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The 0Z Project

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)

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The 0Z Project

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  1. 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)

  2. 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 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

  3. Size of analyzed sample of candidate EMP stars from the HES as f(time) • 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 Jan 2006), all 1700 follow-up spectra in hand analyzed, expect about 1600 stars when duplicates and rejects removed.

  4. 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 H. • 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).

  5. 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 (H) is the most uncertain. It’s the weakest in giants, and located adjacent to other strong features.

  6. Analysis procedures for HIRES spectra • Only freedom is above or below turnoff for stars near MSTO. • Use automatic eq. width program of Ramirez & Cohen, then a well defined procedure of manual checks for weak lines. • Fixed template list of lines, gf values, HFS corrections, damping constants. Fixed list of molecular lines for C and N abundances. • Use MOOG code of Sneden modified for batch operation. • v_t determined for some stars, v_t –Teff relation used for most • Except for the Keck Pilot Project (Sep 2000 run), I have done all the observing, data reduction and analysis thus far myself. • For the Keck Pilot Proj., E. Carretta and R. Gratton did the analysis. I did the stellar parameters and data reduction.

  7. 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

  8. Peculiar Stars among the EMP stars • I am not going to talk about the abundance ratios in the C-normal stars from the HES here, insufficient time • I am ignoring the UMP stars, none in our sample • Highly C-enhanced stars, including Carbon stars • UMP stars ([Fe/H] ~ -5.3 dex) • Extreme r or s-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.

  9. 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.

  10. 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

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

  12. C/Fe and C/N ratio among C-rich stars, HIRES. Suggestion for constant C/H of ~1/7 Solar

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

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

  15. Histogram of log[(C)] (i.e. C/H) for EMP candidate giants from the HES in 2 ranges of [Fe/H].HIRES [Fe/H] used when available.

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

  17. 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 !

  18. 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

  19. 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 !

  20. C12/C13 Ratios for EMP C-stars from C2 and CH

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

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

  23. 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]  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

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

  25. Our Hypothesis for the Origin of C-rich ([C/Fe] > 1.0 dex) Stars Important clue – constant C/H, ~1/7 Sun is consistent with observations in all C-stars analyzed from our sample 85% must be binary mass transfer, high C and high s-process (high Ba and high Ba/Eu) is the signature of AGB stars Remaining 15% tends to be more metal poor, suggesting this is result of mass transfer in more metal poor stars, where the s-process runs all the way to lead, the last stable element, no big enhancement of Ba produced, or insufficient n for s-proc. Predictions: expect Ba-poor C-stars to have low [Fe/H] as is seen. Maybe high Pb in most metal poor C-stars, very hard to detect, At [Fe/H] ~ -1.8 dex, C-stars become CH stars (C up, but C<O), at still higher [Fe/H], CH stars become Ba stars (as observed)

  26. A new short period double-lined spec. binary from HES

  27. Fe/H histogram of HES sample, C-stars black, 663 stars, note absence of C-stars with [Fe/H] -1.8 dex

  28. The Yield of the HES for Metal-Poor StarsCleaned: 3.5 % <-3.01.8% < -3.20.7% < -3.4

  29. C-star Fe/H from analysis of HIRES spectra a) The [Fe/H] values for cool C-stars given by the standard tools used by the HES (and formerly by the HK project) give values too low by ~1.0 dex. b) The sample appears divided into C-normal stars and a much smaller number of C-rich stars, many of which are C-stars. c) When one corrects for (a), the fraction of C-rich stars with [C/Fe] > 1.0 dex is ~14%, and is probably independent of [Fe/H]

  30. C-enhanced stars About 85% of C-enhanced stars also show large enhancements of Ba and lead. The rare earths in these stars appear to be predominantly of s-process origin (high Ba/Eu, high Sr/Ba) About 15% of the highly C-enhanced stars show a “normal” pattern for the heavy elements, no enhancement of Ba nor of the rare earths and no detected lead. No stars seen with high s-process and normal C/Fe Suggest these are ALL mass transfer binaries. At very low [Fe/H], no Ba, just very high Pb or perhaps no n for s-process. At higher Fe/H, CH stars, then Ba stars. A few stars known, almost identical to HE2148-1247, with a large C enh, v. large Ba and rare earths enh, and Ba/Eu intermediate between the pure r- and s-ratios (r+s?)

  31. C-enhancements The C-stars appear to show a fixed (C), about 1/7 Solar We suggest C-enhancement is a sign of binarity. The fraction of C-enhanced (+ Carbon) stars is about 14%, independent of [Fe/H]. Is this consistent with the (unknown) binary fraction ? Is there a more efficient way to determine the binary fraction ? (Big program at Lick/M. Bolte to determine the binary fraction among EMP stars) C isotope ratio from C2 bands is easy ! C12/C13 is LOW.

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