127 research outputs found
Shocks in relativistic transverse stratified jets, a new paradigm for radio-loud AGN
The transverse stratification of active galactic nuclei (AGN) jets is
suggested by observations and theoretical arguments, as a consequence of
intrinsic properties of the central engine (accretion disc + black hole) and
external medium. On the other hand, the one-component jet approaches are
heavily challenged by the various observed properties of plasmoids in radio
jets (knots), often associated with internal shocks. Given that such a
transverse stratification plays an important role on the jets acceleration,
stability, and interaction with the external medium, it should also induce
internal shocks with various strengths and configurations, able to describe the
observed knots behaviours. By establishing a relation between the transverse
stratification of the jets, the internal shock properties, and the multiple
observed AGN jet morphologies and behaviours, our aim is to provide a
consistent global scheme of the various AGN jet structures. Working on a large
sample of AGN radio jets monitored in very long baseline interferometry (VLBI)
by the MOJAVE collaboration, we determined the consistency of a systematic
association of the multiple knots with successive re-collimation shocks. We
then investigated the re-collimation shock formation and the influence of
different transverse stratified structures by parametrically exploring the two
relativistic outflow components with the specific relativistic hydrodynamic
(SRHD) code AMRVAC. We were able to link the different spectral classes of AGN
with specific stratified jet characteristics, in good accordance with their
VLBI radio properties and their accretion regimes.Comment: 16 pages, 12 figures, accepted for publication in A&
Relativistic spine jets from Schwarzschild black holes: "Application to AGN radioloud sources"
The two types of Fanaroff-Riley radio loud galaxies, FRI and FRII, exhibit
strong jets but with different properties. These differences may be associated
to the central engine and/or the external medium. Aims: The AGN classification
FRI and FRII can be linked to the rate of electromagnetic Poynting flux
extraction from the inner corona of the central engine by the jet. The
collimation results from the distribution of the total electromagnetic energy
across the jet, as compared to the corresponding distribution of the thermal
and gravitational energies. We use exact solutions of the fully relativistic
magnetohydrodynamical (GRMHD) equations obtained by a nonlinear separation of
the variables to study outflows from a Schwarzschild black hole corona. A
strong correlation is found between the jet features and the energetic
distribution of the plasma of the inner corona which may be related to the
efficiency of the magnetic rotator. It is shown that observations of FRI and
FRII jets may be partially constrained by our model for spine jets. The
deceleration observed in FRI jets may be associated with a low magnetic
efficiency of the central magnetic rotator and an important thermal confinement
by the hot surrounding medium. Conversely, the strongly collimated and
accelerated FRII outflows may be self collimated by their own magnetic field
because of the high efficiency of the central magnetic rotator.Comment: Accepted for publication in the A&
Magnetohydrodynamic jets from different magnetic field configurations
Using axisymmetric MHD simulations we investigate how the overall jet
formation is affected by a variation in the disk magnetic flux profile and/or
the existence of a central stellar magnetosphere. Our simulations evolve from
an initial, hydrostatic equilibrium state in a force-free magnetic field
configuration. We find a unique relation between the collimation degree and the
disk wind magnetization power law exponent. The collimation degree decreases
for steeper disk magnetic field profiles. Highly collimated outflows resulting
from a flat profile tend to be unsteady. We further consider a magnetic field
superposed of a stellar dipole and a disk field in parallel or anti-parallel
alignment. Both stellar and disk wind may evolve in a pair of outflows,
however, a reasonably strong disk wind component is essential for jet
collimation. Strong flares may lead to a sudden change in mass flux by a factor
two. We hypothesize that such flares may eventually trigger jet knots.Comment: 5 pages, 4 figures; proceedings from conference: Protostellar Jets in
Context, held in Rhodes, July 7-12, 200
Nonradial and nonpolytropic astrophysical outflows VIII. A GRMHD generalization for relativistic jets
Steady axisymmetric outflows originating at the hot coronal magnetosphere of
a Schwarzschild black hole and surrounding accretion disk are studied in the
framework of general relativistic magnetohydrodynamics (GRMHD). The assumption
of meridional self-similarity is adopted for the construction of
semi-analytical solutions of the GRMHD equations describing outflows close to
the polar axis. In addition, it is assumed that relativistic effects related to
the rotation of the black hole and the plasma are negligible compared to the
gravitational and other energetic terms. The constructed model allows us to
extend previous MHD studies for coronal winds from young stars to spine jets
from Active Galactic Nuclei surrounded by disk-driven outflows. The outflows
are thermally driven and magnetically or thermally collimated. The collimation
depends critically on an energetic integral measuring the efficiency of the
magnetic rotator, similarly to the non relativistic case. It is also shown that
relativistic effects affect quantitatively the depth of the gravitational well
and the coronal temperature distribution in the launching region of the
outflow. Similarly to previous analytical and numerical studies, relativistic
effects tend to increase the efficiency of the thermal driving but reduce the
effect of magnetic self-collimation.Comment: 20 page, Accepted in A&A 10/10/200
Infrared Photometry of Red Supergiants in Young Clusters in the Magellanic Clouds
We present broad-band infrared photometry for 52 late-type supergiants in the
young Magellanic Clouds clusters NGC 330, NGC 1818, NGC 2004 and NGC 2100.
Standard models are seen to differ in the temperature they predict for the red
supergiant population on the order of 300K. It appears that these differences
most probably due to the calibration of the mixing-length parameter,
, in the outermost layers of the stellar envelope. Due to the
apparent model dependent nature of we do not quantitatively
compare between models. Qualitatively, we find that
decreases with increased stellar mass within standard models. We do not find
evidence for a metallicity dependence of .Comment: 11 pages, 4 figures. AJ accepte
Large scale magnetic fields in viscous resistive accretion disks. I. Ejection from weakly magnetized disks
Cold steady-state disk wind theory from near Keplerian accretion disks
requires a large scale magnetic field at near equipartition strength. However
the minimum magnetization has never been tested. We investigate the time
evolution of an accretion disk threaded by a weak vertical magnetic field. The
strength of the field is such that the disk magnetization falls off rapidly
with radius. Four 2.5D numerical simulations of viscous resistive accretion
disk are performed using the magnetohydrodynamic code PLUTO. In these
simulations, a mean field approach is used and turbulence is assumed to give
rise to anomalous transport coefficients (alpha prescription). The large scale
magnetic field introduces only a small perturbation to the disk structure, with
accretion driven by the dominant viscous torque. A super fast magnetosonic jet
is observed to be launched from the innermost regions and remains stationary
over more than 953 Keplerian orbits. The self-confined jet is launched from a
finite radial zone in the disk which remains constant over time. Ejection is
made possible because the magnetization reaches unity at the disk surface, due
to the steep density decrease. However, no ejection is reported when the
midplane magnetization becomes too small. The asymptotic jet velocity remains
nevertheless too low to explain observed jets due to the negligible power
carried away by the jet. Astrophysical disks with superheated surface layers
could drive analogous outflows even if their midplane magnetization is low.
Sufficient angular momentum would be extracted by the turbulent viscosity to
allow the accretion process to continue. The magnetized outflows would be no
more than byproducts, rather than a fundamental driver of accretion. However,
if the midplane magnetization increases towards the center, a natural
transition to an inner jet dominated disk could be achieved.Comment: Accepted by Astronomy and Astrophysic
Ca II Triplet Spectroscopy of Giants in SMC Star Clusters: Abundances, Velocities and the Age-Metallicity Relation
We have obtained spectra at the Ca II triplet of individual red giants in
seven SMC star clusters whose ages range from ~4 to 12 Gyr. The spectra have
been used to determine mean abundances for six of the star clusters to a
typical precision of 0.12 dex. When combined with existing data for other
objects, the resulting SMC age-metallicity relation is generally consistent
with that for a simple model of chemical evolution, scaled to the present-day
SMC mean abundance and gas mass fraction. Two of the clusters (Lindsay 113 and
NGC 339), however, have abundances that ~0.5 dex lower than that expected from
the mean age-metallicity relation. It is suggested that the formation of these
clusters, which have ages of ~5 Gyr, may have involved the infall of uneriched
gas, perhaps from the Magellanic Stream. The spectra also yield radial
velocities for the seven clusters. The resulting velocity dispersion is 16 +/-
4 km/sec, consistent with those of the SMC planetary nebula and carbon star
populations.Comment: 28 pages including 4 figure
Nonradial and nonpolytropic astrophysical outflows IX. Modeling T Tauri jets with a low mass-accretion rate
Context: A large sample of T Tauri stars exhibits optical jets, approximately
half of which rotate slowly, only at ten per cent of their breakup velocity.
The disk-locking mechanism has been shown to be inefficient to explain this
observational fact.
Aims: We show that low mass accreting T Tauri stars may have a strong stellar
jet component that can effectively brake the star to the observed rotation
speed.
Methods: By means of a nonlinear separation of the variables in the full set
of the MHD equations we construct semi- analytical solutions describing the
dynamics and topology of the stellar component of the jet that emerges from the
corona of the star.
Results: We analyze two typical solutions with the same mass loss rate but
different magnetic lever arms and jet radii. The first solution with a long
lever arm and a wide jet radius effectively brakes the star and can be applied
to the visible jets of T Tauri stars, such as RY Tau. The second solution with
a shorter lever arm and a very narrow jet radius may explain why similar stars,
either Weak line T Tauri Stars (WTTS) or Classical T Tauri Stars (CTTS) do not
all have visible jets. For instance, RY Tau itself seems to have different
phases that probably depend on the activity of the star.
Conclusions: First, stellar jets seem to be able to brake pre-main sequence
stars with a low mass accreting rate. Second, jets may be visible only part
time owing to changes in their boundary conditions. We also suggest a possible
scenario for explaining the dichotomy between CTTS and WTTS, which rotate
faster and do not have visible jets
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