosynthesis. [a/fe] ratios of all stars with [Fe/ H]<-1 are nearly identical. -> same IMF in different parts of halo or short mixing time scale Halo McWilliam et al Disk

5 1) Galactic Halo A. α-capture abundance ratios Anomalies in α-capture abundance ratios Some low-α stars in the halo -> even have [α/fe] < 0 Brown et al. (1997), Carney et al. (1997), etc. They have orbits that take them to the outer halo regions of the galaxy. They may have formed in ISM regions of abnormally low density, and thus relatively low star-formation rate.

6 1) Galactic Halo B. Ultra-metal-poor stars ( [Fe/H] < -2.5) Abundance Trends in Ultra-Metal-Poor Stars (UMP) [Fe/H] < -2.5 : the lower limit for GCs, and the metallicity at which Fepeak elements in halo stars begin to exhibit severe departures from solar abundance ratios. Non-solar abundance ratios among Fe-peak elements McWilliam et al Filled circles: McWilliam et al Open squares: Gratton & Sneden 1988, 1991, Gratton 1988

7 Timmes, Woosley, and Weaver 1995 Solid line: standard calculation of GCE model, dashed lines: variations in the iron yields from massive stars, Dotted lines: variations in star formation efficiency McWilliam et al Filled circles: McWilliam et al Open squares: Gratton & Sneden 1988, 1991, Gratton 1988 Iron-peak nucleosynthesis in low metallicity Type II super novae is driven toward the heavier elements like Cobalt at the expense of the lighter ones like Chromium

8 1) Galactic Halo B. Ultra-metal-poor stars ( [Fe/H] < -2.5) Constraints on the supernova model and its yields. In core-collapse SNe/HNe, stellar material undergoes shock heating and subsequent explosive nucleosynthesis. Iron-peak element production Nomoto et al complete Si-burning (For Tpeak > 5E9 K) Co, Zn, V Incomplete Si-burning (For 4E9 < Tpeak < 5E9K) after decay products include Cr and Mn. In the bipolar (jet-like) explosions of SNe (Nomoto et al.) The shock wave is stronger along the z-axis(along the jets) and heats up the stellar material to higher temperature. (complete Si-burning) Along the r-axis, temperature are lower (incomplete Siburning) because of weaker shock and densities are higher because of mass accretion. Nomoto et al. 2006

9 1) Galactic Halo B. Ultra-metal-poor stars ( [Fe/H] < -2.5) Constraints on the supernova model and its yields. Nomoto et al The bipolar (jet-like) explosion model preferentially eject the materials experiencing higher temperature in complete Si-burning -> Co, Cr Nomoto et al Cr Cr Co Co Spherical Explosion Aspherical Explosion

10 1) Galactic Halo B. Ultra-metal-poor stars ( [Fe/H] < -2.5) Comparison with model prediction of yields of Pop III stars Co Nomoto et al Cr

11 1) Galactic Halo C. Neutron-Capture Element Abundances at Low Metallicities Sneden et al Ba isotopes <- s-process Eu isotopes <- r-process Their abundances as a function of metallicity can give some information on the relative r/s contributions in the early galaxy. At [Fe/H]~-3, [Ba/Eu]~-0.8 : This is consistent with r-process synthesis from supernovae in the early galaxy, w/o substantial s-process contribution.

12 1) Galactic Halo D. Very Heavy Element Distributions Very heavy element as an age indicator (e.g. Th (Z=90)) Thorium (Z=90) half-life time = 14.0 Gyr Its abundance relative to lighter neutron-capture elements (e.g. [Th/Nd] or [Th/Eu]) can yield an estimate for the age of the galaxy. Ex> CS t ~ Gyr (Sneden et al. 1996) Complications 1. The choice of comparison element (purely r-process element?) 2. Line blending problem for Th II absorption feature 3. The production ratio of Th to the lighter neutron-capture elements (The initial [Th/Eu] ratios)

13 2) Disk A. Solar neighborhood Chiappini et al The G-dwarf metallicity distribution and constraints on the thin disk formation G-dwarf as a fossil record of the SF history We can constraint the timescale for the formation of the disk in the solar vicinity.

14 2) Disk A. Solar neighborhood Chemical evolution of the solar neighborhood Can test the chemical evolution model and compare the model result with observation (MDF, age-metallicity relation.) The total number of SNe Ia can be chosen to meet the MDF. Kobayashi & Nomoto 2009

15 2) Disk A. Solar neighborhood [α/fe] stays constant in the early stage of the galaxy formation but decrease toward 0 later due to the Fe production by SNe Ia. Mn: SNe Ia produce [Mn/ Fe]>0 (Kobayashi et al. 06) Zn: Low Z: Si-burning in HNe. High Z: Neutron-capture in He and C burning and ejected by SNe II. Kobayashi & Nomoto 2009

16 2) Disk B. Galactic Abundance Gradient The metal content decreases from the innermost to the outermost regions. Chiappini et al. (2001) t The theoretical fit to the observed gradients is obtained by the Inside-out formation of the thin disk which implies faster timescales for conversion of gas into stars in the inner regions. Steepening gradient with time <- density threshold for star formation.

17 2) Disk B. Galactic Abundance Gradient Boissier & Prantzos (1999) Flattening gradient with time no threshold

18 3) What we have learned about the MW Some conclusions on the formation and evolution of the MW, derived from chemical abundances. The inner halo formed on a timescale of 1-2Gyr at maximum, and the outer halo formed on longer timescales perhaps from accretion of satellites or gas The disk at the solar ring formed with the gradual accretion of gas. The whole disk formed inside out with timescales of the order of 2 Gyr or less in the inner regions and 10 Gyr or more in the outermost regions. The abundance gradients arise naturally from the assumption of the inside-out formation of the disk.

19 4) Cosmic chemical evolution - The chemical evolution taking place in comoving volumes large enough to be representative of the whole universe. - The cosmic SN Ia and SN II rate - By summing up the supernova rates in spirals and ellipticals with the ratios of the relative mass contribution in the Universe. Type II Type Ia Kobayashi & Nomoto 2009

20 References Boissier, N., & Prantzos, N. 1999, MNRAS, 307, 847 Chiappini, C. et al. 1997, ApJ, 477, 765 Chiappini, C. et al. 2006, A&A, 449, L27 Kobayashi, C. & Nomoto, K. 2009, Submitted to ApJ, astro-ph/ Mattucci, F. 2006, astro-ph/ McWilliam, A. 1997, ARAA, 35, 503 Nomoto K. et al. 2006, astro-ph/ Sneden, C. et al. 1996, ApJ, 327, 298 Sneden, C. 2002, IAUS, 187, 81S Timmes, F. X., Woosley S. E., & Weaver, T. A. 1995, ApJS, 98, 617 Tsujimoto, T., Shigeyama, T., & Yoshii, Y. 1999, ApJ, 519, L63