9,403 research outputs found
Single-Degenerate Type Ia Supernovae Are Preferentially Overluminous
Recent observational and theoretical progress has favored merging and
helium-accreting sub-Chandrasekhar mass white dwarfs in the double-degenerate
and the double-detonation channels, respectively, as the most promising
progenitors of normal Type Ia supernovae (SNe Ia). Thus the fate of
rapidly-accreting Chandrasekhar mass white dwarfs in the single-degenerate
channel remains more mysterious then ever. In this paper, we clarify the nature
of ignition in Chandrasekhar-mass single-degenerate SNe Ia by analytically
deriving the existence of a characteristic length scale which establishes a
transition from central ignitions to buoyancy-driven ignitions. Using this
criterion, combined with data from three-dimensional simulations of convection
and ignition, we demonstrate that the overwhelming majority of ignition events
within Chandrasekhar-mass white dwarfs in the single-degenerate channel are
buoyancy-driven, and consequently lack a vigorous deflagration phase. We thus
infer that single-degenerate SNe Ia are generally expected to lead to
overluminous 1991T-like SNe Ia events. We establish that the rates predicted
from both the population of supersoft X-ray sources and binary population
synthesis models of the single-degenerate channel are broadly consistent with
the observed rates of overluminous SNe Ia, and suggest that the population of
supersoft X-ray sources are the dominant stellar progenitors of SNe 1991T-like
events. We further demonstrate that the single-degenerate channel contribution
to the normal and failed 2002cx-like rates is not likely to exceed 1% of the
total SNe Ia rate. We conclude with a range of observational tests of
overluminous SNe Ia which will either support or strongly constrain the
single-degenerate scenario.Comment: 12 pages, 2 figures, accepted to Astrophysical Journal. Comments
welcom
The Astrochemical Evolution of Turbulent Giant Molecular Clouds : I - Physical Processes and Method of Solution for Hydrodynamic, Embedded Starless Clouds
Contemporary galactic star formation occurs predominantly within
gravitationally unstable, cold, dense molecular gas within supersonic,
turbulent, magnetized giant molecular clouds (GMCs). Significantly, because the
chemical evolution timescale and the turbulent eddy-turnover timescale are
comparable at typical GMC conditions, molecules evolve via inherently
non-equilibrium chemistry which is strongly coupled to the dynamical evolution
of the cloud.
Current numerical simulation techniques, which include at most three decades
in length scale, can just begin to bridge the divide between the global
dynamical time of supersonic turbulent GMCs, and the thermal and chemical
evolution within the thin post-shock cooling layers of their background
turbulence. We address this GMC astrochemical scales problem using a solution
methodology, which permits both complex three-dimensional turbulent dynamics as
well as accurate treatment of non-equilibrium post-shock thermodynamics and
chemistry.
We present the current methodology in the context of the larger scope of
physical processes important in understanding the chemical evolution of GMCs,
including gas-phase chemistry, dust grains and surface chemistry, and turbulent
heating. We present results of a new Lagrangian verification test for
supersonic turbulence. We characterize the evolution of these species according
to the dimensionless local post-shock Damk\"{o}hler number, which quantifies
the ratio of the dynamical time in the post-shock cooling flow to the chemical
reaction time of a given species.
Lastly, we discuss implications of this work to the selection of GMC
molecular tracers, and the zeroing of chemical clocks of GMC cores.Comment: 35 pages, 7 figures, 16 tables. Accepted to MNRAS. Revised to correct
some typographic error
Characteristics of Billfish Anglers in the U.S. Atlantic Ocean
A mail survey of 1,984 U.S. billfish tournament anglers was completed to examine their fishing activity, attitudes, trip expenditures, consumer's surplus, catch levels, and management preferences. A sample of 1,984 anglers was drawn from billfish tournaments in the western Atlantic Ocean (from Maine to Texas, including Puerto Rico and the U.S. Virgin Islands) during 1989. A response rate of 61% was obtained (excluding nondeliverables). Anglers averaged 13 billfish trips per year, catching a billfish 40% of the time while 89% of billfish caught were released with <1 billfish per year per angler retained. Catch and retention rates varied by region. Expenditures averaged 262 per angler, but increased to 179,425,000 in pursuit of billfish in 1989. Anglers opposed management options that would diminish their ability to catch a billfish, but supported options limiting the number of billfish landed
Color homography
We show the surprising result that colors across a change in viewing
condition (changing light color, shading and camera) are related by a
homography. Our homography color correction application delivers improved color
fidelity compared with the linear least-square.Comment: Accepted by Progress in Colour Studies 201
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