22 research outputs found
HII regions in symbiotic binaries and their radio emission
The slow and dense wind from a symbiotic red giant can be significantly
deflected toward the orbital plane by the gravitational pull of the companion
star. In such an environment, the ionizing radiation from the companion creates
a highly asymmetric HII region. We present three-dimensional models of HII
regions in symbiotic S-type systems, for which we calculate radio maps and
radio spectra. We show that the standard assumption of spherically symmetric RG
wind results in wrong shapes, sizes and spectra of ionized regions, which in
turn affects the observational estimates of orbital separation and mass loss
rate. A sample of radio maps and radio spectra of our models is presented and
the results are discussed in relation to observational data.Comment: Accepted for publication in A&
Two-dimensional models of layered protoplanetary discs - II. The effect of a residual viscosity in the dead zone
We study axisymmetric models of layered protoplanetary discs taking radiative
transfer effects into account, and allowing for a residual viscosity in the
dead zone. We also explore the effect of different viscosity prescriptions. In
addition to the ring instability reported in the first paper of the series we
find an oscillatory instability of the dead zone, accompanied by variations of
the accretion rate onto the central star. We provide a simplified analytical
description explaining the mechanism of the oscillations. Finally, we find that
the residual viscosity enables stationary accretion in large regions of layered
discs. Based on results obtained with the help of a simple 1-D hydrocode we
identify these regions, and discuss conditions in which layered discs can give
rise to FU~Orionis phenomena.Comment: 9 pages, 5 figures, accepted for publication in MNRA
UV Emission line shifts of symbiotic binaries
Relative and absolute emission line shifts have been previously found for
symbiotic binaries, but their cause was not clear. This work aims to better
understand the emission line shifts. Positions of strong emission lines were
measured on archival UV spectra of Z And, AG Dra, RW Hya, SY Mus and AX Per and
relative shifts between the lines of different ions compared. Profiles of lines
of RW Hya and Z And were also examined. The reality of the relative shift
between resonance and intercombination lines of several times ionised atoms was
clearly shown except for AG Dra. This redshift shows a well defined variation
with orbital phase for Z And and RW Hya. In addition the intercombination lines
from more ionised atoms and especially OIV are redshifted with respect to those
from less ionised atoms. Other effects are seen in the profiles. The
resonance-intercombination line shift variation can be explained in quiescence
by P Cygni shorter wavelength component absorption, due to the wind of the cool
component, which is specially strong in inferior conjunction of this cool
giant. The velocity stratification permits absorption of line emission. The
relative intercombination line shifts may be connected with varying occultation
of line emission near an accretion disk, which is optically thick in the
continuum.Comment: 14 pages, 11 figures, to appear in A&
The Shapes of AGB Envelopes as Probes of Binary Companions
We describe how the large scale geometry of the circumstellar envelopes of
asymptotic giant branch stars can be used to probe the presence of unseen
stellar companions. A nearby companion modifies the mass loss by
gravitationally focusing the wind towards the orbital plane, and thereby
determines the shape of the envelope at large distances from the star. Using
available simulations, we develop a prescription for the observed shapes of
envelopes in terms of the binary parameters, envelope orientation, and type of
observation. The prescription provides a tool for the analysis of envelope
images at optical, infrared, and millimetre wavelengths, which can be used to
constrain the presence of companions in well observed cases. We illustrate this
approach by examining the possible role of binary companions in triggering the
onset of axi-symmetry in planetary nebula formation. If interaction with the
primary leads to axi-symmetry, the spherical halos widely seen around newly
formed nebulae set limits on the companion mass. Only low mass objects may
orbit close to the primary without observable shaping effects: they remain
invisible until the interaction causes a sudden change in the mass loss
geometry.Comment: 11 pages, 7 figures, to appear in MNRA
Non-spherical core collapse supernovae III. Evolution towards homology and dependence on the numerical resolution
(abridged) We study the hydrodynamic evolution of a non-spherical
core-collapse supernova in two spatial dimensions. We find that our model
displays a strong tendency to expand toward the pole. We demonstrate that this
expansion is a physical property of the low-mode, SASI instability. The SASI
leaves behind a large lateral velocity gradient in the post shock layer which
affects the evolution for minutes and hours later. This results in a prolate
deformation of the ejecta and a fast advection of Ni-rich material from
moderate latitudes to the polar regions. This effect might actually be
responsible for the global asymmetry of the nickel lines in SN 1987A. The
simulations demonstrate that significant radial and lateral motions in the
post-shock region, produced by convective overturn and the SASI during the
early explosion phase, contribute to the evolution for minutes and hours after
shock revival. They lead to both later clump formation, and a significant
prolate deformation of the ejecta which are observed even as late as one week
after the explosion. As pointed out recently by Kjaer et al., such an ejecta
morphology is in good agreement with the observational data of SN 1987A.
Systematic future studies are needed to investigate how the SASI-induced
late-time lateral expansion depends on the dominant mode of the SASI, and to
which extent it is affected by the dimensionality of the simulations. The
impact on and importance of the SASI for the distribution of iron group nuclei
and the morphology of the young SNR argues for future three-dimensional
explosion and post-explosion studies on singularity-free grids that cover the
entire sphere. Given the results of our 2D resolution study, present 3D
simulations must be regarded as underresolved, and their conclusions must be
verified by a proper numerical convergence analysis in three dimensions.Comment: 16 pages, 20 figures, accepted for publication in Astronomy &
Astrophysic
A comparative study of disc-planet interaction
We perform numerical simulations of a disc-planet system using various
grid-based and smoothed particle hydrodynamics (SPH) codes. The tests are run
for a simple setup where Jupiter and Neptune mass planets on a circular orbit
open a gap in a protoplanetary disc during a few hundred orbital periods. We
compare the surface density contours, potential vorticity and smoothed radial
profiles at several times. The disc mass and gravitational torque time
evolution are analyzed with high temporal resolution. There is overall
consistency between the codes. The density profiles agree within about 5% for
the Eulerian simulations while the SPH results predict the correct shape of the
gap although have less resolution in the low density regions and weaker
planetary wakes. The disc masses after 200 orbital periods agree within 10%.
The spread is larger in the tidal torques acting on the planet which agree
within a factor 2 at the end of the simulation. In the Neptune case the
dispersion in the torques is greater than for Jupiter, possibly owing to the
contribution from the not completely cleared region close to the planet.Comment: 32 pages, accepted for publication in MNRA
The Astropy Problem
The Astropy Project (http://astropy.org) is, in its own words, "a community
effort to develop a single core package for Astronomy in Python and foster
interoperability between Python astronomy packages." For five years this
project has been managed, written, and operated as a grassroots,
self-organized, almost entirely volunteer effort while the software is used by
the majority of the astronomical community. Despite this, the project has
always been and remains to this day effectively unfunded. Further, contributors
receive little or no formal recognition for creating and supporting what is now
critical software. This paper explores the problem in detail, outlines possible
solutions to correct this, and presents a few suggestions on how to address the
sustainability of general purpose astronomical software