1,306 research outputs found
On the Spectroscopic Diversity of Type Ia Supernovae
A comparison of the ratio of the depths of two absorption features in the
spectra of TypeIa supernovae (SNe Ia) near the time of maximum brightness with
the blueshift of the deep red Si II absorption feature 10 days after maximum
shows that the spectroscopic diversity of SNe Ia is multi-dimensional. There is
a substantial range of blueshifts at a given value of the depth ratio. We also
find that the spectra of a sample of SNe Ia obtained a week before maximum
brightness can be arranged in a ``blueshift sequence'' that mimics the time
evolution of the pre-maximum-light spectra of an individual SN Ia, the well
observed SN 1994D. Within the context of current SN Ia explosion models, we
suggest that some of the SNe Ia in our sample were delayed-detonations while
others were plain deflagrations.Comment: accepted for publication in ApJ
Analysis of the Type IIn Supernova 1998S: Effects of Circumstellar Interaction on Observed Spectra
We present spectral analysis of early observations of the Type IIn supernova
1998S using the general non-local thermodynamic equilibrium atmosphere code \tt
PHOENIX}. We model both the underlying supernova spectrum and the overlying
circumstellar interaction region and produce spectra in good agreement with
observations. The early spectra are well fit by lines produced primarily in the
circumstellar region itself, and later spectra are due primarily to the
supernova ejecta. Intermediate spectra are affected by both regions. A
mass-loss rate of order \msol yr is inferred
for a wind speed of 100-1000 \kmps. We discuss how future self-consistent
models will better clarify the underlying progenitor structure.Comment: to appear in ApJ, 2001, 54
UAS-SfM for coastal research : geomorphic feature extraction and land cover classification from high-resolution elevation and optical imagery
© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Remote Sensing 9 (2017): 1020, doi:10.3390/rs9101020.The vulnerability of coastal systems to hazards such as storms and sea-level rise is typically characterized using a combination of ground and manned airborne systems that have limited spatial or temporal scales. Structure-from-motion (SfM) photogrammetry applied to imagery acquired by unmanned aerial systems (UAS) offers a rapid and inexpensive means to produce high-resolution topographic and visual reflectance datasets that rival existing lidar and imagery standards. Here, we use SfM to produce an elevation point cloud, an orthomosaic, and a digital elevation model (DEM) from data collected by UAS at a beach and wetland site in Massachusetts, USA. We apply existing methods to (a) determine the position of shorelines and foredunes using a feature extraction routine developed for lidar point clouds and (b) map land cover from the rasterized surfaces using a supervised classification routine. In both analyses, we experimentally vary the input datasets to understand the benefits and limitations of UAS-SfM for coastal vulnerability assessment. We find that (a) geomorphic features are extracted from the SfM point cloud with near-continuous coverage and sub-meter precision, better than was possible from a recent lidar dataset covering the same area; and (b) land cover classification is greatly improved by including topographic data with visual reflectance, but changes to resolution (when <50 cm) have little influence on the classification accuracy.This project was funded by the U.S. Geological Survey (USGS) Coastal and Marine Geology
Program and the Department of the Interior Northeast Climate Science Center
Quantitative analysis of cell types during growth and morphogenesis in Hydra
Tissue maceration was used to determine the absolute number and the distribution of cell types in Hydra. It was shown that the total number of cells per animal as well as the distribution of cells vary depending on temperature, feeding conditions, and state of growth. During head and foot regeneration and during budding the first detectable change in the cell distribution is an increase in the number of nerve cells at the site of morphogenesis. These results and the finding that nerve cells are most concentrated in the head region, diminishing in density down the body column, are discussed in relation to tissue polarity
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