1,122 research outputs found
New perspectives on the supernova remnant Puppis A based on a radio polarization study
We present a polarization study towards the supernova remnant (SNR) Puppis A based on original observations performed with the Australia Telescope Compact Array. Based on the analysis of a feature detected outside the SNR shell (called 'the tail' throughout the paper), it was possible to disentangle the emission with origin in Puppis A itself from that coming from the foreground Vela SNR. We found a very low polarization fraction, of about 3 per cent on average. The upper limit of the magnetic field component parallel to the line of sight is estimated to be B ~ 20 μG. The statistical behaviour of the magnetic vectors shows two preferential directions, almost perpendicular to each other, which are approximately aligned with the flat edges of Puppis A. A third, narrow peak oriented perpendicular to the Galactic plane suggests the existence of an interstellar magnetic field locally aligned in this direction. There is evidence that the magnetic vectors along the shell are aligned with the shock front direction. The low polarization fraction and the statistical behaviour of the magnetic vectors are compatible with a scenario where the SNR evolves inside a stellar wind bubble with a box-like morphology, produced by the interaction of the different stellar winds, one of them magnetized, launched by the SN progenitor. This scenario can furthermore explain the morphology of Puppis A, rendering little support to the previously accepted picture which involved strong density gradients to explain the flat, eastern edge of the shell.Fil: Reynoso, Estela Marta. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Velázquez, P. F.. Universidad Nacional Autónoma de México. Instituto de Ciencias Nucleares; MéxicoFil: Cichowolski, Silvina. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentin
A non-linear degenerate equation for direct aggregation and traveling wave dynamics
The gregarious behavior of individuals of populations is an important factor in avoiding predators or for reproduction. Here, by using a random biased walk approach, we build a model which, after a transformation, takes the general form [u_{t}=[D(u)u_{x}]_{x}+g(u)] . The model involves a density-dependent non-linear diffusion coefficient [D] whose sign changes as the population density [u] increases. For negative values of [D] aggregation occurs, while dispersion occurs for positive values of [D] . We deal with a family of degenerate negative diffusion equations with logistic-like growth rate [g] . We study the one-dimensional traveling wave dynamics for these equations and illustrate our results with a couple of examples. A discussion of the ill-posedness of the partial differential equation problem is included
An MHD study of SN 1006 and determination of the ambient magnetic field direction
In this work we employ an MHD numerical code to reproduce the morphology
observed for SN 1006 in radio synchrotron and thermal X-ray emission. We
introduce a density discontinuity, in the form of a flat cloud parallel to the
Galactic Plane, in order to explain the NW filament observed in optical
wavelengths and in thermal X-rays. We compare our models with observations. We
also perform a test that contrasts the radio emitting bright limbs of the SNR
against the central region, finding additional support to our results. Our main
conclusion is that the most probable direction of the ambient magnetic field is
on average perpendicular to the Galactic Plane.Comment: 7 pages, 5 figures, accepted by MNRA
3D MHD simulation of polarized emission in SN 1006
We use three dimensional magnetohydrodynamic (MHD) simulations to model the
supernova remnant SN 1006. From our numerical results, we have carried out a
polarization study, obtaining synthetic maps of the polarized intensity, the
Stokes parameter , and the polar-referenced angle, which can be compared
with observational results. Synthetic maps were computed considering two
possible particle acceleration mechanisms: quasi-parallel and
quasi-perpendicular. The comparison of synthetic maps of the Stokes parameter
maps with observations proves to be a valuable tool to discern
unambiguously which mechanism is taking place in the remnant of SN 1006, giving
strong support to the quasi-parallel model.Comment: 6 pages, 4 figures, accepted by MNRA
Emission lines from rotating proto-stellar jets with variable velocity profiles. I. Three-dimensional numerical simulation of the non-magnetic case
Using the Yguazu-a three-dimensional hydrodynamic code, we have computed a
set of numerical simulations of heavy, supersonic, radiatively cooling jets
including variabilities in both the ejection direction (precession) and the jet
velocity (intermittence). In order to investigate the effects of jet rotation
on the shape of the line profiles, we also introduce an initial toroidal
rotation velocity profile, in agreement with some recent observational evidence
found in jets from T Tauri stars which seems to support the presence of a
rotation velocity pattern inside the jet beam, near the jet production region.
Since the Yguazu-a code includes an atomic/ionic network, we are able to
compute the emission coefficients for several emission lines, and we generate
line profiles for the H, [O I]6300, [S II]6716 and [N II]6548 lines. Using
initial parameters that are suitable for the DG Tau microjet, we show that the
computed radial velocity shift for the medium-velocity component of the line
profile as a function of distance from the jet axis is strikingly similar for
rotating and non-rotating jet models. These findings lead us to put forward
some caveats on the interpretation of the observed radial velocity distribution
from a few outflows from young stellar objects, and we claim that these data
should not be directly used as a doubtless confirmation of the
magnetocentrifugal wind acceleration models.Comment: 15 pages, 8 figures. Accepted to publication in Astronomy and
Astrophysic
Unveiling shocks in planetary nebulae
The propagation of a shock wave into a medium is expected to heat the
material beyond the shock, producing noticeable effects in intensity line
ratios such as [O III]/Halpha. To investigate the occurrence of shocks in
planetary nebulae (PNe), we have used all narrowband [O III] and Halpha images
of PNe available in the HST archive to build their [O III]/Halpha ratio maps
and to search for regions where this ratio is enhanced. Regions with enhanced
[O III]/Halpha emission ratio can be ascribed to two different types of
morphological structures: bow-shock structures produced by fast collimated
outflows and thin skins enveloping expanding nebular shells. Both collimated
outflows and expanding shells are therefore confirmed to generate shocks in
PNe. We also find regions with depressed values of the [O III]/Halpha ratio
which are found mostly around density bounded PNe, where the local contribution
of [N II] emission into the F656N Halpha filter cannot be neglected.Comment: 13 pages, 9 figures, 3 tables; To appear in Astronomy & Astrophysic
Filaments in Galactic Winds Driven by Young Stellar Clusters
The starburst galaxy M82 shows a system of H-emitting filaments which
extend to each side of the galactic disk. We model these filaments as the
result of the interaction between the winds from a distribution of Super
Stellar Clusters (SSCs). We first derive the condition necessary for producing
a radiative interaction between the cluster winds (a condition which is met by
the SSC distribution of M82). We then compute 3D simulations for SSC wind
distributions which satisfy the condition for a radiative interaction, and also
for distributions which do not satisfy this condition. We find that the highly
radiative models, that result from the interaction of high metallicity cluster
winds, produce a structure of H emitting filaments, which qualitatively
agrees with the observations of the M82, while the non-radiative SSC wind
interaction models do not produce filamentary structures. Therefore, our
criterion for radiative interactions (which depends on the mass loss rate and
the terminal velocity of the SSC winds, and the mean separation between SSCs)
can be used to predict whether or not an observed galaxy should have associated
H emitting filaments.Comment: 10 pages, 6 Figures. ApJ Accepted, August 7, 200
A latitude-dependent wind model for Mira's cometary head
We present a 3D numerical simulation of the recently discovered cometary
structure produced as Mira travels through the galactic ISM. In our simulation,
we consider that Mira ejects a steady, latitude-dependent wind, which interacts
with a homogeneous, streaming environment. The axisymmetry of the problem is
broken by the lack of alignment between the direction of the relative motion of
the environment and the polar axis of the latitude-dependent wind. With this
model, we are able to produce a cometary head with a ``double bow shock'' which
agrees well with the structure of the head of Mira's comet. We therefore
conclude that a time-dependence in the ejected wind is not required for
reproducing the observed double bow shock.Comment: 4 pages, 4 figures, accepted for publication in ApJ
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