40,867 research outputs found
The Best and Brightest Metal-Poor Stars
The chemical abundances of large samples of extremely metal-poor (EMP) stars
can be used to investigate metal-free stellar populations, supernovae, and
nucleosynthesis as well as the formation and galactic chemical evolution of the
Milky Way and its progenitor halos. However, current progress on the study of
EMP stars is being limited by their faint apparent magnitudes. The acquisition
of high signal-to-noise spectra for faint EMP stars requires a major telescope
time commitment, making the construction of large samples of EMP star
abundances prohibitively expensive. We have developed a new, efficient
selection that uses only public, all-sky APASS optical, 2MASS near-infrared,
and WISE mid-infrared photometry to identify bright metal-poor star candidates
through their lack of molecular absorption near 4.6 microns. We have used our
selection to identify 11,916 metal-poor star candidates with V < 14, increasing
the number of publicly-available candidates by more than a factor of five in
this magnitude range. Their bright apparent magnitudes have greatly eased
high-resolution follow-up observations that have identified seven previously
unknown stars with [Fe/H] <~ -3.0. Our follow-up campaign has revealed that
3.8^{+1.3}_{-1.1}% of our candidates have [Fe/H] <~ -3.0 and
32.5^{+3.0}_{-2.9}% have -3.0 <~ [Fe/H] <~ -2.0. The bulge is the most likely
location of any existing Galactic Population III stars, and an infrared-only
variant of our selection is well suited to the identification of metal-poor
stars in the bulge. Indeed, two of our confirmed metal-poor stars with [Fe/H]
<~ -2.7 are within about 2 kpc of the Galactic Center. They are among the most
metal-poor stars known in the bulge.Comment: 28 pages, 6 figures, and 4 tables in emulateapj format; accepted for
publication in Ap
Predicting Success, Preventing Failure: An Investigation of the California High School Exit Exam
Examines early indicators that identify fourth-grade students in San Diego who are at risk of failing the California High School Exit Exam, discusses implications for when and how to intervene to address those factors, and makes policy recommendations
Chemistry of the Most Metal-poor Stars in the Bulge and the z > 10 Universe
Metal-poor stars in the Milky Way are local relics of the epoch of the first
stars and the first galaxies. However, a low metallicity does not prove that a
star formed in this ancient era, as metal-poor stars form over a range of
redshift in different environments. Theoretical models of Milky Way formation
have shown that at constant metallicity, the oldest stars are those closest to
the center of the Galaxy on the most tightly-bound orbits. For that reason, the
most metal-poor stars in the bulge of the Milky Way provide excellent tracers
of the chemistry of the high-redshift universe. We report the dynamics and
detailed chemical abundances of three stars in the bulge with [Fe/H]
, two of which are the most metal-poor stars in the bulge in the
literature. We find that with the exception of scandium, all three stars follow
the abundance trends identified previously for metal-poor halo stars. These
three stars have the lowest [Sc II/Fe] abundances yet seen in -enhanced
giant stars in the Galaxy. Moreover, all three stars are outliers in the
otherwise tight [Sc II/Fe]-[Ti II/Fe] relation observed among metal-poor halo
stars. Theoretical models predict that there is a 30% chance that at least one
of these stars formed at , while there is a 70% chance that at
least one formed at . These observations imply that
by , the progenitor galaxies of the Milky Way had both reached [Fe/H]
and established the abundance pattern observed in extremely
metal-poor stars.Comment: Submitted to ApJ on 2014 December 23, accepted 2015 May 4th after
minor revisions. ArXiv tarball includes referee report and respons
X-ray observations of the galaxy cluster PKS 0745-191: To the virial radius, and beyond
We measure X-ray emission from the outskirts of the cluster of galaxies PKS
0745-191 with Suzaku, determining radial profiles of density, temperature,
entropy, gas fraction, and mass. These measurements extend beyond the virial
radius for the first time, providing new information about cluster assembly and
the diffuse intracluster medium out to ~1.5 r_200, (r_200 ~ 1.7 Mpc ~ 15'). The
temperature is found to decrease by roughly 70 per cent from 0.3-1 r_200. We
also see a flattening of the entropy profile near the virial radius and
consider the implications this has for the assumption of hydrostatic
equilibrium when deriving mass estimates. We place these observations in the
context of simulations and analytical models to develop a better understanding
of non-gravitational physics in the outskirts of the cluster.Comment: 10 pages, 11 figures, accepted to MNRAS; expanded discussion of
analysis and uncertainties, results qualitatively unchange
A short note on the presence of spurious states in finite basis approximations
The genesis of spurious solutions in finite basis approximations to operators
which possess a continuum and a point spectrum is discussed and a simple
solution for identifying these solutions is suggested
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