3,426 research outputs found
Safety hazards associated with the charging of lithium/sulfur dioxide cells
A continuing research program to assess the responses of spirally wound, lithium/sulfur dioxide cells to charging as functions of charging current, temperature, and cell condition prior to charging is described. Partially discharged cells that are charged at currents greater than one ampere explode with the time to explosion inversely proportional to the charging current. Cells charged at currents of less than one ampere may fail in one of several modes. The data allows an empirical prediction of when certain cells will fail given a constant charging current
Chemical analysis of charged Li/SO(sub)2 cells
The initial focus of the program was to confirm that charging can indeed result in explosions and constitute a significant safety problem. Results of this initial effort clearly demonstrated that cells do indeed explode on charge and that charging does indeed constitute a real and severe safety problem. The results of the effort to identify the chemical reactions involved in and responsible for the observed behavior are described
Transmission of Mycobacterium tuberculosis in a Rural Community, Arkansas, 1945–2000
A cluster of tuberculosis cases in a rural community in Arkansas persisted from 1991 to 1999. The cluster had 13 members, 11 linked epidemiologically. Old records identified 24 additional patients for 40 linked case-patients during a 54-year period. Residents of this neighborhood represent a population at high risk who should be considered for tuberculin testing and treatment for latent tuberculosis infection
Detection of Crab Giant Pulses Using the Mileura Widefield Array Low Frequency Demonstrator Field Prototype System
We report on the detection of giant pulses from the Crab Nebula pulsar at a
frequency of 200 MHz using the field deployment system designed for the Mileura
Widefield Array's Low Frequency Demonstrator (MWA-LFD). Our observations are
among the first high-quality detections at such low frequencies. The measured
pulse shapes are deconvolved for interstellar pulse broadening, yielding a
pulse-broadening time of 670100 s, and the implied strength of
scattering (scattering measure) is the lowest that is estimated towards the
Crab nebula from observations made so far. The sensitivity of the system is
largely dictated by the sky background, and our simple equipment is capable of
detecting pulses that are brighter than 9 kJy in amplitude. The brightest
giant pulse detected in our data has a peak amplitude of 50 kJy, and the
implied brightness temperature is K. We discuss the giant pulse
detection prospects with the full MWA-LFD system. With a sensitivity over two
orders of magnitude larger than the prototype equipment, the full system will
be capable of detecting such bright giant pulses out to a wide range of
Galactic distances; from 8 to 30 kpc depending on the frequency.
The MWA-LFD will thus be a highly promising instrument for the studies of giant
pulses and other fast radio transients at low frequencies.Comment: 10 pages, 6 figures, Accepted for publication in the Astrophysical
Journa
The first super-Earth Detection from the High Cadence and High Radial Velocity Precision Dharma Planet Survey
The Dharma Planet Survey (DPS) aims to monitor about 150 nearby very bright
FGKM dwarfs (within 50 pc) during 20162020 for low-mass planet detection and
characterization using the TOU very high resolution optical spectrograph
(R100,000, 380-900nm). TOU was initially mounted to the 2-m Automatic
Spectroscopic Telescope at Fairborn Observatory in 2013-2015 to conduct a pilot
survey, then moved to the dedicated 50-inch automatic telescope on Mt. Lemmon
in 2016 to launch the survey. Here we report the first planet detection from
DPS, a super-Earth candidate orbiting a bright K dwarf star, HD 26965. It is
the second brightest star ( mag) on the sky with a super-Earth
candidate. The planet candidate has a mass of 8.47,
period of d, and eccentricity of . This RV
signal was independently detected by Diaz et al. (2018), but they could not
confirm if the signal is from a planet or from stellar activity. The orbital
period of the planet is close to the rotation period of the star (3944.5 d)
measured from stellar activity indicators. Our high precision photometric
campaign and line bisector analysis of this star do not find any significant
variations at the orbital period. Stellar RV jitters modeled from star spots
and convection inhibition are also not strong enough to explain the RV signal
detected. After further comparing RV data from the star's active magnetic phase
and quiet magnetic phase, we conclude that the RV signal is due to
planetary-reflex motion and not stellar activity.Comment: 13 pages, 17 figures, Accepted for publication in MNRA
Bostonia: The Boston University Alumni Magazine. Volume 32
Founded in 1900, Bostonia magazine is Boston University's main alumni publication, which covers alumni and student life, as well as university activities, events, and programs
Ongoing Assembly of Massive Galaxies by Major Merging in Large Groups and Clusters from the SDSS
We investigate the incidence of major mergers creating >10e11 Msun galaxies
in present-day groups and clusters more massive than 2.5e13 Msun. We identify
38 pairs of massive galaxies with mutual tidal interaction signatures selected
from >5000 galaxies with >5e10 Msun that reside in 845 such groups. We fit the
images of each galaxy pair as the line-of-sight projection of symmetric models
and identify mergers by the presence of residual asymmetries around each
progenitor, such as off-center isophotes, broad tidal tails, and dynamical
friction wakes. At the resolution and sensitivity of the SDSS, such mergers are
found in 16% of high-mass, galaxy-galaxy pairs with magnitude differences of
<1.5 and <30 kpc projected separations. We find that 90% of these mergers have
nearly equal-mass progenitors with red-sequence colors and
centrally-concentrated morphologies, the hallmarks of dissipationless merger
simulations. Mergers at group centers are more common than between 2
satellites, but both are morphologically indistinguishable and we tentatively
conclude that the latter are likely located at the dynamical centers of
recently accreted subhalos. The frequency of central and satellite merging
diminishes with group mass consistent with dynamical friction expectations.
Based on reasonable assumptions, the centers of these massive halos are growing
in stellar mass by 1-9% per Gyr, on average. Compared to all LRG-LRG mergers,
we find a 2-9 times higher rate for their merging when restricted to these
dense environments. Our results imply that the massive end of the galaxy
population continues to evolve hierarchically at a measurable level, and that
the centers of massive groups are the preferred environment for merger-driven
galaxy assembly. (abridged)Comment: 48 pages, 21 figures. Submitted for publication in MNRAS. Version
with full resolution figures at
http://www.astro.umass.edu/~dmac/Preprints/mergers.hires.pd
The Central Mass Distribution in Dwarf and Low Surface Brightness Galaxies
We present Halpha rotation curves for a sample of 15 dwarf and LSB galaxies.
From these, we derive limits on the slopes of the central mass distributions.
Assuming the density distributions of dark matter halos follow a power-law at
small radii, rho(r)~r^(-alpha), we find inner slopes in the range 0<alpha<1 for
most galaxies. In general, halos with constant density cores (\alpha=0) provide
somewhat better fits, but the majority of our galaxies (~75%) are also
consistent with alpha=1, provided that the R-band mass-to-light ratios are
smaller than about 2. Halos with alpha=1.5, however, are ruled out in virtually
every case. To investigate the robustness of these results we discuss and model
several possible causes of systematic errors including non-circular motions,
slit width, seeing, and slit alignment errors. Taking the associated
uncertainties into account, we conclude that even for the 25% of the cases
where alpha=1 seems inconsistent with the rotation curves, we cannot rule out
cusp slopes this steep. Inclusion of literature samples similar to the one
presented here leads to the same conclusion when possible systematic errors are
taken into account. In the ongoing debate on whether the rotation curves of
dwarf and LSB galaxies are consistent with predictions for a CDM universe, we
argue that our sample and the literature samples discussed in this paper
provide insufficient evidence to rule out halos with alpha=1. At the same time,
we note that none of the galaxies in these samples require halos with steep
cusps, as most are equally well or better explained by constant density cores.
(abridged)Comment: 19 pages, accepted for publication in Ap
Structural Properties of Central Galaxies in Groups and Clusters
Using a representative sample of 911 central galaxies (CENs) from the SDSS
DR4 group catalogue, we study how the structure of the most massive members in
groups and clusters depend on (1) galaxy stellar mass (Mstar), (2) dark matter
halo mass of the host group (Mhalo), and (3) their halo-centric position. We
establish and thoroughly test a GALFIT-based pipeline to fit 2D Sersic models
to SDSS data. We find that the fitting results are most sensitive to the
background sky level determination and strongly recommend using the SDSS global
value. We find that uncertainties in the background translate into a strong
covariance between the total magnitude, half-light size (r50), and Sersic index
(n), especially for bright/massive galaxies. We find that n depends strongly on
Mstar for CENs, but only weakly or not at all on Mhalo. Less (more) massive
CENs tend to be disk (spheroid)-like over the full Mhalo range. Likewise, there
is a clear r50-Mstar relation for CENs, with separate slopes for disks and
spheroids. When comparing CENs with satellite galaxies (SATs), we find that low
mass (<10e10.75 Msun/h^2) SATs have larger median n than CENs of similar Mstar.
Low mass, late-type SATs have moderately smaller r50 than late-type CENs of the
same Mstar. However, we find no size differences between spheroid-like CENs and
SATs, and no structural differences between CENs and SATs matched in both mass
and colour. The similarity of massive SATs and CENs shows that this distinction
has no significant impact on the structure of spheroids. We conclude that Mstar
is the most fundamental property determining the basic structure of a galaxy.
The lack of a clear n-Mhalo relation rules out a distinct group mass for
producing spheroids, and the responsible morphological transformation processes
must occur at the centres of groups spanning a wide range of masses. (abridged)Comment: 22 pages, 14 figures, submitted to MNRA
The HIPASS Catalogue - II. Completeness, Reliability, and Parameter Accuracy
The HI Parkes All Sky Survey (HIPASS) is a blind extragalactic HI 21-cm
emission line survey covering the whole southern sky from declination -90 to
+25. The HIPASS catalogue (HICAT), containing 4315 HI-selected galaxies from
the region south of declination +2, is presented in Meyer et al. (2004a, Paper
I). This paper describes in detail the completeness and reliability of HICAT,
which are calculated from the recovery rate of synthetic sources and follow-up
observations, respectively. HICAT is found to be 99 per cent complete at a peak
flux of 84 mJy and an integrated flux of 9.4 Jy km/s. The overall reliability
is 95 per cent, but rises to 99 per cent for sources with peak fluxes >58 mJy
or integrated flux > 8.2 Jy km/s. Expressions are derived for the uncertainties
on the most important HICAT parameters: peak flux, integrated flux, velocity
width, and recessional velocity. The errors on HICAT parameters are dominated
by the noise in the HIPASS data, rather than by the parametrization procedure.Comment: Accepted for publication in MNRAS. 12 pages, 11 figures. Paper with
higher resolution figures can be downloaded from http://hipass.aus-vo.or
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