1,217 research outputs found
Global Nonradial Instabilities of Dynamically Collapsing Gas Spheres
Self-similar solutions provide good descriptions for the gravitational
collapse of spherical clouds or stars when the gas obeys a polytropic equation
of state, (with ). We study the behaviors of
nonradial perturbations in the similarity solutions of Larson, Penston and
Yahil, which describe the evolution of the collapsing cloud prior to core
formation. Our global stability analysis reveals the existence of unstable
bar-modes () when . In particular, for the collapse of
isothermal spheres, which applies to the early stages of star formation, the
density perturbation relative to the background, , increases as ,
where denotes the epoch of core formation, and is the cloud
central density. Thus, the isothermal cloud tends to evolve into an ellipsoidal
shape (prolate bar or oblate disk, depending on initial conditions) as the
collapse proceeds. In the context of Type II supernovae, core collapse is
described by the equation of state, and our analysis
indicates that there is no growing mode (with density perturbation) in the
collapsing core before the proto-neutron star forms, although nonradial
perturbations can grow during the subsequent accretion of the outer core and
envelope onto the neutron star. We also carry out a global stability analysis
for the self-similar expansion-wave solution found by Shu, which describes the
post-collapse accretion (``inside-out'' collapse) of isothermal gas onto a
protostar. We show that this solution is unstable to perturbations of all
's, although the growth rates are unknown.Comment: 28 pages including 7 ps figures; Minor changes in the discussion; To
be published in ApJ (V.540, Sept.10, 2000 issue
Gravitational waves from galaxy encounters
We discuss the emission of gravitational radiation produced in encounters of
dark matter galactic halos. To this aim we perform a number of numerical
simulations of typical galaxy mergers, computing the associated gravitational
radiation waveforms as well as the energy released in the processes. Our
simulations yield dimensionless gravitational wave amplitudes of the order of
and gravitational wave frequencies of the order of Hz,
when the galaxies are located at a distance of 10 Mpc. These values are of the
same order as those arising in the gravitational radiation originated by strong
variations of the gravitational field in the early Universe, and therefore,
such gravitational waves cannot be directly observed by ground-based detectors.
We discuss the feasibility of an indirect detection by means of the B-mode
polarization of the Cosmic Microwave Background (CMB) induced by such waves.
Our results show that the gravitational waves from encounters of dark matter
galactic halos leave much too small an imprint on the CMB polarization to be
actually observed with ongoing and future missions.Comment: 9 pages with revtex style, 3 ps figures; to be published in Physical
Review
Quark-Hadron Duality and Parity Violating Asymmetry of Electroweak Reactions in the Delta Region
A dynamical model of electroweak pion production reactions in the Delta(1232)
region has been extended to include the neutral current contributions for
examining the local Quark-Hadron Duality in neutrino-induced reactions and for
investigating how the axial N-Delta form factor can be determined by the parity
violating asymmetry of N(\vec{e},e') reactions. We first show that the recent
data of (e,e') structure functions F_1 and F_2, which exhibit the Quark-Hadron
Duality, are in good agreement with our predictions. For possible future
experimental tests, we then predict that the structure functions F_1, F_2, and
F_3 for (\nu,e) and (\nu,\nu') processes also show the similar Quark-Hadron
Duality. The spin dependent structure functions g_1 and g_2 of (e,e') have also
been calculated from our model. It is found that the local Quark-Hadron Duality
is not seen in the calculated g_1 and g_2, while our results for g_1 and some
polarization observables associated with the exclusive p(\vec{e},e' pi) and
\vec{p}(\vec{e},e' pi) reactions are in reasonably good agreement with the
recent data. In the investigation of parity violating asymmetry A of
N(\vec{e},e') reactions, it is found that the non-resonant contribution is
small at the Delta peak and a measurement of A can be used to distinguish two
previously determined axial N-Delta transition form factors. The predicted
asymmetry A are also compared with the Parton Model predictions for future
experimental investigations of Quark-Hadron Duality.Comment: 28 pages, 19 figures v2; figures and references adde
Fragmentation of a Molecular Cloud Core versus Fragmentation of the Massive Protoplanetary Disk in the Main Accretion Phase
The fragmentation of molecular cloud cores a factor of 1.1 denser than the
critical Bonnor-Ebert sphere is examined though three-dimensional numerical
simulations. A nested grid is employed to resolve fine structure down to 1 AU
while following the entire structure of the molecular cloud core of radius 0.14
pc. A total of 225 models are shown to survey the effects of initial rotation
speed, rotation law, and amplitude of bar mode perturbation. The simulations
show that the cloud fragments whenever the cloud rotates sufficiently slowly to
allow collapse but fast enough to form a disk before first-core formation. The
latter condition is equivalent to , where
and denote the initial central angular velocity and the
freefall time measured from the central density, respectively. Fragmentation is
classified into six types: disk-bar, ring-bar, satellite, bar, ring, and
dumbbell types according to the morphology of collapse and fragmentation. When
the outward decrease in initial angular velocity is more steep, the cloud
deforms from spherical at an early stage. The cloud deforms into a ring only
when the bar mode m = 2 perturbation is very minor. The ring fragments into two
or three fragments via ring-bar type fragmentation and into at least three
fragments via ring type fragmentation. When the bar mode is significant, the
cloud fragments into two fragments via either bar or dumbbell type
fragmentation. These fragments eventually merge due to their low angular
momenta, after which several new fragments form around the merged fragment via
satellite type fragmentation.Comment: Accepted by ApJ, 53 pages, 27 figures. Document with high quality
figures and movies are available in
http://meric.i.hosei.ac.jp/~matsu/fragment03
Dynamical properties of Ultraluminous Infrared Galaxies. II. Traces of dynamical evolution and end products of local ultraluminous mergers
We present results from our Very Large Telescope large program to study the
dynamical evolution of local Ultraluminous Infrared Galaxies (ULIRGs) and QSOs.
This paper is the second in a series presenting the stellar kinematics of 54
ULIRGs, derived from high resolution, long-slit H- and K-band spectroscopy. The
data presented here, including observations of 17 new targets, are mainly
focused on sources that have coalesced into a single nucleus. The stellar
kinematics, extracted from the CO ro-vibrational bandheads in our spectra,
indicate that ULIRG remnants are dynamically heated systems with a mean
dispersion of 161 km/s. The combination of kinematic, structural, and
photometric properties of the remnants indicate that they mostly originate from
major mergers and that they result in the formation of systems supported by
random motions, therefore, elliptical galaxies. The peak of the velocity
dispersion distribution and the locus of ULIRGs on the fundamental plane of
early-type galaxies indicate that the end products of ultraluminous mergers are
typically moderate-mass ellipticals (of stellar mass ~10^10 - 10^11 M_sun).
Converting the host dispersion into black hole mass with the aid of the
M_BH-sigma relation yields black hole mass estimates of the order 10^7 - 10^8
M_sun and high accretion rates with Eddington efficiencies often >0.5.Comment: Accepted for publication in the Astrophysical Journa
Global Analysis of Data on the Proton Structure Function g1 and Extraction of its Moments
Inspired by recent measurements with the CLAS detector at Jefferson Lab, we
perform a self-consistent analysis of world data on the proton structure
function g1 in the range 0.17 < Q2 < 30 (GeV/c)**2. We compute for the first
time low-order moments of g1 and study their evolution from small to large
values of Q2. The analysis includes the latest data on both the unpolarized
inclusive cross sections and the ratio R = sigmaL / sigmaT from Jefferson Lab,
as well as a new model for the transverse asymmetry A2 in the resonance region.
The contributions of both leading and higher twists are extracted, taking into
account effects from radiative corrections beyond the next-to-leading order by
means of soft-gluon resummation techniques. The leading twist is determined
with remarkably good accuracy and is compared with the predictions obtained
using various polarized parton distribution sets available in the literature.
The contribution of higher twists to the g1 moments is found to be
significantly larger than in the case of the unpolarized structure function F2.Comment: 18 pages, 13 figures, to appear in Phys. Rev.
Pancreatic Resections for Advanced M1-Pancreatic Carcinoma: The Value of Synchronous Metastasectomy
Background. For M1 pancreatic adenocarcinomas pancreatic resection is usually not indicated. However, in highly selected patients synchronous metastasectomy may be appropriate together with pancreatic resection when operative morbidity is low.
Materials and Methods. From January 1, 2004 to December, 2007 a total of 20 patients with pancreatic malignancies were retrospectively evaluated who underwent pancreatic surgery with synchronous resection of hepatic, adjacent organ, or peritoneal metastases for proven UICC stage IV periampullary cancer of the pancreas. Perioperative as well as clinicopathological parameters were evaluated.
Results. There were 20 patients (9 men, 11 women; mean age 58 years) identified. The primary tumor was located in the pancreatic head (n = 9, 45%), in pancreatic tail (n = 9, 45%), and in the papilla Vateri (n = 2, 10%). Metastases were located in the liver (n = 14, 70%), peritoneum (n = 5, 25%), and omentum majus (n = 2, 10%). Lymphnode metastases were present in 16 patients (80%). All patients received resection of their tumors together with metastasectomy. Pylorus preserving duodenopancreatectomy was performed in 8 patients, distal pancreatectomy in 8, duodenopancreatectomy in 2, and total pancreatectomy in 2. Morbidity was 45% and there was no perioperative mortality. Median postoperative survival was 10.7 months (2.6â37.7âmonths) which was not significantly different from a matched-pair group of patients who underwent pancreatic resection for UICC adenocarcinoma of the pancreas (median survival 15.6âmonths; P = .1). Conclusion. Pancreatic resection for M1 periampullary cancer of the pancreas can be performed safely in well-selected patients. However, indication for surgery has to be made on an individual basis
Imaging Performance of the XMM-Newton X-ray telescopes
The in-orbit imaging performance of the three X-ray telescopes on board of
the X-ray astronomy observatory XMM-Newton is presented and compared with the
performance measured on ground at the MPE PANTER test facility. The comparison
shows an excellent agreement between the on ground and in-orbit performance.Comment: 9 pages, 10 Postscript figures, for SPIE 4012, paper 8
Theory Support for the Excited Baryon Program at the Jlab 12 GeV Upgrade
This document outlines major directions in theoretical support for the
measurement of nucleon resonance transition form factors at the JLab 12 GeV
upgrade with the CLAS12 detector. Using single and double meson production,
prominent resonances in the mass range up to 2 GeV will be studied in the range
of photon virtuality up to 12 GeV where quark degrees of freedom are
expected to dominate. High level theoretical analysis of these data will open
up opportunities to understand how the interactions of dressed quarks create
the ground and excited nucleon states and how these interactions emerge from
QCD. The paper reviews the current status and the prospects of QCD based model
approaches that relate phenomenological information on transition form factors
to the non-perturbative strong interaction mechanisms, that are responsible for
resonance formation.Comment: 52 pages, 19 figures, White Paper of the Electromagnetic N-N*
Transition Form Factor Workshop at Jefferson Lab, October 13-15, 2008,
Newport News, VA, US
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