2,008 research outputs found
Supernova Resonance--scattering Line Profiles in the Absence of a Photosphere
In supernova spectroscopy relatively little attention has been given to the
properties of optically thick spectral lines in epochs following the
photosphere's recession. Most treatments and analyses of post-photospheric
optical spectra of supernovae assume that forbidden-line emission comprises
most if not all spectral features. However, evidence exists which suggests that
some spectra exhibit line profiles formed via optically thick
resonance-scattering even months or years after the supernova explosion. To
explore this possibility we present a geometrical approach to supernova
spectrum formation based on the "Elementary Supernova" model, wherein we
investigate the characteristics of resonance-scattering in optically thick
lines while replacing the photosphere with a transparent central core emitting
non-blackbody continuum radiation, akin to the optical continuum provided by
decaying 56Co formed during the explosion. We develop the mathematical
framework necessary for solving the radiative transfer equation under these
conditions, and calculate spectra for both isolated and blended lines. Our
comparisons with analogous results from the Elementary Supernova code SYNOW
reveal several marked differences in line formation. Most notably, resonance
lines in these conditions form P Cygni-like profiles, but the emission peaks
and absorption troughs shift redward and blueward, respectively, from the
line's rest wavelength by a significant amount, despite the spherically
symmetric distribution of the line optical depth in the ejecta. These
properties and others that we find in this work could lead to misidentification
of lines or misattribution of properties of line-forming material at
post-photospheric times in supernova optical spectra.Comment: 37 pages, 24 figures; accepted for publication in ApJ Supplement
Serie
Future research to underpin successful peste des petits ruminants virus (PPRV) eradication
Peste des petits ruminants virus (PPRV) is a significant pathogen of small ruminants and is prevalent in much of Africa, the Near and Middle East and Asia. Despite the availability of an efficacious and cheap live-attenuated vaccine, the virus has continued to spread, with its range stretching from Morocco in the west to China and Mongolia in the east. Some of the worldâs poorest communities rely on small ruminant farming for subsistence and the continued endemicity of PPRV is a constant threat to their livelihoods. Moreover, PPRVâs effects on the worldâs population are felt broadly across many economic, agricultural and social situations. This far-reaching impact has prompted the Food and Agriculture Organization of the United Nations (FAO) and the World Organisation for Animal Health (OIE) to develop a global strategy for the eradication of this virus and its disease. PPRV is a morbillivirus and, given the experience of these organizations in eradicating the related rinderpest virus, the eradication of PPRV should be feasible. However, there are many critical areas where basic and applied virological research concerning PPRV is lacking. The purpose of this review is to highlight areas where new research could be performed in order to guide and facilitate the eradication programme. These areas include studies on disease transmission and epidemiology, the existence of wildlife reservoirs and the development of next-generation vaccines and diagnostics. With the support of the international virology community, the successful eradication of PPRV can be achieved
The ACS Survey of Galactic Globular Clusters. II. Stellar Evolution Tracks, Isochrones, Luminosity Functions, and Synthetic Horizontal-Branch Models
The ACS Survey of Galactic Globular Clusters, an HST Treasury Project, will deliver high-quality, homogeneous photometry of 65 globular clusters. This paper introduces a new collection of stellar evolution tracks and isochrones suitable for analyzing the ACS survey data. Stellar evolution models were computed at [Fe/H] = -2.5, -2.0, -1.5, -1.0, -0.5, and 0; [α/Fe] = -0.2, 0, 0.2, 0.4, 0.6, and 0.8; and three initial He abundances for masses from 0.1 to 1.8 Mâ and ages from 2 to 15 Gyr. Each isochrone spans a wide range in luminosity, from MV ~ 14 up to the tip of the red giant branch. These are complemented by a set of He-burning tracks that extend from the zero-age horizontal branch to the onset of thermal pulsations on the asymptotic giant branch. In addition, a set of computer programs are provided that make it possible to interpolate the isochrones in [Fe/H], generate luminosity functions from the isochrones, and create synthetic horizontal-branch models. The tracks and isochrones have been converted to the observational plane with two different color-Teff transformations, one synthetic and one semiempirical, in ground-based B, V, and I, and F606W and F814W for both ACS WFC and WFPC2 systems. All models and programs presented in this paper are available at the Dartmouth Stellar Evolution Database and the Multimission Archive at the Space Telescope Science Institute
Near-infrared line identification in type Ia supernovae during the transitional phase
We present near-infrared synthetic spectra of a delayed-detonation
hydrodynamical model and compare them to observed spectra of four normal type
Ia supernovae ranging from day +56.5 to day +85. This is the epoch during which
supernovae are believed to be undergoing the transition from the photospheric
phase, where spectra are characterized by line scattering above an optically
thick photosphere, to the nebular phase, where spectra consist of optically
thin emission from forbidden lines. We find that most spectral features in the
near-infrared can be accounted for by permitted lines of Fe II and Co II. In
addition, we find that [Ni II] fits the emission feature near 1.98 {\mu}m,
suggesting that a substantial mass of 58Ni exists near the center of the ejecta
in these objects, arising from nuclear burning at high density. A tentative
identification of Mn II at 1.15 {\mu}m may support this conclusion as well.Comment: accepted to Ap
Stellar Population Models and Individual Element Abundances I: Sensitivity of Stellar Evolution Models
Integrated light from distant galaxies is often compared to stellar
population models via the equivalent widths of spectral features--spectral
indices--whose strengths rely on the abundances of one or more elements. Such
comparisons hinge not only on the overall metal abundance but also on relative
abundances. Studies have examined the influence of individual elements on
synthetic spectra but little has been done to address similar issues in the
stellar evolution models that underlie most stellar population models. Stellar
evolution models will primarily be influenced by changes in opacities. In order
to explore this issue in detail, twelve sets of stellar evolution tracks and
isochrones have been created at constant heavy element mass fraction Z that
self-consistently account for varying heavy element mixtures. These sets
include scaled-solar, alpha-enhanced, and individual cases where the elements
C, N, O, Ne, Mg, Si, S, Ca, Ti, and Fe have been enhanced above their
scaled-solar values. The variations that arise between scaled-solar and the
other cases are examined with respect to the H-R diagram and main sequence
lifetimes.Comment: 33 pages, 13 figures, accepted to Ap
The 2010 Interferometric Imaging Beauty Contest
We present the results of the fourth Optical/IR Interferometry Imaging Beauty
Contest. The contest consists of blind imaging of test data sets derived from
model sources and distributed in the OI-FITS format. The test data consists of
spectral data sets on an object "observed" in the infrared with spectral
resolution. There were 4 different algorithms competing this time: BSMEM the
Bispectrum Maximum Entropy Method by Young, Baron & Buscher; RPR the Recursive
Phase Reconstruction by Rengaswamy; SQUEEZE a Markov Chain Monte Carlo
algorithm by Baron, Monnier & Kloppenborg; and, WISARD the Weak-phase
Interferometric Sample Alternating Reconstruction Device by Vannier & Mugnier.
The contest model image, the data delivered to the contestants and the rules
are described as well as the results of the image reconstruction obtained by
each method. These results are discussed as well as the strengths and
limitations of each algorithm.Comment: To be published in SPIE 2010 "Optical and infrared interferometry II
A Location-based Approach for Distributed Kiosk Design
Electronic kiosk interface design and implementation metrics have been well established. The problem arises when more than one kiosk is utilized in a different location within the same geographic proximity using the same basic informational parameters. This manuscript describes the design implications of a distributed kiosk environment from the standpoint of a field experiment. The log files from 2 kiosks deployed in the same building are analyzed for correlations among kiosk location and information required. The results show that while kiosk systems deployed in âprimary entrancesâ should have a broad view of pertinent information, kiosks deployed in more remote locations should have information pertinent to that area initially presented to the individual. This research provides both confirmatory evidence and a checklist of implementation decision points for those who wish to implement a distributed kiosk architecture
Preliminary Spectral Analysis of the Type II Supernova 1999em
We have calculated fast direct spectral model fits to two early-time spectra
of the Type-II plateau SN 1999em, using the SYNOW synthetic spectrum code. The
first is an extremely early blue optical spectrum and the second a combined HST
and optical spectrum obtained one week later. Spectroscopically this supernova
appears to be a normal Type II and these fits are in excellent agreement with
the observed spectra. Our direct analysis suggests the presence of enhanced
nitrogen. We have further studied these spectra with the full NLTE general
model atmosphere code PHOENIX. While we do not find confirmation for enhanced
nitrogen (nor do we rule it out), we do require enhanced helium. An even more
intriguing possible line identification is complicated Balmer and He I lines,
which we show falls naturally out of the detailed calculations with a shallow
density gradient. We also show that very early spectra such as those presented
here combined with sophisticated spectral modeling allows an independent
estimate of the total reddening to the supernova, since when the spectrum is
very blue, dereddening leads to changes in the blue flux that cannot be
reproduced by altering the ``temperature'' of the emitted radiation. These
results are extremely encouraging since they imply that detailed modeling of
early spectra can shed light on both the abundances and total extinction of SNe
II, the latter improving their utility and reliability as distance indicators.Comment: to appear in ApJ, 2000, 54
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