61 research outputs found
Effect of different stellar galactic environments on planetary discs - I. The solar neighbourhood and the birth cloud of the Sun
We have computed trajectories, distances and times of closest approaches to the Sun by stars in the solar neighbourhood with known position, radial velocity and proper motions. For this purpose, we have used a full potential model of the Galaxy that reproduces the local z-force, the Oort constants, the local escape velocity and the rotation curve of the Galaxy. From our sample, we constructed initial conditions, within observational uncertainties, with a Monte Carlo scheme for the 12 most suspicious candidates because of their small tangential motion. We find that the star Gliese 710 will have the closest approach to the Sun, with a distance of approximately 0.34 pc in 1.36 Myr in the future. We show that the effect of a flyby with the characteristics of Gliese 710 on a 100 au test particle disc representing the Solar system is negligible. However, since there is a lack of 6D data for a large percentage of stars in the solar neighbourhood, closer approaches may exist. We calculate parameters of passing stars that would cause notable effects on the solar disc. Regarding the birth cloud of the Sun, we performed experiments to reproduce roughly the observed orbital parameters such as eccentricities and inclinations of the Kuiper belt. It is now known that in Galactic environments, such as stellar formation regions, the stellar densities of new born stars are high enough to produce close encounters within 200 au. Moreover, in these Galactic environments, the velocity dispersion is relatively low, typically σ∼ 1-3 km s−1. We find that with a velocity dispersion of ∼1 km s−1 and an approach distance of about 150 au, typical of these regions, we obtain approximately the eccentricities and inclinations seen in the current Solar system. Simple analytical calculations of stellar encounters effects on the Oort Cloud are presente
Bondi-Hoyle-Lyttleton Accretion onto a Protoplanetary Disk
Young stellar systems orbiting in the potential of their birth cluster can
accrete from the dense molecular interstellar medium during the period between
the star's birth and the dispersal of the cluster's gas. Over this time, which
may span several Myr, the amount of material accreted can rival the amount in
the initial protoplanetary disk; the potential importance of this `tail-end'
accretion for planet formation was recently highlighted by Throop & Bally
(2008). While accretion onto a point mass is successfully modeled by the
classical Bondi-Hoyle-Lyttleton solutions, the more complicated case of
accretion onto a star-disk system defies analytic solution. In this paper we
investigate via direct hydrodynamic simulations the accretion of dense
interstellar material onto a star with an associated gaseous protoplanetary
disk. We discuss the changes to the structure of the accretion flow caused by
the disk, and vice versa. We find that immersion in a dense accretion flow can
redistribute disk material such that outer disk migrates inwards, increasing
the inner disk surface density and reducing the outer radius. The accretion
flow also triggers the development of spiral density features, and changes to
the disk inclination. The mean accretion rate onto the star remains roughly the
same with and without the presence of a disk. We discuss the potential impact
of this process on planet formation, including the possibility of triggered
gravitational instability; inclination differences between the disk and the
star; and the appearance of spiral structure in a gravitationally stable
system.Comment: Accepted to ApJ. Version 2 replaces a mislabeled figure. Animations
of the simulations and a version of the paper with slightly less-compressed
images can be found at http://origins.colorado.edu/~moeckel/BHLpape
Can photo-evaporation trigger planetesimal formation?
We propose that UV radiation can stimulate the formation of planetesimals in
externally-illuminated protoplanetary disks. We present a numerical model of
disk evolution including vertical sedimentation and photo-evaporation by an
external O or B star. As solid material grows and settles toward the disk
midplane, the outer layers of the disk become dust depleted. When such a disk
is exposed to UV radiation, heating drives photo-evaporative mass-loss from its
surface, generating a dust-depleted outflow. The dust:gas surface density ratio
in the disk interior grows until dust in the disk midplane becomes
gravitationally unstable. Thus, UV radiation fields may induce the rapid
formation of planetesimals in disks where sedimentation has occurred.Comment: 4 pages, 1 figure. Revised and accepted by ApJ
Phase light curves for extrasolar Jupiters and Saturns
We predict how a remote observer would see the brightness variations of giant
planets similar to Jupiter and Saturn as they orbit their central stars. We
model the geometry of Jupiter, Saturn and Saturn's rings for varying orbital
and viewing parameters. Scattering properties for the planets and rings at
wavelenghts 0.6-0.7 microns follow Pioneer and Voyager observations, namely,
planets are forward scattering and rings are backward scattering. Images of the
planet with or without rings are simulated and used to calculate the
disk-averaged luminosity varying along the orbit, that is, a light curve is
generated. We find that the different scattering properties of Jupiter and
Saturn (without rings) make a substantial difference in the shape of their
light curves. Saturn-size rings increase the apparent luminosity of the planet
by a factor of 2-3 for a wide range of geometries. Rings produce asymmetric
light curves that are distinct from the light curve of the planet without
rings. If radial velocity data are available for the planet, the effect of the
ring on the light curve can be distinguished from effects due to orbital
eccentricity. Non-ringed planets on eccentric orbits produce light curves with
maxima shifted relative to the position of the maximum planet's phase. Given
radial velocity data, the amount of the shift restricts the planet's unknown
orbital inclination and therefore its mass. Combination of radial velocity data
and a light curve for a non-ringed planet on an eccentric orbit can also be
used to constrain the surface scattering properties of the planet. To summarize
our results for the detectability of exoplanets in reflected light, we present
a chart of light curve amplitudes of non-ringed planets for different
eccentricities, inclinations, and the viewing azimuthal angles of the observer.Comment: 40 pages, 13 figures, submitted to Ap.
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