66 research outputs found
Planetesimal Formation with Particle Feedback
Proposed mechanisms for the formation of km-sized solid planetesimals face
long-standing difficulties. Robust sticking mechanisms that would produce
planetesimals by coagulation alone remain elusive. The gravitational collapse
of smaller solids into planetesimals is opposed by stirring from turbulent gas.
This proceeding describes recent works showing that "particle feedback," the
back-reaction of drag forces on the gas in protoplanetary disks, promotes
particle clumping as seeds for gravitational collapse. The idealized streaming
instability demonstrates the basic ability of feedback to generate particle
overdensities. More detailed numerical simulations show that the particle
overdensities produced in turbulent flows trigger gravitational collapse to
planetesimals. We discuss surprising aspects of this work, including the large
(super-Ceres) mass of the collapsing bound cluster, and the finding that MHD
turbulence aids gravitational collapse.Comment: 6 pages, to appear in ``Extreme Solar Systems'', D. Fischer, F.
Rasio, S. Thorsett and A. Wolszczan (eds), ASP Conf. Ser., 200
Structure and Evolution of Internally Heated Hot Jupiters
Hot Jupiters receive strong stellar irradiation, producing equilibrium
temperatures of . Incoming irradiation directly
heats just their thin outer layer, down to pressures of $\sim 0.1 \
\mathrm{bars}1 - 10 \ \mathrm{bars}\gtrsim 10\%100 \ \mathrm{bars}1\%1.4 R_{\rm Jup}10^4 \ \mathrm{bars}\approx 99\%$ of the planet's mass -- suppresses planetary cooling
as effectively as heating at the center. In summary, we find that relatively
shallow heating is required to explain the radii of most hot Jupiters, provided
that this heat is applied early and persists throughout their evolution.Comment: Accepted at ApJ, 14 pages, 10 figure
The Exoplanet Census: A General Method, Applied to Kepler
We develop a general method to fit the planetary distribution function (PLDF)
to exoplanet survey data. This maximum likelihood method accommodates more than
one planet per star and any number of planet or target star properties.
Application to \Kepler data relies on estimates of the efficiency of
discovering transits around Solar type stars by Howard et al. (2011). These
estimates are shown to agree with theoretical predictions for an ideal transit
survey. Using announced \Kepler planet candidates, we fit the PLDF as a joint
powerlaw in planet radius, down to 0.5 R_Eart, and orbital period, up to 50
days. The estimated number of planets per star in this sample is ~ 0.7 --- 1.4,
where the broad range covers systematic uncertainties in the detection
efficiency. To analyze trends in the PLDF we consider four planet samples,
divided between shorter and longer periods at 7 days and between large and
small radii at 3 R_Earth. At longer periods, the size distribution of the small
planets, with index \alpha = -1.2 \pm 0.2 steepens to \alpha = -2.0 \pm 0.2 for
the larger planet sample. For shorter periods, the opposite is seen: smaller
planets follow a steep powerlaw, \alpha = -1.9 \pm 0.2 that is much shallower,
\alpha = -0.7 \pm 0.2 at large radii. The observed deficit of
intermediate-sized planets at the shortest periods may arise from the
evaporation and sublimation of Neptune and Saturn-like planets. If the trend
and explanation hold, it would be spectacular observational confirmation of the
core accretion and migration hypotheses, and allow refinement of these
theories.Comment: Submitted to Ap
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