100 research outputs found
The Fifth Data Release of the Sloan Digital Sky Survey
This paper describes the Fifth Data Release (DR5) of the Sloan Digital Sky Survey (SDSS). DR5 includes all survey quality data taken through 2005 June and represents the completion of the SDSS-I project (whose successor, SDSS-II, will continue through mid-2008). It includes five-band photometric data for 217 million objects selected over 8000 deg^2 and 1,048,960 spectra of galaxies, quasars, and stars selected from 5713 deg^2 of that imaging data. These numbers represent a roughly 20% increment over those of the Fourth Data Release; all the data from previous data releases are included in the present release. In addition to "standard" SDSS observations, DR5 includes repeat scans of the southern equatorial stripe, imaging scans across M31 and the core of the Perseus Cluster of galaxies, and the first spectroscopic data from SEGUE, a survey to explore the kinematics and chemical evolution of the Galaxy. The catalog database incorporates several new features, including photometric redshifts of galaxies, tables of matched objects in overlap regions of the imaging survey, and tools that allow precise computations of survey geometry for statistical investigations
A Photometric Metallicity Estimate of the Virgo Stellar Overdensity
We determine photometric metal abundance estimates for individual
main-sequence stars in the Virgo Overdensity (VOD), which covers almost 1000
deg^2 on the sky, based on a calibration of the metallicity sensitivity of
stellar isochrones in the gri filter passbands using field stars with
well-determined spectroscopic metal abundances. Despite the low precision of
the method for individual stars, we derive [Fe/H] = -2.0 +/-0.1 (internal)
+/-0.5 (systematic) for the metal abundance of the VOD from photometric
measurements of 0.7 million stars in the Northern Galactic hemisphere with
heliocentric distances from ~10 kpc to ~20 kpc. The metallicity of the VOD is
indistinguishable, within Delta [Fe/H] < 0.2, from that of field halo stars
covering the same distance range. This initial application suggests that the
SDSS gri passbands can be used to probe the properties of main-sequence stars
beyond ~10 kpc, complementing studies of nearby stars from more
metallicity-sensitive color indices that involve the u passband.Comment: 5 pages, 3 figures, Accepted for publication in ApJ Letter
SEGUE-2 Limits on Metal-Rich Old-Population Hypervelocity Stars In the Galactic Halo
We present new limits on the ejection of metal-rich old-population
hypervelocity stars from the Galactic center (GC) as probed by the SEGUE-2
survey. Our limits are a factor of 3-10 more stringent than previously
reported, depending on stellar type. Compared to the known population of B-star
ejectees, there can be no more than 30 times more metal-rich old-population F/G
stars ejected from the GC. Because B stars comprise a tiny fraction of a normal
stellar population, this places significant limits on a combination of the GC
mass function and the ejection mechanism for hypervelocity stars. In the
presence of a normal GC mass function, our results require an ejection
mechanism that is about 5.5 times more efficient at ejecting B-stars compared
to low-mass F/G stars.Comment: 18 pages including 5 figures; Submitted to Ap
The SEGUE Stellar Parameter Pipeline. IV. Validation with an Extended Sample of Galactic Globular and Open Clusters
Spectroscopic and photometric data for likely member stars of five Galactic
globular clusters (M3, M53, M71, M92, and NGC 5053) and three open clusters
(M35, NGC 2158, and NGC 6791) are processed by the current version of the SEGUE
Stellar Parameter Pipeline (SSPP), in order to determine estimates of
metallicities and radial velocities for the clusters. These results are then
compared to values from the literature. We find that the mean metallicity
() and mean radial velocity () estimates for each cluster are
almost all within 2{\sigma} of the adopted literature values; most are within
1{\sigma}. We also demonstrate that the new version of the SSPP achieves small,
but noteworthy, improvements in estimates at the extrema of the
cluster metallicity range, as compared to a previous version of the pipeline
software. These results provide additional confidence in the application of the
SSPP for studies of the abundances and kinematics of stellar populations in the
Galaxy.Comment: 98 pages, 31 figures; accepted for publication in A
Automatic log analysis with NLP for the CMS workflow handling
The central Monte-Carlo production of the CMS experiment utilizes the WLCG infrastructure and manages daily thousands of tasks, each up to thousands of jobs. The distributed computing system is bound to sustain a certain rate of failures of various types, which are currently handled by computing operators a posteriori. Within the context of computing operations, and operation intelligence, we propose a Machine Learning technique to learn from the operators with a view to reduce the operational workload and delays. This work is in continuation of CMS work on operation intelligence to try and reach accurate predictions with Machine Learning. We present an approach to consider the log files of the workflows as regular text to leverage modern techniques from Natural Language Processing (NLP). In general, log files contain a substantial amount of text that is not human language. Therefore, different log parsing approaches are studied in order to map the log files’ words to high dimensional vectors. These vectors are then exploited as feature space to train a model that predicts the action that the operator has to take. This approach has the advantage that the information of the log files is extracted automatically and the format of the logs can be arbitrary. In this work the performance of the log file analysis with NLP is presented and compared to previous approaches
The SEGUE Stellar Parameter Pipeline. V. Estimation of Alpha-Element Abundance Ratios From Low-Resolution SDSS/SEGUE Stellar Spectra
We present a method for the determination of [alpha/Fe] ratios from
low-resolution (R = 2000) SDSS/SEGUE stellar spectra. By means of a
star-by-star comparison with degraded spectra from the ELODIE spectral library
and with a set of moderately high-resolution (R = 15,000) and medium-resolution
(R = 6000) spectra of SDSS/SEGUE stars, we demonstrate that we are able to
measure [alpha/Fe] from SDSS/SEGUE spectra (with S/N > 20/1) to a precision of
better than 0.1 dex, for stars with atmospheric parameters in the range Teff =
[4500, 7000] K, log g = [1.5, 5.0], and [Fe/H] = [-1.4, +0.3], over the range
[alpha/Fe] = [-0.1, +0.6]. For stars with [Fe/H] < -1.4, our method requires
spectra with slightly higher signal-to-noise to achieve this precision (S/N >
25/1). Over the full temperature range considered, the lowest metallicity star
for which a confident estimate of [alpha/Fe] can be obtained from our approach
is [Fe/H] ~ -2.5; preliminary tests indicate that a metallicity limit as low as
[Fe/H] ~ -3.0 may apply to cooler stars. As a further validation of this
approach, weighted averages of [alpha/Fe] obtained for SEGUE spectra of likely
member stars of Galactic globular clusters (M15, M13, and M71) and open
clusters (NGC 2420, M67, and NGC 6791) exhibit good agreement with the values
of [alpha/Fe] from previous studies. The results of the comparison with NGC
6791 imply that the metallicity range for the method may extend to ~ +0.5.Comment: 47 pages, 11 figures, 5 tables, to appear in A
The SEGUE Stellar Parameter Pipeline. II. Validation with Galactic Globular and Open Clusters
We validate the performance and accuracy of the current SEGUE (Sloan
Extension for Galactic Understanding and Exploration) Stellar Parameter
Pipeline (SSPP), which determines stellar atmospheric parameters (effective
temperature, surface gravity, and metallicity) by comparing derived overall
metallicities and radial velocities from selected likely members of three
globular clusters (M 13, M 15, and M 2) and two open clusters (NGC 2420 and M
67) to the literature values. Spectroscopic and photometric data obtained
during the course of the original Sloan Digital Sky Survey (SDSS-I) and its
first extension (SDSS-II/SEGUE) are used to determine stellar radial velocities
and atmospheric parameter estimates for stars in these clusters. Based on the
scatter in the metallicities derived for the members of each cluster, we
quantify the typical uncertainty of the SSPP values, sigma([Fe/H]) = 0.13 dex
for stars in the range of 4500 K < Teff < 7500 K and 2.0 < log g < 5.0, at
least over the metallicity interval spanned by the clusters studied (-2.3 <
[Fe/H] < 0). The surface gravities and effective temperatures derived by the
SSPP are also compared with those estimated from the comparison of the
color-magnitude diagrams with stellar evolution models; we find satisfactory
agreement. At present, the SSPP underestimates [Fe/H] for
near-solar-metallicity stars, represented by members of M 67 in this study, by
about 0.3 dex.Comment: 56 pages, 8 Tables, 15 figures, submitted to the Astronomical Journa
The fifth data release of the sloan digital sky survey
Astrophysical Journal Supplement Series, 172(2): pp. 634-644.This paper describes the Fifth Data Release (DR5) of the Sloan Digital Sky Survey (SDSS). DR5 includes all survey
quality data taken through 2005 June and represents the completion of the SDSS-I project (whose successor, SDSS-II,
will continue through mid-2008). It includes five-band photometric data for 217 million objects selected over 8000 deg2
and 1,048,960 spectra of galaxies, quasars, and stars selected from 5713 deg2 of that imaging data. These numbers
represent a roughly 20% increment over those of the Fourth Data Release; all the data from previous data releases are
included in the present release. In addition to ‘‘standard’’ SDSS observations,DR5 includes repeat scans of the southern
equatorial stripe, imaging scans across M31 and the core of the Perseus Cluster of galaxies, and the first spectroscopic
data from SEGUE, a survey to explore the kinematics and chemical evolution of the Galaxy. The catalog database
incorporates several new features, including photometric redshifts of galaxies, tables of matched objects in overlap
regions of the imaging survey, and tools that allowprecise computations of survey geometry for statistical investigations
Galactic Globular and Open Clusters in the Sloan Digital Sky Survey. II. Test of Theoretical Stellar Isochrones
We perform an extensive test of theoretical stellar models for main-sequence
stars in ugriz, using cluster fiducial sequences obtained in the previous paper
of this series. We generate a set of isochrones using the Yale Rotating
Evolutionary Code (YREC) with updated input physics, and derive magnitudes and
colors in ugriz from MARCS model atmospheres. These models match cluster main
sequences over a wide range of metallicity within the errors of the adopted
cluster parameters. However, we find a large discrepancy of model colors at the
lower main sequence (Teff < ~4500 K) for clusters at and above solar
metallicity. We also reach similar conclusions using the theoretical isochrones
of Girardi et al. and Dotter et al., but our new models are generally in better
agreement with the data. Using our theoretical isochrones, we also derive
main-sequence fitting distances and turn-off ages for five key globular
clusters, and demonstrate the ability to derive these quantities from
photometric data in the Sloan Digital Sky Survey. In particular, we exploit
multiple color indices (g - r, g - i, and g - z) in the parameter estimation,
which allows us to evaluate internal systematic errors. Our distance estimates,
with an error of sigma(m - M) = 0.03-0.11 mag for individual clusters, are
consistent with Hipparcos-based subdwarf fitting distances derived in the
Johnson-Cousins or Stromgren photometric systems.Comment: 26 pages, 28 figures. Accepted for publication in ApJ. Version with
high resolution figures available at
http://spider.ipac.caltech.edu/~deokkeun/sdss_iso.pd
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