11,521 research outputs found
A GENERAL, DYNAMIC, SUPPLY-RESPONSE MODEL
Resource /Energy Economics and Policy,
Estimating Parameters of a Renewable Resource Model Without Population Data
A general approach to determining parameters of a traditional bioeconomic model is offered for the situation in which knowledge of resource abundance is unknown. Production parameters (such as catchability coefficients) and biological factors (such as natural mortality and recruitment) are included in the model. The general model is articulated for a typical fishery and further specified to obtain estimates of parameters for the St. John's River shad fishery. The results, considering the illustrative nature of the analysis, are promising and suggest avenues of additional research.Environmental Economics and Policy, Research Methods/ Statistical Methods, Resource /Energy Economics and Policy, Risk and Uncertainty,
C^+ distribution around S1 in rho Ophiuchi
We analyze a [C II] 158 micron map obtained with the L2 GREAT receiver on
SOFIA of the emission/reflection nebula illuminated by the early B star S1 in
the rho-OphA cloud core. This data set has been complemented with maps of
CO(3-2), 13CO(3-2) and C18O(3-2), observed as a part of the JCMT Gould Belt
Survey, with archival HCO^+(4-3) JCMT data, as well as with [O I] 63 and 145
micron imaging with Herschel/PACS. The [C II] emission is completely dominated
by the strong PDR emission from the nebula surrounding S1 expanding into the
dense Oph A molecular cloud west and south of S1. The [C II] emission is
significantly blue shifted relative to the CO spectra and also relative to the
systemic velocity, particularly in the northwestern part of the nebula. The [C
II] lines are broader towards the center of the S1 nebula and narrower towards
the PDR shell. The [C II] lines are strongly self-absorbed over an extended
region in the S1 PDR. Based on the strength of the [13C II] F = 2-1 hyperfine
component, [C II] is significantly optically thick over most of the nebula. CO
and 13CO(3-2) spectra are strongly self-absorbed, while C18O(3-2) is single
peaked and centered in the middle of the self-absorption. We have used a simple
two-layer LTE model to characterize the background and foreground cloud
contributing to the [C II] emission. From this analysis we estimate the
extinction due to the foreground cloud to be ~9.9 mag, which is slightly less
than the reddening estimated towards S1. Since some of the hot gas in the PDR
is not traced by low J CO emission, this result appears quite plausible. Using
a plane parallel PDR model with the observed [OI(145)]/[C II] brightness ratio
and an estimated FUV intensity of 3100-5000 G0 suggests that the density of the
[C II] emitting gas is ~3-4x10^3 cm^-3.Comment: Accepted for publication in Astronomy & Astrophysic
Opening the Treasure Chest in Carina
We have mapped the G287.84-0.82 cometary globule (with the Treasure Chest
cluster embedded in it) in the South Pillars region of Carina (i) in [CII],
63micron [OI], and CO(11-10) using upGREAT on SOFIA and (ii) in J=2-1
transitions of CO, 13CO, C18O and J=3-2 transitions of H2CO using the APEX
telescope in Chile. We probe the morphology, kinematics, and physical
conditions of the molecular gas and the photon dominated regions (PDRs) in
G287.84-0.82. The [CII] and [OI] emission suggest that the overall structure of
the pillar (with red-shifted photo evaporating tails) is consistent with the
effect of FUV radiation and winds from eta-Car and O stars in Trumpler 16. The
gas in the head of the pillar is strongly influenced by the embedded cluster,
whose brightest member is an O9.5V star, CPD-59 2661. The emission of the [CII]
and [OI] lines peak at a position close to the embedded star, while all other
tracers peak at another position lying to the north-east consistent with gas
being compressed by the expanding PDR created by the embedded cluster. The
molecular gas inside the globule is probed with the J=2-1 transitions of CO and
isotopologues as well as H2CO, and analyzed using a non-LTE model
(escape-probability approach), while we use PDR models to derive the physical
conditions of the PDR. We identify at least two PDR gas components; the diffuse
part (~10^4 cm^-3) is traced by [CII], while the dense (n~ 2-8x10^5 cm^-3) part
is traced by [CII], [OI], CO(11-10). Using the F=2-1 transition of [13CII]
detected at 50 positions in the region, we derive optical depths (0.9-5),
excitation temperatures of [CII] (80-255 K), and N(C+) of 0.3-1x10^19 cm^-2.
The total mass of the globule is ~1000 Msun, about half of which is traced by
[CII]. The dense PDR gas has a thermal pressure of 10^7-10^8 K cm^-3, which is
similar to the values observed in other regions.Comment: Accepted for publication in Astronomy and Astrophysics (abstract
slightly abridged
A decreased probability of habitable planet formation around low-mass stars
Smaller terrestrial planets (< 0.3 Earth masses) are less likely to retain
the substantial atmospheres and ongoing tectonic activity probably required to
support life. A key element in determining if sufficiently massive "sustainably
habitable" planets can form is the availability of solid planet-forming
material. We use dynamical simulations of terrestrial planet formation from
planetary embryos and simple scaling arguments to explore the implications of
correlations between terrestrial planet mass, disk mass, and the mass of the
parent star. We assume that the protoplanetary disk mass scales with stellar
mass as Mdisk ~ f Mstar^h, where f measures the relative disk mass, and 1/2 < h
< 2, so that disk mass decreases with decreasing stellar mass. We consider
systems without Jovian planets, based on current models and observations for M
stars. We assume the mass of a planet formed in some annulus of a disk with
given parameters is proportional to the disk mass in that annulus, and show
with a suite of simulations of late-stage accretion that the adopted
prescription is surprisingly accurate. Our results suggest that the fraction of
systems with sufficient disk mass to form > 0.3 Earth mass habitable planets
decreases for low-mass stars for every realistic combination of parameters.
This "habitable fraction" is small for stellar masses below a mass in the
interval 0.5 to 0.8 Solar masses, depending on disk parameters, an interval
that excludes most M stars. Radial mixing and therefore water delivery are
inefficient in lower-mass disks commonly found around low-mass stars, such that
terrestrial planets in the habitable zones of most low-mass stars are likely to
be small and dry.Comment: Accepted to ApJ. 11 pages, 6 figure
Planet Formation with Migration
In the core-accretion model, gas-giant planets form solid cores which then
accrete gaseous envelopes. Tidal interactions with disk gas cause a core to
undergo inward type-I migration in 10^4 to 10^5 years. Cores must form faster
than this to survive. Giant planets clear a gap in the disk and undergo inward
type-II migration in <10^6 years if observed disk accretion rates apply to the
disk as a whole. Type-II migration times exceed typical disk lifetimes if
viscous accretion occurs mainly in the surface layers of disks. Low turbulent
viscosities near the midplane may allow planetesimals to form by coagulation of
dust grains. The radius r of such planetesimals is unknown. If r<0.5 km, the
core formation time is shorter than the type-I migration timescale and cores
will survive. Migration is substantial in most cases, leading to a wide range
of planetary orbits, consistent with the observed variety of extrasolar
systems. When r is of order 100m and midplane alpha is of order 3 times 10^-5,
giant planets similar to those in the Solar System can form.Comment: 12 pages including 4 figure
Surface effects on nanowire transport: numerical investigation using the Boltzmann equation
A direct numerical solution of the steady-state Boltzmann equation in a
cylindrical geometry is reported. Finite-size effects are investigated in large
semiconducting nanowires using the relaxation-time approximation. A nanowire is
modelled as a combination of an interior with local transport parameters
identical to those in the bulk, and a finite surface region across whose width
the carrier density decays radially to zero. The roughness of the surface is
incorporated by using lower relaxation-times there than in the interior.
An argument supported by our numerical results challenges a commonly used
zero-width parametrization of the surface layer. In the non-degenerate limit,
appropriate for moderately doped semiconductors, a finite surface width model
does produce a positive longitudinal magneto-conductance, in agreement with
existing theory. However, the effect is seen to be quite small (a few per cent)
for realistic values of the wire parameters even at the highest practical
magnetic fields. Physical insights emerging from the results are discussed.Comment: 15 pages, 7 figure
The (In)Stability of Planetary Systems
We present results of numerical simulations which examine the dynamical
stability of known planetary systems, a star with two or more planets. First we
vary the initial conditions of each system based on observational data. We then
determine regions of phase space which produce stable planetary configurations.
For each system we perform 1000 ~1 million year integrations. We examine
upsilon And, HD83443, GJ876, HD82943, 47UMa, HD168443, and the solar system
(SS). We find that the resonant systems, 2 planets in a first order mean motion
resonance, (HD82943 and GJ876) have very narrow zones of stability. The
interacting systems, not in first order resonance, but able to perturb each
other (upsilon And, 47UMa, and SS) have broad regions of stability. The
separated systems, 2 planets beyond 10:1 resonance, (we only examine HD83443
and HD168443) are fully stable. Furthermore we find that the best fits to the
interacting and resonant systems place them very close to unstable regions. The
boundary in phase space between stability and instability depends strongly on
the eccentricities, and (if applicable) the proximity of the system to perfect
resonance. In addition to million year integrations, we also examined stability
on ~100 million year timescales. For each system we ran ~10 long term
simulations, and find that the Keplerian fits to these systems all contain
configurations which may be regular on this timescale.Comment: 37 pages, 49 figures, 13 tables, submitted to Ap
Ligation of protease-activated receptor 1 enhances alpha(v)beta(6) integrin-dependent TGF-beta activation and promotes acute lung injury
Activation of latent TGF-beta by the alpha(v)beta(6) integrin is a critical step in the development of acute lung injury. However, the mechanism by which a alpha(v)beta(6)-mediated TGF-beta activation is regulated has not been identified. We show that thrombin, and other agonists of protease-activated receptor 1(PAR1), activate TGF-beta in an alpha(v)beta(6) integrin-specific manner. This effect is PART specific and is mediated by RhoA and Rho kinase. Intratracheal instillation of the PART-specific peptide TFLLRN increases lung edema during high-tidal-volume ventilation, and this effect is completely inhibited by a blocking antibody against the alpha(v)beta(6) integrin. Instillation of TFLLRN during high-tidal-volume ventilation is associated with increased pulmonary TGF-beta activation; however, this is not observed in Itgb6(-/-) mice. Furthermore, Itgb6(-/-) mice are also protected from ventilator-induced lung edema. We also demonstrate that pulmonary edema and TGF-beta activity are similarly reduced in Par1(-/-) mice following bleomycin-induced lung injury. These results suggest that PART-mediated enhancement of a alpha(v)beta(6)-dependent TGF-beta activation could be one mechanism by which activation of the coagulation cascade contributes to the development of acute lung injury, and they identify PART and the alpha(v)beta(6) integrin as potential therapeutic targets in this condition
Determining the phonon DOS from specific heat measurements via maximum entropy methods
The maximum entropy and reverse Monte-Carlo methods are applied to the
computation of the phonon density of states (DOS) from heat capacity data. The
approach is introduced and the formalism is described. Simulated data is used
to test the method, and its sensitivity to noise. Heat capacity measurements
from diamond are used to demonstrate the use of the method with experimental
data. Comparison between maximum entropy and reverse Monte-Carlo results shows
the form of the entropy used here is correct, and that results are stable and
reliable. Major features of the DOS are picked out, and acoustic and optical
phonons can be treated with the same approach. The treatment set out in this
paper provides a cost-effective and reliable method for studies of the phonon
properties of materials.Comment: Reprint to improve access. 10 pages, 6 figure
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