211 research outputs found
Momentum transport and torque scaling in Taylor-Couette flow from an analogy with turbulent convection
We generalize an analogy between rotating and stratified shear flows. This
analogy is summarized in Table 1. We use this analogy in the unstable case
(centrifugally unstable flow v.s. convection) to compute the torque in
Taylor-Couette configuration, as a function of the Reynolds number. At low
Reynolds numbers, when most of the dissipation comes from the mean flow, we
predict that the non-dimensional torque , where is the cylinder
length, scales with Reynolds number and gap width , . At larger Reynolds number, velocity
fluctuations become non-negligible in the dissipation. In these regimes, there
is no exact power law dependence the torque versus Reynolds. Instead, we obtain
logarithmic corrections to the classical ultra-hard (exponent 2) regimes: These predictions are found to be in excellent agreement with
available experimental data. Predictions for scaling of velocity fluctuations
are also provided.Comment: revTex, 6 Figure
Sulfur chemistry: 1D modeling in massive dense cores
The main sulfur-bearing molecules OCS, H2S, SO, SO2, and CS have been
observed in four high mass dense cores (W43-MM1, IRAS 18264, IRAS 05358, and
IRAS 18162). Our goal is to put some constraints on the relative evolutionary
stage of these sources by comparing these observations with time-dependent
chemical modeling. We used the chemical model Nahoon, which computes the
gas-phase chemistry and gas-grain interactions of depletion and evaporation.
Mixing of the different chemical compositions shells in a 1D structure through
protostellar envelope has been included since observed lines suggest nonthermal
supersonic broadening. Observed radial profiles of the temperature and density
are used to compute the chemistry as a function of time. With our model, we
underproduce CS by several orders of magnitude compared to the other S-bearing
molecules, which seems to contradict observations, although some uncertainties
in the CS abundance observed at high temperature remain. The OCS/SO2, SO/SO2,
and H2S/SO2 abundance ratios could in theory be used to trace the age of these
massive protostars since they show a strong dependence with time, but the
sources are too close in age compared to the accuracy of chemical models and
observations. Our comparison between observations and modeling may, however,
indicate that W43-MM1 could be chemically younger than the three other sources.
Turbulent diffusivity through the protostellar envelopes has to be less
efficient than 2e14 cm2s-1. Otherwise, it would have smoothed out the abundance
profiles, and this would have been observed. The sulfur chemistry depends
strongly on the 1D physical conditions. In our case, no conclusion can be given
on the relative age of IRAS 18264, IRAS 18162 and IRAS 05358 except that they
are very close. W43-MM1 seems younger than the other sources.Comment: Accepted for publication to A&
A new equation for the mid-plane potential of power law disks
We show that the gravitational potential in the plane of an axisymmetrical
flat disk where the surface density varies as a power of the radius obeys an
inhomogeneous first-order Ordinary Differential Equation (ODE) solvable by
standard techniques. The potential being exactly known at the origin for any
power index (and at infinity as well), the search for solutions consists of a
Two-point Boundary Value Problem (TBVP) with Dirichlet conditions. The
computating time is then linear with the number of grid points, instead of
quadratic from direct summation methods. Complex mass distributions which can
be decomposed into a mixture of power law surface density profiles are easily
accessible through the superposition principle. This ODE definitively takes the
place of the untractable bidimensional Poisson equation for planar
calculations. It opens new horizons to investigate various aspects related to
self-gravity in astrophysical disks (force calculations, stability analysis,
etc.).Comment: 5 pages, 1 figure, accepted for publication in A&
The effect of rotation and tidal heating on the thermal lightcurves of Super Mercuries
Short period (<50 days) low-mass (<10Mearth) exoplanets are abundant and the
few of them whose radius and mass have been measured already reveal a diversity
in composition. Some of these exoplanets are found on eccentric orbits and are
subjected to strong tides affecting their rotation and resulting in significant
tidal heating. Within this population, some planets are likely to be depleted
in volatiles and have no atmosphere. We model the thermal emission of these
"Super Mercuries" to study the signatures of rotation and tidal dissipation on
their infrared light curve. We compute the time-dependent temperature map at
the surface and in the subsurface of the planet and the resulting
disk-integrated emission spectrum received by a distant observer for any
observation geometry. We calculate the illumination of the planetary surface
for any Keplerian orbit and rotation. We include the internal tidal heat flow,
vertical heat diffusion in the subsurface and generate synthetic light curves.
We show that the different rotation periods predicted by tidal models
(spin-orbit resonances, pseudo-synchronization) produce different photometric
signatures, which are observable provided that the thermal inertia of the
surface is high, like that of solid or melted rocks (but not regolith). Tidal
dissipation can also directly affect the light curves and make the inference of
the rotation more difficult or easier depending on the existence of hot spots
on the surface. Infrared light curve measurement with the James Webb Space
Telescope and EChO can be used to infer exoplanets' rotation periods and
dissipation rates and thus to test tidal models. This data will also constrain
the nature of the (sub)surface by constraining the thermal inertia.Comment: 15 pages, 13 figures, accepted for publication in Astronomy &
Astrophysic
Self-gravity at the scale of the polar cell
We present the exact calculus of the gravitational potential and acceleration
along the symmetry axis of a plane, homogeneous, polar cell as a function of
mean radius a, radial extension e, and opening angle f. Accurate approximations
are derived in the limit of high numerical resolution at the geometrical mean
of the inner and outer radii (a key-position in current FFT-based Poisson
solvers). Our results are the full extension of the approximate formula given
in the textbook of Binney & Tremaine to all resolutions. We also clarify
definitely the question about the existence (or not) of self-forces in polar
cells. We find that there is always a self-force at radius except if the
shape factor a.f/e reaches ~ 3.531, asymptotically. Such cells are therefore
well suited to build a polar mesh for high resolution simulations of
self-gravitating media in two dimensions. A by-product of this study is a newly
discovered indefinite integral involving complete elliptic integral of the
first kind over modulus.Comment: 4 pages, 4 figures, A&A accepte
CID: Chemistry In Disks VII. First detection of HC3N in protoplanetary disks
Molecular line emission from protoplanetary disks is a powerful tool to
constrain their physical and chemical structure. Nevertheless, only a few
molecules have been detected in disks so far. We take advantage of the enhanced
capabilities of the IRAM 30m telescope by using the new broad band correlator
(FTS) to search for so far undetected molecules in the protoplanetary disks
surrounding the TTauri stars DM Tau, GO Tau, LkCa 15 and the Herbig Ae star MWC
480. We report the first detection of HC3N at 5 sigma in the GO Tau and MWC 480
disks with the IRAM 30-m, and in the LkCa 15 disk (5 sigma), using the IRAM
array, with derived column densities of the order of 10^{12}cm^{-2}. We also
obtain stringent upper limits on CCS (N < 1.5 x 10^{12} cm^{-3}). We discuss
the observational results by comparing them to column densities derived from
existing chemical disk models (computed using the chemical code Nautilus) and
based on previous nitrogen and sulfur-bearing molecule observations. The
observed column densities of HC3N are typically two orders of magnitude lower
than the existing predictions and appear to be lower in the presence of strong
UV flux, suggesting that the molecular chemistry is sensitive to the UV
penetration through the disk. The CCS upper limits reinforce our model with low
elemental abundance of sulfur derived from other sulfur-bearing molecules (CS,
H2S and SO).Comment: 8 pages, 4 figures, 3 tables, Accepted for publication in Ap
On the pressure of collisionless particle fluids. The case of solids settling in disks
Aims. Collections of dust, grains, and planetesimals are often treated as a
pressureless fluid. We study the validity of neglecting the pressure of such a
fluid by computing it exactly for the case of particles settling in a disk.
Methods. We solve a modified collisionless Boltzmann equation for the particles
and compute the corresponding moments of the phase space distribution: density,
momentum, and pressure. Results. We find that whenever the Stokes number,
defined as the ratio of the gas drag timescale to the orbital timescale, is
more than 1/2, the particle fluid cannot be considered as pressureless. While
we show it only in the simple case of particles settling in a laminar disk,
this property is likely to remain true for most flows, including turbulent
flows.Comment: Accepted for publication as a research note in Astronomy and
Astrophysics. Language edite
The impact of atmospheric circulation on the chemistry of the hot Jupiter HD 209458b
This is the author accepted manuscript. The final version is available from EDP Sciences via the DOI in this record.We investigate the effects of atmospheric circulation on the chemistry of the hot Jupiter HD 209458b. We use a simplified dynamical model and a robust chemical network, as opposed to previous studies which have used a three dimensional circulation model coupled to a simple chemical kinetics scheme. The temperature structure and distribution of the main atmospheric constituents are calculated in the limit of an atmosphere that rotates as a solid body with an equatorial rotation rate of 1 km/s. Such motion mimics a uniform zonal wind which resembles the equatorial superrotation structure found by three dimensional circulation models. The uneven heating of this tidally locked planet causes, even in the presence of such a strong zonal wind, large temperature contrasts between the dayside and nightside, of up to 800 K. This would result in important longitudinal variations of some molecular abundances if the atmosphere were at chemical equilibrium. The zonal wind, however, acts as a powerful disequilibrium process. We identify the existence of a pressure level of transition between two regimes, which may be located between 100 and 0.1 mbar depending on the molecule. Below this transition layer, chemical equilibrium holds, while above it, the zonal wind tends to homogenize the chemical composition of the atmosphere, bringing molecular abundances in the limb and nightside regions close to chemical equilibrium values characteristic of the dayside, i.e. producing an horizontal quenching effect in the abundances. Reasoning based on timescales arguments indicates that horizontal and vertical mixing are likely to compete in HD 209458b's atmosphere, producing a complex distribution where molecular abundances are quenched horizontally to dayside values and vertically to chemical equilibrium values characteristic of deep layers.M.A., O.V., F.S., and E.H. acknowledge support from the European Research Council (ERC Grant 209622: E3ARTHs). Computer time for this study was provided by the computing facilities MCIA (Mésocentre de Calcul Intensif Aquitain) of the Université de Bordeaux and of the Université de Pau et des Pays de l’Adour. We thank the anonymous referee for a constructive report that helped to improve this manuscript
The Newtonian potential of thin disks
The one-dimensional, ordinary differential equation (ODE) by Hur\'e & Hersant
(2007) that satisfies the midplane gravitational potential of truncated, flat
power-law disks is extended to the whole physical space. It is shown that
thickness effects (i.e. non-flatness) can be easily accounted for by
implementing an appropriate "softening length" . The solution of this
"softened ODE" has the following properties: i) it is regular at the edges
(finite radial accelerations), ii) it possesses the correct long-range
properties, iii) it matches the Newtonian potential of a geometrically thin
disk very well, and iv) it tends continuously to the flat disk solution in the
limit . As illustrated by many examples, the ODE,
subject to exact Dirichlet conditions, can be solved numerically with
efficiency for any given colatitude at second-order from center to infinity
using radial mapping. This approach is therefore particularly well-suited to
generating grids of gravitational forces in order to study particles moving
under the field of a gravitating disk as found in various contexts (active
nuclei, stellar systems, young stellar objects). Extension to non-power-law
surface density profiles is straightforward through superposition. Grids can be
produced upon request.Comment: Accepted for publication in A&
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