417 research outputs found
Steady-state simulations using weighted ensemble path sampling
We extend the weighted ensemble (WE) path sampling method to perform rigorous
statistical sampling for systems at steady state. The straightforward
steady-state implementation of WE is directly practical for simple landscapes,
but not when significant metastable intermediates states are present. We
therefore develop an enhanced WE scheme, building on existing ideas, which
accelerates attainment of steady state in complex systems. We apply both WE
approaches to several model systems confirming their correctness and efficiency
by comparison with brute-force results. The enhanced version is significantly
faster than the brute force and straightforward WE for systems with WE bins
that accurately reflect the reaction coordinate(s). The new WE methods can also
be applied to equilibrium sampling, since equilibrium is a steady state
The scale-dependence of relative galaxy bias: encouragement for the halo model description
We investigate the relationship between the colors, luminosities, and
environments of galaxies in the Sloan Digital Sky Survey spectroscopic sample,
using environmental measurements on scales ranging from 0.2 to 6 Mpc/h. We
find: (1) that the relationship between color and environment persists even to
the lowest luminosities we probe (absolute magnitude in the r band of about -14
for h=1); (2) at luminosities and colors for which the galaxy correlation
function has a large amplitude, it also has a steep slope; and (3) in regions
of a given overdensity on small scales (1 Mpc/h), the overdensity on large
scales (6 Mpc/h) does not appear to relate to the recent star formation history
of the galaxies. Of these results, the last has the most immediate application
to galaxy formation theory. In particular, it lends support to the notion that
a galaxy's properties are related only to the mass of its host dark matter
halo, and not to the larger scale environment.Comment: submitted to ApJ; full resolution figures and slide material
available at http://cosmo.nyu.edu/blanton/scale_density.htm
Field testing of World Health Organization (WHO) 2003 recommendations for initiating anti-retroviral therapy (ART) where CD4 is not available revealed low sensitivity among stage I and II patients; a combination of Automated Total Lymphocyte Count (TLC) and Haemoglobin (Hgb) can ration the referral for CD4 testing.
2006 AIDS Conference, Toront
Cosmic homogeneity demonstrated with luminous red galaxies
We test the homogeneity of the Universe at with the Luminous Red
Galaxy (LRG) spectroscopic sample of the Sloan Digital Sky Survey. First, the
mean number of LRGs within completely surveyed LRG-centered spheres of
comoving radius is shown to be proportional to at radii greater than
. The test has the virtue that it does not rely
on the assumption that the LRG sample has a finite mean density; its results
show, however, that there \emph{is} such a mean density. Secondly, the survey
sky area is divided into 10 disjoint solid angular regions and the fractional
rms density variations of the LRG sample in the redshift range
among these () regions is found to be 7
percent of the mean density. This variance is consistent with typical biased
\lcdm models and puts very strong constraints on the quality of SDSS
photometric calibration.Comment: submitted to Ap
Forward modeling the orbits of companions to pulsating stars from their light travel time variations
Mutual gravitation between a pulsating star and an orbital companion leads to
a time-dependent variation in path length for starlight traveling to Earth.
These variations can be used for coherently pulsating stars, such as the
{\delta} Scuti variables, to constrain the masses and orbits of their
companions. Observing these variations for {\delta} Scuti stars has previously
relied on subdividing the light curve and measuring the average pulsation phase
in equally sized subdivisions, which leads to under-sampling near periapsis. We
introduce a new approach that simultaneously forward-models each sample in the
light curve and show that this method improves upon current sensitivity limits
- especially in the case of highly eccentric and short-period binaries. We find
that this approach is sensitive enough to observe Jupiter mass planets around
{\delta} Scuti stars under ideal conditions, and use gravity-mode pulsations in
the subdwarf B star KIC 7668647 to detect its companion without radial velocity
data. We further provide robust detection limits as a function of the SNR of
the pulsation mode and determine that the minimum detectable light travel time
amplitude for a typical Kepler {\delta} Scuti is around 2 s. This new method
significantly enhances the application of light travel time variations to
detecting short period binaries with pulsating components, and pulsating A-type
exoplanet host stars, especially as a tool for eliminating false positives.Comment: 14 pages, accepted for publication in A
Discovery and Characterization of 3000+ Main-Sequence Binaries from APOGEE Spectra
We develop a data-driven spectral model for identifying and characterizing
spatially unresolved multiple-star systems and apply it to APOGEE DR13 spectra
of main-sequence stars. Binaries and triples are identified as targets whose
spectra can be significantly better fit by a superposition of two or three
model spectra, drawn from the same isochrone, than any single-star model. From
an initial sample of 20,000 main-sequence targets, we identify
2,500 binaries in which both the primary and secondary star contribute
detectably to the spectrum, simultaneously fitting for the velocities and
stellar parameters of both components. We additionally identify and fit
200 triple systems, as well as 700 velocity-variable systems in
which the secondary does not contribute detectably to the spectrum. Our model
simplifies the process of simultaneously fitting single- or multi-epoch spectra
with composite models and does not depend on a velocity offset between the two
components of a binary, making it sensitive to traditionally undetectable
systems with periods of hundreds or thousands of years. In agreement with
conventional expectations, almost all the spectrally-identified binaries with
measured parallaxes fall above the main sequence in the color-magnitude
diagram. We find excellent agreement between spectrally and dynamically
inferred mass ratios for the 600 binaries in which a dynamical mass ratio
can be measured from multi-epoch radial velocities. We obtain full orbital
solutions for 64 systems, including 14 close binaries within hierarchical
triples. We make available catalogs of stellar parameters, abundances, mass
ratios, and orbital parameters.Comment: Accepted to MNRAS with minor revisions since v1. 19 pages, 12
figures, plus Appendice
Relationship between environment and the broad-band optical properties of galaxies in the SDSS
We examine the relationship between environment and the luminosities, surface
brightnesses, colors, and profile shapes of luminous galaxies in the Sloan
Digital Sky Survey (SDSS). For the SDSS sample, galaxy color is the galaxy
property most predictive of the local environment. Galaxy color and luminosity
jointly comprise the most predictive pair of properties. At fixed luminosity
and color, density is not closely related to surface brightness or to Sersic
index -- the parameter in this study that astronomers most often associate with
morphology. In the text, we discuss what measureable residual relationships
exist, generally finding that at red colors and fixed luminosity, the mean
density decreases at the highest surface brightnesses and Sersic indices. In
general, these results suggest that the morphological properties of galaxies
are less closely related to galaxy environment than are their masses and
star-formation histories.Comment: submitted to ApJ, pedagogy and bitmapped figures for presentations
available at http://cosmo.nyu.edu/blanton/full_density.htm
Chemical tagging can work: Identification of stellar phase-space structures purely by chemical-abundance similarity
Chemical tagging promises to use detailed abundance measurements to identify
spatially separated stars that were in fact born together (in the same
molecular cloud), long ago. This idea has not yielded much practical success,
presumably because of the noise and incompleteness in chemical-abundance
measurements. We have succeeded in substantially improving spectroscopic
measurements with The Cannon, which has now delivered 15 individual abundances
for ~100,000 stars observed as part of the APOGEE spectroscopic survey, with
precisions around 0.04 dex. We test the chemical-tagging hypothesis by looking
at clusters in abundance space and confirming that they are clustered in phase
space. We identify (by the k-means algorithm) overdensities of stars in the
15-dimensional chemical-abundance space delivered by The Cannon, and plot the
associated stars in phase space. We use only abundance-space information (no
positional information) to identify stellar groups. We find that clusters in
abundance space are indeed clusters in phase space. We recover some known
phase-space clusters and find other interesting structures. This is the
first-ever project to identify phase-space structures at survey-scale by blind
search purely in abundance space; it verifies the precision of the abundance
measurements delivered by The Cannon; the prospects for future data sets appear
very good.Comment: accepted for publication in the Ap
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