arXiv subjects

T. C. Beers

Publications and source records attributed to T. C. Beers.

At least 91 records · Page 5Linked to original sources

arXiv2007

The SEGUE Stellar Parameter Pipeline. III. Comparison with High-Resolution Spectroscopy of SDSS/SEGUE Field Stars

We report high-resolution spectroscopy of 125 field stars previously observed as part of the Sloan Digital Sky Survey and its program for Galactic studies, the Sloan Extension for Galactic Understanding and Exploration (SEGUE). These spectra are used to measure radial velocities and to derive atmospheric parameters, which we compare with those reported by the SEGUE Stellar Parameter Pipeline (SSPP). The SSPP obtains estimates of these quantities based on SDSS ugriz photometry and low-resolution (R = 2000) spectroscopy. For F- and G-type stars observed with high signal-to-noise ratios (S/N), we empirically determine the typical random uncertainties in the radial velocities, effective temperatures, surface gravities, and metallicities delivered by the SSPP to be 2.4 km/s, 130 K (2.2%), 0.21 dex, and 0.11 dex, respectively, with systematic uncertainties of a similar magnitude in the effective temperatures and metallicities. We estimate random errors for lower S/N spectra based on numerical simulations.

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arXiv2007

The SEGUE Stellar Parameter Pipeline. I. Description and Initial Validation Tests

We describe the development and implementation of the SEGUE (Sloan Extension for Galactic Exploration and Understanding) Stellar Parameter Pipeline (SSPP). The SSPP derives, using multiple techniques, radial velocities and the fundamental stellar atmospheric parameters (effective temperature, surface gravity, and metallicity) for AFGK-type stars, based on medium-resolution spectroscopy and $ugriz$ photometry obtained during the course of the original Sloan Digital Sky Survey (SDSS-I) and its Galactic extension (SDSS-II/SEGUE). The SSPP also provides spectral classification for a much wider range of stars, including stars with temperatures outside of the window where atmospheric parameters can be estimated with the current approaches. This is Paper I in a series of papers on the SSPP; it provides an overview of the SSPP, and initial tests of its performance using multiple data sets. Random and systematic errors are critically examined for the current version of the SSPP, which has been used for the sixth public data release of the SDSS (DR-6).

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arXiv2007

Distances to Galactic high-velocity clouds. Complex C

We report the first determination of a distance bracket for the high-velocity cloud (HVC) complex C. Combined with previous measurements showing that this cloud has a metallicity of 0.15 times solar, these results provide ample evidence that complex C traces the continuing accretion of intergalactic gas falling onto the Milky Way. Accounting for both neutral and ionized hydrogen as well as He, the distance bracket implies a mass of 3-14x10^6 M_sun, and the complex represents a mass inflow of 0.1-0.25 M_sun/yr. We base our distance bracket on the detection of CaII absorption in the spectrum of the blue horizontal branch star SDSS J120404.78+623345.6, in combination with a significant non-detection toward the BHB star BS 16034-0114. These results set a strong distance bracket of 3.7-11.2 kpc on the distance to complex C. A more weakly supported lower limit of 6.7 kpc may be derived from the spectrum of the BHB star BS 16079-0017.

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arXiv2007

First Stars VIII -- Enrichment of the neutron-capture elements in the early Galaxy

Our aim is to measure accurate, homogeneous neutron-capture element abundances for the sample of 32 EMP giant stars studied earlier in this series, including 22 stars with [Fe/H] $< -$3.0. Based on high-resolution, high S/N spectra from the ESO VLT/UVES, 1D, LTE model atmospheres, and synthetic spectrum fits, we determine abundances or upper limits for the 16 elements Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, and Yb in all stars. As found earlier, [Sr/Fe], [Y/Fe], [Zr/Fe] and [Ba/Fe] are below Solar in the EMP stars, with very large scatter. However, we find a tight anti-correlation of [Sr/Ba], [Y/Ba], and [Zr/Ba] with [Ba/H] for $-4.5 <$ [Ba/H] $< -2.5$, also when subtracting the contribution of the main $r$-process as measured by [Ba/H]. The huge, well-characterised scatter of the [n-capture/Fe] ratios in our EMP stars is in stark contrast to the negligible dispersion in the [$α$/Fe] and [Fe-peak/Fe] ratios for the same stars found in Paper V. These results demonstrate that a second (``weak'' or LEPP) $r$-process dominates the production of the lighter neutron-capture elements for [Ba/H] $< -2.5$. The combination of very consistent [$α$/Fe] and erratic [n-capture/Fe] ratios indicates that inhomogeneous models for the early evolution of the halo are needed. Our accurate data provide strong constraints on future models of the production and mixing of the heavy elements in the early Galaxy.

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arXiv2007

Distances to Galactic high-velocity clouds. I. Cohen Stream, complex GCP, cloud g1

The high- and intermediate-velocity interstellar clouds (HVCs/IVCs) are tracers of energetic processes in and around the Milky Way. Clouds with near-solar metallicity about one kpc above the disk trace the circulation of material between disk and halo (the Galactic Fountain). The Magellanic Stream consists of gas tidally extracted from the SMC, tracing the dark matter potential of the Milky Way. Several other HVCs have low-metallicity and appear to trace the continuing accretion of infalling intergalactic gas. These assertions are supported by the metallicities (0.1 to 1 solar) measured for about ten clouds in the past decade. Direct measurements of distances to HVCs have remained elusive, however. In this paper we present four new distance brackets, using VLT observations of interstellar \CaII H and K absorption toward distant Galactic halo stars. We derive distance brackets of 5.0 to 11.7 kpc for the Cohen Stream (likely to be an infalling low-metallicity cloud), 9.8 to 15.1 kpc for complex GCP (also known as the Smith Cloud or HVC40-15+100 and with still unknown origin), 1.0 to 2.7 kpc for an IVC that appears associated with the return flow of the Fountain in the Perseus Arm, and 1.8 to 3.8 kpc for cloud g1, which appears to be in the outflow phase of the Fountain. Our measurements further demonstrate that the Milky Way is accreting substantial amounts of gaseous material, which influences the Galaxy's current and future dynamical and chemical evolution.

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arXiv2007

Fluorine in a Carbon-Enhanced Metal-Poor Star

The fluorine abundance of the Carbon-Enhanced Metal-Poor (CEMP) star HE 1305+0132 has been derived by analysis of the molecular HF (1-0) R9 line at 2.3357 microns in a high-resolution (R = 50,000) spectrum obtained with the Phoenix spectrometer and Gemini-South telescope. Our abundance analysis makes use of a CNO-enhanced ATLAS12 model atmosphere characterized by a metallicity and CNO enhancements determined utilizing medium-resolution (R = 3,000) optical and near-IR spectra. The effective iron abundance is found to be [Fe/H] = -2.5, making HE 1305+0132 the most Fe-deficient star, by more than an order of magnitude, for which the abundance of fluorine has been measured. Using spectral synthesis, we derive a super-solar fluorine abundance of A(19F) = 4.96 +/- 0.21, corresponding to a relative abundance of [F/Fe] = 2.90. A single line of the Phillips C_2 system is identified in our Phoenix spectrum, and along with multiple lines of the first-overtone vibration-rotation CO (3-1) band head, C and O abundances of A(12C) = 8.57 +/- 0.11 and A(16O) = 7.04 +/- 0.14 are derived. We consider the striking fluorine overabundance in the framework of the nucleosynthetic processes thought to be responsible for the C-enhancement of CEMP stars and conclude that the atmosphere of HE 1305+0132 was polluted via mass transfer by a primary companion during its asymptotic giant branch phase. This is the first study of fluorine in a CEMP star, and it demonstrates that this rare nuclide can be a key diagnostic of nucleosynthetic processes in the early Galaxy.

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arXiv2007

Nucleosynthesis in the Early Galaxy

Recent observations of r-process-enriched metal-poor star abundances reveal a non-uniform abundance pattern for elements $Z\leq47$. Based on non-correlation trends between elemental abundances as a function of Eu-richness in a large sample of metal-poor stars, it is shown that the mixing of a consistent and robust light element primary process (LEPP) and the r-process pattern found in r-II metal-poor stars explains such apparent non-uniformity. Furthermore, we derive the abundance pattern of the LEPP from observation and show that it is consistent with a missing component in the solar abundances when using a recent s-process model. As the astrophysical site of the LEPP is not known, we explore the possibility of a neutron capture process within a site-independent approach. It is suggested that scenarios with neutron densities $n_{n}\leq10^{13}$ $cm^{-3}$ or in the range $n_{n}\geq10^{24}$ $cm^{-3}$ best explain the observations.

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arXiv2007

Estimation of stellar atmospheric parameters from SDSS/SEGUE spectra

We present techniques for the estimation of stellar atmospheric parameters (Teff,logg,[Fe/H]) for stars from the SDSS/SEGUE survey. The atmospheric parameters are derived from the observed medium-resolution (R=2000) stellar spectra using non-linear regression models trained either on (1) pre-classified observed data or (2) synthetic stellar spectra. In the first case we use our models to automate and generalize parametrization produced by a preliminary version of the SDSS/SEGUE Spectroscopic Parameter Pipeline (SSPP). In the second case we directly model the mapping between synthetic spectra (derived from Kurucz model atmospheres) and the atmospheric parameters, independently of any intermediate estimates. After training, we apply our models to various samples of SDSS spectra to derive atmospheric parameters, and compare our results with those obtained previously by the SSPP for the same samples. We obtain consistency between the two approaches, with RMS deviations of 150K in Teff, 0.35dex in logg, and 0.22dex in [Fe/H]. The models are applied to pre-processed spectra, either via Principal Components Analysis or a Wavelength Range Selection method, which employs a subset of the full 3850-9000A spectral range. This is both for computational reasons, and because it delivers higher accuracy. From an analysis of cluster candidates with available SDSS spectroscopy (M15, M13, M2, and NGC2420), we find evidence for small systematic offsets in Teff and/or logg for the atmospheric parameter estimates from the model trained on real data with the SSPP. Thus, this model turns out to derive more precise, but less accurate, atmospheric parameters than the model trained on synthetic data.

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arXiv2007

The Hercules-Aquila Cloud

We present evidence for a substantial overdensity of stars in the direction of the constellations of Hercules and Aquila. The Cloud is centered at a Galactic longitude of about 40 degrees and extends above and below the Galactic plane by at least 50 degrees. Given its off-centeredness and height, it is unlikely that the Hercules-Aquila Cloud is related to the bulge or thick disk. More likely, this is a new structural component of the Galaxy that passes through the disk. The Cloud stretches about 80 degrees in longitude. Its heliocentric distance lies between 10 and 20 kpc so that the extent of the Cloud in projection is roughly 20 kpc by 15 kpc. It has an absolute magnitude of -13 and its stellar population appears to be comparable to, but somewhat more metal-rich than, M92.

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arXiv2007

Discovery of an Unusual Dwarf Galaxy in the Outskirts of the Milky Way

In this Letter, we announce the discovery of a new dwarf galaxy, Leo T, in the Local Group. It was found as a stellar overdensity in the Sloan Digital Sky Survey Data Release 5 (SDSS DR5). The color-magnitude diagram of Leo T shows two well-defined features, which we interpret as a red giant branch and a sequence of young, massive stars. As judged from fits to the color-magnitude diagram, it lies at a distance of about 420 kpc and has an intermediate-age stellar population with a metallicity of [Fe/H]= -1.6, together with a young population of blue stars of age of 200 Myr. There is a compact cloud of neutral hydrogen with mass roughly 10^5 solar masses and radial velocity 35 km/s coincident with the object visible in the HIPASS channel maps. Leo T is the smallest, lowest luminosity galaxy found to date with recent star-formation. It appears to be a transition object similar to, but much lower luminosity than, the Phoenix dwarf.

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arXiv2006

An Orphan in the "Field of Streams"

We use Sloan Digital Sky Survey Data Release 5 photometry and spectroscopy to study a tidal stream that extends over 50 degrees in the North Galactic Cap. From the analysis of the path of the stream and the colors and magnitudes of its stars, the stream is about 20 kpc away at its nearest detection (the celestial equator). We detect a distance gradient -- the stream is farther away from us at higher declination. The contents of the stream are made up from a predominantly old and metal-poor population that is similar to the globular clusters M13 and M92. The integrated absolute magnitude of the stream stars is estimated to be M_r = -7.5. There istentative evidence for a velocity signature, with the stream moving at -40 km/s at low declinations and +100 km/s at high declinations. The stream lies on the same great circle as Complex A, a roughly linear association of HI high velocity clouds stretching over 30 degrees on the sky, and as Ursa Major II, a recently discovered dwarf spheroidal galaxy. Lying close to the same great circle are a number of anomalous, young and metal-poor globular clusters, including Palomar 1 and Ruprecht 106.

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arXiv2006

First stars VII. Lithium in extremely metal poor dwarfs

Aims. This study aims to determine the level and constancy of the Spite plateau as definitively as possible from homogeneous high-quality VLT-UVES spectra of 19 of the most metal-poor dwarf stars known. Methods. Our high-resolution (R ~ 43000), high S/N spectra are analysed with OSMARCS 1D LTE model atmospheres and turbospectrum synthetic spectra to determine effective temperatures, surface gravities, and metallicities, as well as Li abundances for our stars. Results. Eliminating a cool subgiant and a spectroscopic binary, we find 8 stars to have -3.5 < [Fe/H] < -3.0 and 9 stars with -3.0 < [Fe/H] < -2.5. Our best value for the mean level of the plateau is A(Li) =2.10 +- 0.09. The scatter around the mean is entirely explained by our estimate of the observational error and does not allow for any intrinsic scatter in the Li abundances. In addition, we conclude that a systematic error of the order of 200 K in any of the current temperature scales remains possible. The iron excitation equilibria in our stars support our adopted temperature scale, which is based on a fit to wings of the Halpha line, and disfavour hotter scales, which would lead to a higher Li abundance, but fail to achieve excitation equilibrium for iron. Conclusions. We confirm the previously noted discrepancy between the Li abundance measured in extremely metal-poor turnoff stars and the primordial Li abundance predicted by standard Big-Bang nucleosynthesis models adopting the baryonic density inferred from WMAP. We discuss recent work explaining the discrepancy in terms of diffusion and find that uncertain temperature scales remain a major question. (abridged)

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arXiv2006

Cats and Dogs, Hair and A Hero: A Quintet of New Milky Way Companions

We present five new satellites of the Milky Way discovered in Sloan Digital Sky Survey (SDSS) imaging data, four of which were followed-up with either the Subaru or the Isaac Newton Telescopes. They include four probable new dwarf galaxies -- one each in the constellations of Coma Berenices, Canes Venatici, Leo and Hercules -- together with one unusually extended globular cluster, Segue 1. We provide distances, absolute magnitudes, half-light radii and color-magnitude diagrams for all five satellites. The morphological features of the color-magnitude diagrams are generally well described by the ridge line of the old, metal-poor globular cluster M92. In the last two years, a total of ten new Milky Way satellites with effective surface brightness mu_v >~ 28 mag/sq. arcsec have been discovered in SDSS data. They are less luminous, more irregular and appear to be more metal-poor than the previously-known nine Milky Way dwarf spheroidals. The relationship between these objects and other populations is discussed. We note that there is a paucity of objects with half-light radii between ~40 pc and ~ 100 pc. We conjecture that this may represent the division between star clusters and dwarf galaxies.

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arXiv2006

Bright Metal-Poor Stars from the Hamburg/ESO Survey. I. Selection and Follow-up Observations from 329 Fields

We present a sample of 1777 bright (9 1.0) metal-poor ([Fe/H]<-2.0) giants of 9% +- 2%, which is lower than previously reported. However, the frequency raises to similar (>20%) and higher values with increasing distance from the Galactic plane. Although the numbers of stars at low metallicity are falling rapidly at the lowest metallicities, there is evidence that the fraction of carbon-enhanced metal-poor stars is increasing rapidly as a function of declining metallicity. For ~60 objects, high-resolution data have already been obtained; one of these, HE 1327-2326, is the new record holder for the most iron-deficient star known.

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arXiv2006

From First Stars to the Spite Plateau: a Possible Reconciliation of Halo Stars Observations with Predictions from Big Bang Nucleosynthesis

Since the pioneering observations of Spite & Spite in 1982, the constant lithium abundance of metal-poor ([Fe/H]<-1.3) halo stars near the turnoff has been attributed a cosmological origin. Closer analysis revealed that the observed abundance lies at $Δ$ Li~0.4 dex below the predictions of Big Bang Nucleosynthesis. The measurements of deuterium abundances on the lines of sight toward quasars and the recent data from the Wilkinson Microwave Anisotropy Probe independently confirmed this gap. We suggest here that part of the discrepancy is explained by the first generation of stars that efficiently depleted lithium. Assuming that the models for lithium evolution in the halo turnoff stars and $Δ$ Li estimates are correct, we infer that between 1/3 and 1/2 of the baryonic matter of the early halo (~10^9 Mo) was processed through Population III stars. This new paradigm proposes a very economical solution to the lingering difficulty of understanding the properties of the Spite Plateau and its lack of star-to-star scatter down to [Fe/H]=-2.5. It is moreover in agreement both with the absence of lithium detection in the most metal-poor star presently known (HE 1327-2326), and also with new trends of the Plateau suggesting its low metallicity edge may be reached around [Fe/H]=-2.5. We discuss the turbulent mixing associated with enhanced supernovae explosions in the early interstellar medium in this picture. Finally we show that other chemical properties of the extremely metal-poor stars are in agreement with a significant Population III processing in the halo, provided these models include mass-loss and rotationally-induced mixing.

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arXiv2006

First stars X. The nature of three unevolved Carbon-Enhanced Metal-Poor stars

From high-resolution VLT/UVES spectra (R~43,000), we determine abundances or upper limits for Li, C, N, O, and other important elements, as well as 12C/13C isotopic ratio for three Carbon enhanced metal poor stars. All three stars have -3.30 <= [Fe/H] <= -2.85 and moderate to high CNO abundances. CS 22958-042 is one of the most carbon-rich CEMP stars known ([C/Fe] = +3.2), while CS 29528-041 (one of the few N-enhanced metal-poor stars known) is one of the most nitrogen rich ([N/Fe] = +3.0). Oxygen is very high in CS 31080-095 ([O/Fe] = +2.35) and in CS 22958-042 ([O/Fe] = +1.35). All three stars exhibit [Sr/Fe] < 0; Ba is not detected in CS 22958-042 ([Ba/Fe] < -0.53), but it is moderately enhanced ([Ba/Fe] ~ 1) in the other two stars. CS 22958-042 displays one of the largest sodium overabundances yet found in CEMP stars ([Na/Fe] = +2.8). CS 22958-042 has 12C/13C = 9, similar to most other CEMP stars without enhanced neutron-capture elements, while 12C/13C <= 40 in CS 31080-095. CS 31080-095 and CS 29528-041 have A(Li) ~ 1.7, below the Spite Plateau, while Li is not detected in CS 22958-042. CS 22958-042 is a CEMP-no star, but the other two stars are in no known class of CEMP star and thus either constitute a new class or are a link between the CEMP-no and CEMP-s classes, adding complexity to the abundance patterns for CEMP stars. We interpret the abundance patterns in our stars to imply that current models for the presumed AGB binary progenitors lack an extra-mixing process, similar to those apparently operating in RGB stars.

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arXiv2006

A Curious New Milky Way Satellite in Ursa Major

In this Letter, we study a localized stellar overdensity in the constellation of Ursa Major, first identified in Sloan Digital Sky Survey (SDSS) data and subsequently followed up with Subaru imaging. Its color-magnitude diagram (CMD) shows a well-defined sub-giant branch, main sequence and turn-off, from which we estimate a distance of ~30 kpc and a projected size of ~250 x 125 pc. The CMD suggests a composite population with some range in metallicity and/or age. Based on its extent and stellar population, we argue that this is a previously unknown satellite galaxy of the Milky Way, hereby named Ursa Major II (UMa II) after its constellation. Using SDSS data, we find an absolute magnitude of M_V \~ -3.8, which would make it the faintest known satellite galaxy. UMa II's isophotes are irregular and distorted with evidence for multiple concentrations; this suggests that the satellite is in the process of disruption.

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arXiv2006

A Faint New Milky Way Satellite in Bootes

In this Letter, we announce the discovery of a new satellite of the Milky Way in the constellation of Bootes at a distance of 60 kpc. It was found in a systematic search for stellar overdensities in the North Galactic Cap using Sloan Digital Sky Survey Data Release 5 (SDSS DR5). The color-magnitude diagram shows a well-defined turn-off, red giant branch, and extended horizontal branch. Its absolute magnitude is -5.8, which makes it one of the faintest galaxies known. The half-light radius is 220 pc. The isodensity contours are elongated and have an irregular shape, suggesting that Boo may be a disrupted dwarf spheroidal galaxy.

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