21 research outputs found
Exchange of ejecta between Telesto and Calypso: Tadpoles, horseshoes, and passing orbits
We have numerically integrated the orbits of ejecta from Telesto and Calypso,
the two small Trojan companions of Saturn's major satellite Tethys. Ejecta were
launched with speeds comparable to or exceeding their parent's escape velocity,
consistent with impacts into regolith surfaces. We find that the fates of
ejecta fall into several distinct categories, depending on both the speed and
direction of launch.
The slowest ejecta follow sub-orbital trajectories and re-impact their source
moon in less than one day. Slightly faster debris barely escape their parent's
Hill sphere and are confined to tadpole orbits, librating about Tethys'
triangular Lagrange points L4 (leading, near Telesto) or L5 (trailing, near
Calypso) with nearly the same orbital semi-major axis as Tethys, Telesto, and
Calypso. These ejecta too eventually re-impact their source moon, but with a
median lifetime of a few dozen years. Those which re-impact within the first
ten years or so have lifetimes near integer multiples of 348.6 days (half the
tadpole period).
Still faster debris with azimuthal velocity components >~ 10 m/s enter
horseshoe orbits which enclose both L4 and L5 as well as L3, but which avoid
Tethys and its Hill sphere. These ejecta impact either Telesto or Calypso at
comparable rates, with median lifetimes of several thousand years. However,
they cannot reach Tethys itself; only the fastest ejecta, with azimuthal
velocities >~ 40 m/s, achieve "passing orbits" which are able to encounter
Tethys. Tethys accretes most of these ejecta within several years, but some 1 %
of them are scattered either inward to hit Enceladus or outward to strike
Dione, over timescales on the order of a few hundred years
Architecture and Dynamics of Kepler's Candidate Multiple Transiting Planet Systems
About one-third of the ~1200 transiting planet candidates detected in the
first four months of \ik data are members of multiple candidate systems. There
are 115 target stars with two candidate transiting planets, 45 with three, 8
with four, and one each with five and six. We characterize the dynamical
properties of these candidate multi-planet systems. The distribution of
observed period ratios shows that the vast majority of candidate pairs are
neither in nor near low-order mean motion resonances. Nonetheless, there are
small but statistically significant excesses of candidate pairs both in
resonance and spaced slightly too far apart to be in resonance, particularly
near the 2:1 resonance. We find that virtually all candidate systems are
stable, as tested by numerical integrations that assume a nominal mass-radius
relationship. Several considerations strongly suggest that the vast majority of
these multi-candidate systems are true planetary systems. Using the observed
multiplicity frequencies, we find that a single population of planetary systems
that matches the higher multiplicities underpredicts the number of
singly-transiting systems. We provide constraints on the true multiplicity and
mutual inclination distribution of the multi-candidate systems, revealing a
population of systems with multiple super-Earth-size and Neptune-size planets
with low to moderate mutual inclinations.Comment: 27 pages, 19 figures, 8 tables, emulateapj style. Accepted to ApJ.
This version includes several minor changes to the tex