104 research outputs found
Nonthermal Radio Emission from Planetary Nebulae
In a recent analysis of the radio emission from the planetary nebula A30,
Dgani, Evans & White (1998) claim that the emission, located in the inner
region, is probably dominated by nonthermal emission.
We propose a model to explain this.
We assume that the fast wind, blown by the central star of A30 carries a very
weak magnetic field. The interaction of this wind with a cluster of dense
condensations traps the magnetic field lines for a long time and stretches
them, leading to a strong magnetic field.
If relativistic particles are formed as the fast wind is shocked, then the
enhanced magnetic field will result in nonthermal radio emission.
The typical nonthermal radio flux at 1 GHz can be up to several milli-Jansky.
In order to detect the nonthermal emission, the emitting region should be
spatially resolved from the main optical nebula.
We list other planetary nebulae which may possess nonthermal radio emission.Comment: 11 page
MHD numerical simulations of colliding winds in massive binary systems - I. Thermal vs non-thermal radio emission
In the past few decades detailed observations of radio and X-rays emission
from massive binary systems revealed a whole new physics present in such
systems. Both thermal and non-thermal components of this emission indicate that
most of the radiation at these bands originates in shocks. OB and WR stars
present supersonic and massive winds that, when colliding, emit largely due to
the free-free radiation. The non-thermal radio and X-ray emissions are due to
synchrotron and inverse compton processes, respectively. In this case, magnetic
fields are expected to play an important role on the emission distribution. In
the past few years the modeling of the free-free and synchrotron emissions from
massive binary systems have been based on purely hydrodynamical simulations,
and ad hoc assumptions regarding the distribution of magnetic energy and the
field geometry. In this work we provide the first full MHD numerical
simulations of wind-wind collision in massive binary systems. We study the
free-free emission characterizing its dependence on the stellar and orbital
parameters. We also study self-consistently the evolution of the magnetic field
at the shock region, obtaining also the synchrotron energy distribution
integrated along different lines of sight. We show that the magnetic field in
the shocks is larger than that obtained when the proportionality between
and the plasma density is assumed. Also, we show that the role of the
synchrotron emission relative to the total radio emission has been
underestimated.Comment: MNRAS accepte
Numerical Simulation of Rotating Accretion Disk Around the Schwarzschild Black Hole Using GRH Code
The 2D time dependent solution of thin accretion disk in a close binary
system have been presented on the equatorial plane around the Schwarzschild
black hole. To do that, the special part of the General Relativistic
Hydrodynamical(GRH) equations are solved using High Resolution Shock Capturing
(HRSC) schemes. The spiral shock waves on the accretion disk are modeled using
perfect fluid equation of state with adiabatic indices and
5/3. The results show that the spiral shock waves are created for gammas except
the case . These results consistent with results from Newtonian
hydrodynamic code except close to black hole. Newtonian approximation does not
give good solution while matter closes to black hole. Our simulations
illustrate that the spiral shock waves are created close to black hole and the
location of inner radius of spiral shock wave is around and it depends on
the specific heat rates. We also find that the smaller is the more
tightly the spiral winds.Comment: 19 pages 11 figure
The interaction of planetary nebulae and their AGB progenitors with the interstellar medium
Interaction with the Interstellar Medium (ISM) cannot be ignored in
understanding planetary nebula (PN) evolution and shaping. In an effort to
understand the range of shapes observed in the outer envelopes of PNe, we have
run a comprehensive set of three-dimensional hydrodynamic simulations, from the
beginning of the asymptotic giant branch (AGB) superwind phase until the end of
the post--AGB/PN phase. A 'triple-wind' model is used, including a slow AGB
wind, fast post--AGB wind and third wind reflecting the linear movement through
the ISM. A wide range of stellar velocities, mass-loss rates and ISM densities
have been considered. We find ISM interaction strongly affects outer PN
structures, with the dominant shaping occuring during the AGB phase. The
simulations predict four stages of PN--ISM interaction whereby the PN is
initially unaffected (1), then limb-brightened in the direction of motion (2),
then distorted with the star moving away from the geometric centre (3) and
finally so distorted that the object is no longer recognisable as a PN and may
not be classed as such (4). Parsec-size shells around PN are predicted to be
common. The structure and brightness of ancient PNe is largely determined by
the ISM interaction, caused by rebrightening during the second stage; this
effect may address the current discrepancies in Galactic PN abundance. The
majority of PNe will have tail structures. Evidence for strong interaction is
found for all known planetary nebulae in globular clusters.Comment: 22 pages, 16 figures, accepted by MNRAS (consists of 14 page journal
paper and 8 page online-only appendix). Email C Wareing for high quality PDF
versio
Metastasizing placental site trophoblastic tumor: Immunohistochemical and DNA analysis 2 case reports and a review of the literature
Placental-site trophoblastic tumor (PSTT) is a rare form of gestational trophoblastic neoplasia. The clinical behaviour of PSTT is usually benign, but sometimes it can be highly malignant with late recurrence and metastasis. We describe two cases of PSTT with pulmonary metastasis in patients aged 35 and 29 years respectively. The mitotic rate was elevated to 9 and 13 mitotic figures per 10 high-power fields respectively. Immunohistochemical staining showed a predominance of human placental lactogen (hPL) positive cells when compared with human chorionic gonadotropin (hCG) reactive cells in one case, and a reverse pattern in the other one. DNA measurement in one case showed an aneuploid tumor with a tetraploid DNA peak. The clinical behaviour of PSTT remains unpredictable, and there are no reliable means of predicting clinical outcom
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
On the Exchange of Kinetic and Magnetic Energy Between Clouds and the Interstellar Medium
We investigate, through 2D MHD numerical simulations, the interaction of a
uniform magnetic field oblique to a moving interstellar cloud. In particular we
explore the transformation of cloud kinetic energy into magnetic energy as a
result of field line stretching. Some previous simulations have emphasized the
possible dynamical importance of a ``magnetic shield'' formed around clouds
when the magnetic field is perpendicular to the cloud motion (Jones et al.
1996, Miniati et al. 1998). It was not clear, however, how dependent those
findings were to the assumed field configuration and cloud properties. To
expand our understanding of this effect, we examine several new cases by varing
the magnetic field orientation angle with respect to the cloud motion (\theta),
the cloud-background density contrast, and the cloud Mach number.
We show that in 2D and with \theta large enough, the magnetic field tension
can become dominant in the dynamics of the motion of high density contrast, low
Mach number clouds. In such cases a significant fraction of cloud kinetic
energy can be transformed into magnetic energy with the magnetic pressure at
the cloud nose exceeding the ram pressure of the impinging flow. We derive a
characteristic timescale for this process of energy ``conversion''. We find
also that unless the cloud motion is highly aligned to the magnetic field,
reconnection through tearing mode instabilities in the cloud wake limit the
formation of a strong flux rope feature following the cloud. Finally we attempt
to interpret some observational properties of the magnetic field in view of our
results.Comment: 24 pages in aaspp4 Latex and 7 figures. Accepted for publication in
The Astrophysical Journa
Discovery of a halo around the Helix Nebula NGC 7293 in the WISE all-sky survey
We report the discovery of an extended halo (40' in diameter) around the
planetary nebula NGC7293 (the Helix Nebula) observed in 12micron band from the
Wide-field Infrared Survey Explorer all-sky survey. The mid-infrared halo has
an axisymmetric structure with a sharp boundary to the northeast and a more
diffuse boundary to the southwest, suggesting an interaction between the
stellar wind and the interstellar medium (ISM). The symmetry axis of the halo
is well aligned with that of a northeast arc, suggesting that the two
structures are physically associated. We have attempted to fit the observed
geometry with a model of a moving steady-state stellar wind interacting with
the ISM. Possible combinations of the ISM density and the stellar velocity are
derived from these fittings. The discrepancies between the model and the
observations suggest that the stellar mass loss has a more complicated history,
including possible time and angle dependences.Comment: 15 pages, 2 figures, accepted by Ap
X Her and TX Psc: Two cases of ISM interaction with stellar winds observed by Herschel
The asymptotic giant branch (AGB) stars X Her and TX Psc have been imaged at
70 and 160 microns with the PACS instrument onboard the Herschel satellite, as
part of the large MESS (Mass loss of Evolved StarS) Guaranteed Time Key
Program. The images reveal an axisymmetric extended structure with its axis
oriented along the space motion of the stars. This extended structure is very
likely to be shaped by the interaction of the wind ejected by the AGB star with
the surrounding interstellar medium (ISM). As predicted by numerical
simulations, the detailed structure of the wind-ISM interface depends upon the
relative velocity between star+wind and the ISM, which is large for these two
stars (108 and 55 km/s for X Her and TX Psc, respectively). In both cases,
there is a compact blob upstream whose origin is not fully elucidated, but that
could be the signature of some instability in the wind-ISM shock. Deconvolved
images of X Her and TX Psc reveal several discrete structures along the
outermost filaments, which could be Kelvin-Helmholtz vortices. Finally, TX Psc
is surrounded by an almost circular ring (the signature of the termination
shock?) that contrasts with the outer, more structured filaments. A similar
inner circular structure seems to be present in X Her as well, albeit less
clearly.Comment: 11 pages, Astronomy & Astrophysics, in pres
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