4,905 research outputs found
The Exciting Lives of Giant Molecular Clouds
We present a detailed study of the evolution of GMCs in a galactic disc
simulation. We follow individual GMCs (defined in our simulations by a total
column density criterion), including their level of star formation, from their
formation to dispersal. We find the evolution of GMCs is highly complex. GMCs
often form from a combination of smaller clouds and ambient ISM, and similarly
disperse by splitting into a number of smaller clouds and ambient ISM. However
some clouds emerge as the result of the disruption of a more massive GMC,
rather than from the assembly of smaller clouds. Likewise in some cases, clouds
accrete onto more massive clouds rather than disperse. Because of the
difficulty of determining a precursor or successor of a given GMC, determining
GMC histories and lifetimes is highly non-trivial. Using a definition relating
to the continuous evolution of a cloud, we obtain lifetimes typically of 4-25
Myr for >10^5 M GMCs, over which time the star formation efficiency
is about 1 %. We also relate the lifetime of GMCs to their crossing time. We
find that the crossing time is a reasonable measure of the actual lifetime of
the cloud, although there is considerable scatter. The scatter is found to be
unavoidable because of the complex and varied shapes and dynamics of the
clouds. We study cloud dispersal in detail and find both stellar feedback and
shear contribute to cloud disruption. We also demonstrate that GMCs do not
behave as ridge clouds, rather massive spiral arm GMCs evolve into smaller
clouds in inter-arm spurs.Comment: 15 pages, 16 figures, accepted for publication in MNRA
Giant Molecular clouds: what are they made from, and how do they get there?
We analyse the results of four simulations of isolated galaxies: two with a
rigid spiral potential of fixed pattern speed, but with different degrees of
star-formation induced feedback, one with an axisymmetric galactic potential
and one with a `live' self-gravitating stellar component. Since we use a
Lagrangian method we are able to select gas that lies within giant molecular
clouds (GMCs) at a particular timeframe, and to then study the properties of
this gas at earlier and later times. We find that gas which forms GMCs is not
typical of the interstellar medium at least 50 Myr before the clouds form and
reaches mean densities within an order of magnitude of mean cloud densities by
around 10 Myr before. The gas in GMCs takes at least 50 Myr to return to
typical ISM gas after dispersal by stellar feedback, and in some cases the gas
is never fully recycled. We also present a study of the two-dimensional,
vertically-averaged velocity fields within the ISM. We show that the velocity
fields corresponding to the shortest timescales (that is, those timescales
closest to the immediate formation and dissipation of the clouds) can be
readily understood in terms of the various cloud formation and dissipation
mechanisms. Properties of the flow patterns can be used to distinguish the
processes which drive converging flows (e.g.\ spiral shocks, supernovae) and
thus molecular cloud formation, and we note that such properties may be
detectable with future observations of nearby galaxies.Comment: 13 pages, 8 figures, accepted for publication in MNRA
Cloud angular momentum and effective viscosity in global SPH simulations with feedback
We examine simulations of isolated galaxies to analyse the effects of localized feedback on the formation and evolution of molecular clouds. Feedback contributes to turbulence and the destruction of clouds, leading to a population of clouds that is younger, less massive, and with more retrograde rotation. We investigate the evolution of clouds as they interact with each other and the diffuse interstellar medium, and determine that the role of cloud interactions differs strongly with the presence of feedback: in models without feedback, scattering events dramatically increase the retrograde fraction, but in models with feedback, mergers between clouds may slightly increase the prograde fraction. We also produce an estimate of the viscous time-scale due to cloud–cloud collisions, which increases with increasing strength of feedback (tν ∼ 20 Gyr versus tν ∼ 10 Gyr), but is still much smaller than previous estimates (tν ∼ 1000 Gyr); although collisions become more frequent with feedback, less energy is lost in each collision than in the models without feedback
Circulation and Dissipation on Hot Jupiters
Many global circulation models predict supersonic zonal winds and large
vertical shears in the atmospheres of short-period jovian exoplanets. Using
linear analysis and nonlinear local simulations, we investigate hydrodynamic
dissipation mechanisms to balance the thermal acceleration of these winds. The
adiabatic Richardson criterion remains a good guide to linear stability,
although thermal diffusion allows some modes to violate it at very long
wavelengths and very low growth rates. Nonlinearly, wind speeds saturate at
Mach numbers and Richardson numbers for a broad
range of plausible diffusivities and forcing strengths. Turbulence and vertical
mixing, though accompanied by weak shocks, dominate the dissipation, which
appears to be the outcome of a recurrent Kelvin-Helmholtz instability. An
explicit shear viscosity, as well as thermal diffusivity, is added to ZEUS to
capture dissipation outside of shocks. The wind speed is not monotonic nor
single valued for shear viscosities larger than about of the sound
speed times the pressure scale height. Coarsening the numerical resolution can
also increase the speed. Hence global simulations that are incapable of
representing vertical turbulence and shocks, either because of reduced physics
or because of limited resolution, may overestimate wind speeds. We recommend
that such simulations include artificial dissipation terms to control the Mach
and Richardson numbers and to capture mechanical dissipation as heat.Comment: 34 pages, 10 figure
Potential for the Conservation Security Program to Induce More Ecologically Diverse Crop Rotations in the Western Corn Belt
The potential of the USDA’s new Conservation Security Program (CSP) for inducing farmers to adopt more ecologically diverse crop rotations in the Western Corn Belt is examined. Simulations are conducted for a representative farm model in southeastern South Dakota, using different assumptions about commodity policies and CSP payments.CRP, Conservation, crop rotation, crop diversity
Long term evolution of planetary systems with a terrestrial planet and a giant planet
We study the long term orbital evolution of a terrestrial planet under the
gravitational perturbations of a giant planet. In particular, we are interested
in situations where the two planets are in the same plane and are relatively
close. We examine both possible configurations: the giant planet orbit being
either outside or inside the orbit of the smaller planet. The perturbing
potential is expanded to high orders and an analytical solution of the
terrestrial planetary orbit is derived. The analytical estimates are then
compared against results from the numerical integration of the full equations
of motion and we find that the analytical solution works reasonably well. An
interesting finding is that the new analytical estimates improve greatly the
predictions for the timescales of the orbital evolution of the terrestrial
planet compared to an octupole order expansion. Finally, we briefly discuss
possible applications of the analytical estimates in astrophysical problems.Comment: Accepted for publication in MNRA
Shocks, cooling and the origin of star formation rates in spiral galaxies
Understanding star formation is problematic as it originates in the large
scale dynamics of a galaxy but occurs on the small scale of an individual star
forming event. This paper presents the first numerical simulations to resolve
the star formation process on sub-parsec scales, whilst also following the
dynamics of the interstellar medium (ISM) on galactic scales. In these models,
the warm low density ISM gas flows into the spiral arms where orbit crowding
produces the shock formation of dense clouds, held together temporarily by
their external pressure. Cooling allows the gas to be compressed to
sufficiently high densities that local regions collapse under their own gravity
and form stars. The star formation rates follow a Schmidt-Kennicutt
\Sigma_{SFR} ~ \Sigma_{gas}^{1.4} type relation with the local surface density
of gas while following a linear relation with the cold and dense gas. Cooling
is the primary driver of star formation and the star formation rates as it
determines the amount of cold gas available for gravitational collapse. The
star formation rates found in the simulations are offset to higher values
relative to the extragalactic values, implying a constant reduction, such as
from feedback or magnetic fields, is likely to be required. Intriguingly, it
appears that a spiral or other convergent shock and the accompanying thermal
instability can explain how star formation is triggered, generate the physical
conditions of molecular clouds and explain why star formation rates are tightly
correlated to the gas properties of galaxies.Comment: 13 pages, 12 figures. MNRAS in pres
Effects on muscle tension and tracking task performance of simulated sonic booms with low and high intensity vibrational components
Effects of simulated sonic booms with high and low intensity vibrational components on tracking task performance and muscle tension in human subject
Age distributions of star clusters in spiral and barred galaxies as a test for theories of spiral structure
We consider models of gas flow in spiral galaxies in which the spiral
structure has been excited by various possible mechanisms: a global steady
density wave, self-gravity of the stellar disc and an external tidal
interaction, as well as the case of a galaxy with a central rotating bar. In
each model we estimate in a simple manner the likely current positions of star
clusters of a variety of ages, ranging from ~ 2 Myr to around 130 Myr,
depending on the model. We find that the spatial distribution of cluster of
different ages varies markedly depending on the model, and propose that
observations of the locations of age-dated stellar clusters is a possible
discriminant between excitation mechanisms for spiral structure in an
individual galaxy.Comment: 10 pages, 4 figures, accepted for publication in MNRA
Two-photon width of the charmonium state X_(c2)
The two-photon width of X_(c2)^3P_2 state of charmonium has been measured using 14.4 fb^(-1) of e^+e^-data taken at √s
=9.46–11.30 GeV with the CLEO III detector. The yy-fusion reaction studied is e^+e^- → e^+e^-yy, → yy X_(c2) → yJ/Ψ → ye^+e^-(µ^+µ^-). We measure Г_(yy) (X_(c2))B(X_(c2)) → y
J/Ψ)B(J/Ψ → e^+e^- + µ^+µ^-)= 13.2 ± 1.4(stat)± 1.1(syst) eV, and obtain Г yy(Xc2)= 559 ± 57(stat) ± 48(syst) ± 36(br) eV. This result is in excellent agreement with the result of -fusion measurement by Belle and is consistent with that of the pp → X_(c2) → yy measurement, when they are both reevaluated using the recent CLEO result for the radiative decay X_(c2) → J/Ψ
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