263 research outputs found
The Effects of Metallicity, and Grain Growth and Settling on the Early Evolution of Gaseous Protoplanets
Giant protoplanets formed by gravitational instability in the outer regions
of circumstellar disks go through an early phase of quasi-static contraction
during which radii are large and internal temperatures are low. The main source
of opacity in these objects is dust grains. We investigate two problems
involving the effect of opacity on the evolution of planets of 3, 5, and 7 M_J.
First, we pick three different overall metallicities for the planet and simply
scale the opacity accordingly. We show that higher metallicity results in
slower contraction as a result of higher opacity. It is found that the
pre-collapse time scale is proportional to the metallicity. In this scenario,
survival of giant planets formed by gravitational instability is predicted to
be more likely around low-metallicity stars, since they evolve to the point of
collapse to small size on shorter time scales. But metal-rich planets, as a
result of longer contraction times, have the best opportunity to capture
planetesimals and form heavy-element cores. Second, we investigate the effects
of opacity reduction as a result of grain growth and settling, for the same
three planetary masses and for three different values of overall metallicity.
When these processes are included, the pre-collapse time scale is found to be
of order 1000 years for the three masses, significantly shorter than the time
scale calculated without these effects. In this case the time scale is found to
be relatively insensitive to planetary mass and composition. However, the
effects of planetary rotation and accretion of gas and dust, which could
increase the timescale, are not included in the calculation. The short time
scale we find would preclude metal enrichment by planetesimal capture, as well
as heavy-element core formation, over a large range of planetary masses and
metallicities.Comment: 22 pages, accepted to Icaru
Numerical two-dimensional calculations of the formation of the solar nebula
Numerical two dimensional calculations of the formation of the solar nebula are presented. The following subject areas are covered: (1) observational constraints of the properties of the initial solar nebula; (2) the physical problem; (3) review if two dimensional calculations of the formation phase; (4) recent models with hydrodynamics and radiative transport; and (5) further evolution of the system
Determination of the Interior Structure of Transiting Planets in Multiple-Planet Systems
Tidal dissipation within a short-period transiting extrasolar planet
perturbed by a companion object can drive orbital evolution of the system to a
so-called tidal fixed point, in which the apsidal lines of the transiting
planet and its perturber are aligned, and for which variations in the orbital
eccentricities of both planet and perturber are damped out. Significant
contributions to the apsidal precession rate are made by the secular
planet-planet interaction, by general relativity, and by the gravitational
quadropole fields created by the transiting planet's tidal and rotational
distortions. The fixed-point orbital eccentricity of the inner planet is
therefore a strong function of the planet's interior structure. We illustrate
these ideas in the specific context of the recently discovered HAT-P-13
exo-planetary system, and show that one can already glean important insights
into the physical properties of the inner transiting planet. We present
structural models of the planet, which indicate that its observed radius can be
maintained for a one-parameter sequence of models that properly vary core mass
and tidal energy dissipation in the interior. We use an octopole-order secular
theory of the orbital dynamics to derive the dependence of the inner planet's
eccentricity, on its tidal Love number. We find that the currently measured
eccentricity, implies 0.116 < k2_{b} < 0.425, 0 M_{Earth}<M_{core}<120
M_{Earth}$, and Q_{b} < 300,000. Improved measurement of the eccentricity will
soon allow for far tighter limits to be placed on all three of these
quantities, and will provide an unprecedented probe into the interior structure
of an extrasolar planet.Comment: 13 pages, 2 figures, submitted to ApJ Letter
Tidal and Magnetic Interactions between a Hot Jupiter and its Host Star in the Magnetospheric Cavity of a Protoplanetary Disk
We present a simplified model to study the orbital evolution of a young hot
Jupiter inside the magnetospheric cavity of a proto-planetary disk. The model
takes into account the disk locking of stellar spin as well as the tidal and
magnetic interactions between the star and the planet. We focus on the orbital
evolution starting from the orbit in the 2:1 resonance with the inner edge of
the disk, followed by the inward and then outward orbital migration driven by
the tidal and magnetic torques as well as the Roche-lobe overflow of the
tidally inflated planet. The goal in this paper is to study how the orbital
evolution inside the magnetospheric cavity depends on the cavity size, planet
mass, and orbital eccentricity. In the present work, we only target the mass
range from 0.7 to 2 Jupiter masses. In the case of the large cavity
corresponding to the rotational period ~ 7 days, the planet of mass >1 Jupiter
mass with moderate initial eccentricities (>~ 0.3) can move to the region <
0.03 AU from its central star in 10^7 years, while the planet of mass <1
Jupiter mass cannot. We estimate the critical eccentricity beyond which the
planet of a given mass will overflow its Roche radius and finally lose all of
its gas onto the star due to runaway mass loss. In the case of the small cavity
corresponding to the rotational period ~ 3 days, all of the simulated planets
lose all of their gas even in circular orbits. Our results for the orbital
evolution of young hot Jupiters may have the potential to explain the absence
of low-mass giant planets inside ~ 0.03 AU from their dwarf stars revealed by
transit surveys.Comment: 29 pages, 6 figures, 1 table. accepted for publication by Ap
In Situ Formation and Dynamical Evolution of Hot Jupiter Systems
Hot Jupiters, giant extrasolar planets with orbital periods shorter than ~10
days, have long been thought to form at large radial distances, only to
subsequently experience long-range inward migration. Here, we propose that in
contrast with this picture, a substantial fraction of the hot Jupiter
population formed in situ via the core accretion process. We show that under
conditions appropriate to the inner regions of protoplanetary disks, rapid gas
accretion can be initiated by Super-Earth type planets, comprising 10-20 Earth
masses of refractory composition material. An in situ formation scenario leads
to testable consequences, including the expectation that hot Jupiters should
frequently be accompanied by additional low-mass planets with periods shorter
than ~100 days. Our calculations further demonstrate that dynamical
interactions during the early stages of planetary systems' lifetimes should
increase the inclinations of such companions, rendering transits rare.
High-precision radial velocity monitoring provides the best prospect for their
detection.Comment: 19 pages, 10 figures, accepted to Ap
Evolution of Ohmically Heated Hot Jupiters
We present calculations of thermal evolution of Hot Jupiters with various
masses and effective temperatures under Ohmic dissipation. The resulting
evolutionary sequences show a clear tendency towards inflated radii for
effective temperatures that give rise to significant ionization of alkali
metals in the atmosphere, compatible with the trend of the data. The degree of
inflation shows that Ohmic dissipation, along with the likely variability in
heavy element content can account for all of the currently detected radius
anomalies. Furthermore, we find that in absence of a massive core, low-mass hot
Jupiters can over-flow their Roche-lobes and evaporate on Gyr time-scales,
possibly leaving behind small rocky cores.Comment: Accepted to The Astrophysical Journal (2011) 735-2, 9 pages, 8
figures, updated figures 2-
Effects of FU Orionis Outbursts on Protoplanetary Disks
In the early stages of work under this grant, we developed simulations to match the light curves of the three best studied systems: FU Ori, V1515 Cyg, & V1057 Cyg (Bell et al. 1995). We compared the details of model results to observations to test the validity of the thermal ionization instability model for outburst. In this paper, we were able to answer several of the key objections to the accretion disk outburst model for the FU Orionis phenomenon (eg. Herbig 1989). The declines in line width and reddening observed in V1057 Cyg following peak light had been used as arguments against the disk instability model. We showed these effects to be natural consequences of the slow outward progression and limited radial excursion of the ionization front during outburst. By the end of the grant period, we had begun combining the inner and outer disk models to derive the radiation expected in the planet forming region of the disk. A crucial step in this was the development of a radiative transfer model of the complete inner and outer disk system (Turner, Bodenheimer, & Bell 1997). In this work, wavelength dependent opacities were used to synthesize images and spectral energy distributions of FU Ori objects. New detailed opacities provided by Alexander (1995) allowed the resolution of coarse features such as silicate emission lines. Data for the fits were taken from the time dependent simulations in Bell et al. (1995) to which was added the effect of disk to disk or "self"-reprocessing which accounts for the illumination of the outer disk by the inner disk (Bell 1998). Through the course of work on this grant we have made considerable progress in computing detailed models of both the active outburst region of the disk and the outer quiescent disk. We have begun an investigation into the effects of a cocooning envelope. Direct comparisons with observations lend confidence that the basic underlying assumptions of the outburst model are reasonable. We are now ready to build upon these results to investigate the effects of outbursts on the central star and on the accumulation of small particles in the planet-forming regions
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