1,265 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
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
Computing Fast and Reliable Gravitational Waveforms of Binary Neutron Star Merger Remnants
Gravitational waves have been detected from the inspiral of a binary
neutron-star, GW170817, which allowed constraints to be placed on the neutron
star equation of state. The equation of state can be further constrained if
gravitational waves from a post-merger remnant are detected. Post-merger
waveforms are currently generated by numerical-relativity simulations, which
are computationally expensive. Here we introduce a hierarchical model trained
on numerical-relativity simulations, which can generate reliable post-merger
spectra in a fraction of a second. Our spectra have mean fitting factors of
0.95, which compares to fitting factors of 0.76 and 0.85 between different
numerical-relativity codes that simulate the same physical system. This method
is the first step towards generating large template banks of spectra for use in
post-merger detection and parameter estimation.Comment: Submitted to PRL. 6 pages, 4 figure
Exposure to the Dental Environment and Prevalence of Respiratory Illness in Dental Student Populations
Objective: To determine if the prevalence of respiratory disease among dental students and dental residents varies with their exposure to the clinical dental environment.
Methods: A detailed questionnaire was administered to 817 students at 3 dental schools. The questionnaire sought information concerning demographic characteristics, school year, exposure to the dental environment and dental procedures, and history of respiratory disease. The data obtained were subjected to bivariate and multiple logistic regression analysis.
Results: Respondents reported experiencing the following respiratory conditions during the previous year: asthma (26 cases), bronchitis (11 cases), chronic lung disease (6 cases), pneumonia (5 cases) and streptococcal pharyngitis (50 cases). Bivariate statistical analyses indicated no significant associations between the prevalence of any of the respiratory conditions and year in dental school, except for asthma, for which there was a significantly higher prevalence at 1 school compared to the other 2 schools. When all cases of respiratory disease were combined as a composite variable and subjected to multivariate logistic regression analysis controlling for age, sex, race, dental school, smoking history and alcohol consumption, no statistically significant association was observed between respiratory condition and year in dental school or exposure to the dental environment as a dental patient.
Conclusion: No association was found between the prevalence of respiratory disease and a student\u27s year in dental school or previous exposure to the dental environment as a patient. These results suggest that exposure to the dental environment does not increase the risk for respiratory infection in healthy dental health care workers
Rethinking Tiebout: The Contribution of Political Fragmentation and Racial/Economic Segregation to the Flint Water Crisis
The water crisis that has embroiled Flint, Michigan, since 2014 is often explained via the proximate causes of government oversight and punitive emergency management. While these were critical elements in the decision to switch the city's water source, many other forces helped precipitate the crisis. One such force has been an enduring support for Charles Tiebout's model of interlocal competition, through which a region is presumed stronger when fragmented, independent municipalities compete for residents and investment. However, the Tiebout model fails to account for spillover effects, particularly regarding questions of social and regional equity. In this sense, the fragmentation of the Flint metropolitan region?supported through a variety of housing and land use policies over many decades?created the conditions through which suburbs were absolved of responsibility for Flint's decades-long economic crisis. Because of the Tiebout model's inability to address imbalances in population shifts arising from disparities in municipal services, Flint's more affluent suburbs continued to prosper, while Flint grew poorer and experienced infrastructure decline. Underlying this pattern of inequality has been a long history of racial segregation and massive deindustrialization, which concentrated the region's black population in the economically depressed central city. The Flint Water Crisis is thus a classic example of an environmental injustice, as policies were set in motion, which led to the creation of a politically separate and majority-black municipality with concentrated poverty, while nearby municipalities?most of them overwhelmingly white?accepted little responsibility for the legacy costs created by the region's starkly uneven patterns of metropolitan development.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/140089/1/env.2016.0015.pd
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