706 research outputs found
Generation of radiative knots in a randomly pulsed protostellar jet I. Dynamics and energetics
HH objects are characterized by a complex knotty morphology detected mainly
along the axis of protostellar jets in a wide range of bands. Evidence of
interactions between knots formed in different epochs have been found,
suggesting that jets may result from the ejection of plasma blobs from the
source. We aim at investigating the physical mechanism leading to the irregular
knotty structure observed in jets in different bands and the complex
interactions occurring among blobs of plasma ejected from the stellar source.
We perform 2D axisymmetric HD simulations of a randomly ejected pulsed jet. The
jet consists of a train of blobs which ram with supersonic speed into the
ambient medium. The initial random velocity of each blob follows an exponential
distribution. We explore the ejection rate parameter to derive constraints on
the physical properties of protostellar jets by comparison of model results
with observations. Our model takes into account radiative losses and thermal
conduction. We find that the mutual interactions of blobs ejected at different
epochs and with different speed lead to a variety of plasma components not
described by current models. The main features characterizing the random pulsed
jet scenario are: single high speed knots, showing a measurable proper motion
in nice agreement with observations; irregular chains of knots aligned along
the jet axis and possibly interacting with each other; reverse shocks
interacting with outgoing knots; oblique shocks produced by the reflection of
shocks at the jet cocoon. All these structures concur to determine the
morphology of the jet in different bands. We also find that the thermal
conduction plays a crucial role in damping out HD instabilities that would
develop within the cocoon and that contribute to the jet breaking.Comment: 10 pages, 10 figures, accepted for publication in A&
Redshifted X-rays from the material accreting onto TW Hya: evidence of a low-latitude accretion spot
High resolution spectroscopy, providing constraints on plasma motions and
temperatures, is a powerful means to investigate the structure of accretion
streams in CTTS. In particular, the accretion shock region, where the accreting
material is heated to temperatures of a few MK as it continues its inward bulk
motion, can be probed by X-ray spectroscopy. To attempt to detect for the first
time the motion of this X-ray-emitting post-shock material, we searched for a
Doppler shift in the deep Chandra/HETGS observation of the CTTS TW Hya. This
test should unveil the nature of this X-ray emitting plasma component in CTTS,
and constrain the accretion stream geometry. We searched for a Doppler shift in
the X-ray emission from TW Hya with two different methods, by measuring the
position of a selected sample of emission lines, and by fitting the whole TW
Hya X-ray spectrum, allowing the line-of-sight velocity to vary. We found that
the plasma at T~2-4 MK has a line-of-sight velocity of 38.3+/-5.1 km/s with
respect to the stellar photosphere. This result definitively confirms that this
X-ray-emitting material originates in the post-shock region, at the base of the
accretion stream, and not in coronal structures. The comparison of the observed
velocity along the line of sight, 38.3+/-5.1 km/s, with the inferred intrinsic
velocity of the post shock of TW Hya, v_post~110-120 km/s, indicates that the
footpoints of the accretion streams on TW Hya are located at low latitudes on
the stellar surface. Our results indicate that complex magnetic field
geometries, such as that of TW Hya, permit low-latitude accretion spots.
Moreover, since on TW Hya the redshift of the soft X-ray emission is very
similar to that of the narrow component of the CIV resonance doublet at 1550
Ang, as found by Ardila et al. (2013), then the plasma at 2-4 MK and that at
0.1 MK likely originate in the same post-shock regions.Comment: Accepted for publication in Astronomy & Astrophysics; 2nd version
after language editor corrections; 16 pages, 8 figures, 6 table
Pensar a educação: contributos da filosofia na procura da qualidade
Neste trabalho abordaremos o papel da reflexão como um dos importantes contributos que a Filosofia presta à Educação. Como fio condutor exploramos a relação entre a Filosofia e o conhecimento, focando a atenção no acto de conhecer, enquanto gerador de conhecimento, pelo qual o homem se faz homem, aliando a virtude à sabedoria. Imbuir a educação desta sageza é um imperativo actual. Só uma educação pensada poderá orientar o homem para si mesmo e para o seu semelhante, no entanto, o que de facto se efectiva rumo à excelência educativa nem sempre coincide com o proclamado, pelo que fazemos também uma aproximação aos conceitos de qualidade e qualidade do ensino, apontando algumas das variáveis que concorrem para a educação com a qualidade que se preconiza
Modeling non-thermal emission from stellar bow shocks
Runaway O- and early B-type stars passing throughout the interstellar medium
at supersonic velocities and characterized by strong stellar winds may produce
bow shocks that can serve as particle acceleration sites. Previous theoretical
models predict the production of high energy photons by non-thermal radiative
processes, but their efficiency is still debated. We aim to test and explain
the possibility of emission from the bow shocks formed by runaway stars
traveling through the interstellar medium by using previous theoretical models.
We apply our model to AE Aurigae, the first reported star with an X-ray
detected bow shock, to BD+43 3654, in which the observations failed in
detecting high energy emission, and to the transition phase of a supergiant
star in the late stages of its life.From our analysis, we confirm that the
X-ray emission from the bow shock produced by AE Aurigae can be explained by
inverse Compton processes involving the infrared photons of the heated dust. We
also predict low high energy flux emission from the bow shock produced by BD+43
3654, and the possibility of high energy emission from the bow shock formed by
a supergiant star during the transition phase from blue to red supergiant.Bow
shock formed by different type of runaway stars are revealed as a new possible
source of high energy photons in our neighbourhood
High-Order AFEM for the Laplace-Beltrami Operator: Convergence Rates
We present a new AFEM for the Laplace-Beltrami operator with arbitrary
polynomial degree on parametric surfaces, which are globally and
piecewise in a suitable Besov class embedded in with . The idea is to have the surface sufficiently well resolved in
relative to the current resolution of the PDE in . This gives
rise to a conditional contraction property of the PDE module. We present a
suitable approximation class and discuss its relation to Besov regularity of
the surface, solution, and forcing. We prove optimal convergence rates for AFEM
which are dictated by the worst decay rate of the surface error in
and PDE error in .Comment: 51 pages, the published version contains an additional glossar
Accretion-ejection connection in the young brown dwarf candidate ISO-Cha1 217
As the number of observed brown dwarf outflows is growing it is important to
investigate how these outflows compare to the well studied jets from young
stellar objects. A key point of comparison is the relationship between outflow
and accretion activity and in particular the ratio between the mass outflow and
accretion rates (/). The brown dwarf candidate
ISO-ChaI 217 was discovered by our group, as part of a spectro-astrometric
study of brown dwarfs, to be driving an asymmetric outflow with the
blue-shifted lobe having a position angle of 20. The aim here
is to further investigate the properties of ISO-ChaI 217, the morphology and
kinematics of its outflow, and to better constrain
(/). The outflow is spatially resolved in the
lines and is detected out to 1\farcs6
in the blue-shifted lobe and ~ 1" in the red-shifted lobe. The asymmetry
between the two lobes is confirmed although the velocity asymmetry is less
pronounced with respect to our previous study. Using thirteen different
accretion tracers we measure log() [M/yr]= -10.6
0.4. As it was not possible to measure the effect of extinction on the ISO-ChaI
217 outflow was derived for a range of values of A, up to
a value of A = 2.5 mag estimated for the source extinction. The logarithm
of the mass outflow () was estimated in the range -11.7 to -11.1
for both jets combined. Thus / [\Msun/yr] lies
below the maximum value predicted by magneto-centrifugal jet launching models.
Finally, both model fitting of the Balmer decrements and spectro-astrometric
analysis of the H line show that the bulk of the H I emission comes
from the accretion flow.Comment: accepted by Astronomy & Astrophysic
Evidence of non-thermal X-ray emission from HH 80
Protostellar jets appear at all stages of star formation when the accretion
process is still at work. Jets travel at velocities of hundreds of km/s,
creating strong shocks when interacting with interstellar medium. Several cases
of jets have been detected in X-rays, typically showing soft emission. For the
first time, we report evidence of hard X-ray emission possibly related to
non-thermal processes not explained by previous models of the post-shock
emission predicted in the jet/ambient interaction scenario. HH 80 is located at
the south head of the jet associated to the massive protostar IRAS 18162-2048.
It shows soft and hard X-ray emission in regions that are spatially separated,
with the soft X-ray emission region situated behind the region of hard X-ray
emission. We propose a scenario for HH 80 where soft X-ray emission is
associated to thermal processes from the interaction of the jet with denser
ambient matter and the hard X-ray emission is produced by synchrotron radiation
at the front shock.Comment: Accepted for publication in ApJ
Formation of X-ray emitting stationary shocks in magnetized protostellar jets
X-ray observations of protostellar jets show evidence of strong shocks
heating the plasma up to temperatures of a few million degrees. In some cases,
the shocked features appear to be stationary. They are interpreted as shock
diamonds. We aim at investigating the physics that guides the formation of
X-ray emitting stationary shocks in protostellar jets, the role of the magnetic
field in determining the location, stability, and detectability in X-rays of
these shocks, and the physical properties of the shocked plasma. We performed a
set of 2.5-dimensional magnetohydrodynamic numerical simulations modelling
supersonic jets ramming into a magnetized medium and explored different
configurations of the magnetic field. The model takes into account the most
relevant physical effects, namely thermal conduction and radiative losses. We
compared the model results with observations, via the emission measure and the
X-ray luminosity synthesized from the simulations. Our model explains the
formation of X-ray emitting stationary shocks in a natural way. The magnetic
field collimates the plasma at the base of the jet and forms there a magnetic
nozzle. After an initial transient, the nozzle leads to the formation of a
shock diamond at its exit which is stationary over the time covered by the
simulations (~ 40 - 60 yr; comparable with time scales of the observations).
The shock generates a point-like X-ray source located close to the base of the
jet with luminosity comparable with that inferred from X-ray observations of
protostellar jets. For the range of parameters explored, the evolution of the
post-shock plasma is dominated by the radiative cooling, whereas the thermal
conduction slightly affects the structure of the shock.Comment: Accepted for publication in Astronomy and Astrophysic
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