2,739 research outputs found
Population Studies. XIII. A New Analysis of the Bidelman-MacConnell "Weak-Metal" Stars - Confirmation of Metal-Poor Stars in the Thick Disk of the Galaxy
A new set of very high signal-to-noise (S/N > 100/1), medium-resolution
(R~3000) optical spectra have been obtained for 302 of the candidate
"weak-metal" stars selected by Bidelman & MacConnell. We use these data to
calibrate the recently developed generalization of the SEGUE Stellar Parameter
Pipeline, and obtain estimates of the atmospheric parameters (Teff, log g , and
[Fe/H]) for these non-SDSS/SEGUE data; we also obtain estimates of [C/Fe]. The
new abundance measurements are shown to be consistent with available
high-resolution spectroscopic determinations, and represent a substantial
improvement over the accuracies obtained from the previous photometric
estimates reported in Paper I of this series. The apparent offset in the
photometric abundances of the giants in this sample noted by several authors is
confirmed by our new spectroscopy; no such effect is found for the dwarfs. The
presence of a metal-weak thick-disk (MWTD) population is clearly supported by
these new abundance data. Some 25% of the stars with metallicities -1.8 <
[Fe/H] <= -0.8 exhibit orbital eccentricities e < 0.4, yet are clearly
separated from members of the inner-halo population with similar metallicities
by their location in a Lindblad energy vs. angular momentum diagram. A
comparison is made with recent results for a similar-size sample of RAVE stars
from Ruchti et al. We conclude, based on both of these samples, that the MWTD
is real, and must be accounted for in discussions of the formation and
evolution of the disk system of the Milky Way.Comment: 45 pages, 14 figures; accepted for publication in Ap
Empirical ugri-UBVRc Transformations for Galaxies
We present empirical color transformations between Sloan Digital Sky Survey
ugri and Johnson-Cousins UBVRc photometry for nearby galaxies (D < 11 Mpc). We
use the Local Volume Legacy (LVL) galaxy sample where there are 90 galaxies
with overlapping observational coverage for these two filter sets. The LVL
galaxy sample consists of normal, non-starbursting galaxies. We also examine
how well the LVL galaxy colors are described by previous transformations
derived from standard calibration stars and model-based galaxy templates. We
find significant galaxy color scatter around most of the previous
transformation relationships. In addition, the previous transformations show
systematic offsets between transformed and observed galaxy colors which are
visible in observed color-color trends. The LVL-based transformations
show no systematic color offsets and reproduce the observed color-color galaxy
trends.Comment: Accepted for publication in MNRAS (9 pages, 6 figures, 4 tables
Spitzer Local Volume Legacy (LVL) SEDs and Physical Properties
We present the panchromatic spectral energy distributions (SEDs) of the Local
Volume Legacy (LVL) survey which consists of 258 nearby galaxies (11 Mpc).
The wavelength coverage spans the ultraviolet to the infrared (1500
to 24 m) which is utilized to derive global physical
properties (i.e., star formation rate, stellar mass, internal extinction due to
dust.). With these data, we find color-color relationships and correlated
trends between observed and physical properties (i.e., optical magnitudes and
dust properties, optical color and specific star formation rate, and
ultraviolet-infrared color and metallicity). The SEDs are binned by different
galaxy properties to reveal how each property affects the observed shape of
these SEDs. In addition, due to the volume-limited nature of LVL, we utilize
the dwarf-dominated galaxy sample to test star formation relationships
established with higher-mass galaxy samples. We find good agreement with the
star-forming "main-sequence" relationship, but find a systematic deviation in
the infrared "main-sequence" at low luminosities. This deviation is attributed
to suppressed polycyclic aromatic hydrocarbon (PAH) formation in low
metallicity environments and/or the destruction of PAHs in more intense
radiation fields occurring near a suggested threshold in sSFR at a value of
log() 10.2.Comment: Accepted for publication in MNRAS (15 pages, 14 figures, 1 table
Carbon-enhanced Metal-poor Stars in SDSS/SEGUE. I. Carbon Abundance Estimation and Frequency of CEMP Stars
We describe a method for the determination of stellar [C/Fe] abundance ratios
using low-resolution (R = 2000) stellar spectra from the SDSS and SEGUE. By
means of a star-by-star comparison with a set of SDSS/SEGUE spectra with
available estimates of [C/Fe] based on published high-resolution analyses, we
demonstrate that we can measure [C/Fe] from SDSS/SEGUE spectra with S/N > 15 to
a precision better than 0.35 dex. Using the measured carbon-to-iron abundance
ratios obtained by this technique, we derive the frequency of carbon-enhanced
stars ([C/Fe] > +0.7) as a function of [Fe/H], for both the SDSS/SEGUE stars
and other samples from the literature. We find that the differential frequency
slowly rises from almost zero to about 14% at [Fe/H] ~ -2.4, followed by a
sudden increase, by about a factor of three, to 39% from [Fe/H] ~ -2.4 to
[Fe/H] ~ -3.7. We also examine how the cumulative frequency of CEMP stars
varies across different luminosity classes. The giant sample exhibits a
cumulative CEMP frequency of 32% for [Fe/H] < -2.5, 31% for [Fe/H] < -3.0, and
33% for [Fe/H] < -3.5. For the main-sequence turnoff stars, we obtain a lower
cumulative CEMP frequency, around 10% for [Fe/H] < -2.5. The dwarf population
displays a large change in the cumulative frequency for CEMP stars below [Fe/H]
= -2.5, jumping from 15% for [Fe/H] < -2.5 to about 75% for [Fe/H] < -3.0. When
we impose a restriction with respect to distance from the Galactic mid-plane
(|Z| < 5 kpc), the frequency of the CEMP giants does not increase at low
metallicity ([Fe/H] < -2.5), but rather, decreases, due to the dilution of
C-rich material in stars that have undergone mixing with CNO-processed material
from their interiors. The frequency of CEMP stars near the main-sequence
turnoff, which are not expected to have experienced mixing, increases for
[Fe/H] < -3.0. [abridged]Comment: 19 pages, 10 figures, 6 tables, accepted for publication in AJ on
August 20, 201
The Spitzer Local Volume Legacy (LVL) Global Optical Photometry
We present the global optical photometry of 246 galaxies in the Local Volume
Legacy (LVL) survey. The full volume-limited sample consists of 258 nearby (D <
11 Mpc) galaxies whose absolute B-band magnitude span a range of -9.6 < M_B <
-20.7 mag. A composite optical (UBVR) data set is constructed from observed
UBVR and SDSS ugriz imaging, where the ugriz magnitudes are transformed into
UBVR. We present photometry within three galaxy apertures defined at UV,
optical, and IR wavelengths. Flux comparisons between these apertures reveal
that the traditional optical R25 galaxy apertures do not fully encompass
extended sources. Using the larger IR apertures we find color-color
relationships where later-type spiral and irregular galaxies tend to be bluer
than earlier-type galaxies. These data provide the missing optical emission
from which future LVL studies can construct the full panchromatic
(UV-optical-IR) spectral energy distributions.Comment: Accepted for publication in MNRAS (9 pages, 5 figures, 5 tables
Metallicity Mapping with gri Photometry: The Virgo Overdensity and the Halos of the Galaxy
We describe the methodology required for estimation of photometric estimates
of metallicity based on the SDSS gri passbands, which 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. As a first application of this approach, we determine photometric
metal abundance estimates for individual main-sequence stars in the Virgo
Overdensity, which covers almost 1000 square degrees 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,
internal errors of in [Fe/H] ~ +/- 0.1 dex can be achieved for bulk stellar
populations. The global metal abundance of the Virgo Overdensity determined in
this way is = -2.0 +/- 0.1 (internal) +/- 0.5 (systematic), from
photometric measurements of 0.7 million stars with heliocentric distances from
~ 10 kpc to ~ 20 kpc. A preliminary metallicity map, based on results for 2.9
million stars in the northern SDSS DR-7 footprint, exhibits a shift to lower
metallicities as one proceeds from the inner- to the outer-halo population,
consistent with recent interpretation of the kinematics of local samples of
stars with spectroscopically available metallicity estimates and full space
motions.Comment: 4 pages, 2 figures, to appear in IAU Symp. 26
A Rewriting-Logic-Based Technique for Modeling Thermal Systems
This paper presents a rewriting-logic-based modeling and analysis technique
for physical systems, with focus on thermal systems. The contributions of this
paper can be summarized as follows: (i) providing a framework for modeling and
executing physical systems, where both the physical components and their
physical interactions are treated as first-class citizens; (ii) showing how
heat transfer problems in thermal systems can be modeled in Real-Time Maude;
(iii) giving the implementation in Real-Time Maude of a basic numerical
technique for executing continuous behaviors in object-oriented hybrid systems;
and (iv) illustrating these techniques with a set of incremental case studies
using realistic physical parameters, with examples of simulation and model
checking analyses.Comment: In Proceedings RTRTS 2010, arXiv:1009.398
Lymphoid follicle colonization by Bcl-2bright+CD10+ B-cells (“follicular lymphoma in situ”) at nodal and extranodal sites can be a manifestation of follicular homing of lymphoma
Follicular lymphoma (FL) in situ (FLIS) was first described and proposed as a distinct entity associated with an indolent clinical course in 2002. To gain further insight into the biology of this enigmatic lymphoproliferation, we analyzed morphologic, phenotypic, cytogenetic and molecular features of tissue specimens manifesting a pattern of follicular colonization by Bcl-2bright+CD10+ B-cells and associated lymphomas from 13 adults and evaluated their clinical outcomes. We observed this immunoarchitectural pattern in lymph nodes (n = 8), at extranodal sites (n = 4), or at both locations (n = 1) at diagnosis. All except 3 cases showed concomitant bright CD10 expression. Six (46%) patients had synchronous and 2 (15%) developed metachronous B-cell lymphomas, with 5 representing high-grade lymphomas. The Bcl-2bright+CD10+ B-cells colonizing reactive follicles and synchronous lymphomas were clonally related in 4/5 (80%) cases analyzed and 5/6 (83%) displayed BCL2 translocations. Two cases exhibited complex karyotypes in both components; a genetic “triple hit” was detected in one instance and 2 copies of t(14,18) were observed in a lymph node biopsy lacking evidence of lymphoma from an individual with stage 4 disease, suspected on imaging, who subsequently displayed a mantle zone/perifollicular infiltrate of Bcl-2bright+CD10+ B-cells in the adenoids. Our findings suggest that bright Bcl-2, and often bright CD10 expression, by B-cells colonizing reactive follicles might represent a phenomenon related to follicular homing of lymphoma, rather than being an attribute of preneoplastic FL precursors. Furthermore, due to the relatively high frequency of overt lymphomas observed, complete staging workup is recommended for patients exhibiting a Bcl-2bright+CD10+ B-cell follicular colonization pattern on biopsy
BRIGHT METAL-POOR STARS FROM THE HAMBURG/ESO SURVEY. II. A CHEMODYNAMICAL ANALYSIS
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[subscript 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.National Science Foundation (U.S.) (CAREER Grant AST 12-55160)Massachusetts Institute of Technology. Silverman (1968) Family Career Development Professorshi
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