1,206 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&
A hybrid model for mapping simplified seismic response via a GIS-metamodel approach
In earthquake-prone areas, site seismic response due to lithostratigraphic sequence plays a key role in seismic hazard assessment. A hybrid model, consisting of GIS and metamodel (model of model) procedures, was introduced aimed at estimating the 1-D spatial seismic site response in accordance with spatial variability of sediment parameters. Inputs and outputs are provided and processed by means of an appropriate GIS model, named GIS Cubic Model (GCM). This consists of a block-layered parametric structure aimed at resolving a predicted metamodel by means of pixel to pixel vertical computing. The metamodel, opportunely calibrated, is able to emulate the classic shape of the spectral acceleration response in relation to the main physical parameters that characterize the spectrum itself. Therefore, via the GCM structure and the metamodel, the hybrid model provides maps of normalized acceleration response spectra. The hybrid model was applied and tested on the built-up area of the San Giorgio del Sannio village, located in a high-risk seismic zone of southern Italy. Efficiency tests showed a good correspondence between the spectral values resulting from the proposed approach and the 1-D physical computational models. Supported by lithology and geophysical data and corresponding accurate interpretation regarding modelling, the hybrid model can be an efficient tool in assessing urban planning seismic hazard/risk. © Author(s) 2014
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
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
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
The X-ray puzzle of the L1551 IRS 5 jet
Protostars are actively accreting matter and they drive spectacular, dynamic
outflows, which evolve on timescales of years. X-ray emission from these jets
has been detected only in a few cases and little is known about its time
evolution. We present a new Chandra observation of L1551 IRS 5's jet in the
context of all available X-ray data of this object. Specifically, we perform a
spatially resolved spectral analysis of the X-ray emission and find that (a)
the total X-ray luminosity is constant over almost one decade, (b) the majority
of the X-rays appear to be always located close to the driving source, (c)
there is a clear trend in the photon energy as a function of the distance to
the driving source indicating that the plasma is cooler at larger distances and
(d) the X-ray emission is located in a small volume which is unresolved
perpendicular to the jet axis by Chandra. A comparison of our X-ray data of the
L1551 IRS 5 jet both with models as well as X-ray observations of other
protostellar jets shows that a base/standing shock is a likely and plausible
explanation for the apparent constancy of the observed X-ray emission. Internal
shocks are also consistent with the observed morphology if the supply of jet
material by the ejection of new blobs is sufficiently constant. We conclude
that the study of the X-ray emission of protostellar jet sources allows us to
diagnose the innermost regions close to the acceleration region of the
outflows.Comment: A&A accepted, 14 pages, 9 figure
Assembly of the Auditory Circuitry by a Hox Genetic Network in the Mouse Brainstem
Rhombomeres (r) contribute to brainstem auditory nuclei during development. Hox genes are determinants of rhombomere-derived fate and neuronal connectivity. Little is known about the contribution of individual rhombomeres and their associated Hox codes to auditory sensorimotor circuitry. Here, we show that r4 contributes to functionally linked sensory and motor components, including the ventral nucleus of lateral lemniscus, posterior ventral cochlear nuclei (VCN), and motor olivocochlear neurons. Assembly of the r4-derived auditory components is involved in sound perception and depends on regulatory interactions between Hoxb1 and Hoxb2. Indeed, in Hoxb1 and Hoxb2 mutant mice the transmission of low-level auditory stimuli is lost, resulting in hearing impairments. On the other hand, Hoxa2 regulates the Rig1 axon guidance receptor and controls contralateral projections from the anterior VCN to the medial nucleus of the trapezoid body, a circuit involved in sound localization. Thus, individual rhombomeres and their associated Hox codes control the assembly of distinct functionally segregated sub-circuits in the developing auditory brainstem
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
Pathway Weathering in Granitoid Rocks from Central Region of Angola: Geochemical and Mineralogical Data
The Central Region of Angola is characterized by the abundance of granitoid rocks, whose weathering “in situ” originated the so-called residual soils. The textural, geochemical and mineralogical properties of these soils depend not only on the chemical composition of parent rock, but mainly on the local climatic and geomorphological characteristics.
In the study area, sampling sites were selected, which extend from the region of Kwanza- Norte (Kassenda, Dondo) through Kwanza-Sul (Cangulo, Quibala and Waco Kungo) until the plateau of Huambo, where samples of fresh rock, weathered rock and its residual soil were collected along each weathering profile.
Chemical analytical data were determined using X-ray fluorescence (XRF) analysis of the major and minor elements, whereas mineralogical data were determined using X-ray diffraction (XRD), on the samples of rock and on the respective residual soil.
The results obtained and their comparative analysis between the sampling sites, as well as along each weathering profile is presented.
This paper allows contributing to the knowledge of the geochemical weathering in tropical areas, as is the case of Angola
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
- …