172 research outputs found
A radiation-hydrodynamics scheme valid from the transport to the diffusion limit
We present in this paper the numerical treatment of the coupling between
hydrodynamics and radiative transfer. The fluid is modeled by classical
conservation laws (mass, momentum and energy) and the radiation by the grey
moment system. The scheme introduced is able to compute accurate
numerical solution over a broad class of regimes from the transport to the
diffusive limits. We propose an asymptotic preserving modification of the HLLE
scheme in order to treat correctly the diffusion limit. Several numerical
results are presented, which show that this approach is robust and have the
correct behavior in both the diffusive and free-streaming limits. In the last
numerical example we test this approach on a complex physical case by
considering the collapse of a gas cloud leading to a proto-stellar structure
which, among other features, exhibits very steep opacity gradients.Comment: 29 pages, submitted to Journal of Computational physic
Review of Solar and Reactor Neutrinos
Over the last several years, experiments have conclusively demonstrated that
neutrinos are massive and that they mix. There is now direct evidence for
s from the Sun transforming into other active flavors while en route to
the Earth. The disappearance of reactor s, predicted under the
assumption of neutrino oscillation, has also been observed. In this paper,
recent results from solar and reactor neutrino experiments and their
implications are reviewed. In addition, some of the future experimental
endeavors in solar and reactor neutrinos are presented.Comment: Proceedings of the XXII International Symposium on Lepton and Photon
Interactions at High Energy (Lepton-Photon 2005, June 30 to July 5, 2005,
Uppsala, Sweden). 11 figures, 5 table
Temporal evolution of magnetic molecular shocks I. Moving grid simulations
We present time-dependent 1D simulations of multifluid magnetic shocks with
chemistry resolved down to the mean free path. They are obtained with an
adaptive moving grid implemented with an implicit scheme. We examine a broad
range of parameters relevant to conditions in dense molecular clouds, with
preshock densities between 10^3 and 10^5 cm-3, velocities between 10 and 40
km/s, and three different scalings for the transverse magnetic field: B=0,0.1,1
\mu G \sqrt{n.cm3}. We first use this study to validate the results of
Chi\`eze, Pineau des For\^ets & Flower (1998), in particular the long delays
necessary to obtain steady C-type shocks, and we provide evolutionary
time-scales for a much greater range of parameters. We also present the first
time-dependent models of dissociative shocks with a magnetic precursor,
including the first models of stationary CJ shocks in molecular conditions. We
find that the maximum speed for steady C-type shocks is reached before the
occurrence of a sonic point in the neutral fluid, unlike previously thought. As
a result, the maximum speed for C-shocks is lower than previously believed.
Finally, we find a large amplitude bouncing instability in J-type fronts near
the H2 dissociation limit (u ~ 25-30 km/s), driven by H2
dissociation/reformation. At higher speeds, we find an oscillatory behaviour of
short period and small amplitude linked to collisional ionisation of H. Both
instabilities are suppressed after some time when a magnetic field is present.
In a companion paper, we use the present simulations to validate a new
semi-analytical construction method for young low-velocity magnetic shocks
based on truncated steady-state models.Comment: A&A in pres
Temporal evolution of magnetic molecular shocks II. Analytics of the steady state and semi-analytical construction of intermediate ages
In the first paper of this series (Paper I) we computed time dependent
simulations of multifluid shocks with chemistry and a transverse magnetic field
frozen in the ions, using an adaptive moving grid. In this paper, we present
new analytical results on steady-state molecular shocks. Relationships between
density and pressure in the neutral fluid are derived for the cold magnetic
precursor, hot magnetic precursor, adiabatic shock front, and the following
cooling layer. The compression ratio and temperature behind a fully
dissociative adiabatic shock is also derived. To prove that these results may
even hold for intermediate ages, we design a test to locally characterise the
validity of the steady state equations in a time-dependent shock simulation.
Applying this tool to the results of Paper I, we show that most of these shocks
(all the stable ones) are indeed in a quasi-steady state at all times, i.e. : a
given snapshot is composed of one or more truncated steady shock. Finally, we
use this property to produce a construction method of any intermediate time of
low velocity shocks (u < 20 km/s) with only a steady-state code. In particular,
this method allows one to predict the occurrence of steady CJ-type shocks more
accurately than previously proposed criteria.Comment: A&A in pres
Coriolis force corrections to g-mode spectrum in 1D MHD model
The corrections to g-mode frequencies caused by the presence of a central
magnetic field and rotation of the Sun are calculated. The calculations are
carried out in the simple one dimensional magnetohydrodynamical model using the
approximations which allow one to find the purely analytical spectra of
magneto-gravity waves beyond the scope of the JWKB approximation and avoid in a
small background magnetic field the appearance of the cusp resonance which
locks a wave within the radiative zone. These analytic results are compared
with the satellite observations of the g-mode frequency shifts which are of the
order one per cent as given in the GOLF experiment at the SoHO board. The main
contribution turns out to be the magnetic frequency shift in the strong
magnetic field which obeys the used approximations. In particular, the fixed
magnetic field strength 700 KG results in the mentioned value of the frequency
shift for the g-mode of the radial order n=-10. The rotational shift due to the
Coriolis force appears to be small and does not exceed a fracton of per cent,
\alpha_\Omega < 0.003.Comment: RevTeX4, 9 pages, 4 eps figures; accepted for publication in
Astronomy Reports (Astronomicheskii Zhurnal
Seismic and dynamical solar models i-the impact of the solar rotation history on neutrinos and seismic indicators
Solar activity and helioseismology show the limitation of the standard solar
model and call for the inclusion of dynamical processes in both convective and
radiative zones. We concentrate here on the radiative zone and first show the
sensitivity of boron neutrinos to the microscopic physics included in solar
models. We confront the neutrino predictions of the seismic model to all the
detected neutrino fluxes. Then we compute new models of the Sun including a
detailed transport of angular momentum and chemicals due to internal rotation
that includes meridional circulation and shear induced turbulence. We use two
stellar evolution codes: CESAM and STAREVOL to estimate the different terms. We
follow three temporal evolutions of the internal rotation differing by their
initial conditions: very slow, moderate and fast rotation, with magnetic
braking at the arrival on the main sequence for the last two. We find that the
meridional velocity in the present solar radiative zone is extremely small in
comparison with those of the convective zone, smaller than 10^-6 cm/s instead
of m/s. All models lead to a radial differential rotation profile but with a
significantly different contrast. We compare these profiles to the presumed
solar internal rotation and show that if meridional circulation and shear
turbulence were the only mechanisms transporting angular momentum within the
Sun, a rather slow rotation in the young Sun is favored. The transport by
rotation slightly influence the sound speed profile but its potential impact on
the chemicals in the transition region between radiation and convective zones.
This work pushes us to pursue the inclusion of the other dynamical processes to
better reproduce the present observable and to describe the young active Sun.
We also need to get a better knowledge of solar gravity mode splittings to use
their constraints.Comment: 39 pages, 9 figures, accepted in Astrophysical Journa
Seismic diagnostics for transport of angular momentum in stars 1. Rotational splittings from the PMS to the RGB
Rotational splittings are currently measured for several main sequence stars
and a large number of red giants with the space mission Kepler. This will
provide stringent constraints on rotation profiles. Our aim is to obtain
seismic constraints on the internal transport and surface loss of angular
momentum of oscillating solar-like stars. To this end, we study the evolution
of rotational splittings from the pre-main sequence to the red-giant branch for
stochastically excited oscillation modes. We modified the evolutionary code
CESAM2K to take rotationally induced transport in radiative zones into account.
Linear rotational splittings were computed for a sequence of
models. Rotation profiles were derived from our evolutionary models and
eigenfunctions from linear adiabatic oscillation calculations. We find that
transport by meridional circulation and shear turbulence yields far too high a
core rotation rate for red-giant models compared with recent seismic
observations. We discuss several uncertainties in the physical description of
stars that could have an impact on the rotation profiles. For instance, we find
that the Goldreich-Schubert-Fricke instability does not extract enough angular
momentum from the core to account for the discrepancy. In contrast, an increase
of the horizontal turbulent viscosity by 2 orders of magnitude is able to
significantly decrease the central rotation rate on the red-giant branch. Our
results indicate that it is possible that the prescription for the horizontal
turbulent viscosity largely underestimates its actual value or else a mechanism
not included in current stellar models of low mass stars is needed to slow down
the rotation in the radiative core of red-giant stars.Comment: 15 pages, 13 figures, accepted for publication in A&
Helioseismology with PICARD
PICARD is a CNES micro-satellite launched in June 2010 (Thuillier at al.
2006). Its main goal is to measure the solar shape, total and spectral
irradiance during the ascending phase of the activity cycle. The SODISM
telescope onboard PICARD also allows us to conduct a program for
helioseismology in intensity at 535.7 nm (Corbard et al. 2008). One-minute
cadence low-resolution full images are available for a so-called medium-
program, and high-resolution images of the limb recorded every 2 minutes are
used to study mode amplification near the limb in the perspective of g-mode
search. First analyses and results from these two programs are presented here.Comment: 6 pages, 6 figures, Eclipse on the Coral Sea: Cycle 24 Ascending,
GONG 2012 / LWS/SDO-5 / SOHO 27, November 12 - 16, 2012, Palm Cove,
Queensland. Accepted for publication in Journal of Physics Conference Series
on March 1st 201
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
- …