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THE FRACTIONS OF INNER- AND OUTER-HALO STARS IN THE LOCAL VOLUME
An, Deokkeun,Beers, Timothy C.,Santucci, Rafael M.,Carollo, Daniela,Placco, Vinicius M.,Lee, Young Sun,Rossi, Silvia IOP Publishing 2015 ASTROPHYSICAL JOURNAL LETTERS - Vol.813 No.2
<P>We obtain a new determination of the metallicity distribution function (MDF) of stars within similar to 5-10 kpc of the Sun, based on recently improved co-adds of ugriz photometry for Stripe. 82 from the Sloan Digital Sky Survey. Our new estimate uses the methodology developed previously by An et al. to study in situ halo stars, but is based on a factor of two larger sample than available before, with much-improved photometric errors and zero-points. The newly obtained MDF can be divided into multiple populations of halo stars, with peak metallicities at [Fe/H] approximate to 1.4 and 1.9, which we associate with the inner-halo and outer-halo populations of the Milky Way, respectively. We find that the kinematics of these stars (based on proper-motion measurements at high Galactic latitude) supports the proposed dichotomy of the halo, as stars with retrograde motions in the rest frame of the Galaxy are generally more metal-poor than stars with prograde motions, consistent with previous claims. In addition, we generate mock catalogs of stars from a simulated Milk Way halo system, and demonstrate for the first time that the chemically and kinematically distinct properties of the inner-and outer-halo populations are qualitatively in agreement with our observations. The decomposition of the observed MDF and our comparison with the mock catalog results suggest that the outer-halo population contributes on the order of similar to 35%-55% of halo stars in the local volume.</P>
THE STELLAR METALLICITY DISTRIBUTION FUNCTION OF THE GALACTIC HALO FROM SDSS PHOTOMETRY
An, Deokkeun,Beers, Timothy C.,Johnson, Jennifer A.,Pinsonneault, Marc H.,Lee, Young Sun,Bovy, Jo,Ivezić,, Ž,eljko,Carollo, Daniela,Newby, Matthew IOP Publishing 2013 The Astrophysical journal Vol.763 No.1
<P>We explore the stellar metallicity distribution function of the Galactic halo based on SDSS ugriz photometry. A set of stellar isochrones is calibrated using observations of several star clusters and validated by comparisons with medium-resolution spectroscopic values over a wide range of metal abundance. We estimate distances and metallicities for individual main-sequence stars in the multiply scanned SDSS Stripe 82, at heliocentric distances in the range 5-8 kpc and |b| > 35 degrees, and find that the in situ photometric metallicity distribution has a shape that matches that of the kinematically selected local halo stars from Ryan & Norris. We also examine independent kinematic information from proper-motion measurements for high Galactic latitude stars in our sample. We find that stars with retrograde rotation in the rest frame of the Galaxy are generally more metal poor than those exhibiting prograde rotation, which is consistent with earlier arguments by Carollo et al. that the halo system comprises at least two spatially overlapping components with differing metallicity, kinematics, and spatial distributions. The observed photometric metallicity distribution and that of Ryan & Norris can be described by a simple chemical evolution model by Hartwick (or by a single Gaussian distribution); however, the suggestive metallicity-kinematic correlation contradicts the basic assumption in this model that the Milky Way halo consists primarily of a single stellar population. When the observed metallicity distribution is deconvolved using two Gaussian components with peaks at [Fe/H] approximate to -1.7 and -2.3, the metal-poor component accounts for approximate to 20%-35% of the entire halo population in this distance range.</P>
An, Deokkeun,Terndrup, Donald M.,Pinsonneault, Marc H.,Lee, Jae-Woo IOP Publishing 2015 The Astrophysical journal Vol.811 No.1
<P>We extend our effort to calibrate stellar isochrones in the Johnson-Cousins (BVIC) and the 2MASS (JHK(s)) filter systems based on observations of well-studied open clusters. Using cool main-sequence (MS) stars in Praesepe, we define empirical corrections to the Lejeune et al. color-effective temperature (T-eff) relations down to T-eff similar to 3600 K, complementing our previous work based on the Hyades and the Pleiades. We apply empirically corrected isochrones to existing optical and near-infrared photometry of cool (T-eff less than or similar to 5500 K) and metal-rich ([Fe/H] = +0.37) MS stars in NGC 6791. The current methodology relies on an assumption that color-T-eff corrections are independent of metallicity, but we find that estimates of color excess and distance from color-magnitude diagrams with different color indices converge on each other at the precisely known metallicity of the cluster. Along with a satisfactory agreement with eclipsing binary data in the cluster, we view the improved internal consistency as a validation of our calibrated isochrones at super-solar metallicities. For very cool stars (T-eff less than or similar to 4800 K), however, we find that B - V colors of our models are systematically redder than the cluster photometry by similar to 0.02 mag. We use color-T-eff transformations from the infrared flux method and alternative photometry to examine a potential color-scale error in the input cluster photometry. After excluding B - V photometry of these cool MS stars, we derive E(B - V) = 0.105 +/- 0.014, [M/H] = +0.42 +/- 0.07, (m - M)(0) = 13.04 +/- 0.08, and the age of 9.5 +/- 0.3 Gyr for NGC 6791.</P>