1,023 research outputs found
Recollimation shocks and radiative losses in extragalactic relativistic jets
We present the results of state-of-the-art simulations of recollimation
shocks induced by the interaction of a relativistic jet with an external
medium, including the effect of radiative losses of the shocked gas. Our
simulations confirm that -- as suggested by earlier semi-analytical models --
the post-shock pressure loss induced by radiative losses may lead to a
stationary equilibrium state characterized by a very strong focusing of the
flow, with the formation of quite narrow nozzles, with cross-sectional radii as
small as times the length scale of the jet. We also study the
time-dependent evolution of the jet structure induced of a density perturbation
injected at the flow base. The set-up and the results of the simulations are
particularly relevant for the interpretation of the observed rapid variability
of the -ray emission associated to flat spectrum radio quasars. In
particular, the combined effects of jet focusing and Doppler beaming of the
observed radiation make it possible to explain the sub-hour flaring events such
as that observed in the FSRQ PKS 1222+216 by MAGIC.Comment: 8 pages, 8 figures, Astronomy and Astrophysics accepte
Collisional Quenching at Ultralow Energies: Controlling Efficiency with Internal State Selection
Calculations have been carried out for the vibrational quenching of excited
H molecules which collide with Li ions at ultralow energies. The
dynamics has been treated exactly using the well known quantum coupled-channel
expansions over different initial vibrational levels. The overall interaction
potential has been obtained from the calculations carried out earlier in our
group using highly correlated ab initio methods. The results indicate that
specific features of the scattering observables, e.g. the appearance of
Ramsauer-Townsend minima in elastic channel cross sections and the marked
increase of the cooling rates from specific initial states, can be linked to
potential properties at vanishing energies (sign and size of scattering
lengths) and to the presence of either virtual states or bound states. The
suggestion is made that by selecting the initial state preparation of the
molecular partners, the ionic interactions would be amenable to controlling
quenching efficiency at ultralow energies
Fully Convective Magnetorotational Turbulence in Stratified Shearing Boxes
We present a numerical study of turbulence and dynamo action in stratified
shearing boxes with zero magnetic flux. We assume that the fluid obeys the
perfect gas law and has finite (constant) thermal diffusivity. We choose
radiative boundary conditions at the vertical boundaries in which the heat flux
is propor- tional to the fourth power of the temperature. We compare the
results with the corresponding cases in which fixed temperature boundary
conditions are applied. The most notable result is that the formation of a
fully convective state in which the density is nearly constant as a function of
height and the heat is transported to the upper and lower boundaries by
overturning motions is robust and persists even in cases with radiative
boundary conditions. Interestingly, in the convective regime, although the
diffusive transport is negligible the mean stratification does not relax to an
adiabatic state.Comment: 11 pages, 4 figures, accepted for publication in ApJ Letter
On the convergence of Magnetorotational turbulence in stratified isothermal shearing boxes
We consider the problem of convergence in stratified isothermal shearing
boxes with zero net magnetic flux. We present results with the highest
resolution to-date--up to 200 grid-point per pressure scale height--that show
no clear evidence of convergence. Rather, the Maxwell stresses continue to
decrease with increasing resolution. We propose some possible scenarios to
explain the lack of convergence based on multi-layer dynamo systems.Comment: 10 pages, 4 figures, accepted for publication in ApJ Letter
Velocity Fields of Spiral Galaxies in z~0.5 Clusters
Spiral galaxies can be affected by interactions in clusters, that also may
distort the internal velocity field. If unrecognized from single-slit
spectroscopy, this could lead to a wrong determination of the maximum rotation
velocity as pointed out by Ziegler et al. (2003). This parameter directly
enters into the Tully-Fisher relation, an important tool to investigate the
evolution of spiral galaxies. To overcome this problem, we measure the
2D-velocity fields by observing three different slit positions per galaxy using
FORS2 at the VLT providing us with full coverage of each galaxy and an adequate
spatial resolution. The kinematic properties are compared to structural
features determined on the HST/ACS images to assess possible interaction
processes. As a next step, the whole analysis will be performed for three more
clusters, so that we will be able to establish a high-accuracy TFR for spirals
at z~0.5.Comment: 2 pages, 2 figures, going to be published in the proceedings of the
IAU Symp. 241, "Stellar Populations as Building Blocks of Galaxies
Magnetic Helicities and Dynamo Action in Magneto-rotationally Driven Turbulence
We examine the relationship between magnetic flux generation, taken as an
indicator of large-scale dynamo action, and magnetic helicity, computed as an
integral over the dynamo volume, in a simple dynamo. We consider dynamo action
driven by Magneto-Rotational Turbulence (MRT) within the shearing-box
approximation. We consider magnetically open boundary conditions that allow a
flux of helicity in or out of the computational domain. We circumvent the
problem of the lack of gauge invariance in open domains by choosing a
particular gauge -- the winding gauge -- that provides a natural interpretation
in terms of average winding number of pairwise field lines. We use this gauge
precisely to define and measure the helicity and helicity flux for several
realizations of dynamo action. We find in these cases, that the system as a
whole does not break reflectional symmetry and the total helicity remains small
even in cases when substantial magnetic flux is generated. We find no
particular connection between the generation of magnetic flux and the helicity
or the helicity flux through the boundaries. We suggest that this result may be
due to the essentially nonlinear nature of the dynamo processes in MRT.Comment: 26 pages, 10 figures, ApJ accepte
Neutral and ionic dopants in helium clusters: interaction forces for the and
The potential energy surface (PES) describing the interactions between
and and an extensive
study of the energies and structures of a set of small clusters,
, have been presented by us in a previous series of
publications [1-3]. In the present work we want to extend the same analysis to
the case of the excited and of the
ionized Li moiety. We thus show here calculated
interaction potentials for the two title systems and the corresponding fitting
of the computed points. For both surfaces the MP4 method with cc-pV5Z basis
sets has been used to generate an extensive range of radial/angular coordinates
of the two dimensional PES's which describe rigid rotor molecular dopants
interacting with one He partner
Bosonic Helium droplets with cationic impurities: onset of electrostriction and snowball effects from quantum calculations
Variational MonteCarlo and Diffusion MonteCarlo calculations have been
carried out for cations like Li, Na and K as dopants of small
helium clusters over a range of cluster sizes up to about 12 solvent atoms. The
interaction has been modelled through a sum-of-potential picture that
disregards higher order effects beyond atom-atom and atom-ion contributions.
The latter were obtained from highly correlated ab-initio calculations over a
broad range of interatomic distances.
This study focuses on two of the most striking features of the microsolvation
in a quantum solvent of a cationic dopant: electrostriction and snowball
effects. They are here discussed in detail and in relation with the nanoscopic
properties of the interaction forces at play within a fully quantum picture of
the clusters features
- …