679 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
Spectroscopic observations of blue stars with infrared excesses in NGC 6611
Context. The young open cluster NGC 6611 includes a group of peculiar objects with interesting properties among its candidate members: blue stars with infrared (IR) excesses. These stars show excesses in IR bands, a signature of the presence of a circumstellar disk, but optical colors typical of older field stars. To confirm their membership in the cluster, it is therefore important to use new spectroscopic observations, together with previous photometric data. Aims. We aim to confirm the membership of these objects and investigate their physical properties to verify whether the observed colors are intrinsic or altered by the disk or by the accretion processes. Methods. We analyzed the intermediate-resolution spectroscopic data obtained for a subsample of blue stars in NGC 6611 with FLAMES. In particular, we focused on the study of 1) the profile of the Hα emission line, to select stars with accretion and outflow activity; 2) the Li absorption line, used as a youth indicator; 3) the radial velocity. Results. Using the spectroscopic analysis, it has been possible to investigate the Li absorption line, as well as to distinguish between stars with inert or active disks. In particular, from the analysis of the Hα emission line we were able to infer the activity due to the accretion and outflow processes and the variability of the emission. We also investigated the binarity of the blue stars and their membership to NGC 6611. Conclusions. From our spectroscopic analysis, we conclude that half of the sample of blue stars (10/20) are confirmed members of NGC 6611 (with 6 more stars that could also be possible members). In conclusion, our results indicate that members of young clusters can also be found in an anomalous region of the color-magnitude diagram, i.e., outside of the pre-main sequence locus where most of the cluster members lie
Pre-main-sequence stars older than 8 Myr in the Eagle nebula
Attention is given to a population of 110 stars in the NGC 6611 cluster of
the Eagle Nebula that have prominent near-infrared (NIR) excess and optical
colours typical of pre-main sequence (PMS) stars older than 8 Myr. At least
half of those for which spectroscopy exists have a Halpha emission line profile
revealing active accretion. In principle, the V-I colours of all these stars
would be consistent with those of young PMS objects (< 1 Myr) whose radiation
is heavily obscured by a circumstellar disc seen at high inclination and in
small part scattered towards the observer by the back side of the disc.
However, using theoretical models it is shown here that objects of this type
can only account for a few percent of this population. In fact, the spatial
distribution of these objects, their X-ray luminosities, their optical
brightness, their positions in the colour-magnitude diagram and the weak Li
absorption lines of the stars studied spectroscopically suggest that most of
them are at least 8 times older than the ~1 Myr-old PMS stars already known in
this cluster and could be as old as ~30 Myr. This is the largest homogeneous
sample to date of Galactic PMS stars considerably older than 8 Myr that are
still actively accreting from a circumstellar disc and it allows us to set a
lower limit of 7% to the disc frequency at ~16 Myr in NGC 6611. These values
imply a characteristic exponential lifetime of ~6 Myr for disc dissipation.Comment: 12 pages, 5 figures, accepted for publication in Monthly Notices of
the Astronomical Societ
AE Aurigae: first detection of non-thermal X-ray emission from a bow shock produced by a runaway star
Runaway stars produce shocks when passing through interstellar medium at
supersonic velocities. Bow shocks have been detected in the mid-infrared for
several high-mass runaway stars and in radio waves for one star. Theoretical
models predict the production of high-energy photons by non-thermal radiative
processes in a number sufficiently large to be detected in X-rays. To date, no
stellar bow shock has been detected at such energies. We present the first
detection of X-ray emission from a bow shock produced by a runaway star. The
star is AE Aur, which was likely expelled from its birthplace by the encounter
of two massive binary systems and now is passing through the dense nebula IC
405. The X-ray emission from the bow shock is detected at 30" to the northeast
of the star, coinciding with an enhancement in the density of the nebula. From
the analysis of the observed X-ray spectrum of the source and our theoretical
emission model, we confirm that the X-ray emission is produced mainly by
inverse Compton upscattering of infrared photons from dust in the shock front.Comment: Accepted for publication in the Astrophysical Journal with number
ApJ, 757, L6. Four figure
Radiative accretion shocks along nonuniform stellar magnetic fields in classical T Tauri stars
(abridged) AIMS. We investigate the dynamics and stability of post-shock
plasma streaming along nonuniform stellar magnetic fields at the impact region
of accretion columns. We study how the magnetic field configuration and
strength determine the structure, geometry, and location of the shock-heated
plasma. METHODS. We model the impact of an accretion stream onto the
chromosphere of a CTTS by 2D axisymmetric magnetohydrodynamic simulations. Our
model takes into account the gravity, the radiative cooling, and the
magnetic-field-oriented thermal conduction. RESULTS. The structure, stability,
and location of the shocked plasma strongly depend on the configuration and
strength of the magnetic field. For weak magnetic fields, a large component of
B may develop perpendicular to the stream at the base of the accretion column,
limiting the sinking of the shocked plasma into the chromosphere. An envelope
of dense and cold chromospheric material may also develop around the shocked
column. For strong magnetic fields, the field configuration determines the
position of the shock and its stand-off height. If the field is strongly
tapered close to the chromosphere, an oblique shock may form well above the
stellar surface. In general, a nonuniform magnetic field makes the distribution
of emission measure vs. temperature of the shocked plasma lower than in the
case of uniform magnetic field. CONCLUSIONS. The initial strength and
configuration of the magnetic field in the impact region of the stream are
expected to influence the chromospheric absorption and, therefore, the
observability of the shock-heated plasma in the X-ray band. The field strength
and configuration influence also the energy balance of the shocked plasma, its
emission measure at T > 1 MK being lower than expected for a uniform field. The
above effects contribute in underestimating the mass accretion rates derived in
the X-ray band.Comment: 11 pages, 11 Figures; accepted for publication on A&A. Version with
full resolution images can be found at
http://www.astropa.unipa.it/~orlando/PREPRINTS/sorlando_accretion_shocks.pd
X-ray emission from protostellar jet HH 154: the first evidence of a diamond shock?
X-ray emission from about ten protostellar jets has been discovered and it
appears as a feature common to the most energetic jets. Although X-ray emission
seems to originate from shocks internal to jets, the mechanism forming these
shocks remains controversial. One of the best studied X-ray jet is HH 154 that
has been observed by Chandra over a time base of about 10 years. We analyze the
Chandra observations of HH 154 by investigating the evolution of its X-ray
source. We show that the X-ray emission consists of a bright stationary
component and a faint elongated component. We interpret the observations by
developing a hydrodynamic model describing a protostellar jet originating from
a nozzle and compare the X-ray emission synthesized from the model with the
X-ray observations. The model takes into account the thermal conduction and
radiative losses and shows that the jet/nozzle leads to the formation of a
diamond shock at the nozzle exit. The shock is stationary over the period
covered by our simulations and generates an X-ray source with luminosity and
spectral characteristics in excellent agreement with the observations. We
conclude that the X-ray emission from HH 154 is consistent with a diamond shock
originating from a nozzle through which the jet is launched into the ambient
medium. We suggest that the physical origin of the nozzle could be related to
the dense gas in which the HH 154 driving source is embedded and/or to the
magnetic field at the jet launching/collimation region.Comment: Accepted for publication in The Astrophysical Journa
Magnetohydrodynamic modeling of the accretion shocks in classical T Tauri stars: the role of local absorption on the X-ray emission
We investigate the properties of X-ray emission from accretion shocks in
classical T Tauri stars (CTTSs), generated where the infalling material impacts
the stellar surface. Both observations and models of the accretion process
reveal several aspects that are still unclear: the observed X-ray luminosity in
accretion shocks is below the predicted value, and the density versus
temperature structure of the shocked plasma, with increasing densities at
higher temperature, deduced from the observations, is at odds with that
proposed in the current picture of accretion shocks. To address these open
issues we investigate whether a correct treatment of the local absorption by
the surrounding medium is crucial to explain the observations. To this end, we
describe the impact of an accretion stream on a CTTS by considering a
magnetohydrodynamic model. From the model results we synthesize the X-ray
emission from the accretion shock by producing maps and spectra. We perform
density and temperature diagnostics on the synthetic spectra, and we directly
compare the results with the observations. Our model shows that the X-ray
fluxes inferred from the emerging spectra are lower than expected because of
the complex local absorption by the optically thick material of the
chromosphere and of the unperturbed stream. Moreover, our model including the
effects of local absorption explains in a natural way the apparently puzzling
pattern of density versus temperature observed in the X-ray emission from
accretion shocks.Comment: Accepted for publication in Astrophysical Journal Letters; 5 pages, 4
figure
Agenesia lumbosacra
La agenesia lumbosacra es una malformación congénita muy rara que forma parte del síndrome de regresión caudal; su incidencia es de un caso por cada 25,000 nacidos vivos y es más frecuente en los hijos de madres diabéticas. Se comunica el caso de un neonato masculino de dos días de vida extrauterina atendido en el Hospital Universitario Dr. José Eleuterio González, con malformaciones en el segmento corporal inferior. Con el estudio radiográfico se comprobó el diagnóstico de agenesia lumbosacra. La familia desconocía los tipos de malformaciones o enfermedades hereditarias en su árbol genealógico; el estudio de cariotipo no demostró alteraciones (46 XY), al igual que la determinación de hemoglobina glucosilada a la madre. En el ecocardiograma se identificó foramen oval permeable y miocardiopatía hipertrófica sin repercusión hemodinámica. Debido a su complejidad clínica, el tratamiento para la agenesia lumbosacacra debe abordarse con un equipo médico multidisciplinario para identificar cualquier alteración anatómica y funcional
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