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Very high quality spectra of 24 metal-poor halo dwarfs and subgiants have been acquired with ESO's VLT/UVES for the purpose of determining Li isotopic abundances. The derived one-dimensional, non-LTE 7 Li abundances from the Li i 670.8 nm line reveal a pronounced dependence on metallicity but with negligible scatter around this trend. Very good agreement is found between the abundances from the Li i 670.8 nm line and the Li i 610.4 nm line. The estimated primordial 7 Li abundance is 7 Li /H ¼ (1:1 1:5) ; 10 À10 , which is a factor of 3-4 lower than predicted from standard big bang nucleosynthesis with the baryon density inferred from the cosmic microwave background. Interestingly, 6 Li is detected in 9 of our 24 stars at the !2 significance level. Our observations suggest the existence of a 6 Li plateau at the level of log 6 Li % 0:8; however, taking into account predictions for 6 Li destruction during the pre-main-sequence evolution tilts the plateau such that the 6 Li abundances apparently increase with metallicity. Our most noteworthy result is the detection of 6 Li in the very metal-poor star LP 815À43. Such a high 6 Li abundance during these early Galactic epochs is very difficult to achieve by Galactic cosmic-ray spallation and -fusion reactions. It is concluded that both Li isotopes have a pre-Galactic origin. Possible 6 Li production channels include protogalactic shocks and late-decaying or annihilating supersymmetric particles during the era of big bang nucleosynthesis. The presence of 6 Li limits the possible degree of stellar 7 Li depletion and thus sharpens the discrepancy with standard big bang nucleosynthesis.

We describe the Sloan

Using a sample of 69,919 red giants from the SDSS-III/APOGEE Data Release 12, we measure the distribution of stars in the [α/Fe] vs. [Fe/H] plane and the metallicity distribution functions (MDF) across an unprecedented volume of the Milky Way disk, with radius 3 < R < 15 kpc and height |z| < 2 kpc. Stars in the inner disk (R < 5 kpc) lie along a single track in [α/Fe] vs. [Fe/H] , starting with α-enhanced, metal-poor stars and ending at [α/Fe] ∼ 0 and [Fe/H]∼ +0.4. At larger radii we find two distinct sequences in [α/Fe] vs. [Fe/H] space, with a roughly solar-α sequence that spans a decade in metallicity and a high-α sequence that merges with the low-α sequence at super-solar [Fe/H].The location of the high-α sequence is nearly constant across the disk, however there are very few high-α stars at R > 11 kpc. The peak of the midplane MDF shifts to lower metallicity at larger R, reflecting the Galactic metallicity gradient. Most strikingly, the shape of the midplane MDF changes systematically with radius, with a negatively skewed distribution at 3 < R < 7 kpc, to a roughly Gaussian distribution at the solar annulus, to a positively skewed shape in the outer Galaxy. For stars with |z| > 1 kpc or [α/Fe] > 0.18, the MDF shows little dependence on R. The positive skewness of the outer disk MDF may be a signature of radial migration; we show that blurring of stellar populations by orbital eccentricities is not enough to explain the reversal of MDF shape but a simple model of radial migration can do so.