8,092 research outputs found
Buoyancy-driven plumes in a layered porous medium
Thin, roughly horizontal low-permeability layers are a common form of large-scale
heterogeneity in geological porous formations. In this paper, the dynamics of a buoyancydriven plume in a two-dimensional layered porous medium is studied theoretically, with
the aid of high-resolution numerical simulations. The medium is uniform apart from
a thin, horizontal layer of a much lower permeability, located a dimensionless distance
L 1 below the dense plume source. If the dimensionless thickness 2εL and permeability
Î of the low-permeability layer are small, the effect of the layer is found to be well
parameterized by its impedance Ω = 2εL/Π. Five different regimes of flow are identified
and characterized. For Ω L
1/3
, the layer has no effect on the plume, but as Ω is
increased the plume widens and spreads over the layer as a gravity current. For still
larger Ω, the flow becomes destabilized by convective instabilities both below and above
the layer, until, for Ω L, the spread of the plume is dominated by convective mixing
and buoyancy is transported across the layer by diffusion alone. Analytical models for
the spread of the plume over the layer in the various different regimes are presented
An HI census of Loose Groups of Galaxies
We present results from our Parkes Multibeam HI survey of 3 loose groups of
galaxies that are analogous to the Local Group. This is a survey of groups
containing only spiral galaxies with mean separations of a few hundred kpc, and
total areas of approximately 1 sq. Mpc; groups similar to our own Local Group.
We present a census of the HI-rich objects in these groups down to an M(HI),
1-sigma sensitivity ~7x10^5 M(sun), as well as the detailed properties of these
detections from follow-up Compact Array observations. We found 7 new HI-rich
members in the 3 groups, all of which have stellar counterparts and are,
therefore, typical dwarf galaxies. The ratio of low-mass to high-mass gas-rich
galaxies in these groups is less than in the Local Group meaning that the
``missing satellite'' problem is not unique. No high-velocity cloud analogs
were found in any of the groups. If HVCs in these groups are the same as in the
Local Group, this implies that HVCs must be located within ~300-400 kpc of the
Milky Way.Comment: 6 pages, to appear in the ASP proceedings of IAU Symposium 217,
"Recycling intergalactic and interstellar matter", eds. Pierre-Alain Duc,
Jonathan Braine, Elias Brink
The Parsec-scale Structure, Kinematics, and Polarization of Radio-Loud Narrow-Line Seyfert 1 Galaxies
Several narrow-line Seyfert 1 galaxies (NLS1s) have now been detected in
gamma rays, providing firm evidence that at least some of this class of active
galactic nuclei (AGN) produce relativistic jets. The presence of jets in NLS1s
is surprising, as these sources are typified by comparatively small black hole
masses and near- or super-Eddington accretion rates. This challenges the
current understanding of the conditions necessary for jet production. Comparing
the properties of the jets in NLS1s with those in more familiar jetted systems
is thus essential to improve jet production models. We present early results
from our campaign to monitor the kinematics and polarization of the
parsec-scale jets in a sample of 15 NLS1s through multifrequency observations
with the Very Long Baseline Array. These observations are complemented by
fast-cadence 15 GHz monitoring with the Owens Valley Radio Observatory 40m
telescope and optical spectroscopic monitoring with with the 2m class telescope
at the Guillermo Haro Astrophysics Observatory in Cananea, Mexico.Comment: 4 pages, 1 figure. To appear in the Proceedings of the IAU Symposium
No. 313: "Extragalactic jets from every angle," Galapagos, Ecuador, 15-19
September 2014, F. Massaro, C. C. Cheung, E. Lopez, and A. Siemiginowska
(Eds.), Cambridge University Pres
Faraday rotation in the MOJAVE blazars: 3C 273 a case study
Radio polarimetric observations of Active Galactic Nuclei can reveal the
magnetic field structure in the parsec-scale jets of these sources. We have
observed the gamma-ray blazar 3C 273 as part of our multi-frequency survey with
the Very Long Baseline Array to study Faraday rotation in a large sample of
jets. Our observations re-confirm the transverse rotation measure gradient in
3C 273. For the first time the gradient is seen to cross zero which is further
indication for a helical magnetic field and spine-sheath structure in the jet.
We believe the difference to previous epochs is due to a different part of the
jet being illuminated in our observations.Comment: 6 pages, 3 figures. To appear in the proceedings of "Beamed and
Unbeamed Gamma-rays from Galaxies", held in Muonio, Finland, April 11-15,
2011. Journal of Physics: Conference Serie
Gamma-ray Loudness, Synchrotron Peak Frequency, and Parsec-Scale Properties of Blazars Detected by the Fermi Large Area Telescope
The parsec-scale radio properties of 232 active galactic nuclei (AGNs), most
of which are blazars, detected by the Large Area Telescope (LAT) on board the
Fermi Gamma-ray Space Telescope have been observed contemporaneously by the
Very Long Baseline Array (VLBA) at 5 GHz. Data from both the first 11 months
(1FGL) and the first 2 years (2FGL) of the Fermi mission were used to
investigate these sources' gamma-ray properties. We use the ratio of the
gamma-ray to radio luminosity as a measure of gamma-ray loudness. We
investigate the relationship of several radio properties to gamma-ray loudness
and to the synchrotron peak frequency. There is a tentative correlation between
gamma-ray loudness and synchrotron peak frequency for BL Lac objects in both
1FGL and 2FGL, and for flat-spectrum radio quasars (FSRQs) in 2FGL. We find
that the apparent opening angle tentatively correlates with gamma-ray loudness
for FSRQs, but only when we use the 2FGL data. We also find that the total VLBA
flux density correlates with the synchrotron peak frequency for BL Lac objects
and FSRQs. The core brightness temperature also correlates with synchrotron
peak frequency, but only for the BL Lac objects. The low-synchrotron peaked
(LSP) BL Lac object sample shows indications of contamination by FSRQs which
happen to have undetectable emission lines. There is evidence that the LSP BL
Lac objects are more strongly beamed than the rest of the BL Lac object
population.Comment: 38 pages, 2 tables, 12 figures. Accepted for publication in The
Astrophysical Journa
Coupled-Channels Approach for Dissipative Quantum Dynamics in Near-Barrier Collisions
A novel quantum dynamical model based on the dissipative quantum dynamics of
open quantum systems is presented. It allows the treatment of both
deep-inelastic processes and quantum tunneling (fusion) within a fully quantum
mechanical coupled-channels approach. Model calculations show the transition
from pure state (coherent) to mixed state (decoherent and dissipative) dynamics
during a near-barrier nuclear collision. Energy dissipation, due to
irreversible decay of giant-dipole excitations of the interacting nuclei,
results in hindrance of quantum tunneling.Comment: 8 pages, 4 figures, Invited talk by A. Diaz-Torres at the FUSION08
Conference, Chicago, September 22-26, 2008, To appear in AIP Conference
Proceeding
WASP-29b: A Saturn-sized transiting exoplanet
We report the discovery of a Saturn-sized planet transiting a V = 11.3, K4
dwarf star every 3.9 d. WASP-29b has a mass of 0.24+/-0.02 M_Jup and a radius
of 0.79+/-0.05 R_Jup, making it the smallest planet so far discovered by the
WASP survey, and the exoplanet most similar in mass and radius to Saturn. The
host star WASP-29 has an above-Solar metallicity and fits a possible
correlation for Saturn-mass planets such that planets with higher-metallicity
host stars have higher core masses and thus smaller radii.Comment: 6 pages, submitted to ApJ
Pores in Bilayer Membranes of Amphiphilic Molecules: Coarse-Grained Molecular Dynamics Simulations Compared with Simple Mesoscopic Models
We investigate pores in fluid membranes by molecular dynamics simulations of
an amphiphile-solvent mixture, using a molecular coarse-grained model. The
amphiphilic membranes self-assemble into a lamellar stack of amphiphilic
bilayers separated by solvent layers. We focus on the particular case of
tension less membranes, in which pores spontaneously appear because of thermal
fluctuations. Their spatial distribution is similar to that of a random set of
repulsive hard discs. The size and shape distribution of individual pores can
be described satisfactorily by a simple mesoscopic model, which accounts only
for a pore independent core energy and a line tension penalty at the pore
edges. In particular, the pores are not circular: their shapes are fractal and
have the same characteristics as those of two dimensional ring polymers.
Finally, we study the size-fluctuation dynamics of the pores, and compare the
time evolution of their contour length to a random walk in a linear potential
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