2,191 research outputs found
Improved Ion Heating in Fast Ignition by Pulse Shaping
The fast ignition paradigm for inertial fusion offers increased gain and
tolerance of asymmetry by compressing fuel at low entropy and then quickly
igniting a small region. Because this hotspot rapidly disassembles, the ions
must be heated to ignition temperature as quickly as possible, but most ignitor
designs directly heat electrons. A constant-power ignitor pulse, which is
generally assumed, is suboptimal for coupling energy from electrons to ions.
Using a simple model of a hotspot in isochoric plasma, a novel pulse shape to
maximize ion heating is presented in analytical form. Bounds are derived on the
maximum ion temperature attainable by electron heating only. Moreover,
arranging for faster ion heating allows a smaller hotspot, improving fusion
gain. Under representative conditions, the optimized pulse can reduce ignition
energy by over 20%.Comment: 8 pages, 4 figure
Discovery of Three Distant, Cold Brown Dwarfs in the WFC3 Infrared Spectroscopic Parallels Survey
We present the discovery of three late type (>T4) brown dwarfs, including a
probable Y dwarf, in the WFC3 Infrared Spectroscopic Parallels (WISP) Survey.
We use the G141 grism spectra to determine the spectral types of the dwarfs and
derive distance estimates based on a comparison with nearby T dwarfs with known
parallaxes. These are the most distant spectroscopically confirmed T/Y dwarfs,
with the farthest at an estimated distance of ~400 pc. We compare the number of
cold dwarfs found in the WISP survey with simulations of the brown dwarf mass
function. The number found is generally consistent with an initial stellar mass
function dN/dM \propto M^{-\alpha} with \alpha = 0.0--0.5, although the
identification of a Y dwarf is somewhat surprising and may be indicative of
either a flatter absolute magnitude/spectral type relation than previously
reported or an upturn in the number of very late type brown dwarfs in the
observed volume.Comment: Accepted for publication by ApJ Letters. 10 pages, 2 figure
Optical to near-infrared transmission spectrum of the warm sub-Saturn HAT-P-12b
We present the transmission spectrum of HAT-P-12b through a joint analysis of
data obtained from the Hubble Space Telescope Space Telescope Imaging
Spectrograph (STIS) and Wide Field Camera 3 (WFC3) and Spitzer, covering the
wavelength range 0.3-5.0 m. We detect a muted water vapor absorption
feature at 1.4 m attenuated by clouds, as well as a Rayleigh scattering
slope in the optical indicative of small particles. We interpret the
transmission spectrum using both the state-of-the-art atmospheric retrieval
code SCARLET and the aerosol microphysics model CARMA. These models indicate
that the atmosphere of HAT-P-12b is consistent with a broad range of
metallicities between several tens to a few hundred times solar, a roughly
solar C/O ratio, and moderately efficient vertical mixing. Cloud models that
include condensate clouds do not readily generate the sub-micron particles
necessary to reproduce the observed Rayleigh scattering slope, while models
that incorporate photochemical hazes composed of soot or tholins are able to
match the full transmission spectrum. From a complementary analysis of
secondary eclipses by Spitzer, we obtain measured depths of
and at 3.6 and 4.5 m, respectively, which are
consistent with a blackbody temperature of K and indicate
efficient day-night heat recirculation. HAT-P-12b joins the growing number of
well-characterized warm planets that underscore the importance of clouds and
hazes in our understanding of exoplanet atmospheres.Comment: 25 pages, 19 figures, accepted for publication in AJ, updated with
proof correction
Physical Properties of Emission-Line Galaxies at z ~ 2 from Near-Infrared Spectroscopy with Magellan FIRE
We present results from near-infrared spectroscopy of 26 emission-line
galaxies at z ~ 2 obtained with the FIRE spectrometer on the Magellan Baade
telescope. The sample was selected from the WISP survey, which uses the
near-infrared grism of the Hubble Space Telescope Wide Field Camera 3 to detect
emission-line galaxies over 0.3 < z < 2.3. Our FIRE follow-up spectroscopy
(R~5000) over 1.0-2.5 micron permits detailed measurements of physical
properties of the z~2 emission-line galaxies. Dust-corrected star formation
rates for the sample range from ~5-100 M_sun yr-1. We derive a median
metallicity for the sample of ~0.45 Z_sun, and the estimated stellar masses
range from ~10^8.5 - 10^9.5 M_sun. The average ionization parameters measured
for the sample are typically much higher than what is found for local
star-forming galaxies. We derive composite spectra from the FIRE sample, from
which we infer typical nebular electron densities of ~100-400 cm^-3. Based on
the location of the galaxies and composite spectra on BPT diagrams, we do not
find evidence for significant AGN activity in the sample. Most of the galaxies
as well as the composites are offset in the BPT diagram toward higher [O
III]/H-beta at a given [N II]/H-alpha, in agreement with other observations of
z > 1 star-forming galaxies, but composite spectra derived from the sample do
not show an appreciable offset from the local star-forming sequence on the [O
III]/H-beta versus [S II]/H-alpha diagram. We infer a high nitrogen-to-oxygen
abundance ratio from the composite spectrum, which may contribute to the offset
of the high-redshift galaxies from the local star-forming sequence in the [O
III]/H-beta versus [N II]/H-alpha diagram. We speculate that the elevated
nitrogen abundance could result from substantial numbers of Wolf-Rayet stars in
starbursting galaxies at z~2. (Abridged)Comment: Accepted for publication in Ap
Rosetta-Alice Observations of Exospheric Hydrogen and Oxygen on Mars
The European Space Agency's Rosetta spacecraft, en route to a 2014 encounter
with comet 67P/Churyumov-Gerasimenko, made a gravity assist swing-by of Mars on
25 February 2007, closest approach being at 01:54UT. The Alice instrument on
board Rosetta, a lightweight far-ultraviolet imaging spectrograph optimized for
in situ cometary spectroscopy in the 750-2000 A spectral band, was used to
study the daytime Mars upper atmosphere including emissions from exospheric
hydrogen and oxygen. Offset pointing, obtained five hours before closest
approach, enabled us to detect and map the HI Lyman-alpha and Lyman-beta
emissions from exospheric hydrogen out beyond 30,000 km from the planet's
center. These data are fit with a Chamberlain exospheric model from which we
derive the hydrogen density at the 200 km exobase and the H escape flux. The
results are comparable to those found from the the Ultraviolet Spectrometer
experiment on the Mariner 6 and 7 fly-bys of Mars in 1969. Atomic oxygen
emission at 1304 A is detected at altitudes of 400 to 1000 km above the limb
during limb scans shortly after closest approach. However, the derived oxygen
scale height is not consistent with recent models of oxygen escape based on the
production of suprathermal oxygen atoms by the dissociative recombination of
O2+.Comment: 17 pages, 8 figures, accepted for publication in Icaru
Euarchontan opsin variation brings new focus to primate origins
Debate on the adaptive origins of primates has long focused on the functional ecology of the primate visual system. For example, it is hypothesized that variable expression of short- (SWS1) and middle-to-long-wavelength sensitive (M/LWS) opsins, which confer color vision, can be used to infer ancestral activity patterns and therefore selective ecological pressures. A problem with this approach is that opsin gene variation is incompletely known in the grandorder Euarchonta, i.e., the orders Scandentia (treeshrews), Dermoptera (colugos), and Primates. The ancestral state of primate color vision is therefore uncertain. Here we report on the genes (OPN1SW and OPN1LW) that encode SWS1 and M/LWS opsins in seven species of treeshrew, including the sole nocturnal scandentian Ptilocercus lowii. In addition, we examined the opsin genes of the Central American woolly opossum (Caluromys derbianus), an enduring ecological analogue in the debate on primate origins. Our results indicate: 1) retention of ultraviolet (UV) sensitivity in C. derbianus and a shift from UV to blue spectral sensitivities at the base of Euarchonta; 2) ancient pseudogenization of OPN1SW in the ancestors of P. lowii, but a signature of purifying selection in those of C. derbianus; and, 3) the absence of OPN1LW polymorphism among diurnal treeshrews. These findings suggest functional variation in color vision of nocturnal mammals and a distinctive visual ecology of early primates, perhaps one that demanded greater spatial resolution under light levels that could support cone-mediated color discrimination
Epidemiology of injuries from fire, heat and hot substances: global, regional and national morbidity and mortality estimates from the Global Burden of Disease 2017 study
Background Past research has shown how fires, heat and hot substances are important causes of health loss globally. Detailed estimates of the morbidity and mortality from these injuries could help drive preventative measures and improved access to care.
Methods We used the Global Burden of Disease 2017 framework to produce three main results. First, we produced results on incidence, prevalence, years lived with disability, deaths, years of life lost and disability-adjusted life years from 1990 to 2017 for 195 countries and territories. Second, we analysed these results to measure mortality-to-incidence ratios by location. Third, we reported the measures above in terms of the cause of fire, heat and hot substances and the types of bodily injuries that result.
Results Globally, there were 8 991 468 (7 481 218 to 10 740 897) new fire, heat and hot substance injuries in 2017 with 120 632 (101 630 to 129 383) deaths. At the global level, the age-standardised mortality caused by fire, heat and hot substances significantly declined from 1990 to 2017, but regionally there was variability in age-standardised incidence with some regions experiencing an increase (eg, Southern Latin America) and others experiencing a significant decrease (eg, High-income North America).
Conclusions The incidence and mortality of injuries that result from fire, heat and hot substances affect every region of the world but are most concentrated in middle and lower income areas. More resources should be invested in measuring these injuries as well as in improving infrastructure, advancing safety measures and ensuring access to care.
This is an open access article distributed in accordance with the Creative Commons Attribution 4.0 Unported (CC BY 4.0) license, which permits others to copy, redistribute, remix, transform and build upon this work for any purpose, provided the original work is properly cited, a link to the licence is given, and indication of whether changes were made
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