1,304 research outputs found
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
Gyroscopic pumping of large-scale flows in stellar interiors, and application to Lithium Dip stars
The maintenance of large-scale differential rotation in stellar convective
regions by rotationally influenced convective stresses also drives large-scale
meridional flows by angular--momentum conservation. This process is an example
of ``gyroscopic pumping'', and has recently been studied in detail in the solar
context. An important question concerns the extent to which these
gyroscopically pumped meridional flows penetrate into nearby stably stratified
(radiative) regions, since they could potentially be an important source of
non-local mixing. Here we present an extensive study of the gyroscopic pumping
mechanism, using a combination of analytical calculations and numerical
simulations both in Cartesian geometry and in spherical geometry. The various
methods, when compared with one another, provide physical insight into the
process itself, as well as increasingly sophisticated means of estimating the
gyroscopic pumping rate. As an example of application, we investigate the
effects of this large-scale mixing process on the surface abundances of the
light elements Li and Be for stars in the mass range 1.3-1.5 solar masses
(so-called ``Li-dip stars''). We find that gyroscopic pumping is a very
efficient mechanism for circulating material between the surface and the deep
interior, so much in fact that it over-estimates Li and Be depletion by orders
of magnitude for stars on the hot side of the dip.However, when the diffusion
of chemical species back into the surface convection zone is taken into
account, a good fit with observed surface abundances of Li and Be as a function
of stellar mass in the Hyades cluster can be found for reasonable choices of
model parameters.Comment: Submitted to Ap
Models of the formation of the planets in the 47 UMa system
Formation of planets in the 47 UMa system is followed in an evolving
protoplanetary disk composed of gas and solids. The evolution of the disk is
calculated from an early stage, when all solids, assumed to be high-temperature
silicates, are in the dust form, to the stage when most solids are locked in
planetesimals. The simulation of planetary evolution starts with a solid embryo
of ~1 Earth mass, and proceeds according to the core accretion -- gas capture
model. Orbital parameters are kept constant, and it is assumed that the
environment of each planet is not perturbed by the second planet. It is found
that conditions suitable for both planets to form within several Myr are easily
created, and maintained throughout the formation time, in disks with . In such disks, a planet of 2.6 Jupiter masses (the minimum for
the inner planet of the 47 UMa system) may be formed at 2.1 AU from the star in
\~3 Myr, while a planet of 0.89 Jupiter masses (the minimum for the outer
planet) may be formed at 3.95 AU from the star in about the same time. The
formation of planets is possible as a result of a significant enhancement of
the surface density of solids between 1.0 and 4.0 AU, which results from the
evolution of a disk with an initially uniform gas-to-dust ratio of 167 and an
initial radius of 40 AU.Comment: Accepted for publication in A&A. 10 pages, 10 figure
Numerical studies of collapsing interstellar clouds
Numerical simulation of the structure and evolution of interstellar clouds was initiated. Steps were taken toward an integrated treatment of the dynamical, thermal, and chemical processes entering model calculations. A detailed study was made of radiative transfer in molecular lines to allow model predictions to be tested against empirical data. The calculations have successfully reproduced and explained several observed cloud properties, including abundances of complex molecular species and the apparent depletion of CO in dense cores
Fragmentation in a centrally condensed protostar
Hydrodynamical calculations in three space dimensions of the collapse of an isothermal, centrally condensed, rotating 1 M\sol protostellar cloud are presented. A numerical algorithm involving nested subgrids is used to resolve the region where fragmentation occurs in the central part of the protostar. A previous calculation by Boss, which produced a hierarchical multiple system, is evolved further, at comparable numerical resolution, and the end result is a binary, with more than half of the mass of the original cloud, whose orbital separation increases with time as a result of accretion of high-angular momentum material and as a result of merging with fragments that have formed farther out. Repeating the calculation with significantly higher resolution, we find that a sequence of binaries can be induced by fragmentation of circumbinary disks. The stability of the resulting multiple system is investigated Using n-body calculations, which indicate that such a system would transform on a short time scale into a more stable hierarchical structure. The outermost and most massive binary which forms in the high-resolution run has properties similar to that of the binary found in the low-resolution calculation. Thus the basic outcome is shown to be independent of the numerical spatial resolution. The high-resolution run, in addition, leads to the formation of a system of smaller fragments, which might be important for the understanding of the origin of close binaries with low-mass components and of low mass single stars
Protoplanetary Formation and the FU Orionis Outburst
The following three publications which reference the above grant from the NASA Origins of Solar Systems program are attached and form the final technical report for this project. The research involved comparisons of the spectral energy distributions of FU Orionis objects with theoretical models and associated studies of the structure of the outbursting accretion disks, as well as related studies on the effects of magnetic fields in disks, which will lead in the future to models of FU Orionis outbursts which include the effects of magnetic fields. The project was renewed under a new grant NAGW-4456, entitled 'Effects of FU Orionis Outbursts on Protoplanetary Disks'. Work now being prepared for publication deals more specifically with the issue of the effects of the outbursts on protoplanetary formation. Models of the spectral energy distribution of FU Orionis stars. A simple model of a buoyant magnetic dynamo in accretion disks and a numerical study of magnetic buoyancy in an accretion disk have been submitted
Formation of Giant Planets by Concurrent Accretion of Solids and Gas inside an Anti-Cyclonic Vortex
We study the formation of a giant gas planet by the core--accretion
gas--capture process, with numerical simulations, under the assumption that the
planetary core forms in the center of an anti-cyclonic vortex. The presence of
the vortex concentrates particles of centimeter to meter size from the
surrounding disk, and speeds up the core formation process. Assuming that a
planet of Jupiter mass is forming at 5 AU from the star, the vortex enhancement
results in considerably shorter formation times than are found in standard
core--accretion gas--capture simulations. Also, formation of a gas giant is
possible in a disk with mass comparable to that of the minimum mass solar
nebula.Comment: 27 pages, 4 figures, ApJ in pres
Protostellar collapse: rotation and disk formation
We present some important conclusions from recent calculations pertaining to
the collapse of rotating molecular cloud cores with axial symmetry,
corresponding to evolution of young stellar objects through classes 0 and begin
of class I. Three main issues have been addressed: (1) The typical timescale
for building up a preplanetary disk - once more it turned out that it is of the
order of one free-fall time which is decisively shorter than the widely assumed
timescale related to the so-called 'inside-out collapse'; (2) Redistribution of
angular momentum and the accompanying dissipation of kinetic (rotational)
energy - together these processes govern the mechanical and thermal evolution
of the protostellar core to a large extent; (3) The origin of
calcium-aluminium-rich inclusions (CAIs) - due to the specific pattern of the
accretion flow, material that has undergone substantial chemical and
mineralogical modifications in the hot (exceeding 900 K) interior of the
protostellar core may have a good chance to be advectively transported outward
into the cooler remote parts (beyond 4 AU, say) of the growing disk and to
survive there until it is incorporated into a meteoritic body.Comment: 4 pages, 4 figure
- …