2,462 research outputs found
Collisions and close encounters involving massive main-sequence stars
We study close encounters involving massive main sequence stars and the
evolution of the exotic products of these encounters as common--envelope
systems or possible hypernova progenitors. We show that parabolic encounters
between low-- and high--mass stars and between two high--mass stars with small
periastrons result in mergers on timescales of a few tens of stellar freefall
times (a few tens of hours). We show that such mergers of unevolved low--mass
stars with evolved high--mass stars result in little mass loss (
M) and can deliver sufficient fresh hydrogen to the core of the
collision product to allow the collision product to burn for several million
years. We find that grazing encounters enter a common--envelope phase which may
expel the envelope of the merger product. The deposition of energy in the
envelopes of our merger products causes them to swell by factors of .
If these remnants exist in very densely-populated environments
( pc), they will suffer further collisions which may
drive off their envelopes, leaving behind hard binaries. We show that the
products of collisions have cores rotating sufficiently rapidly to make them
candidate hypernova/gamma--ray burst progenitors and that of massive
stars may suffer collisions, sufficient for such events to contribute
significantly to the observed rates of hypernovae and gamma--ray bursts.Comment: 15 pages, 13 figures, LaTeX, to appear in MNRAS (in press
Capacity And Capacity Utilization In Fishing Industries
The definition and measurement of capacity in fishing and other natural resource industries possess unique problems because of the stock-flow production technology, in which inputs are applied to the natural resource stock to produce a flow of output. In addition, there are often multiple resource stocks, corresponding to different species, with a mobile stock of capital that can exploit one or more of these stocks. In turn, this leads to three unique issues: (1) multiple stocks of capital and the resource; (2) that of aggregation or how to define the industry and resource stocks to consider; and (3), that of latent capacity or how to include stocks of capital that are currently inactive or exploit the resource stock only at low levels of variable input utilization. This paper presents appropriate definitions of capacity and methods for measuring capacity in fishing industries taking into consideration these issues.https://scholarworks.wm.edu/vimsbooks/1060/thumbnail.jp
The fragmentation of expanding shells III: Oligarchic accretion and the mass spectrum of fragments
We use SPH simulations to investigate the gravitational fragmentation of
expanding shells through the linear and non--linear regimes. The results are
analysed using spherical harmonic decomposition to capture the initiation of
structure during the linear regime; the potential-based method of Smith et al.
(2009) to follow the development of clumps in the mildly non-linear regime; and
sink particles to capture the properties of the final bound objects during the
highly non-linear regime. In the early, mildly non--linear phase of
fragmentation, we find that the clump mass function still agrees quite well
with the mass function predicted by the analytic model. However, the sink mass
function is quite different, in the sense of being skewed towards high-mass
objects. This is because, once the growth of a condensation becomes non-linear,
it tends to be growing non-competitively from its own essentially separate
reservoir; we call this Oligarchic Accretion.Comment: 14 pages, accepted for publication in MNRA
THE PROFITABILITY OF SUSTAINABLE AGRICULTURE ON A REPRESENTATIVE GRAIN FARM IN THE MID-ATLANTIC REGION, 1981-89
A long-term whole-farm analysis compared conventional and low-input farming systems. Data from a nine-year agronomic study at the Rodale Research Farm, Kutztown, Pennsylvania, were used to analyze profitability, liquidity, solvency, and risk on a representative commercial grain farm. Conventional and low-input farms participating in government programs are the most profitable scenarios, followed by conventional and low-input farms not participating in government programs. All farms increased their net worth. The low-input approach is advantageous for risk-averse farmers using a safety-first criterion.Crop Production/Industries, Environmental Economics and Policy,
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