78 research outputs found
Counterrotation in magnetocentrifugally driven jets and other winds
Rotation measurement in jets from T Tauri stars is a rather difficult task.
Some jets seem to be rotating in a direction opposite to that of the underlying
disk, although it is not yet clear if this affects the totality or part of the
outflows. On the other hand, Ulysses data also suggest that the solar wind may
rotate in two opposite ways between the northern and southern hemispheres. We
show that this result is not as surprising as it may seem and that it emerges
naturally from the ideal MHD equations. Specifically, counterrotating jets
neither contradict the magnetocentrifugal driving of the flow nor prevent
extraction of angular momentum from the disk. The demonstration of this result
is shown by combining the ideal MHD equations for steady axisymmetric flows.
Provided that the jet is decelerated below some given threshold beyond the
Alfven surface, the flow will change its direction of rotation locally or
globally. Counterrotation is also possible for only some layers of the outflow
at specific altitudes along the jet axis. We conclude that the counterrotation
of winds or jets with respect to the source, star or disk, is not in
contradiction with the magnetocentrifugal driving paradigm. This phenomenon may
affect part of the outflow, either in one hemisphere, or only in some of the
outflow layers. From a time-dependent simulation, we illustrate this effect and
show that it may not be permanent.Comment: To appear in ApJ
FUV variability of HD 189733. Is the star accreting material from its hot Jupiter?
Hot Jupiters are subject to strong irradiation from the host stars and, as a
consequence, they do evaporate. They can also interact with the parent stars by
means of tides and magnetic fields. Both phenomena have strong implications for
the evolution of these systems. Here we present time resolved spectroscopy of
HD~189733 observed with the Cosmic Origin Spectrograph (COS) on board to HST.
The star has been observed during five consecutive HST orbits, starting at a
secondary transit of the planet ( ~0.50-0.63). Two main episodes of
variability of ion lines of Si, C, N and O are detected, with an increase of
line fluxes. Si IV lines show the highest degree of variability. The FUV
variability is a signature of enhanced activity in phase with the planet
motion, occurring after the planet egress, as already observed three times in
X-rays. With the support of MHD simulations, we propose the following
interpretation: a stream of gas evaporating from the planet is actively and
almost steadily accreting onto the stellar surface, impacting at
ahead of the sub-planetary point.Comment: 35 pages, 19 Figures. Accepted for publication to Ap
Counter-rotation in relativistic magnetohydrodynamic jets
Young stellar object observations suggest that some jets rotate in the
opposite direction with respect to their disk. In a recent study, Sauty et al.
(2012) have shown that this does not contradict the magnetocentrifugal
mechanism that is believed to launch such outflows. Signatures of motions
transverse to the jet axis and in opposite directions have recently been
measured in M87 (Meyer et al. 2013). One possible interpretation of this motion
is the one of counter rotating knots. Here, we extend our previous analytical
derivation of counter-rotation to relativistic jets, demonstrating that
counter-rotation can indeed take place under rather general conditions. We show
that both the magnetic field and a non-negligible enthalpy are necessary at the
origin of counter-rotating outflows, and that the effect is associated with a
transfer of energy flux from the matter to the electromagnetic field. This can
be realized in three cases : if a decreasing enthalpy causes an increase of the
Poynting flux, if the flow decelerates, or, if strong gradients of the magnetic
field are present. An illustration of the involved mechanism is given by an
example of relativistic MHD jet simulation.Comment: Accepted for publication in ApJ
Two-component jet simulations: II. Combining analytical disk and stellar MHD outflow solutions
Theoretical arguments along with observational data of YSO jets suggest the
presence of two steady components: a disk wind type outflow needed to explain
the observed high mass loss rates and a stellar wind type outflow probably
accounting for the observed stellar spin down.
Each component's contribution depends on the intrinsic physical properties of
the YSO-disk system and its evolutionary stage. The main goal of this paper is
to understand some of the basic features of the evolution, interaction and
co-existence of the two jet components over a parameter space and when time
variability is enforced. Having studied separately the numerical evolution of
each type of the complementary disk and stellar analytical wind solutions in
Paper I of this series, we proceed here to mix together the two models inside
the computational box. The evolution in time is performed with the PLUTO code,
investigating the dynamics of the two-component jets, the modifications each
solution undergoes and the potential steady state reached.Comment: accepted for publication in A&
Stability and structure of analytical MHD jet formation models with a finite outer disk radius
(Abridged) Finite radius accretion disks are a strong candidate for launching
astrophysical jets from their inner parts and disk-winds are considered as the
basic component of such magnetically collimated outflows. The only available
analytical MHD solutions for describing disk-driven jets are those
characterized by the symmetry of radial self-similarity. Radially self-similar
MHD models, in general, have two geometrical shortcomings, a singularity at the
jet axis and the non-existence of an intrinsic radial scale, i.e. the jets
formally extend to radial infinity. Hence, numerical simulations are necessary
to extend the analytical solutions towards the axis and impose a physical
boundary at finite radial distance. We focus here on studying the effects of
imposing an outer radius of the underlying accreting disk (and thus also of the
outflow) on the topology, structure and variability of a radially self-similar
analytical MHD solution. The initial condition consists of a hybrid of an
unchanged and a scaled-down analytical solution, one for the jet and the other
for its environment. In all studied cases, we find at the end steady
two-component solutions.Comment: 14 pages, 15 figures, accepted for publication in A &
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
Young stellar object jet models: From theory to synthetic observations
Astronomical observations, analytical solutions and numerical simulations
have provided the building blocks to formulate the current theory of young
stellar object jets. Although each approach has made great progress
independently, it is only during the last decade that significant efforts are
being made to bring the separate pieces together. Building on previous work
that combined analytical solutions and numerical simulations, we apply a
sophisticated cooling function to incorporate optically thin energy losses in
the dynamics. On the one hand, this allows a self-consistent treatment of the
jet evolution and on the other, it provides the necessary data to generate
synthetic emission maps. Firstly, analytical disk and stellar outflow solutions
are properly combined to initialize numerical two-component jet models inside
the computational box. Secondly, magneto-hydrodynamical simulations are
performed in 2.5D, following properly the ionization and recombination of a
maximum of ions. Finally, the outputs are post-processed to produce
artificial observational data. The first two-component jet simulations, based
on analytical models, that include ionization and optically thin radiation
losses demonstrate promising results for modeling specific young stellar object
outflows. The generation of synthetic emission maps provides the link to
observations, as well as the necessary feedback for the further improvement of
the available models.Comment: accepted for publication A&A, 20 pages, 11 figure
Velocity asymmetries in YSO jets: Intrinsic and extrinsic mechanisms
It is a well established fact that some YSO jets (e.g. RW Aur) display
different propagation speeds between their blue and red shifted parts, a
feature possibly associated with the central engine or the environment in which
the jet propagates. In order to understand the origin of asymmetric YSO jet
velocities, we investigate the efficiency of two candidate mechanisms, one
based on the intrinsic properties of the system and one based on the role of
the external medium. In particular, a parallel or anti-parallel configuration
between the protostellar magnetosphere and the disk magnetic field is
considered and the resulting dynamics are examined both in an ideal and a
resistive magneto-hydrodynamical (MHD) regime. Moreover, we explore the effects
of a potential difference in the pressure of the environment, as a consequence
of the non-uniform density distribution of molecular clouds. Ideal and
resistive axisymmetric numerical simulations are carried out for a variety of
models, all of which are based on a combination of two analytical solutions, a
disk wind and a stellar outflow. We find that jet velocity asymmetries can
indeed occur both when multipolar magnetic moments are present in the star-disk
system as well as when non-uniform environments are considered. The latter case
is an external mechanism that can easily explain the large time scale of the
phenomenon, whereas the former one naturally relates it to the YSO intrinsic
properties. [abridged]Comment: accepted for publication in A&
- …