How Many SN Contribute to an EMP Star’s Chemical Inventory ?

1. THE LOCAL HIGH REDSHIFT UNIVERSE – THE EXTREMELY METAL POOR STARS IN THE GALACTIC HALOWhy study very metal poor stars ?We can study the early epochs of the Galaxy, the local equivalent of the z ~ 5 Universe, with objects that are bright enough to be found in large numbers and to be analyzed in considerable detail.We can study the kinematics of the early Galaxy and its halo.We can study the onset of chemical evolution in the Galaxy, thepossible stellar sources which produced many elements at very early epochs (very massive stars and SNII).We can probe the age of the galaxy (Th and U dating) and the relationship between the halo field stars and the Galactic globular clusters.

2. Binding energy/nucleon – up to Fe/Ni, fusion releases energy. Heavier than Fe/Ni, fusion is energetically prohibited; fission releases energy. Most of the available energy is released when fusing 4 H atoms to He.

3. The onion-layer characteristic of highly evolved massive stars, a consequence of the ever increasing T required to burn various nuclear fuels, H to He, He to C, C to Si …to Fe..Only SNII contribute to EMP stars, no SNI.

4. r and s-process neutron captures produce specific isotopes. From Sneden & Cowan, 2003, Science, 299, 70

5. Solar Abundances Beyond the Fe-peak, produced by n-addition, Separated by s and r processes of n-addition.

6. How Many SN Contribute to an EMP Star’s Chemical Inventory ?

7. Timescale to reach 1/3000 Solar Metallicity • Assume SN rate at least as large as now, 1 SN/100 yr in our galaxy • Assume solar abundance ratios. • All material for < -3.5 dex is then made by t << 10**6 yr. • The EMP stars thus represent the relics of the early stages of the formation of our Galaxy. They are very old. • Their effective redshifts are very high (z>4). • The EMP stars represent the local high redshift Universe (near field cosmology)

8. Energy Considerations Assume 1Msun of material (mostly Fe) is ejected per SNII at ejection velocity of 10,000 km/sec. The amount of mass (assumed to be pristine H) swept up when the ejecta has decelerated to the level of typical clouds in the proto-Galaxy (assumed to be vf = 50 km/sec) is then (by conservation of energy) M = 4 x10^4 Msun x (Mej/1 Msun) x (50/vf)^2, and the resulting [Fe/H] is So 1 SN cannot pollute the entire halo mass. Within this swept up mass, [Fe/H] is Solar, so there must be considerable mixing or overlap between the ejecta of individual SNII.

9. The Holy Grail of Cosmochronology – Does It Exist ? • Use radioactive decays to age date EMP halo stars, get age of halo, add a small amount for the initial formation of the proto-halo, get age of Universe • Must measure N(decay)(mostly lead)(now) / N(parent in the past), but lead is made in other ways as well • Easiest: Th/Eu ratio, but these are far apart in the periodic table, do we understand their production well enough ? Probably no • Results from Th/Eu from different EMP stars are inconsistent • Th/U is better, much closer in periodic table • Th lines are hard to detect, U lines are almost impossible to detect • Half life – Th 13.5 Gyr, U 4.5 Gyr

10. Galactic chemical evolution models for EMP stars • EMP stars are very old, so fewer production sources contribute (at high metallicity, many production sources with varying timescales can contribute, much more complex to model). • Only SNII and any pre-galactic stars (first stars, VMS, Pop III) contribute, lifetimes and mass distributions for these sources • Metal production and ejection into the ISM by each relevant source • Issues: volume of pollution, mixing of ejecta, mass outflows from the galaxy, continued accretion into the proto-galaxy halo • Stellar IMF for first generation, star formation rate f(t), mass loss • Role of binaries

11. Solar abundances, atomic numbers 1-32. Isotopic ratios mostly measured just for SunSum for all stable isotopes of an element.Heavier than Fe-peak, very low abunds.

12. The 0Z Project The core group: Judy Cohen (Caltech), Andy McWilliam, Steve Shectman, Ian Thompson (Carnegie), Norbert Christlieb (Hamburg) Current and former postdocs: Inese Ivans, Jorge Melendez, Solange Ramirez (Caltech), Innocenza Busa, Franz-Josef Zickgraf (Hamburg) Undergraduate students: Amber Swenson (Caltech), Berit Behnke (Hamburg)

13. The 0Z Project Plan • Goal: to find and study a large sample of extremely metal poor stars • We use the Hamburg/ESO survey to generate candidate lists. We have exclusive access to half of the HES fields. • We have taken about 1700 mod. resolution follow-up spectra, highest quality class candidates first, and by brightness. About 600 from 200-inch at Palomar, about 1100 from 6.5 m Magellan Telescope at Las Campanas. • We are 99.9% complete (to B=17.5 mag, i.e. the HES limit) in the fall-north fields (about 900 deg sq). • Those with [Fe/H](HES) < -2.9 dex from P200 spectra are to be observed at HIRES/Keck – about 75 such stars observed to date, at present only 8 such stars have not yet been observed

14. Finding EMP stars (EMP: Fe abund. < 1/1000 Solar) • Rare: roughly 1/square degree to B(lim) ~ 17 mag • Use objective prism spectra to search for candidate EMP stars. These are low quality, low disp spectra. • Since contamination by higher [Fe/H] stars is severe, candidates must be checked with mod. res. spectra. So 3 stage process, HES, then follow up, then HIRES for most interesting • HK Survey, late 1980s, Preston & Shectman • Hamburg/ESO Survey, 2000+, digitzed and fainter. Original purpose – find bright QSOs. Norbert Christlieb, in charge of stellar survey.

15. Examples of HES objective prism spectra of candidate metal poor stars from the HES (digitized photographic plates) (from Christlieb 2003)

16. Sample Mod. Res. Follow Up Spectra of HES EMP candidates from 200-inch Hale Telescope. Measure CaII and Balmer line indices.

17. 3933/CaII index versus Hdelta index for ~800 stars from the HES with mod. res. specta from Magellan (from Ian Thompson) with Carnegie Fe/H calibration

18. The assignment of [Fe/H](HES) • We use the algorithm of Beers et al (1999) to assign [Fe/H](HES) from analysis of the moderate resolution spectra. • Two indices are used, KP, measuring the absorption in the 3933 A line of CaII, and HP2, measuring the strength of Hd. • An index measuring the strength of the G band of CH (GP) is also measured. • We use the definitions of these indices from Beers et al (1999).

19. Size of analyzed sample of candidate EMP stars from the HES as f(time) (1700 Follow Up Spectra In Hand) • Papers through 2004, abundance analyses of individual stars of interest, no attempt at a statistical sample • ApJL now in press, early 2005, 497 stars, all DBSP/P200 through spring 2004) • Current sample shown here : 732 spectra of EMP candidates (~600 DBSP/P200 + 120 Magellan), remove duplicates and rejects, yields 663 different EMP/VMP stars with ~21 rejects (galaxies or M dwarfs, dMe) • Final sample (expected ready summer 2006), all 1700 follow-up spectra in hand analyzed, expect about 1600 stars when duplicates and rejects removed. Now assembled, working on this.

20. Accuracy checks of our results from the moderate resolution spectra: • 57 stars with duplicate spectra (runs in 2 different months on P200 or P200+Mag obs.) • “Standard stars” – comparison of values inferred from our spectra with published indices and [Fe/H] values. • The HP2 index (Hd) is the most uncertain. It’s the weakest in giants, and located adjacent to other strong features.

21. HIRES/Keck Spectra obtained for the most interesting EMP stars only: Spectra for EMP dwarfs (and 1 EMP giant) in the region of the 4215 A SrII line

22. Spectral synthesis of the region of the CH and C2 bandheads in the dwarf carbon star HE0007-1832

23. Teff, log(g) for 62 EMP candidates from the HES analyzed HIRES spectra so far. Note: log(g) is from isochrone and Teff.Includes 16 C*, 3 C-enh., 24 C-normal dwarfs, 19 C-normal giants

24. Errors in our detailed abundance analysis using1d model stellar atmosphere, (mostly) LTE, line by line analysis • Random errors: Uncertainty in Teff (from broad band colors) is the biggest contributor, surface gravity and vt also Unc. in the measurements of the strength of the absorption features in the spectra Random errors in transition probs. and other atomic data • Systematic errors: Does the model atmosphere adequately represent T(depth) ? Is the assumption of non-LTE or the specific non-LTE corrections adopted for several elements adequate ? Is the absolute scale of the gf values OK (lab measure) ?

25. A check of the analysis for Fe/H in a star using the many FeI lines in its spectrum. Is the deducedFe abundance independent of the EP of the line ? of the strength of the line ? of the wavelength of the line ?If not, something is wrong in the analysis.

26. Fe ionization equilibrium – a stringent test of the validity of our abundance analyses

27. Deviations from the mean for Ca/Fe versus Ti/Fe. Circular distribution suggests random errors. From Cohen et al (2004)

28. Deviations from the mean for Cr/Fe versus those for Ti/Fe. Circular distribution suggests random errors. (Cohen et al 2004) One star is a probable real outlier.

29. Current measurements of s[X/Fe]by all groups are very small ! (examples from Arnone et al 2005)

30. Comparison of our results with the SNII yields of Umeda & Nomoto for a range of progenitor mass and explosion energy, from Cohen et al (2004). Note odd-even problems.

31. Ti/Fe for a sample of halo field stars and galactic globular clusters. Red – GCs, green – stars from the HES

32. Ba/Fe as a function of Fe/H for halo field stars, stars from the HES, and globular clusters. Ba production is disconnected from Fe production at low metallicity.

33. Ways to explain the low s values for [X/Fe] in EMP starsand, very recently, also found for DLAs at z 2 to 4 • All early SN (all SNII) were essentially identical, same mass, same production and ejection yields • Many SNII contributed to the gas which became a single low mass EMP star • But the observed abundance patterns require SNII of relatively high mass, where yields depend a lot on M, on the mass cut • The proto-galactic halo was very well mixed

34. Peculiar Stars among the EMP stars • Highly C-enhanced stars, including Carbon stars • UMP stars ([Fe/H] ~ -5.3 dex) • Extreme s-process stars (all of these are C-rich stars) • Extreme r-process stars • Stars with symptoms of substantial r and s-process • Occasional genuine outlier in some Fe-peak element, strong Cr and Mn enhancements seen in 1 star.

35. Roads to Peculiar Abundances These stars are very metal poor. A very small amount of additional material of a relatively rare element can produce a big change in its abundance, assuming the material does not diffuse into the interior. Mass transfer in a binary system where the primary is more massive, in the AGB phase, and transferring material onto the low mass secondary. Today the primary is a WD, and the secondary, which we see, has a surface contaminated with enhanced C and s-process elements.

36. Problem with C-star [Fe/H](HES). For coolest C-stars its too low by a factor of 10 compared to results of HIRES analyses.Offset for dwarfs due to lower Teff scale we adopt

37. Spectra of 2 C-stars and a C-normal star, KP and HP2 index feature and continuum bandpasses

38. e(C) and C/N Ratio Among C-rich Stars, HIRESSuggestion for a Constant C/H of ~ 1/5 Solar

39. Strength of G Band of CH versus V-K color (Teff) (663 stars)

40. GP indices (G band of CH) measured from synthetic spectra of M. Briley.Grid calculated for 2 values of [Fe/H]

41. Histogram of C/Fe for candidate EMP giants from the HES for 2 ranges of [Fe/H]. HIRES [Fe/H] used when available.

42. Fraction of C-enhanced giants as a function of [Fe/H], with and without HIRES Fe.Much smaller when correct [Fe/H] is used for C-stars !

43. C/H vs Teff for EMP giants. note trend of C-depletion for cooler, more luminous, more evolved giantsStars at bottom, GP index too low for calibration

44. Large isotopic separation for 4740 A band of C2 (but not for stronger (0,0) band at 5160 A.Easy to derive C12/C13 ratio !

45. C12/C13 For EMP C-stars from CH and C2

46. Ba/Fe for EMP C-stars, 85% high, 15% low, C-normal stars from HES also shown

47. Ba/C abundances for the C-rich stars, with 10 EMP C-stars from the literature added, 85% high Ba (s-process)

48. Are elements besides CNO and s-process affected in C-stars ? HIRES results say NO for Na through Ni, median [X/Fe] Species N(C-stars) [X/Fe] s EMP C-normal dwarfs [Na/Fe] 3 0.27 0.22 0.41 [Mg/Fe] 12 0.55 0.27 0.56 [Al/Fe] 10 0.27 0.39 -0.09 *** [Ca/Fe] 14 0.54 0.36 0.31 [Sc/Fe] 5 0.39 0.26 0.24 *** [Ti/Fe] 15 0.43 0.26 0.36 [Cr/Fe] 14 0.43 0.21 0.36 [Mn/Fe] 12 -0.30 0.21 -0.23 *** only 1 line used, possible blending by CH or CN

49. CMD diagram, C-stars, Ba-poor stars, known binaries marked

50. Mass Transfer in Binary Systems Abundances Afterward