423,040 research outputs found
Models of the circumstellar medium of evolving, massive runaway stars moving through the Galactic plane
At least 5 per cent of the massive stars are moving supersonically through
the interstellar medium (ISM) and are expected to produce a stellar wind bow
shock. We explore how the mass loss and space velocity of massive runaway stars
affect the morphology of their bow shocks. We run two-dimensional axisymmetric
hydrodynamical simulations following the evolution of the circumstellar medium
of these stars in the Galactic plane from the main sequence to the red
supergiant phase. We find that thermal conduction is an important process
governing the shape, size and structure of the bow shocks around hot stars, and
that they have an optical luminosity mainly produced by forbidden lines, e.g.
[OIII]. The Ha emission of the bow shocks around hot stars originates from near
their contact discontinuity. The H emission of bow shocks around cool
stars originates from their forward shock, and is too faint to be observed for
the bow shocks that we simulate. The emission of optically-thin radiation
mainly comes from the shocked ISM material. All bow shock models are brighter
in the infrared, i.e. the infrared is the most appropriate waveband to search
for bow shocks. Our study suggests that the infrared emission comes from near
the contact discontinuity for bow shocks of hot stars and from the inner region
of shocked wind for bow shocks around cool stars. We predict that, in the
Galactic plane, the brightest, i.e. the most easily detectable bow shocks are
produced by high-mass stars moving with small space velocities.Comment: 22 pages, 24 figure
E-BOSS: An Extensive stellar BOw Shock Survey. II. Catalogue second release
Context. Stellar bow shocks have been studied not only observationally, but
also theoretically since the late 1980s. Only a few catalogues of them exist.
The bow shocks show emission along all the electromagnetic spectrum, but they
are detected more easily in infrared wavelengths. The release of new and
high-quality infrared data eases the discovery and subsequent study of new
objects. Aims. We search stellar bow-shock candidates associated with nearby
runaway stars, and gather them together with those found elsewhere, to enlarge
the list of the E-BOSS first release. We aim to characterize the bow-shock
candidates and provide a database suitable for statistical studies. We
investigate the low-frequency radio emission at the position of the bow-shock
features, that can contribute to further studies of high-energy emission from
these objects. Methods. We considered samples from different literature sources
and searched for bow-shaped structures associated with stars in the Wide-field
Infrared Survey Explorer (WISE) images. We looked for each bow-shock candidate
on centimeter radio surveys. Results. We reunited 45 bow-shock candidates and
generated composed WISE images to show the emission in different infrared
bands. Among them there are new sources, previously studied objects, and bow
shocks found serendipitously. Five bow shocks show evidence of radio emission.
Conclusions. Stellar bow shocks constitute an active field with open questions
and enormous amounts of data to be analyzed. Future research at all wavelengths
databases, and use of instruments like Gaia, will provide a more complete
picture of these objects. For instance, infrared spectral energy distributions
can give information about physical parameters of the bow shock matter. In
addition, dedicated high-sensitivity radio observations can help to understand
the radio- connection.Comment: 20 pages, 11 figures, and 8 tables. Accepted for publication in A&
On the stability of bow shocks generated by red supergiants: the case of IRC-10414
In this Letter, we explore the hypothesis that the smooth appearance of bow
shocks around some red supergiants (RSGs) might be caused by the ionization of
their winds by external sources of radiation. Our numerical simulations of the
bow shock generated by IRC-10414 (the first-ever RSG with an optically detected
bow shock) show that the ionization of the wind results in its acceleration by
a factor of two, which reduces the difference between the wind and space
velocities of the star and makes the contact discontinuity of the bow shock
stable for a range of stellar space velocities and mass-loss rates. Our best
fit model reproduces the overall shape and surface brightness of the observed
bow shock and suggests that the space velocity and mass-loss rate of IRC-10414
are 50 and , respectively, and that the number density of the local ISM is
3 . It also shows that the bow shock emission comes
mainly from the shocked stellar wind. This naturally explains the enhanced
nitrogen abundance in the line-emitting material, derived from the spectroscopy
of the bow shock. We found that photoionized bow shocks are 1550
times brighter in optical line emission than their neutral counterparts, from
which we conclude that the bow shock of IRC-10414 must be photoionized.Comment: 5 pages, 5 figures. Accepted for publication in MNRAS Letter
Systematic search for high-energy gamma-ray emission from bow shocks of runaway stars
Context. It has been suggested that the bow shocks of runaway stars are
sources of high-energy gamma rays (E > 100 MeV). Theoretical models predicting
high-energy gamma-ray emission from these sources were followed by the first
detection of non-thermal radio emission from the bow shock of BD+43^\deg 3654
and non-thermal X-ray emission from the bow shock of AE Aurigae. Aims. We
perform the first systematic search for MeV and GeV emission from 27 bow shocks
of runaway stars using data collected by the Large Area Telescope (LAT) onboard
the Fermi Gamma-ray Space Telescope (Fermi). Methods. We analysed 57 months of
Fermi-LAT data at the positions of 27 bow shocks of runaway stars extracted
from the Extensive stellar BOw Shock Survey catalogue (E-BOSS). A likelihood
analysis was performed to search for gamma-ray emission that is not compatible
with diffuse background or emission from neighbouring sources and that could be
associated with the bow shocks. Results. None of the bow shock candidates is
detected significantly in the Fermi-LAT energy range. We therefore present
upper limits on the high-energy emission in the energy range from 100 MeV to
300 GeV for 27 bow shocks of runaway stars in four energy bands. For the three
cases where models of the high-energy emission are published we compare our
upper limits to the modelled spectra. Our limits exclude the model predictions
for Zeta Ophiuchi by a factor 5.Comment: 5 pages, 5 figures, 1 table, accepted by A&
E-BOSS: an Extensive stellar BOw Shock Survey. I: Methods and First Catalogue
Context: Bow shocks are produced by many astrophysical objects where shock
waves are present. Stellar bow shocks, generated by runaway stars, have been
previously detected in small numbers and well-studied. Along with progress in
model development and improvements in observing instruments, our knowledge of
the emission produced by these objects and its origin can now be more clearly
understood. Aims: We produce a stellar bow-shock catalogue by applying uniform
search criteria and a systematic search process. This catalogue is a starting
point for statistical studies, to help us address fundamental questions such
as, for instance, the conditions under wich a stellar bow shock is detectable.
Methods: By using the newest infrared data releases, we carried out a search
for bow shocks produced by early-type runaway stars. We first explored whether
a set of known IRAS bow shock candidates are visible in the most recently
available IR data, which has much higher resolution and sensitivity. We then
carried out a selection of runaway stars from the latest, large runaway
catalogue available. In this first release, we focused on OB stars and searched
for bow-shaped features in the vicinity of these stars. Results: We provide a
bow-shock candidate survey that gathers a total of 28 members which we call the
Extensive stellar BOw Shock Survey (E-BOSS). We derive the main bow-shock
parameters, and present some preliminary statistical results on the detected
objects. Conclusions: Our analysis of the initial sample and the newly detected
objects yields a bow-shock detectability around OB stars of 10 per cent.
The detections do not seem to depend particularly on either stellar mass, age
or position. The extension of the E-BOSS sample, with upcoming IR data, and by
considering, for example, other spectral types as well, will allow us to
perform a more detailed study of the findings.Comment: A&A accepted (25-NOV-2011), 15 pages, 4 tables, 11 figure
Image classification by visual bag-of-words refinement and reduction
This paper presents a new framework for visual bag-of-words (BOW) refinement
and reduction to overcome the drawbacks associated with the visual BOW model
which has been widely used for image classification. Although very influential
in the literature, the traditional visual BOW model has two distinct drawbacks.
Firstly, for efficiency purposes, the visual vocabulary is commonly constructed
by directly clustering the low-level visual feature vectors extracted from
local keypoints, without considering the high-level semantics of images. That
is, the visual BOW model still suffers from the semantic gap, and thus may lead
to significant performance degradation in more challenging tasks (e.g. social
image classification). Secondly, typically thousands of visual words are
generated to obtain better performance on a relatively large image dataset. Due
to such large vocabulary size, the subsequent image classification may take
sheer amount of time. To overcome the first drawback, we develop a graph-based
method for visual BOW refinement by exploiting the tags (easy to access
although noisy) of social images. More notably, for efficient image
classification, we further reduce the refined visual BOW model to a much
smaller size through semantic spectral clustering. Extensive experimental
results show the promising performance of the proposed framework for visual BOW
refinement and reduction
Shock excitation of the knots of Hen 3-1475
We present new optical STIS HST spectroscopic observations of the jets of the
proto-planetary nebula Hen 3-1475. The excitation conditions of the knots of
Hen 3-1475 are derived from the observed optical spectra, confirming that the
knots are shock excited. The shocked spectra are qualitatively reproduced by
simple ``3/2''D bow shock models. We present a set of bow shock models devoted
to planetary nebulae, and discuss the effects of the pre-ionization conditions,
the bow shock velocity, the bow shock shape and the chemical abundances on the
predicted spectra.
To explore the reliability of the ``3/2''D bow shock models, we also compare
the observed spectra of other three proto-planetary nebulae (M 1-92, M 2-56 and
CRL 618) to the predicted spectra.Comment: 13 pages. A&A (in press
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