3,049 research outputs found
Low-Mass Binary Induced Outflows from Asymptotic Giant Branch Stars
A significant fraction of planetary nebulae (PNe) and proto-planetary nebulae
(PPNe) exhibit aspherical, axisymmetric structures, many of which are highly
collimated. The origin of these structures is not entirely understood, however
recent evidence suggests that many observed PNe harbor binary systems, which
may play a role in their shaping. In an effort to understand how binaries may
produce such asymmetries, we study the effect of low-mass (< 0.3 M_sun)
companions (planets, brown dwarfs and low-mass main sequence stars) embedded
into the envelope of a 3.0 M_sun star during three epochs of its evolution (Red
Giant Branch, Asymptotic Giant Branch (AGB), interpulse AGB). We find that
common envelope evolution can lead to three qualitatively different
consequences: (i) direct ejection of envelope material resulting in a
predominately equatorial outflow, (ii) spin-up of the envelope resulting in the
possibility of powering an explosive dynamo driven jet and (iii) tidal
shredding of the companion into a disc which facilitates a disc driven jet. We
study how these features depend on the secondary's mass and discuss
observational consequences.Comment: 24 pages, 6 figures, submitted to MNRA
Planetesimal growth in turbulent discs before the onset of gravitational instability
It is difficult to imagine a planet formation model that does not at some
stage include a gravitationally unstable disc. Initially unstable gas-dust
discs may form planets directly, but the high surface density required has
motivated the alternative that gravitational instability occurs in a dust
sub-layer only after grains have grown large enough by electrostatic sticking.
Although such growth up to the instability stage is efficient for laminar
discs, concern has mounted as to whether realistic disc turbulence
catastrophically increases the settling time, thereby requiring additional
processes to facilitate planet formation on the needed time scales. To evaluate
this concern, we develop a model for grain growth that accounts for the
influence of turbulence on the collisional velocity of grains and on the scale
height of the dust layer. The relative effect on these quantities depends on
the grain size. The model produces a disc-radius dependent time scale to reach
a gravitationally unstable phase of planet formation. For a range of dust
sticking and disc parameters, we find that for viscosity parameters , this time scale is short enough over a significant range in radii
that turbulence does not catastrophically slow the early phases of planet
formation, even in the absence of agglomeration enhancement agents like
vortices.Comment: Submitted to New Astronom
Simulated emergence of cyclic sexual-asexual reproduction
Motivated by the cyclic pattern of reproductive regimes observed in some
species of green flies (``{\it aphids}''), we simulate the evolution of a
population enduring harsh seasonal conditions for survival. The reproductive
regime of each female is also seasonal in principle and genetically acquired,
and can mutate for each newborn with some small probability. The results show a
sharp transition at a critical value of the survival probability in the winter,
between a reproductive regime in the fall that is predominantly sexual, for low
values of this probability, or asexual, for high values.Comment: 9 pages, 4 figures, requires RevTe
Magnetic helicity in primordial and dynamo scenarios of galaxies
Some common properties of helical magnetic fields in decaying and driven
turbulence are discussed. These include mainly the inverse cascade that
produces fields on progressively larger scales. Magnetic helicity also
restricts the evolution of the large scale field: the field decays less rapidly
than a non-helical field, but it also saturates more slowly, i.e. on a
resistive time scale if there are no magnetic helicity fluxes. The former
effect is utilized in primordial field scenarios, while the latter is important
for successfully explaining astrophysical dynamos that saturate faster than
resistively. Dynamo action is argued to be important not only in the galactic
dynamo, but also in accretion discs in active galactic nuclei and around
protostars, both of which contribute to producing a strong enough seed magnetic
field. Although primordial magnetic fields may be too weak to compete with
these astrophysical mechanisms, such fields could perhaps still be important in
producing polarization effects in the cosmic background radiation.Comment: 8 pages, 5 figures, talk given in Bologna, August 2006, proceedings
of "The Origin and Evolution of Cosmic Magnetism", Astron. Nachr. (in press
Low-Frequency Oscillations in Global Simulations of Black Hole Accretion
We have identified the presence of large-scale, low-frequency dynamo cycles
in a long-duration, global, magnetohydrodynamic (MHD) simulation of black hole
accretion. Such cycles had been seen previously in local shearing box
simulations, but we discuss their evolution over 1,500 inner disk orbits of a
global pi/4 disk wedge spanning two orders of magnitude in radius and seven
scale heights in elevation above/below the disk midplane. The observed cycles
manifest themselves as oscillations in azimuthal magnetic field occupying a
region that extends into a low-density corona several scale heights above the
disk. The cycle frequencies are ten to twenty times lower than the local
orbital frequency, making them potentially interesting sources of low-frequency
variability when scaled to real astrophysical systems. Furthermore, power
spectra derived from the full time series reveal that the cycles manifest
themselves at discrete, narrow-band frequencies that often share power across
broad radial ranges. We explore possible connections between these simulated
cycles and observed low-frequency quasi-periodic oscillations (LFQPOs) in
galactic black hole binary systems, finding that dynamo cycles have the
appropriate frequencies and are located in a spatial region associated with
X-ray emission in real systems. Derived observational proxies, however, fail to
feature peaks with RMS amplitudes comparable to LFQPO observations, suggesting
that further theoretical work and more sophisticated simulations will be
required to form a complete theory of dynamo-driven LFQPOs. Nonetheless, this
work clearly illustrates that global MHD dynamos exhibit quasi-periodic
behavior on timescales much longer than those derived from test particle
considerations.Comment: Version accepted to The Astrophysical Journal, 8 pages, 7 figure
Analysis of the distribution of HII regions in external galaxies. IV The new galaxy sample. Position and Inclination angles
We have compiled a new sample of galaxies with published catalogs of HII
region coordinates. This sample, together with the former catalog of
Garcia-Gomez and Athanassoula (1991) will form the basis for subsequent studies
of the spiral structure in disc galaxies. In this paper we address the problem
of the deprojection of the galaxy images. For this purpose we use two
deprojection methods based on the HII region distribution and compare the
results with the values found in the literature using other deprojection
methods. Taking into account the results of all the methods, we propose optimum
values for the position and inclination angles of all the galaxies in our
sample.Comment: TeX file with 16 postscript figure
Disordered Correlated Kondo-lattice model
We propose a self-consistent approximate solution of the disordered
Kondo-lattice model (KLM) to get the interconnected electronic and magnetic
properties of 'local-moment' systems like diluted ferromagnetic semiconductors.
Aiming at compounds, where magnetic (M) and non-magnetic (A)
atoms distributed randomly over a crystal lattice, we present a theory which
treats the subsystems of itinerant charge carriers and localized magnetic
moments in a homologous manner. The coupling between the localized moments due
to the itinerant electrons (holes) is treated by a modified RKKY-theory which
maps the KLM onto an effective Heisenberg model. The exchange integrals turn
out to be functionals of the electronic selfenergy guaranteeing selfconsistency
of our theory. The disordered electronic and magnetic moment systems are both
treated by CPA-type methods.
We discuss in detail the dependencies of the key-terms such as the long range
and oscillating effectice exchange integrals, 'the local-moment' magnetization,
the electron spin polarization, the Curie temperature as well as the electronic
and magnonic quasiparticle densities of states on the concentration of
magnetic ions, the carrier concentration , the exchange coupling , and
the temperature. The shape and the effective range of the exchange integrals
turn out to be strongly -dependent. The disorder causes anomalies in the
spin spectrum especially in the low-dilution regime, which are not observed in
the mean field approximation.Comment: Accepted by JMM
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