15,148 research outputs found
The role of accretion disks in the formation of massive stars
We present radiation hydrodynamics simulations of the collapse of massive
pre-stellar cores. We treat frequency dependent radiative feedback from stellar
evolution and accretion luminosity at a numerical resolution down to 1.27 AU.
In the 2D approximation of axially symmetric simulations, it is possible for
the first time to simulate the whole accretion phase of several 10^5 yr for the
forming massive star and to perform a comprehensive scan of the parameter
space. Our simulation series show evidently the necessity to incorporate the
dust sublimation front to preserve the high shielding property of massive
accretion disks. Our disk accretion models show a persistent high anisotropy of
the corresponding thermal radiation field, yielding to the growth of the
highest-mass stars ever formed in multi-dimensional radiation hydrodynamics
simulations. Non-axially symmetric effects are not necessary to sustain
accretion. The radiation pressure launches a stable bipolar outflow, which
grows in angle with time as presumed from observations. For an initial mass of
the pre-stellar host core of 60, 120, 240, and 480 Msol the masses of the final
stars formed in our simulations add up to 28.2, 56.5, 92.6, and at least 137.2
Msol respectively.Comment: 4 pages, 2 figures, Computational Star Formation Proceedings IAU
Symposium No. 270, 2010, Ed.: J. Alves, B. Elmegreen, J. Girart & V. Trimbl
Space transportation nodes assumptions and requirements: Lunar base systems study task 2.1
The Space Transportation Nodes Assumptions and Requirements task was performed as part of the Advanced Space Transportation Support Contract, a NASA Johnson Space Center (JSC) study intended to provide planning for a Lunar Base near the year 2000. The original task statement has been revised to satisfy the following queries: (1) What vehicles are to be processed at the transportation node; (2) What is the flow of activities involved in a vehicle passing through the node; and (3) What node support resources are necessary to support a lunar scenario traffic model composed of a mix of vehicles in an active flight schedule. The Lunar Base Systems Study is concentrating on the initial years of the Phase 2 Lunar Base Scenario. The study will develop the first five years of that phase in order to define the transportation and surface systems (including mass, volumes, power requirements, and designs)
Dipole formation at metal/PTCDA interfaces: Role of the Charge Neutrality Level
The formation of a metal/PTCDA (3, 4, 9, 10-perylenetetracarboxylic
dianhydride) interface barrier is analyzed using weak-chemisorption theory. The
electronic structure of the uncoupled PTCDA molecule and of the metal surface
is calculated. Then, the induced density of interface states is obtained as a
function of these two electronic structures and the interaction between both
systems. This induced density of states is found to be large enough (even if
the metal/PTCDA interaction is weak) for the definition of a Charge Neutrality
Level for PTCDA, located 2.45 eV above the highest occupied molecular orbital.
We conclude that the metal/PTCDA interface molecular level alignment is due to
the electrostatic dipole created by the charge transfer between the two solids.Comment: 6 page
AC Magnetic Fields in the Vicinity of a Crack Calculated by Analytic and Numerical Methods
We report calculations of the impedance of a long solenoid which surrounds a cylinder of conducting material containing a radial surface crack. The calculation is accomplished by two independent methods. The first method expresses the field in the interior of the cracked cylinder as an infinite series of cylindrical Bessel functions. The coefficients in the series are determined in principle by boundary conditions; the most significant terms are calculated by solving the finite set of equations obtained by truncation of the series. The second method, applicable to any uniform geometric cross-section, obtains the impedance from the normal derivative of the field on the boundary of the conductor. This normal derivative satisfies a (boundary) Fredholm integral equation of the first kind; a solution is obtained by discretizing and solving the resulting linear system of algebraic equations. The impedance is calculated for a wide range of values of the ratios of crack depth-to-radius and radius-to-skin depth. The results are displayed in graphical form giving the fractional charges of the real and imaginary parts of the complex impedance induced by the presence of the crack
A DMRG Study of Low-Energy Excitations and Low-Temperature Properties of Alternating Spin Systems
We use the density matrix renormalization group (DMRG) method to study the
ground and low-lying excited states of three kinds of uniform and dimerized
alternating spin chains. The DMRG procedure is also employed to obtain
low-temperature thermodynamic properties of these systems. We consider a 2N
site system with spins and alternating from site to site and
interacting via a Heisenberg antiferromagnetic exchange. The three systems
studied correspond to being equal to and
; all of them have very similar properties. The ground state is found
to be ferrimagnetic with total spin . We find that there is
a gapless excitation to a state with spin , and a gapped excitation to
a state with spin . Surprisingly, the correlation length in the ground
state is found to be very small for this gapless system. The DMRG analysis
shows that the chain is susceptible to a conditional spin-Peierls instability.
Furthermore, our studies of the magnetization, magnetic susceptibility
and specific heat show strong magnetic-field dependences. The product
shows a minimum as a function of temperature T at low magnetic fields; the
minimum vanishes at high magnetic fields. This low-field behavior is in
agreement with earlier experimental observations. The specific heat shows a
maximum as a function of temperature, and the height of the maximum increases
sharply at high magnetic fields. Although all the three systems show
qualitatively similar behavior, there are some notable quantitative differences
between the systems in which the site spin difference, , is large
and small respectively.Comment: 16 LaTeX pages, 13 postscript figure
Electronic structure of the molecule based magnet Cu PM(NO3)2 (H2O)2
We present density functional calculations on the molecule based S=1/2
antiferromagnetic chain compound Cu PM(NO3)2 (H2O)2; PM = pyrimidine. The
properties of the ferro- and antiferromagnetic state are investigated at the
level of the local density approximation and with the hybrid functional B3LYP.
Spin density maps illustrate the exchange path via the pyrimidine molecule
which mediates the magnetism in the one-dimensional chain. The computed
exchange coupling is antiferromagnetic and in reasonable agreement with the
experiment. It is suggested that the antiferromagnetic coupling is due to the
possibility of stronger delocalization of the charges on the nitrogen atoms,
compared to the ferromagnetic case. In addition, computed isotropic and
anisotropic hyperfine interaction parameters are compared with recent NMR
experiments
A High-Mass Protobinary System in the Hot Core W3(H2O)
We have observed a high-mass protobinary system in the hot core W3(H2O) with
the BIMA Array. Our continuum maps at wavelengths of 1.4mm and 2.8mm both
achieve sub-arcsecond angular resolutions and show a double-peaked morphology.
The angular separation of the two sources is 1.19" corresponding to 2.43X10^3
AU at the source distance of 2.04 kpc. The flux densities of the two sources at
1.4mm and 2.8mm have a spectral index of 3, translating to an opacity law of
kappa ~ nu. The small spectral indices suggest that grain growth has begun in
the hot core. We have also observed 5 K components of the CH3CN (12-11)
transitions. A radial velocity difference of 2.81 km/s is found towards the two
continuum peaks. Interpreting these two sources as binary components in orbit
about one another, we find a minimum mass of 22 Msun for the system. Radiative
transfer models are constructed to explain both the continuum and methyl
cyanide line observations of each source. Power-law distributions of both
density and temperature are derived. Density distributions close to the
free-fall value, r^-1.5, are found for both components, suggesting continuing
accretion. The derived luminosities suggest the two sources have equivalent
zero-age main sequence (ZAMS) spectral type B0.5 - B0. The nebular masses
derived from the continuum observations are about 5 Msun for source A and 4
Msun for source C. A velocity gradient previously detected may be explained by
unresolved binary rotation with a small velocity difference.Comment: 38 pages, 9 figures, accepted by The Astrophysical Journa
Barrier formation at metal/organic interfaces: dipole formation and the Charge Neutrality Level
The barrier formation for metal/organic semiconductor interfaces is analyzed
within the Induced Density of Interface States (IDIS) model. Using weak
chemisorption theory, we calculate the induced density of states in the organic
energy gap and show that it is high enough to control the barrier formation. We
calculate the Charge Neutrality Levels of several organic molecules (PTCDA,
PTCBI and CBP) and the interface Fermi level for their contact with a Au(111)
surface. We find an excellent agreement with the experimental evidence and
conclude that the barrier formation is due to the charge transfer between the
metal and the states induced in the organic energy gap.Comment: 7 pages, Proceedings of ICFSI-9, Madrid, Spain (September 2003),
special issue of Applied Surface Science (in press
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