493 research outputs found
Consideration of the relationship between Kepler and cyclotron dynamics leading to prediction of a non-MHD gravity-driven Hamiltonian dynamo
Conservation of canonical angular momentum shows that charged particles are
typically constrained to stay within a poloidal Larmor radius of a poloidal
magnetic flux surface. However, more detailed consideration shows that
particles with a critical charge to mass ratio can have zero canonical angular
momentum and so be both immune from centrifugal force and not constrained to
stay in the vicinity of a specific flux surface. Suitably charged dust grains
can have zero canonical angular momentum and in the presence of a gravitational
field will spiral inwards across poloidal magnetic surfaces toward the central
object and accumulate. This accumulation results in a gravitationally-driven
dynamo, i.e., a mechanism for converting gravitational potential energy into a
battery-like electric power source.Comment: 14 pages, 1 figur
Planet Signatures in Collisionally Active Debris Discs: scattered light images
Planet perturbations are often invoked as a potential explanation for many
spatial structures that have been imaged in debris discs. So far this issue has
been mostly investigated with collisionless N-body numerical models. We
numerically investigate how the coupled effect of collisions and radiation
pressure can affect the formation and survival of radial and azimutal
structures in a disc perturbed by a planet. We consider two set-ups: a planet
embedded within an extended disc and a planet exterior to an inner debris ring.
We use the DyCoSS code of Thebault(2012) and derive synthetic images of the
system in scattered light. The planet's mass and orbit, as well as the disc's
collisional activity are explored as free parameters.
We find that collisions always significantly damp planet-induced structures.
For the case of an embedded planet, the planet's signature, mostly a density
gap around its radial position, should remain detectable in head-on images if
M_planet > M_Saturn. If the system is seen edge-on, however, inferring the
presence of the planet is much more difficult, although some planet-induced
signatures might be observable under favourable conditions.
For the inner-ring/external-planet case, planetary perturbations cannot
prevent collision-produced small fragments from populating the regions beyond
the ring: The radial luminosity profile exterior to the ring is close to the
one it should have in the absence of the planet. However, a Jovian planet on a
circular orbit leaves precessing azimutal structures that can be used to
indirectly infer its presence. For a planet on an eccentric orbit, the ring is
elliptic and the pericentre glow effect is visible despite of collisions and
radiation pressure, but detecting such features in real discs is not an
unambiguous indicator of the presence of an outer planet.Comment: Accepted for Publication in A&A (NOTE: Abridged abstract and
(very)LowRes Figures. Better version, with High Res figures and full abstract
can be found at http://lesia.obspm.fr/perso/philippe-thebault/planpapph.pdf
Outer edges of debris discs: how sharp is sharp?
Ring-like features have been observed in several debris discs. Outside the
main ring, while some systems exhibit smooth surface brightness profiles (SB)
that fall off roughly as r**-3.5, others display large luminosity drops at the
ring's outer edge and steeper radial SB profiles. We seek to understand this
diversity of outer edge profiles under the ``natural'' collisional evolution of
the system, without invoking external agents such as planets or gas. We use a
statistical code to follow the evolution of a collisional population, ranging
from dust grains (submitted to radiation pressure) to planetesimals and
initially confined within a belt (the 'birth ring'). The system typically
evolves toward a "standard" steady state, with no sharp edge and SB \propto
r**-3.5 outside the birth ring. Deviations from this standard profile, in the
form of a sharp outer edge and a steeper fall-off, occur only when two
parameters take their extreme values: 1) When the birth ring is so massive that
it becomes radially optically thick for the smallest grains. However, the
required disc mass is here probably too high to be realistic. 2) When the
dynamical excitation of the dust-producing planetesimals is so low ( <0.01)
that the smallest grains, which otherwise dominate the total optical depth, are
preferentially depleted. This low-excitation case, although possibly not
generic, cannot be ruled out by observations. Our "standard" profile provides a
satisfactory explanation for a large group of debris discs with outer edges and
SB falling as r**-3.5. Systems with sharper outer edges, barring other
confining agents, could still be explained by ``natural'' collisional evolution
if their dynamical excitation is very low. We show that such a dynamically-cold
case provides a satisfactory fit for HR4796AComment: Accepted for publication in A&A (abstract truncated here, full
version in the pdf file); v2: typos corrected + rephrasing title of Section
5.1.2; v3 :final technical change
Multiple spiral patterns in the transitional disk of HD 100546
Protoplanetary disks around young stars harbor many structures related to
planetary formation. Of particular interest, spiral patterns were discovered
among several of these disks and are expected to be the sign of gravitational
instabilities leading to giant planets formation or gravitational perturbations
caused by already existing planets. In this context, the star HD100546 presents
some specific characteristics with a complex gas and dusty disk including
spirals as well as a possible planet in formation. The objective of this study
is to analyze high contrast and high angular resolution images of this
emblematic system to shed light on critical steps of the planet formation. We
retrieved archival images obtained at Gemini in the near IR (Ks band) with the
instrument NICI and processed the data using advanced high contrast imaging
technique taking advantage of the angular differential imaging. These new
images reveal the spiral pattern previously identified with HST with an
unprecedented resolution, while the large-scale structure of the disk is mostly
erased by the data processing. The single pattern at the southeast in HST
images is now resolved into a multi-armed spiral pattern. Using two models of a
gravitational perturber orbiting in a gaseous disk we attempted to bring
constraints on the characteristics of this perturber assuming each spiral being
independent and we derived qualitative conclusions. The non-detection of the
northeast spiral pattern observed in HST allows to put a lower limit on the
intensity ratio between the two sides of the disk, which if interpreted as
forward scattering yields a larger anisotropic scattering than derived in the
visible. Also, we found that the spirals are likely spatially resolved with a
thickness of about 5-10AU. Finally, we did not detect the candidate forming
planet recently discovered in the Lp band, with a mass upper limit of 16-18 MJ.Comment: Accepted for publication in Astronomy and Astrophysics, 10 pages, 8
figure
Collisional processes and size distribution in spatially extended debris discs
We present a new multi-annulus code for the study of collisionally evolving
extended debris discs. We first aim to confirm results obtained for a
single-annulus system, namely that the size distribution in "real" debris discs
always departs from the theoretical collisional equilibrium
dN\proptoR^{-3.5}dR power law, especially in the crucial size range of
observable particles (<1cm), where it displays a characteristic wavy pattern.
We also aim at studying how debris discs density distributions, scattered light
luminosity profiles, and SEDs are affected by the coupled effect of collisions
and radial mixing due to radiation pressure affected small grains. The size
distribution evolution is modeled from micron-sized grains to 50km-sized
bodies. The model takes into account the crucial influence of radiation
pressure-affected small grains. We consider the collisional evolution of a
fiducial a=120AU radius disc with an initial surface density in
. We show that the system's radial extension plays
a crucial role: in most regions the collisional and size evolution of the dust
is imposed by small particles on eccentric or unbound orbits produced further
inside the disc. The spatial distribution of small grains strongly departs from
the initial profile, while the bigger objects, containing most of the system's
mass, still follow the initial distribution. This has consequences on the
scattered--light radial profiles which get significantly flatter, and we
propose an empirical law to trace back the distribution of large unseen parent
bodies from the observed profiles. We finally provide empirical formula for the
collisional size distribution and collision timescale that can be used for
future debris disc modeling.Comment: Accepted for publication in Astronomy and Astrophysics (with better
figures) (note: full abstract in the *.pdf file
Molecular hydrogen in the disk of the Herbig Ae star HD97048
We present high-resolution spectroscopic mid-infrared observations of the
circumstellar disk around the Herbig Ae star HD97048 obtained with the VLT
Imager and Spectrometer for the mid-InfraRed (VISIR). We conducted observations
of mid-infrared pure rotational lines of molecular hydrogen (H2) as a tracer of
warm gas in the disk surface layers. In a previous paper, we reported the
detection of the S(1) pure rotational line of H2 at 17.035 microns and argued
it is arising from the inner regions of the disk around the star. We used VISIR
on the VLT for a more comprehensive study based on complementary observations
of the other mid-infrared molecular transitions, namely S(2) and S(4) at 12.278
microns and 8.025 microns respectively, to investigate the physical properties
of the molecular gas in the circumstellar disk around HD97048. We do not detect
neither the S(2) line nor the S(4) H2 line from the disk of HD97048, but we
derive upper limits on the integrated line fluxes which allows us to estimate
an upper limit on the gas excitation temperature, T_ex < 570 K. This limit on
the temperature is consistent with the assumptions previously used in the
analysis of the S(1) line, and allows us to set stronger contraints on the mass
of warm gas in the inner regions of the disk. Indeed, we estimate the mass of
warm gas to be lower than 0.1 M_Jup. We also discuss the probable physical
mechanisms which could be responsible of the excitation of H2 in the disk of
HD97048.Comment: accepted for publication in Ap
Morphology of the very inclined debris disk around HD 32297
Direct imaging of circumstellar disks at high angular resolution is mandatory
to provide morphological information that bring constraints on their
properties, in particular the spatial distribution of dust. New techniques
combining observing strategy and data processing now allow very high contrast
imaging with 8-m class ground-based telescopes (10^-4 to 10^-5 at ~1") and
complement space telescopes while improving angular resolution at near infrared
wavelengths. We carried out a program at the VLT with NACO to image known
debris disks with higher angular resolution in the near IR than ever before in
order to study morphological properties and ultimately to detect signpost of
planets. The observing method makes use of advanced techniques: Adaptive
Optics, Coronagraphy and Differential Imaging, a combination designed to
directly image exoplanets with the upcoming generation of "planet finders" like
GPI (Gemini Planet Imager) and SPHERE (Spectro-Polarimetric High contrast
Exoplanet REsearch). Applied to extended objects like circumstellar disks, the
method is still successful but produces significant biases in terms of
photometry and morphology. We developed a new model-matching procedure to
correct for these biases and hence to bring constraints on the morphology of
debris disks. From our program, we present new images of the disk around the
star HD 32297 obtained in the H (1.6mic) and Ks (2.2mic) bands with an
unprecedented angular resolution (~65 mas). The images show an inclined thin
disk detected at separations larger than 0.5-0.6". The modeling stage confirms
a very high inclination (i=88{\deg}) and the presence of an inner cavity inside
r_0~110AU. We also found that the spine (line of maximum intensity along the
midplane) of the disk is curved and we attributed this feature to a large
anisotropic scattering factor (g~0.5, valid for an non-edge on disk). Abridged
...Comment: 12 pages, 10 figures, accepted for publication in Astronomy and
Astrophysic
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