2,222 research outputs found
Composition of Massive Giant Planets
The two current models for giant planet formation are core accretion and disk
instability. We discuss the core masses and overall planetary enrichment in
heavy elements predicted by the two formation models, and show that both models
could lead to a large range of final compositions. For example, both can form
giant planets with nearly stellar compositions. However, low-mass giant
planets, enriched in heavy elements compared to their host stars, are more
easily explained by the core accretion model. The final structure of the
planets, i.e., the distribution of heavy elements, is not firmly constrained in
either formation model.Comment: 6 pages, Proceedings of IAU Symposium 276 (Invited talk), The
Astrophysics of Planetary Systems: Formation, Structure, and Dynamical
Evolution. Turin, Italy, Oct. 201
Terrestrial Planet Formation Surrounding Close Binary Stars
Disk material has been observed around both components of some young close
binary star systems. It has been shown that if planets form at the right places
within such disks, they can remain dynamically stable for very long times.
Herein, we numerically simulate the late stages of terrestrial planet growth in
circumbinary disks around 'close' binary star systems with stellar separations
between 0.05 AU and 0.4 AU and binary eccentricities up to 0.8. In each
simulation, the sum of the masses of the two stars is 1 solar mass, and giant
planets are included. Our results are statistically compared to a set of planet
formation simulations in the Sun-Jupiter-Saturn system that begin with
essentially the same initial disk of protoplanets. The planetary systems formed
around binaries with apastron distances less than ~ 0.2 AU are very similar to
those around single stars, whereas those with larger maximum separations tend
to be sparcer, with fewer planets, especially interior to 1 AU. We also provide
formulae that can be used to scale results of planetary accretion simulations
to various systems with different total stellar mass, disk sizes, and
planetesimal masses and densities.Comment: 60 pages, 4 tables, and 11 low resolution eps figures. Article with
high resolution figures is available at
http://www-personal.umich.edu/~equintan/publications.html . Accepted for
publication in Icaru
A scaling law for accretion zone sizes
Current theories of runaway planetary accretion require small random velocities of the accreted particles. Two body gravitational accretion cross sections which ignore tidal perturbations of the Sun are not valid for the slow encounters which occur at low relative velocities. Wetherill and Cox have studied accretion cross sections for rocky protoplanets orbiting at 1 AU. Using analytic methods based on Hill's lunar theory, one can scale these results for protoplanets that occupy the same fraction of their Hill sphere as does a rocky body at 1 AU. Generalization to bodies of different sizes is achieved here by numerical integrations of the three-body problem. Starting at initial positions far from the accreting body, test particles are allowed to encounter the body once, and the cross section is computed. A power law is found relating the cross section to the radius of the accreting body (of fixed mass)
Planets Formed in Habitable Zones of M Dwarf Stars Probably are Deficient in Volatiles
Dynamical considerations, presented herein via analytic scalings and
numerical experiments, imply that Earth-mass planets accreting in regions that
become habitable zones of M dwarf stars form within several million years.
Temperatures in these regions during planetary accretion are higher than those
encountered by the material that formed the Earth. Collision velocities during
and after the prime accretionary epoch are larger than for Earth. These factors
suggest that planets orbiting low mass main sequence stars are likely to be
either too distant (and thus too cold) for carbon/water based life on their
surfaces or have abundances of the required volatiles that are substantially
less than on Earth.Comment: 11 pages, 1 figure, Astrophysical Journal Letters, in pres
New Formation Models for the Kepler-36 System
Formation of the planets in the Kepler-36 system is modeled by detailed
numerical simulations according to the core-nucleated accretion scenario. The
standard model is updated to include the dissolution of accreting rocky
planetesimals in the gaseous envelope of the planet, leading to substantial
enrichment of the envelope mass in heavy elements and a non-uniform composition
with depth. For Kepler-36 c, models involving in situ formation and models
involving orbital migration are considered. The results are compared with
standard formation models. The calculations include the formation (accretion)
phase, as well as the subsequent cooling phase, up to the age of Kepler-36 (7
Gyr). During the latter phase, mass loss induced by stellar XUV radiation is
included. In all cases, the results fit the measured mass, 7.84 M, and
radius, 3.68 R, of Kepler-36 c. Two parameters are varied to obtain
these fits: the disk solid surface density at the formation location, and the
"efficiency" factor in the XUV mass loss rate. The updated models are hotter
and therefore less dense in the silicate portion of the planet and in the
overlying layers of H/He, as compared with standard models. The lower densities
mean that only about half as much H/He is needed to be accreted to fit the
present-day mass and radius constraints. For Kepler-36 b, an updated in situ
calculation shows that the entire H/He envelope is lost, early in the cooling
phase, in agreement with observation.Comment: 21 pages, 18 figures, 1 table. Accepted for publication in The
Astrophysical Journa
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