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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.
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.
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.
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.
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.
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)
candidates must be checked with mod. res. spectra. So 3 stage process, HES, then follow up, then HIRES for most interesting
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
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
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
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
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) ?
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.
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.
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.
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.
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.
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
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
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
Important clue – constant C/H, ~1/5 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 is the signature of ~4 Msun 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)
Suggested phenomenological model:N(elem E)=A x p(E,light) + B x p(E,Fe-peak) + C x p(E,r,Fe-seed) + D x p(E,s,Fe-seed)A, B, C, and D are independent !Key elements in each process:Light – CNO through Mg - SNIISNIa, Fe-peak - Fe, Ni(Ca to Zn)s process – Ba, Lar process – Eu (hard to observe all r-process elements)
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) 85% of the C-rich stars are also s-process rich with low C12/C13
d) 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]
Ca/Fe for 40 C-normal stars, HIRES, red = giants, blue =dwarfs.s(dwarfs) = 0.10 dex, <[Ca/Fe]> = +0.25 dexGiant anal. not full checked yet.
Sc, Ti, and Mn/Fe versus Fe/H for normal-C candidate EMP stars (dwarfs blue, giants red) Each s(dwarfs) < 0.15 dexSc, Ti, Mn/Fe 35 to 39 stars, s 0.21, 0.18, 0.28(trend ?) dex<X/Fe]> 0.20, 0.28, -0.63 dexGiant analyses not checked yet.