1,248 research outputs found
H-alpha Survey of the Local Volume: Isolated Southern Galaxies
We present our H-alpha observations of 11 isolated southern galaxies: SDIG,
PGC 51659, E 222-010, E 272-025, E 137-018, IC 4662, Sag DIG, IC 5052, IC 5152,
UGCA 438, and E149-003, with distances from 1 to 7 Mpc. We have determined the
total H-alpha fluxes from these galaxies. The star formation rates in these
galaxies range from 10^{-1} (IC 4662) to 10^{-4}_{\odot}/yr (SDIG) and the gas
depletion time at the observed star formation rates lies within the range from
1/6 to 24 Hubble times H_0^{-1} .Comment: 9 pages, 3 figure
A disk of dust and molecular gas around a high-mass protostar
The processes leading to the birth of low-mass stars such as our Sun have
been well studied, but the formation of high-mass (> 8 x Sun's mass) stars has
heretofore remained poorly understood. Recent observational studies suggest
that high-mass stars may form in essentially the same way as low-mass stars,
namely via an accretion process, instead of via merging of several low-mass (<
8 Msun) stars. However, there is as yet no conclusive evidence. Here, we report
the discovery of a flattened disk-like structure observed at submillimeter
wavelengths, centered on a massive 15 Msun protostar in the Cepheus-A region.
The disk, with a radius of about 330 astronomical units (AU) and a mass of 1 to
8 Msun, is detected in dust continuum as well as in molecular line emission.
Its perpendicular orientation to, and spatial coincidence with the central
embedded powerful bipolar radio jet, provides the best evidence yet that
massive stars form via disk accretion in direct analogy to the formation of
low-mass stars
Femtosecond x-ray diffraction from an aerosolized beam of protein nanocrystals
We demonstrate near-atomic-resolution Bragg diffraction from aerosolized
single granulovirus crystals using an x-ray free-electron laser. The form of
the aerosol injector is nearly identical to conventional liquid-microjet
nozzles, but the x-ray-scattering background is reduced by several orders of
magnitude by the use of helium carrier gas rather than liquid. This approach
provides a route to study the weak diffuse or lattice-transform signal arising
from small crystals. The high speed of the particles is particularly well
suited to upcoming MHz-repetition-rate x-ray free-electron lasers
The development of a biologically inspired propulsor for unmanned underwater vehicles
IEEE Journal of Oceanic Engineering, 32(3): pp. 533-550Fish are remarkable in their ability to maneuver
and to control their body position. This ability is the result of the
coordinated movement of fins which extend from the body and
form control surfaces that can create and vector forces in 3-D.
We have embarked on a research program designed to develop a
maneuvering propulsor for unmanned undersea vehicles (UUVs)
that is based on the pectoral fin of the bluegill sunfish. For this,
the anatomy, kinematics, and hydrodynamics of the sunfish pectoral
fin were investigated experimentally and through the use of
computational fluid dynamics (CFD) simulations. These studies
identified that the kinematics of the sunfish pectoral fin are very
complex and are not easily described by traditional “rowing”-
and “flapping”-type kinematics. A consequence of the complex
motion is that the pectoral fin can produce forward thrust during
both its outstroke (abduction) and instroke (adduction), and while
doing so generates only small lateral and lift forces. The results
of the biological studies were used to guide the design of robotic
pectoral fins which were built as experimental devices and used
to investigate the mechanisms of thrust production and control.
Because of a design that was based heavily on the anatomy of the
sunfish fin, the robotic pectoral fins had the level of control and
degrees of freedom necessary to reproduce many of the complex
fin motions used by the sunfish during steady swimming. These
robotic fins are excellent experimental tools, and are an important
first step towards developing propulsive devices that will give the
next generation of UUVs the ability to produce and control thrust
like highly maneuverable fish
Modeling DNA Structure, Elasticity and Deformations at the Base-pair Level
We present a generic model for DNA at the base-pair level. We use a variant
of the Gay-Berne potential to represent the stacking energy between neighboring
base-pairs. The sugar-phosphate backbones are taken into account by semi-rigid
harmonic springs with a non-zero spring length. The competition of these two
interactions and the introduction of a simple geometrical constraint leads to a
stacked right-handed B-DNA-like conformation. The mapping of the presented
model to the Marko-Siggia and the Stack-of-Plates model enables us to optimize
the free model parameters so as to reproduce the experimentally known
observables such as persistence lengths, mean and mean squared base-pair step
parameters. For the optimized model parameters we measured the critical force
where the transition from B- to S-DNA occurs to be approximately . We
observe an overstretched S-DNA conformation with highly inclined bases that
partially preserves the stacking of successive base-pairs.Comment: 15 pages, 25 figures. submitted to PR
Star forming dwarf galaxies
Star forming dwarf galaxies (SFDGs) have a high gas content and low
metallicities, reminiscent of the basic entities in hierarchical galaxy
formation scenarios. In the young universe they probably also played a major
role in the cosmic reionization. Their abundant presence in the local volume
and their youthful character make them ideal objects for detailed studies of
the initial stellar mass function (IMF), fundamental star formation processes
and its feedback to the interstellar medium. Occasionally we witness SFDGs
involved in extreme starbursts, giving rise to strongly elevated production of
super star clusters and global superwinds, mechanisms yet to be explored in
more detail. SFDGs is the initial state of all dwarf galaxies and the relation
to the environment provides us with a key to how different types of dwarf
galaxies are emerging. In this review we will put the emphasis on the exotic
starburst phase, as it seems less important for present day galaxy evolution
but perhaps fundamental in the initial phase of galaxy formation.Comment: To appear in JENAM Symposium "Dwarf Galaxies: Keys to Galaxy
Formation and Evolution", P. Papaderos, G. Hensler, S. Recchi (eds.). Lisbon,
September 2010, Springer Verlag, in pres
Indirect Dark Matter Detection from Dwarf Satellites: Joint Expectations from Astrophysics and Supersymmetry
We present a general methodology for determining the gamma-ray flux from
annihilation of dark matter particles in Milky Way satellite galaxies, focusing
on two promising satellites as examples: Segue 1 and Draco. We use the
SuperBayeS code to explore the best-fitting regions of the Constrained Minimal
Supersymmetric Standard Model (CMSSM) parameter space, and an independent MCMC
analysis of the dark matter halo properties of the satellites using published
radial velocities. We present a formalism for determining the boost from halo
substructure in these galaxies and show that its value depends strongly on the
extrapolation of the concentration-mass (c(M)) relation for CDM subhalos down
to the minimum possible mass. We show that the preferred region for this
minimum halo mass within the CMSSM with neutralino dark matter is ~10^-9-10^-6
solar masses. For the boost model where the observed power-law c(M) relation is
extrapolated down to the minimum halo mass we find average boosts of about 20,
while the Bullock et al (2001) c(M) model results in boosts of order unity. We
estimate that for the power-law c(M) boost model and photon energies greater
than a GeV, the Fermi space-telescope has about 20% chance of detecting a dark
matter annihilation signal from Draco with signal-to-noise greater than 3 after
about 5 years of observation
The BLAST View of the Star Forming Region in Aquila (ell=45deg,b=0deg)
We have carried out the first general submillimeter analysis of the field
towards GRSMC 45.46+0.05, a massive star forming region in Aquila. The
deconvolved 6 deg^2 (3\degree X 2\degree) maps provided by BLAST in 2005 at
250, 350, and 500 micron were used to perform a preliminary characterization of
the clump population previously investigated in the infrared, radio, and
molecular maps. Interferometric CORNISH data at 4.8 GHz have also been used to
characterize the Ultracompact HII regions (UCHIIRs) within the main clumps. By
means of the BLAST maps we have produced an initial census of the submillimeter
structures that will be observed by Herschel, several of which are known
Infrared Dark Clouds (IRDCs). Our spectral energy distributions of the main
clumps in the field, located at ~7 kpc, reveal an active population with
temperatures of T~35-40 K and masses of ~10^3 Msun for a dust emissivity index
beta=1.5. The clump evolutionary stages range from evolved sources, with
extended HII regions and prominent IR stellar population, to massive young
stellar objects, prior to the formation of an UCHIIR.The CORNISH data have
revealed the details of the stellar content and structure of the UCHIIRs. In
most cases, the ionizing stars corresponding to the brightest radio detections
are capable of accounting for the clump bolometric luminosity, in most cases
powered by embedded OB stellar clusters
Dynamics of Barred Galaxies
Some 30% of disc galaxies have a pronounced central bar feature in the disc
plane and many more have weaker features of a similar kind. Kinematic data
indicate that the bar constitutes a major non-axisymmetric component of the
mass distribution and that the bar pattern tumbles rapidly about the axis
normal to the disc plane. The observed motions are consistent with material
within the bar streaming along highly elongated orbits aligned with the
rotating major axis. A barred galaxy may also contain a spheroidal bulge at its
centre, spirals in the outer disc and, less commonly, other features such as a
ring or lens. Mild asymmetries in both the light and kinematics are quite
common. We review the main problems presented by these complicated dynamical
systems and summarize the effort so far made towards their solution,
emphasizing results which appear secure. (Truncated)Comment: This old review appeared in 1993. Plain tex with macro file. 82 pages
18 figures. A pdf version with figures at full resolution (3.24MB) is
available at http://www.physics.rutgers.edu/~sellwood/bar_review.pd
1967: Abilene Christian College Bible Lectures - Full Text
LIFTING UP THE CHRIST”
Being the Abilene Christian College Annual Bible Lectures 1967
$3.95
Published by
ABILENE CHRISTIAN COLLEGE STUDENTS EXCHANGE
ACC Station Abilene, Texa
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