51,979 research outputs found
The Formation of Globules in Planetary Nebulae
We discuss the formation of globules in planetary nebulae, typified by those
observed in the Helix Nebula. We show that the properties of the globules,
their number, mass, separation, and overall geometry strongly support a
scenario in which globules are formed by the fragmentation of a swept-up shell
as opposed to models in which the knots form in the AGB wind. We show that the
RT or other instabilities which lead to the break-up of shells formed in the
nebulae by fast winds or ionization fronts can produce arrays of globules with
the overall geometry and within the mass range observed. We also show that the
presence of a magnetic field in the circumstellar gas may play an important
role in controlling the fragmentation process. Using field strengths measured
in the precursor AGB envelopes, we find that close to the central star where
the fields are relatively strong, the wavelengths of unstable MRT modes are
larger than the shell dimensions, and the fragmentation of the shell is
suppressed. The wavelength of the most unstable MRT mode decreases with
increasing distance from the star, and when it becomes comparable to the shell
thickness, it can lead to the sudden, rapid break-up of an accelerating shell.
For typical nebula parameters, the model results in numerous fragments with a
mass scale and a separation scale similar to those observed. Our results
provide a link between global models of PN shaping in which shells form via
winds and ionization fronts, and the formation of small scale structures in the
nebulae.Comment: 4 pages, 2 figures, to appear in IAU Symp. 234, Planetary Nebulae in
Our Galaxy and Beyond, eds. M. J. Barlow, R. H. Mende
Computational tasks in robotics and factory automation
The design of Manufacturing Planning and Control Systems (MPCSs) — systems that negotiate with Customers and Suppliers to exchange products in return for money in order to generate profit, is discussed.\ud
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The computational task of MPCS components are systematically specified as a starting point for the development of computational engines, as computer systems and programs, that execute the specified computation. Key issues are the overwhelming complexity and frequently changing application of MPCSs
Wave packet approach to periodically driven scattering
For autonomous systems it is well known how to extract tunneling
probabilities from wavepacket calculations. Here we present a corresponding
approach for periodically time-dependent Hamiltonians, valid at all
frequencies, field strengths, and transition orders. After mapping the
periodically driven system onto a time-independent one with an additional
degree of freedom, use is made of the correlation function formulation of
scattering [J. Chem. Phys. {\bf 98}, 3884 (1993)]. The formalism is then
applied to study the transmission properties of a resonant tunneling double
barrier structure under the influence of a sinusoidal laser field, revealing an
unexpected antiresonance in the zero photon transition for large field
strengths.Comment: 4 pages, 2 figure
Squeezars: Tidally powered stars orbiting a massive black hole
We propose that there exists a class of transient sources, "squeezars", which
are stars caught in highly eccentric orbits around a massive (m<10^8 Mo) black
hole (MBH), whose atypically high luminosity (up to a significant fraction of
their Eddington luminosity) is powered by tidal interactions with the MBH.
Their existence follows from the presence of a mass sink, the MBH, in the
galactic center, which drives a flow of stars into nearly radial orbits to
replace those it has destroyed. We consider two limits for the stellar response
to tidal heating: surface heating with radiative cooling ("hot squeezars") and
bulk heating with adiabatic expansion ("cold squeezars"), and calculate the
evolution of the squeezar orbit, size, luminosity and effective temperature.
The squeezar formation rate is only ~0.05 that of tidal disruption flares, but
squeezar lifetimes are many orders of magnitude longer, and so future
observations of squeezars in nearby galaxies can probe the tidal process that
feeds MBHs and the effects of extreme tides on stars. The mean number of
squeezars orbiting the Galactic MBH is estimated at 0.1-1.Comment: ApJ Lett. accepted. 4 pp. 1 fi
Subluminal OPERA Neutrinos
The OPERA collaboration has announced to have observed superluminal neutrinos
with a mean energy 17.5 GeV, but afterward the superluminal interpretation of
the OPERA results has been refuted theoretically by Cherenkov-like radiation
and pion decay. In a recent work, we have proposed a kinematical resolution to
this problem. A key idea in our resolution is that the OPERA neutrinos are not
superluminal but subluminal since they travel faster than the observed speed of
light in vacuum on the earth while they do slower than the true speed of light
in vacuum determining the causal structure of events. In this article, we dwell
upon our ideas and present some concrete models, which realize our ideas, based
on spin 0, 1 and 2 bosonic fields. We also discuss that the principle of
invariant speed of light in special relativity can be replaced with the
principle of a universal limiting speed.Comment: 17 page
Simulating Radiative Magnetohydrodynamical Flows with AstroBEAR: Implementation and Applications of Non-equilibrium Cooling
Radiative cooling plays a crucial role in the dynamics of many astrophysical
flows, and is particularly important in the dense shocked gas within
Herbig-Haro (HH) objects and stellar jets. Simulating cooling processes
accurately is necessary to compare numerical simulations with existing and
planned observations of HH objects, such as those from the Hubble Space
Telescope and the James Webb Space Telescope. In this paper we discuss a new,
non-equilibrium cooling scheme we have implemented into the 3-D
magnetohydrodynamic (MHD) code AstroBEAR. The new cooling function includes
ionization, recombination, and excitation of all the important atomic species
that cool below 10000 K. We tested the routine by comparing its predictions
with those from the well-tested 1-D Cox-Raymond shock code (Raymond 1979). The
results show thatAstroBEAR accurately tracks the ionization fraction,
temperature, and other MHD variables for all low-velocity (.90 km/s) magnetized
radiative shock waves. The new routine allows us to predict synthetic emission
maps in all the bright forbidden and permitted lines observed in stellar jets,
including H{\alpha}, [NII], [OI], and [SII]. We present an example as to how
these synthetic maps facilitate a direct comparison with narrowband images of
HH objects.Comment: 8 figure
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