34,383 research outputs found
What Controls the Star Formation in Luminous Starburst Mergers ?
In order to understand what controls the star formation process in luminous
starburst mergers (e.g., NGC 6240, Arp 220, and so on), we investigate
observational properties of two samples of high-luminosity starburst galaxies
mapped in CO(=1--0) independently using both the Owens Valley Radio
Observatory (Scoville et al. 1991) and the IRAM interferometer (Downes &
Solomon 1998). We find that the surface density of far-infrared luminosity,
(FIR), is proportional linearly to the H surface mass density,
(H), for the two samples; (FIR) (H) with a correlation coefficient of 0.96. It is
often considered that (FIR) provides a good measure of the star
formation rate per unit area, (SFR). It is also known that molecular
gas is dominated in circumnuclear regions in the luminous starburst mergers;
i.e., (gas) (H). Therefore, the above relationship
suggests a star formation law; (SFR) (gas). We suggest
that this star formation law favors the gravitational instability scenario
rather than the cloud-cloud collision one.Comment: 14 pages, 2 figures. The Astrophysical Journal (Letters), in pres
The Geant4 Hadronic Verification Suite for the Cascade Energy Range
A Geant4 hadronic process verification suite has been designed to test and
optimize Geant4 hadronic models in the cascade energy range. It focuses on
quantities relevant to the LHC radiation environment and spallation source
targets. The general structure of the suite is presented, including the user
interface, stages of verification, management of experimental data, event
generation, and comparison of results to data. Verification results for the
newly released Binary cascade and Bertini cascade models are presented.Comment: Talk from the 2003 Computing in High Energy and Nuclear Physics
(CHEP03), La Jolla, Ca, USA, March 2003, 5 pages, LaTeX, 3 eps figures. PSN
MOMT00
Form Factors from a Relativistic Dynamical Model of Pion Electroproduction
We obtain the electromagnetic form factors of the transition
by analyzing recent pion-electroproduction data using a fully relativistic
dynamical model. Special care is taken to satisfy Ward-Takahashi identities for
the Born term in the presence of form factors thereby allowing the use of
realistic electromagnetic form factors of the nucleon and pion. We parametrize
the dependence of the {\it bare} form factors by a
three-parameter form which is consistent with the asymptotic behavior inferred
from QCD. The parameters of the bare form factors are the
only free parameters of the model and are fitted to the differential
cross-section and multipole-analysis data up to (GeV/c) in the
-resonance region. This analysis emphasizes the significance of
the pion-cloud effects in the extraction of the resonance parameters.Comment: 9 pages, 5 figures, several small corrections, to be published in
Phys. Rev.
Ill-posedness of degenerate dispersive equations
In this article we provide numerical and analytical evidence that some
degenerate dispersive partial differential equations are ill-posed.
Specifically we study the K(2,2) equation and
the "degenerate Airy" equation . For K(2,2) our results are
computational in nature: we conduct a series of numerical simulations which
demonstrate that data which is very small in can be of unit size at a
fixed time which is independent of the data's size. For the degenerate Airy
equation, our results are fully rigorous: we prove the existence of a compactly
supported self-similar solution which, when combined with certain scaling
invariances, implies ill-posedness (also in )
The Bleaching of Soy Bean Oil with Peat
The color in soy bean oil is removed by four or five treatments at 120°C with 10% - 20% of its weight of north Iowa peat or peat ash. The removal of the color is accomplished with fewer treatments by first mixing the oil with an equal volume of solvent naphtha which is distilled off after the bleaching treatment. Exposure of the oil to strong sunlight, or better to ultraviolet rays, assists in removing the last traces of color
Infrared Emission from the Radio Supernebula in NGC 5253: A Proto-Globular Cluster?
Hidden from optical view in the starburst region of the dwarf galaxy NGC 5253
lies an intense radio source with an unusual spectrum which could be
interpreted variously as nebular gas ionized by a young stellar cluster or
nonthermal emission from a radio supernova or an AGN. We have obtained 11.7 and
18.7 micron images of this region at the Keck Telescope and find that it is an
extremely strong mid-infrared emitter. The infrared to radio flux ratio rules
out a supernova and is consistent with an HII region excited by a dense cluster
of young stars. This "super nebula" provides at least 15% of the total
bolometric luminosity of the galaxy. Its excitation requires 10^5-10^6 stars,
giving it the total mass and size (1-2 pc diameter) of a globular cluster.
However, its high obscuration, small size, and high gas density all argue that
it is very young, no more than a few hundred thousand years old. This may be
the youngest globular cluster yet observed.Comment: 6 pages, 2 color figures, Submitted to the ApJL, Revised 4/6/01 based
on referee's comment
Optical photometric GTC/OSIRIS observations of the young massive association Cygnus OB2
In order to fully understand the gravitational collapse of molecular clouds,
the star formation process and the evolution of circumstellar disks, these
phenomena must be studied in different Galactic environments with a range of
stellar contents and positions in the Galaxy. The young massive association
Cygnus OB2, in the Cygnus-X region, is an unique target to study how star
formation and the evolution of circumstellar disks proceed in the presence of a
large number of massive stars. We present a catalog obtained with recent
optical observations in r,i,z filters with OSIRIS, mounted on the GTC
telescope, which is the deepest optical catalog of Cyg OB2 to date.
The catalog consist of 64157 sources down to M=0.15 solar masses at the
adopted distance and age of Cyg OB2. A total of 38300 sources have good
photometry in all three bands. We combined the optical catalog with existing
X-ray data of this region, in order to define the cluster locus in the optical
diagrams. The cluster locus in the r-i vs. i-z diagram is compatible with an
extinction of the optically selected cluster members in the 2.64<AV<5.57 range.
We derive an extinction map of the region, finding a median value of AV=4.33 in
the center of the association, decreasing toward the north-west. In the
color-magnitude diagrams, the shape of the distribution of main sequence stars
is compatible with the presence of an obscuring cloud in the foreground at
about 850+/-25 pc from the Sun.Comment: Accepted for publication ApJS 201
A cohort study of influences, health outcomes and costs of patients' health-seeking behaviour for minor ailments from primary and emergency care settings
To compare health-related and cost-related outcomes of consultations for symptoms suggestive of minor ailments in emergency departments (EDs), general practices and community pharmacies
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