356 research outputs found
Three-dimensional modeling of radiative disks in binaries
Circumstellar disks in binaries are perturbed by the companion gravity
causing significant alterations of the disk morphology. Spiral waves due to the
companion tidal force also develop in the vertical direction and affect the
disk temperature profile. These effects may significantly influence the process
of planet formation. We perform 3D numerical simulations of disks in binaries
with different initial dynamical configurations and physical parameters. Our
goal is to investigate their evolution and their propensity to grow planets. We
use an improved version of the SPH code VINE modified to better account for
momentum and energy conservation. The energy equation includes a flux--limited
radiative transfer algorithm and the disk cooling is obtained via "boundary
particles". We model a system made of star/disk + star/disk where the secondary
star (and relative disk) is less massive than the primary. The numerical
simulations performed for different values of binary separation and disk
density show that the disk morphology is substantially affected by the
companion perturbations. Trailing spiral shock waves develop when the stars
approach their pericenter. Strong hydraulic jumps occur at the shock front
creating breaking waves and a consistent mass stream between the two disks,
significantly heating them. The high gas temperature may prevent the ice
condensation by moving outward the "snow line". The hydraulic jumps may slow
down or even halt the dust coagulation process. At apocenter these
perturbations are reduced and the disks are cooled down and less eccentric. The
strength of the hydraulic jumps, disk heating, and mass exchange depends on the
binary separation, and for larger semi-major axes, the tidal spiral pattern is
substantially reduced.Comment: 15 pages, 17 figures, accepted for publication in A&
Circumstellar disks can erase the effects of stellar fly-bys on planetary systems
Most stars form in embedded clusters. Stellar flybys may affect the orbital
architecture of the systems by exciting the eccentricity and causing dynamical
instability. Since, incidentally, the timescale over which a cluster loses its
gaseous component and begins to disperse is comparable to the circumstellar
disk lifetime, we expect that closer, and more perturbing, stellar flybys occur
when the planets are still embedded in their birth disk. We investigate the
effects of the disk on the dynamics of planets after the stellar encounter to
test whether it can damp the eccentricity and return the planetary system to a
non-excited state. We use the hydrodynamical code FARGO to study the
disk+planet(s) system during and after the stellar encounter in the context of
evolved disk models whose superficial density is 10 times lower than that of
the Minimum Mass Solar Nebula. The numerical simulations show that the planet
eccentricity, excited during a close stellar flyby, is damped on a short
timescale (~ 10 Kyr) in spite of the disk low initial density and subsequent
tidal truncation. This damping is effective also for a system of 3 giant
planets and the effects of the dynamical instability induced by the passing
star are quickly absorbed. If the circumstellar disk is still present around
the star during a stellar flyby, a planet (or a planetary system) is returned
to a non-excited state on a short timescale. This does not mean that stellar
encounters do not affect the evolution of planets, but they do it in a subtle
way with a short period of agitated dynamical evolution. At the end of it, the
system resumes a quiet evolution and the planetary orbits are circularized by
the interaction with the disk.Comment: 7 pages, 8 figures, accepted for publication in Astronomy &
Astrophysic
Circumstellar dust distribution in systems with two planets in resonance
We investigate via numerical modeling the effects of two planets locked in
resonance, and migrating outward, on the dust distribution of the natal
circumstellar disk. We aim to test whether the dust distribution exhibits
peculiar features arising from the interplay among the gravitational
perturbations of the planets in resonance, the evolution of the gas, and its
influence on the dust grains' dynamics. We focus on the 3:2 and 2:1 resonance,
where the trapping may be caused by the convergent migration of a Jupiter- and
Saturn-mass planet, preceding the common gap formation and ensuing outward (or
inward) migration. Models show that a common gap also forms in the dust
component -- similarly to what a single, more massive planet would generate --
and that outward migration leads to a progressive widening of the dust gap and
to a decoupling from the gas gap. As the system evolves, a significantly wider
gap is observed in the dust distribution, which ceases to overlap with the gas
gap in the inner disk regions. At the outer edge of the gas gap, outward
migration of the planets produces an over-density of dust particles, which
evolve differently in the 3:2 and 2:1 resonances. For the 3:2, the dust trap at
the gap's outer edge is partly efficient and a significant fraction of the
grains filters through the gap. For the 2:1 resonance, the trap is more
efficient and very few grains cross the gap, while the vast majority accumulate
at the outer edge of the gap.Comment: 16 pages, 12 figures. Accepted for publicatio
Ring dynamics around an oblate body with an inclined satellite: The case of Haumea
The recent discovery of rings and massive satellites around minor bodies and
dwarf planets suggests that they may often coexist, as for example around
Haumea. A ring perturbed by an oblate central body and by an inclined satellite
may disperse on a short timescale. The conditions under which a ring may
survive are explored both analytically and numerically. The trajectories of
ring particles are integrated under the influence of the gravitational field of
a triaxial ellipsoid and (a) massive satellite(s), including the effects of
collisions. A ring initially formed in the equatorial plane of the central body
will be disrupted if the satellite has an inclination in the Kozai Lidov regime
(39.2 < i < 144.8). For lower inclinations, the ring may relax to the satellite
orbital plane thanks to an intense collisional damping. On the other hand, a
significant J2 term easily suppresses the perturbations of an inclined
satellite within a critical semimajor axis, even in the case of Kozai Lidov
cycles. However, if the ring is initially inclined with respect to the
equatorial plane, the same J2 perturbations are not a protective factor but
instead disrupt the ring on a short timescale. The ring found around Haumea is
stable despite the rise in the impact velocities that is due to the asymmetric
shape of the the body and the presence of a 3:1 resonance with the rotation of
the central body. A ring close to an oblate central body should be searched for
in the proximity of the equatorial plane, where the J2 perturbations protect it
against the perturbations of an external inclined satellites. In an inclined
configuration, the J2 term is itself disruptive.Comment: Accepted for publication on Astronomy and Astrophysic
Phonon anharmonicities in graphite and graphene
We determine from first-principles the finite-temperature
properties--linewidths, line shifts, and lifetimes--of the key vibrational
modes that dominate inelastic losses in graphitic materials. In graphite, the
phonon linewidth of the Raman-active E2g mode is found to decrease with
temperature; such anomalous behavior is driven entirely by electron-phonon
interactions, and does not appear in the nearly-degenerate infrared-active E1u
mode. In graphene, the phonon anharmonic lifetimes and decay channels of the
A'1 mode at K dominate over E2g at G and couple strongly with acoustic phonons,
highlighting how ballistic transport in carbon-based interconnects requires
careful engineering of phonon decays and thermalization.Comment: 5 pages, 4 figures; typos corrected and reference adde
Effects of stellar flybys on planetary systems: 3D modeling of the circumstellar disks damping effects
Stellar flybys in star clusters are suspected to affect the orbital
architecture of planetary systems causing eccentricity excitation and orbital
misalignment between the planet orbit and the equatorial plane of the star. We
explore whether the impulsive changes in the orbital elements of planets,
caused by an hyperbolic stellar flyby, can be fully damped by the circumstellar
disk surrounding the star. The time required to disperse stellar clusters is in
fact comparable to circumstellar disk's lifetime. We have modelled in 3D a
system made of a solar type star surrounded by a low density disk with a giant
planet embedded in it approached on a hyperbolic encounter trajectory by a
second star, of similar mass and with its own disk. We focus on extreme
configurations where a very deep stellar flyby perturbs a Jovian planet on an
external orbit. This allows to test in full the ability of the disk to erase
the effects of the stellar encounter. We find that the amount of mass lost by
the disk during the stellar flyby is less than in 2D models where a single disk
was considered due to the mass exchange between the two disks at the encounter.
The damping in eccentricity is slightly faster than in 2D models and it occurs
on timescales of the order of a few kyr. The only trace of the flyby left in
the planet system, after about 10^4 yr, is a small misalignment, lower than 9
degrees, between the star equatorial plane and the planet orbit. In a realistic
model based on 3D simulations of star--planet--disk interactions, we find that
stellar flybys cannot excite significant eccentricities and inclinations of
planets in stellar clusters. The circumstellar disks hosting the planets damp
on a short timescale all the step changes in the two orbital parameters
produced during any stellar encounter. All records of past encounters are
erased.Comment: 9 pages, 11 figures, accepted for publication in A&
Planets in Binaries: Formation and Dynamical Evolution
Binary systems are very common among field stars. While this relatively small
number of planets in binaries is probably partly due to strong observational
biases, there is, however, statistical evidence that planets are indeed less
frequent in binaries with separations smaller than 100 au, strongly suggesting
that the presence of a close in companion star has an adverse effect on planet
formation. It is indeed possible for the gravitational pull of the second star
to affect all the different stages of planet formation, from proto-planetary
disk formation to dust accumulation into planetesimals, to the accretion of
these planetesimals into large planetary embryos and, eventually, the final
growth of these embryos into planets. For the crucial planetesimal accretion
phase, the complex coupling between dynamical perturbations from the binary and
friction due to gas in the protoplanetary disk suggests that planetesimal
accretion might be hampered due to increased, accretion hostile impact
velocities. Likewise, the interplay between the binary secular perturbations
and mean motion resonances lead to unstable regions, where not only planet
formation is inhibited, but where a massive body would be ejected from the
system on a hyperbolic orbit. The amplitude of these two main effects is
different for S and P type planets, so that a comparison between the two
populations might outline the influence of the companion star on the planet
formation process. Unfortunately, at present the two populations (circumstellar
or circumbinary) are not known equally well and different biases and
uncertainties prevent a quantitative comparison. We also highlight the long
term dynamical evolution of both S and P type systems and focus on how these
different evolutions influence the final architecture of planetary systems in
binaries.Comment: Published on Galaxies, vol. 7, issue 4, p. 8
Dust distribution around low-mass planets on converging orbits
Super-Earths can form at large orbital radii and migrate inward due to tidal
interactions with the circumstellar disk. In this scenario, convergent
migration may occur and lead to the formation of resonant pairs of planets. We
explore the conditions under which convergent migration and resonance capture
take place, and what dynamical consequences can be expected on the dust
distribution surrounding the resonant pair. We combine hydrodynamic
planet--disk interaction models with dust evolution calculations to investigate
the signatures produced in the dust distribution by a pair of planets in
mean-motion resonances. We find that convergent migration takes place when the
outer planet is the more massive. However, convergent migration also depends on
the local properties of the disk, and divergent migration may result as well.
For similar disk parameters, the capture in low degree resonances (e.g., 2:1 or
3:2) is preferred close to the star where the resonance strength can more
easily overcome the tidal torques exerted by the gaseous disk. Farther away
from the star, convergent migration may result in capture in high degree
resonances. The dust distribution shows potentially observable features
typically when the planets are trapped in a 2:1 resonance. In other cases, with
higher degree resonances (e.g., 5:4 or 6:5) dust features may not be
sufficiently pronounced to be easily observable. The degree of resonance
established by a pair of super-Earths may be indicative of the location in the
disk where capture occurred. There can be significant differences in the dust
distribution around a single super-Earth and a pair of super-Earths in
resonance.Comment: Accepted for publication on Astronomy and Astrophysic
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