100 research outputs found
Local variations in spatial synchrony of influenza epidemics
Background: Understanding the mechanism of influenza spread across multiple geographic scales is not complete. While the mechanism of dissemination across regions and states of the United States has been described, understanding the determinants of dissemination between counties has not been elucidated. The paucity of high resolution spatial-temporal influenza incidence data to evaluate disease structure is often not available. Methodology and Findings: We report on the underlying relationship between the spread of influenza and human movement between counties of one state. Significant synchrony in the timing of epidemics exists across the entire state and decay with distance (regional correlation = 62%). Synchrony as a function of population size display evidence of hierarchical spread with more synchronized epidemics occurring among the most populated counties. A gravity model describing movement between two populations is a stronger predictor of influenza spread than adult movement to and from workplaces suggesting that non-routine and leisure travel drive local epidemics. Conclusions: These findings highlight the complex nature of influenza spread across multiple geographic scales. © 2012 Stark et al
High-resolution, H band Spectroscopy of Be Stars with SDSS-III/APOGEE: I. New Be Stars, Line Identifications, and Line Profiles
APOGEE has amassed the largest ever collection of multi-epoch,
high-resolution (R~22,500), H-band spectra for B-type emission line (Be) stars.
The 128/238 APOGEE Be stars for which emission had never previously been
reported serve to increase the total number of known Be stars by ~6%. We focus
on identification of the H-band lines and analysis of the emission peak
velocity separations (v_p) and emission peak intensity ratios (V/R) of the
usually double-peaked H I and non-hydrogen emission lines. H I Br11 emission is
found to preferentially form in the circumstellar disks at an average distance
of ~2.2 stellar radii. Increasing v_p toward the weaker Br12--Br20 lines
suggests these lines are formed interior to Br11. By contrast, the observed IR
Fe II emission lines present evidence of having significantly larger formation
radii; distinctive phase lags between IR Fe II and H I Brackett emission lines
further supports that these species arise from different radii in Be disks.
Several emission lines have been identified for the first time including
~16895, a prominent feature in the spectra for almost a fifth of the sample
and, as inferred from relatively large v_p compared to the Br11-Br20, a tracer
of the inner regions of Be disks. Unlike the typical metallic lines observed
for Be stars in the optical, the H-band metallic lines, such as Fe II 16878,
never exhibit any evidence of shell absorption, even when the H I lines are
clearly shell-dominated. The first known example of a quasi-triple-peaked Br11
line profile is reported for HD 253659, one of several stars exhibiting intra-
and/or extra-species V/R and radial velocity variation within individual
spectra. Br11 profiles are presented for all discussed stars, as are full
APOGEE spectra for a portion of the sample.Comment: accepted in A
A Cautionary Tale: MARVELS Brown Dwarf Candidate Reveals Itself To Be A Very Long Period, Highly Eccentric Spectroscopic Stellar Binary
We report the discovery of a highly eccentric, double-lined spectroscopic
binary star system (TYC 3010-1494-1), comprising two solar-type stars that we
had initially identified as a single star with a brown dwarf companion. At the
moderate resolving power of the MARVELS spectrograph and the spectrographs used
for subsequent radial-velocity (RV) measurements (R ~ <30,000), this particular
stellar binary mimics a single-lined binary with an RV signal that would be
induced by a brown dwarf companion (Msin(i)~50 M_Jup) to a solar-type primary.
At least three properties of this system allow it to masquerade as a single
star with a very low-mass companion: its large eccentricity (e~0.8), its
relatively long period (P~238 days), and the approximately perpendicular
orientation of the semi-major axis with respect to the line of sight (omega~189
degrees). As a result of these properties, for ~95% of the orbit the two sets
of stellar spectral lines are completely blended, and the RV measurements based
on centroiding on the apparently single-lined spectrum is very well fit by an
orbit solution indicative of a brown dwarf companion on a more circular orbit
(e~0.3). Only during the ~5% of the orbit near periastron passage does the
true, double-lined nature and large RV amplitude of ~15 km/s reveal itself. The
discovery of this binary system is an important lesson for RV surveys searching
for substellar companions; at a given resolution and observing cadence, a
survey will be susceptible to these kinds of astrophysical false positives for
a range of orbital parameters. Finally, for surveys like MARVELS that lack the
resolution for a useful line bisector analysis, it is imperative to monitor the
peak of the cross-correlation function for suspicious changes in width or
shape, so that such false positives can be flagged during the candidate vetting
process.Comment: 16 pages, 11 figures, 6 table
Very Low Mass Stellar and Substellar Companions to Solar-Like Stars From MARVELS V: A Low Eccentricity Brown Dwarf from the Driest Part of the Desert, MARVELS-6b
We describe the discovery of a likely brown dwarf (BD) companion with a
minimum mass of 31.7 +/- 2.0 M_Jup to GSC 03546-01452 from the MARVELS radial
velocity survey, which we designate as MARVELS-6b. For reasonable priors, our
analysis gives a probability of 72% that MARVELS-6b has a mass below the
hydrogen-burning limit of 0.072 M_Sun, and thus it is a high-confidence BD
companion. It has a moderately long orbital period of 47.8929 +0.0063/-0.0062
days with a low eccentricty of 0.1442 +0.0078/-0.0073, and a semi-amplitude of
1644 +12/-13 m/s. Moderate resolution spectroscopy of the host star has
determined the following parameters: T_eff = 5598 +/- 63, log g = 4.44 +/-
0.17, and [Fe/H] = +0.40 +/- 0.09. Based upon these measurements, GSC
03546-01452 has a probable mass and radius of M_star = 1.11 +/- 0.11 M_Sun and
R_star = 1.06 +/- 0.23 R_Sun with an age consistent with less than ~6 Gyr at a
distance of 219 +/- 21 pc from the Sun. Although MARVELS-6b is not observed to
transit, we cannot definitively rule out a transiting configuration based on
our observations. There is a visual companion detected with Lucky Imaging at
7.7 arcsec from the host star, but our analysis shows that it is not bound to
this system. The minimum mass of MARVELS-6b exists at the minimum of the mass
functions for both stars and planets, making this a rare object even compared
to other BDs.Comment: 15 pages, 15 figures, 5 tables. Accepted for publication in The
Astronomical Journa
Impact of changing the measles vaccine vial size on Niger's vaccine supply chain: a computational model
<p>Abstract</p> <p>Background</p> <p>Many countries, such as Niger, are considering changing their vaccine vial size presentation and may want to evaluate the subsequent impact on their supply chains, the series of steps required to get vaccines from their manufacturers to patients. The measles vaccine is particularly important in Niger, a country prone to measles outbreaks.</p> <p>Methods</p> <p>We developed a detailed discrete event simulation model of the vaccine supply chain representing every vaccine, storage location, refrigerator, freezer, and transport device (e.g., cold trucks, 4 × 4 trucks, and vaccine carriers) in the Niger Expanded Programme on Immunization (EPI). Experiments simulated the impact of replacing the 10-dose measles vial size with 5-dose, 2-dose and 1-dose vial sizes.</p> <p>Results</p> <p>Switching from the 10-dose to the 5-dose, 2-dose and 1-dose vial sizes decreased the average availability of EPI vaccines for arriving patients from 83% to 82%, 81% and 78%, respectively for a 100% target population size. The switches also changed transport vehicle's utilization from a mean of 58% (range: 4-164%) to means of 59% (range: 4-164%), 62% (range: 4-175%), and 67% (range: 5-192%), respectively, between the regional and district stores, and from a mean of 160% (range: 83-300%) to means of 161% (range: 82-322%), 175% (range: 78-344%), and 198% (range: 88-402%), respectively, between the district to integrated health centres (IHC). The switch also changed district level storage utilization from a mean of 65% to means of 64%, 66% and 68% (range for all scenarios: 3-100%). Finally, accounting for vaccine administration, wastage, and disposal, replacing the 10-dose vial with the 5 or 1-dose vials would increase the cost per immunized patient from 0.71US and $1.26US, respectively.</p> <p>Conclusions</p> <p>The switch from the 10-dose measles vaccines to smaller vial sizes could overwhelm the capacities of many storage facilities and transport vehicles as well as increase the cost per vaccinated child.</p
Very Low-Mass Stellar and Substellar Companions to Solar-Like Stars from MARVELS I: A Low Mass Ratio Stellar Companion to TYC 4110-01037-1 in a 79-day Orbit
TYC 4110-01037-1 has a low-mass stellar companion, whose small mass ratio and
short orbital period are atypical amongst solar-like (Teff ~< 6000 K) binary
systems. Our analysis of TYC 4110-01037-1 reveals it to be a moderately aged
(~<5 Gyr) solar-like star having a mass of 1.07 +/- 0.08 MSun and radius of
0.99 +/- 0.18 RSun. We analyze 32 radial velocity measurements from the
SDSS-III MARVELS survey as well as 6 supporting radial velocity measurements
from the SARG spectrograph on the 3.6m TNG telescope obtained over a period of
~2 years. The best Keplerian orbital fit parameters were found to have a period
of 78.994 +/- 0.012 days, an eccentricity of 0.1095 +/- 0.0023, and a
semi-amplitude of 4199 +/- 11 m/s. We determine the minimum companion mass (if
sin i = 1) to be 97.7 +/- 5.8 MJup. The system's companion to host star mass
ratio, >0.087 +/- 0.003, places it at the lowest end of observed values for
short period stellar companions to solar-like (Teff ~< 6000 K) stars. One
possible way to create such a system would be if a triple-component stellar
multiple broke up into a short period, low q binary during the cluster
dispersal phase of its lifetime. A candidate tertiary body has been identified
in the system via single-epoch, high contrast imagery. If this object is
confirmed to be co-moving, we estimate it would be a dM4 star. We present these
results in the context of our larger-scale effort to constrain the statistics
of low mass stellar and brown dwarf companions to FGK-type stars via the
MARVELS survey.Comment: 22 pages; accepted in A
The Ninth Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the SDSS-III Baryon Oscillation Spectroscopic Survey
The Sloan Digital Sky Survey III (SDSS-III) presents the first spectroscopic
data from the Baryon Oscillation Spectroscopic Survey (BOSS). This ninth data
release (DR9) of the SDSS project includes 535,995 new galaxy spectra (median
z=0.52), 102,100 new quasar spectra (median z=2.32), and 90,897 new stellar
spectra, along with the data presented in previous data releases. These spectra
were obtained with the new BOSS spectrograph and were taken between 2009
December and 2011 July. In addition, the stellar parameters pipeline, which
determines radial velocities, surface temperatures, surface gravities, and
metallicities of stars, has been updated and refined with improvements in
temperature estimates for stars with T_eff<5000 K and in metallicity estimates
for stars with [Fe/H]>-0.5. DR9 includes new stellar parameters for all stars
presented in DR8, including stars from SDSS-I and II, as well as those observed
as part of the SDSS-III Sloan Extension for Galactic Understanding and
Exploration-2 (SEGUE-2).
The astrometry error introduced in the DR8 imaging catalogs has been
corrected in the DR9 data products. The next data release for SDSS-III will be
in Summer 2013, which will present the first data from the Apache Point
Observatory Galactic Evolution Experiment (APOGEE) along with another year of
data from BOSS, followed by the final SDSS-III data release in December 2014.Comment: 9 figures; 2 tables. Submitted to ApJS. DR9 is available at
http://www.sdss3.org/dr
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