BRIGHT METAL-POOR STARS FROM THE HAMBURG/ESO SURVEY. II. A CHEMODYNAMICAL ANALYSIS

Beers, Timothy C.,Placco, Vinicius M.,Carollo, Daniela,Rossi, Silvia,Lee, Young Sun,Frebel, Anna,Norris, John E.,Dietz, Sarah,Masseron, Thomas American Astronomical Society 2017 The Astrophysical journal Vol.835 No.1

<P>We obtain estimates of stellar atmospheric parameters for a previously published sample of 1777 relatively bright (9 < B < 14) metal-poor candidates from the Hamburg/ESO Survey. The original Frebel et al. analysis of these stars was able to derive estimates of [Fe/H] and [C/Fe] only for a subset of the sample, due to limitations in the methodology then available. A new spectroscopic analysis pipeline has been used to obtain estimates of T-eff, log g, [Fe/H], and [C/Fe] for almost the entire data set. This sample is very local-about 90% of the stars are located within 0.5 kpc of the Sun. We consider the chemodynamical properties of these stars in concert with a similarly local sample of stars from a recent analysis of the Bidelman and MacConnell 'weak metal' candidates by Beers et al. We use this combined sample to identify possible members of the halo stream of stars suggested by Helmi et al. and Chiba & Beers, as well as stars that may be associated with stripped debris from the putative parent dwarf of the globular cluster Omega Centauri, suggested to exist by previous authors. We identify a clear increase in the cumulative frequency of carbon-enhanced metal-poor (CEMP) stars with declining metallicity, as well as an increase in the fraction of CEMP stars with distance from the Galactic plane, consistent with previous results. We also identify a relatively large number of CEMP stars with kinematics consistent with the metal-weak thick-disk population, with possible implications for its origin.</P>

The Hamburg/ESO R-process Enhanced Star survey (HERES): Overview of the project and new strongly r-process enhanced stars

Christlieb, N.,Beers, T.C.,Barklem, P.S.,Bessell, M.S.,Hill, V.,Holmberg, J.,Korn, A.J.,Marsteller, B.,Mashonkina, L.,Rossi, S. Kluwer Academic Publishers 2005 Symposium Vol.228 No.-

Galactic Archeology with the AEGIS Survey: The Evolution of Carbon and Iron in the Galactic Halo

Yoon, Jinmi,Beers, Timothy C.,Dietz, Sarah,Lee, Young Sun,Placco, Vinicius M.,Costa, Gary Da,Keller, Stefan,Owen, Christopher I.,Sharma, Mahavir American Astronomical Society 2018 The Astrophysical journal Vol.861 No.2

<P>Understanding the evolution of carbon and iron in the Milky Way's halo is of importance because these two elements play crucial roles in constraining star formation, Galactic assembly, and chemical evolution in the early universe. Here we explore the spatial distributions of the carbonicity, [C/Fe], and metallicity, [Fe/H], of the halo system based on medium-resolution (R similar to 1300)spectroscopy of similar to 58,000 stars in the southern hemisphere from the AAOmega Evolution of Galactic Structure (AEGIS) survey. The AEGIS carbonicity map exhibits a positive gradient with distance, as similarly found for the Sloan Digital Sky Survey carbonicity map of Lee et al. The metallicity map confirms that [Fe/H] decreases with distance from the inner halo to the outer halo. We also explore the formation and chemical evolution history of the halo by considering the populations of carbon-enhanced metalpoor (CEMP) stars present in the AEGIS sample. The cumulative and differential frequency of CEMP-no stars (as classified by their characteristically lower levels of absolute carbon abundance, A(C) <= 7.1, for subgiants and giants) increases with decreasing metallicity and is substantially higher than previous determinations for CEMP stars as a whole. In contrast, that of CEMP-s stars (with higher A(C)) remains almost flat, at a value of similar to 10% in the range -4.0 less than or similar to [Fe/H] less than or similar to -2.0. The distinctly different behaviors of the CEMP-no and CEMP-s stars relieve the tension with population synthesis models assuming a binary mass-transfer origin, which previously struggled to account for the higher reported frequencies of CEMP stars, taken as a whole, at low metallicity.</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>

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>

[Erratum] Diverse Chemo-Dynamical Properties of Nitrogen-Rich Stars Identified from Low-Resolution Spectra (2023, JKAS, 56, 59)

김창민,이영선,Timothy C. Beers,김영광 한국천문학회 2024 Journal of The Korean Astronomical Society Vol.57 No.1

The following sentence was erroneously dropped from the acknowledgment: This work was supported by Chungnam National University. The corrected text is listed below; it should replace the Acknowledgements section.

Diverse Chemo-Dynamical Properties of Nitrogen-Rich Stars Identified from Low-Resolution Spectra

김창민,이영선,Timothy C. Beers,김영광 한국천문학회 2023 Journal of The Korean Astronomical Society Vol.56 No.1

The second generation of stars in the globular clusters (GCs) of the Milky Way (MW) exhibit unusually high N, Na, or Al, compared to typical Galactic halo stars at similar metallicities. The halo field stars enhanced with such elements are believed to have originated in disrupted GCs or escaped from existing GCs. We identify such stars in the metallicity range -3.0 < [Fe/H] < 0.0 from a sample of ~36,800 giant stars observed in the Sloan Digital Sky Survey and Large Sky Area Multi-Object Fiber Spectroscopic Telescope survey, and present their dynamical properties. The N-rich population (NRP) and N-normal population (NNP) among our giant sample do not exhibit similarities in either in their metallicity distribution function (MDF) or dynamical properties. We find that, even though the MDF of the NRP looks similar to that of the MW's GCs in the range of [Fe/H] < -1.0, our analysis of the dynamical properties does not indicate similarities between them in the same metallicity range, implying that the escaped members from existing GCs may account for a small fraction of our N-rich stars, or the orbits of the present GCs have been altered by the dynamical friction of the MW. We also find a significant increase in the fraction of N-rich stars in the halo field in the very metal-poor (VMP; [Fe/H] < -2.0) regime, comprising up to ~20% of the fraction of the N-rich stars below [Fe/H] = -2.5, hinting that partially or fully destroyed VMP GCs may have in some degree contributed to the Galactic halo. A more detailed dynamical analysis of the NRP reveals that our sample of N-rich stars do not share a single common origin. Although a substantial fraction of the N-rich stars seem to originate from the GCs formed in situ, more than 60% of them are not associated with those of typical Galactic populations, but probably have extragalactic origins associated with Gaia Sausage/Enceladus, Sequoia, and Sagittarius dwarf galaxies, as well as with presently unrecognized progenitors.

Chemical Cartography. I. A Carbonicity Map of the Galactic Halo

Lee, Young Sun,Beers, Timothy C.,Kim, Young Kwang,Placco, Vinicius,Yoon, Jinmi,Carollo, Daniela,Masseron, Thomas,Jung, Jaehun American Astronomical Society 2017 The Astrophysical journal Vol.836 No.1

<P>We present the first map of carbonicity, [C/Fe], for the halo system of the Milky Way, based on a sample of over 100,000 main-sequence turnoff stars with available spectroscopy from the Sloan Digital Sky Survey. This map, which explores distances up to 15 kpc from the Sun, reveals clear evidence for the dual nature of the Galactic halo, based on the spatial distribution of stellar carbonicity. The metallicity distribution functions of stars in the inner-and outer-halo regions of the carbonicity map reproduce those previously argued to arise from contributions of the inner-and outer-halo populations, with peaks at [Fe/H] = -1.5 and -2.2, respectively. From consideration of the absolute carbon abundances for our sample, A(C), we also confirm that the carbon-enhanced metal-poor (CEMP) stars in the outer- halo region exhibit a higher frequency of CEMP-no stars (those with no overabundances of heavy neutron-capture elements) than of CEMP-s stars (those with strong overabundances of elements associated with the s-process), whereas the stars in the inner-halo region exhibit a higher frequency of CEMP-s stars. We argue that the contrast in the behavior of the CEMP-no and CEMP-s fractions in these regions arises from differences in the mass distributions of the mini-halos from which the stars of the inner- and outer-halo populations formed, which gives rise in turn to the observed dichotomy of the Galactic halo.</P>

APPLICATION OF THE SEGUE STELLAR PARAMETER PIPELINE TO LAMOST STELLAR SPECTRA

Lee, Young Sun,Beers, Timothy C.,Carlin, Jeffrey L.,Newberg, Heidi J.,Hou, Yonghui,Li, Guangwei,Luo, A.-Li,Wu, Yue,Yang, Ming,Zhang, Haotong,Zhang, Wei,Zhang, Yong IOP Publishing 2015 The Astronomical journal Vol.150 No.6

CHRONOGRAPHY OF THE MILKY WAY’S HALO SYSTEM WITH FIELD BLUE HORIZONTAL-BRANCH STARS

Santucci, Rafael M.,Beers, Timothy C.,Placco, Vinicius M.,Carollo, Daniela,Rossi, Silvia,Lee, Young Sun,Denissenkov, Pavel,Tumlinson, Jason,Tissera, Patricia B. IOP Publishing 2015 ASTROPHYSICAL JOURNAL LETTERS - Vol.813 No.1

<P>In a pioneering effort, Preston et al. reported that the colors of blue horizontal-branch (BHB) stars in the halo of the Galaxy shift with distance, from regions near the Galactic center to about 12 kpc away, and interpreted this as a correlated variation in the ages of halo stars, from older to younger, spanning a range of a few Gyrs. We have applied this approach to a sample of some 4700 spectroscopically confirmed BHB stars selected from the Sloan Digital Sky Survey to produce the first 'chronographic map' of the halo of the Galaxy. We demonstrate that the mean de-reddened g - r color, <(g - r)(0)>, increases outward in the Galaxy from -0.22 to -0.08 (over a color window spanning [-0.3: 0.0]) from regions close to the Galactic center to similar to 40 kpc, independent of the metallicity of the stars. Models of the expected shift in the color of the field BHB stars based on modern stellar evolutionary codes confirm that this color gradient can be associated with an age difference of roughly 2-2.5 Gyr, with the oldest stars concentrated in the central similar to 15 kpc of the Galaxy. Within this central region, the age difference spans a mean color range of about 0.05 mag (similar to 0.8 Gyr). Furthermore, we show that chronographic maps can be used to identify individual substructures, such as the Sagittarius Stream, and overdensities in the direction of Virgo and Monoceros, based on the observed contrast in their mean BHB colors with respect to the foreground/background field population.</P>