340 research outputs found
Planet formation in highly inclined binaries
We explore planet formation in binary systems around the central star where
the protoplanetary disk plane is highly inclined with respect to the companion
star orbit. This might be the most frequent scenario for binary separations
larger than 40 AU, according to Hale (1994). We focus on planetesimal accretion
and compute average impact velocities in the habitable region and up to 6 AU
from the primary.Comment: Accepted for publication on A&
On the eccentricity of self-gravitating circumstellar disks in eccentric binary systems
We study the evolution of circumstellar massive disks around the primary star
of a binary system focusing on the computation of disk eccentricity. In
particular, we concentrate on its dependence on the binary eccentricity.
Self-gravity is included in our numerical simulations. Our standard model
assumes a semimajor axis for the binary of 30 AU, the most probable value
according to the present binary statistics.Comment: Accepted for publication on A&
Debris discs in binaries: a numerical study
Debris disc analysis and modelling provide crucial information about the
structure and the processes at play in extrasolar planetary systems. In binary
systems, this issue is more complex because the disc should in addition respond
to the companion star's perturbations. We explore the dynamical evolution of a
collisionally active debris disc for different initial parent body populations,
diverse binary configurations and optical depths. We focus on the radial extent
and size distribution of the disc at a stationary state. We numerically follow
the evolution of massless small grains, initially produced from a
circumprimary disc of parent bodies following a size distribution in ds . Grains are submitted to both stars' gravity as well as
radiation pressure. In addition, particles are assigned an empirically derived
collisional lifetime. For all the binary configurations the disc extends far
beyond the critical semimajor axis for orbital stability. This is due
to the steady production of small grains, placed on eccentric orbits reaching
beyond by radiation pressure. The amount of matter beyond acrit
depends on the balance between collisional production and dynamical removal
rates: it increases for more massive discs as well as for eccentric binaries.
Another important effect is that, in the dynamically stable region, the disc is
depleted from its smallest grains. Both results could lead to observable
signatures. We have shown that a companion star can never fully truncate a
collisionally active disc. For eccentric companions, grains in the unstable
regions can significantly contribute to the thermal emission in the mid-IR.
Discs with sharp outer edges, especially bright ones such as HR4796A, are
probably shaped by other mechanisms.Comment: accepted for publication in A&
Relative velocities among accreting planetesimals in binary systems: the circumbinary case
We numerically investigate the possibility of planetesimal accretion in
circumbinary disks, under the coupled influence of both stars' secular
perturbations and friction due to the gaseous component of the protoplanetary
disk. We focus on one crucial parameter: the distribution of encounter
velocities between planetesimals in the 0.5 to 100km size range. An extended
range of binary systems with differing orbital parameters is explored. The
resulting encounter velocities are compared to the threshold velocities below
which the net outcome of a collision is accumulation into a larger body instead
of mass erosion. For each binary configuration, we derive the critical radial
distance from the binary barycenter beyond which planetesimal accretion is
possible. This critical radial distance is smallest for equal-mass binaries on
almost circular orbits. It shifts to larger values for increasing
eccentricities and decreasing mass ratio. The importance of the planetesimals'
orbital alignments of planetesimals due to gas drag effects is discussed.Comment: accepted for publication in MNRA
Planets in binary systems: is the present configuration indicative of the formation process?
The present dynamical configuration of planets in binary star systems may not
reflect their formation process since the binary orbit may have changed in the
past after the planet formation process was completed. An observed binary
system may have been part of a former hierarchical triple that became unstable
after the planets completed their growth around the primary star.
Alternatively, in a dense stellar environment even a single stellar encounter
between the star pair and a singleton may singificantly alter the binary orbit.
In both cases the planets we observe at present would have formed when the
dynamical environment was different from the presently observed one.
We have numerically integrated the trajectories of the stars (binary plus
singleton) and of test planets to investigate the abovementioned mechanisms.
Our simulations show that the circumstellar environment during planetary
formation around the primary was gravitationally less perturbed when the binary
was part of a hierarchical triple because the binary was necessarely wider and,
possibly, less eccentric. This circumstance has consequences for the planetary
system in terms of orbital spacing, eccentricity, and mass of the individual
planets. Even in the case of a single stellar encounter the present appearance
of a planetary system in a binary may significantly differ from what it had
while planet formation was ongoing. However, while in the case of instability
of a triple the trend is always towards a tighter and more eccentric binary
system, when a single stellar encounter affects the system the orbit of the
binary can become wider and be circularized.Comment: 5 pages, 5 figures Accepted for publication on A&
Eccentricity of radiative discs in close binary-star systems
Discs in binaries have a complex behavior because of the perturbations of the
companion star. Planet formation in binary-star systems both depend on the
companion star parameters and on the properties of the circumstellar disc. An
eccentric disc may increase the impact velocity of planetesimals and therefore
jeopardize the accumulation process. We model the evolution of discs in close
binaries including the effects of self-gravity and adopting different
prescriptions to model the disc's radiative properties. We focus on the
dynamical properties and evolutionary tracks of the discs. We use the
hydrodynamical code FARGO and we include in the energy equation heating and
cooling effects. Radiative discs have a lower disc eccentricity compared to
locally isothermal discs with same temperature profile. As a consequence, we do
not observe the formation of an internal elliptical low density region as in
locally isothermal disc models. However, the disc eccentricity depends on the
disc mass through the opacities. Akin to locally isothermal disc models,
self-gravity forces the disc's longitude of pericenter to librate about a fixed
orientation with respect to the binary apsidal line (). The disc's
radiative properties play an important role in the evolution of discs in
binaries. A radiative disc has an overall shape and internal structure that are
significantly different compared to a locally isothermal disc with same
temperature profile. This is an important finding both for describing the
evolutionary track of the disc during its progressive mass loss, and for planet
formation since the internal structure of the disc is relevant for
planetesimals growth in binary systems. The non-symmetrical distribution of
mass in these discs causes large eccentricities for planetesimals that may
affect their growth.Comment: accepted for publication in A&A (abstract truncated to comply with
astro-ph rules
Collisional Velocities and Rates in Resonant Planetesimal Belts
We consider a belt of small bodies around a star, captured in one of the
external or 1:1 mean-motion resonances with a massive perturber. The objects in
the belt collide with each other. Combining methods of celestial mechanics and
statistical physics, we calculate mean collisional velocities and collisional
rates, averaged over the belt. The results are compared to collisional
velocities and rates in a similar, but non-resonant belt, as predicted by the
particle-in-a-box method. It is found that the effect of the resonant lock on
the velocities is rather small, while on the rates more substantial. The
collisional rates between objects in an external resonance are by about a
factor of two higher than those in a similar belt of objects not locked in a
resonance. For Trojans under the same conditions, the collisional rates may be
enhanced by up to an order of magnitude. Our results imply, in particular,
shorter collisional lifetimes of resonant Kuiper belt objects in the solar
system and higher efficiency of dust production by resonant planetesimals in
debris disks around other stars.Comment: 31 pages, 11 figures (some of them heavily compressed to fit into
arxiv-maximum filesize), accepted for publication at "Celestial Mechanics and
Dynamical Astronomy
On how the optical depth tunes the effects of ISM neutral atom flow on debris disks
The flux of ISM neutral atoms surrounding stars and their environment affects
the motion of dust particles in debris disks, causing a significant dynamical
evolution. Large values of eccentricity and inclination can be excited and
strong correlations settle in among the orbital angles. This dynamical
behaviour, in particular for bound dust grains, can potentially cause
significant asymmetries in dusty disks around solar type stars which might be
detected by observations. However, the amount of orbital changes due to this
non--gravitational perturbation is strongly limited by the collisional lifetime
of dust particles. We show that for large values of the disk's optical depth
the influence of ISM flow on the disk shape is almost negligible because the
grains are collisionally destroyed before they can accumulate enough orbital
changes due to the ISM perturbations. On the other hand, for values smaller
than , peculiar asymmetric patterns appear in the density profile of
the disk when we consider 1-10 mum grains, just above the blow-out threshold.
The extent and relevance of these asymmetries grow for lower values of the
optical depth. An additional sink mechanism, which may prevent the formation of
large clumps and warping in the disks is related to the fast inward migration
due to the drag component of the forces. When a significant eccentricity is
pumped up by the ISM perturbations, the drag forces (Poynting-Robertson and in
particular ISM drag) drive the disk particles on fast migrating tracks leading
them into the star on a short timescale. It is then expected that disks with
small optical depth expand inside the parent body ring all the way towards the
star while disks with large optical depth would not significantly extend
inside.Comment: accepted for publication in MNRA
Can gas in young debris disks be constrained by their radial brightness profiles?
Disks around young stars are known to evolve from optically thick,
gas-dominated protoplanetary disks to optically thin, almost gas-free debris
disks. It is thought that the primordial gas is largely removed at ages of ~10
Myr, but it is difficult to discern the true gas densities from gas
observations. This suggests using observations of dust: it has been argued that
gas, if present with higher densities, would lead to flatter radial profiles of
the dust density and surface brightness than those actually observed. However,
here we show that these profiles are surprisingly insensitive to variation of
the parameters of a central star, location of the dust-producing planetesimal
belt, dustiness of the disk and - most importantly - the parameters of the
ambient gas. This result holds for a wide range of gas densities (three orders
of magnitude), for different radial distributions of the gas temperature, and
different gas compositions. The brightness profile slopes of -3...-4 we find
are the same that were theoretically found for gas-free debris disks, and they
are the same as actually retrieved from observations of many debris disks. Our
specific results for three young (10-30 Myr old), spatially resolved, edge-on
debris disks (beta Pic, HD 32297, and AU Mic) show that the observed radial
profiles of the surface brightness do not pose any stringent constraints on the
gas component of the disk. We cannot exclude that outer parts of the systems
may have retained substantial amounts of primordial gas which is not evident in
the gas observations (e.g. as much as 50 Earth masses for beta Pic). However,
the possibility that gas, most likely secondary, is only present in little to
moderate amounts, as deduced from gas detections (e.g. ~0.05 Earth masses in
the beta Pic disk), remains open, too.Comment: Accepted for publication in Astronomy and Astrophysic
Planet formation in Alpha Centauri A revisited: not so accretion-friendly after all
We numerically explore planet formation around alpha Cen A by focusing on the
crucial planetesimals-to-embryos phase. Our code computes the relative velocity
distribution, and thus the accretion vs. fragmentation trend, of planetesimal
populations having any given size distribution. This is a critical aspect of
planet formation in binaries since the pericenter alignment of planetesimal
orbits due to the gravitational perturbations of the companion star and to gas
friction strongly depends on size. We find that, for the nominal case of a MMSN
gas disc, the region beyond 0.5AU from the primary is hostile to planetesimal
accretion. In this area, impact velocities between different-size bodies are
increased, by the differential orbital phasing, to values too high to allow
mutual accretion. For any realistic size distribution for the planetesimal
population, this accretion-inhibiting effect is the dominant collision outcome
and the accretion process is halted. Results are robust with respect to the
profile and density of the gas disc: except for an unrealistic almost gas-free
case, the inner accretion safe area never extends beyond 0.75AU. We conclude
that planet formation is very difficult in the terrestrial region around alpha
Cen A, unless it started from fast-formed very large (>30km) planetesimals.
Notwithstanding these unlikely initial conditions, the only possible
explanation for the presence of planets around 1 AU from the star would be the
hypothetical outward migration of planets formed closer to the star or a
different orbital configuration in the binary's early history. Our conclusions
differ from those of several studies focusing on the later embryos-to-planets
stage, confirming that the planetesimals-to-embryos phase is more affected by
binary perturbations.Comment: accepted for publication in MNRAS (Note: abstract truncated. Full
abstract in the pdf file
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