68 research outputs found
Toward a Deterministic Model of Planetary Formation IV: Effects of Type-I Migration
In a further development of a deterministic planet-formation model (Ida & Lin
2004), we consider the effect of type-I migration of protoplanetary embryos due
to their tidal interaction with their nascent disks. During the early embedded
phase of protostellar disks, although embryos rapidly emerge in regions
interior to the ice line, uninhibited type-I migration leads to their efficient
self-clearing. But, embryos continue to form from residual planetesimals at
increasingly large radii, repeatedly migrate inward, and provide a main channel
of heavy element accretion onto their host stars. During the advanced stages of
disk evolution (a few Myr), the gas surface density declines to values
comparable to or smaller than that of the minimum mass nebula model and type-I
migration is no longer an effective disruption mechanism for mars-mass embryos.
Over wide ranges of initial disk surface densities and type-I migration
efficiency, the surviving population of embryos interior to the ice line has a
total mass several times that of the Earth. With this reservoir, there is an
adequate inventory of residual embryos to subsequently assemble into rocky
planets similar to those around the Sun. But, the onset of efficient gas
accretion requires the emergence and retention of cores, more massive than a
few M_earth, prior to the severe depletion of the disk gas. The formation
probability of gas giant planets and hence the predicted mass and semimajor
axis distributions of extrasolar gas giants are sensitively determined by the
strength of type-I migration. We suggest that the observed fraction of
solar-type stars with gas giant planets can be reproduced only if the actual
type-I migration time scale is an order of magnitude longer than that deduced
from linear theories.Comment: 32 pages, 8 figures, 1 table, accepted for publication in Ap
The Evolution of Protoplanetary Disks Around Millisecond Pulsars: The PSR 1257 +12 System
We model the evolution of protoplanetary disks surrounding millisecond
pulsars, using PSR 1257+12 as a test case. Initial conditions were chosen to
correspond to initial angular momenta expected for supernova-fallback disks and
disks formed from the tidal disruption of a companion star. Models were run
under two models for the viscous evolution of disks: fully viscous and layered
accretion disk models. Supernova-fallback disks result in a distribution of
solids confined to within 1-2 AU and produce the requisite material to form the
three known planets surrounding PSR 1257+12. Tidal disruption disks tend to
slightly underproduce solids interior to 1 AU, required for forming the pulsar
planets, while overproducing the amount of solids where no body, lunar mass or
greater, exists. Disks evolving under 'layered' accretion spread somewhat less
and deposit a higher column density of solids into the disk. In all cases,
circumpulsar gas dissipates on year timescales, making
formation of gas giant planets highly unlikely.Comment: 16 pages, 17 figures, Accepted for publication in The Astrophysical
Journal (September 20, 2007 issue
Dust Size Growth and Settling in a Protoplanetary Disk
We have studied dust evolution in a quiescent or turbulent protoplanetary
disk by numerically solving coagulation equation for settling dust particles,
using the minimum mass solar nebular model. As a result, if we assume an
ideally quiescent disk, the dust particles settle toward the disk midplane to
form a gravitationally unstable layer within 2x10^3 - 4x10^4 yr at 1 - 30 AU,
which is in good agreement with an analytic calculation by Nakagawa, Sekiya, &
Hayashi (1986) although they did not take into account the particle size
distribution explicitly. In an opposite extreme case of a globally turbulent
disk, on the other hand, the dust particles fluctuate owing to turbulent motion
of the gas and most particles become large enough to move inward very rapidly
within 70 - 3x10^4 yr at 1 - 30 AU, depending on the strength of turbulence.
Our result suggests that global turbulent motion should cease for the
planetesimal formation in protoplanetary disks.Comment: 27 pages, 8 figures, accepted for publication in the Ap
Radial Flow of Dust Particles in Accretion Disks
We study the radial migration of dust particles in accreting protostellar
disks analogous to the primordial solar nebula. This study takes account of the
two dimensional (radial and normal) structure of the disk gas, including the
effects of the variation in the gas velocity as a function of distance from the
midplane. It is shown that the dust component of disks accretes slower than the
gas component. At high altitude from the disk midplane, the gas rotates faster
than particles because of the inward pressure gradient force, and its drag
force causes particles to move outward in the radial direction. Viscous torque
induces the gas within a scale height from the disk midplane to flow outward,
carrying small (size < 100 micron at 10 AU) particles with it. Only particles
at intermediate altitude or with sufficiently large sizes (> 1 mm at 10 AU)
move inward. When the particles' radial velocities are averaged over the entire
vertical direction, particles have a net inward flux. At large distances from
the central star, particles migrate inward with a velocity much faster than the
gas accretion velocity. However, their inward velocity is reduced below that of
the gas in the inner regions of the disk. The rate of velocity decrease is a
function of the particles' size. While larger particles retain fast accretion
velocity until they approach closer to the star, 10 micron particles have
slower velocity than the gas in the most part of the disk (r < 100 AU). This
differential migration of particles causes the size fractionation. Dust disks
composed mostly of small particles (size < 10 micron) accrete slower than gas
disks, resulting in the increase in the dust-gas ratio during the gas accretion
phase.Comment: ApJ, accepted, 17 pages, 14 figure
HD 80606 b, a planet on an extremely elongated orbit
We report the detection of a planetary companion orbiting the solar-type star
HD 80606, the brighter component of a wide binary with a projected separation
of about 2000 AU. Using high-signal spectroscopic observations of the two
components of the visual binary, we show that they are nearly identical. The
planet has an orbital period of 111.8 days and a minimum mass of 3.9 M_Jup.
With e=0.927, this planet has the highest orbital eccentricity among the
extrasolar planets detected so far. We finally list several processes this
extreme eccentricity could result from.Comment: 4 pages, 1 figure included, submitted to A&A, final versio
The CORALIE survey for southern extra-solar planets VIII. The very low-mass companions of HD141937, HD162020, HD168443, HD202206: brown dwarfs or superplanets?
Doppler CORALIE measurements of the solar-type stars HD141937, HD162020,
HD168443 and HD202206 show Keplerian radial-velocity variations revealing the
presence of 4 new companions with minimum masses close to the
planet/brown-dwarf transition, namely with m_2sin(i) = 9.7, 14.4, 16.9, and
17.5 M_Jup, respectively. The orbits present fairly large eccentricities
(0.22<e<0.43). Except for HD162020, the parent stars are metal rich compared to
the Sun, as are most of the detected extra-solar planet hosts. Considerations
of tidal dissipation in the short-period HD162020 system points towards a
brown-dwarf nature for the low-mass companion. HD168443 is a multiple system
with two low-mass companions being either brown dwarfs or formed simultaneously
in the protoplanetary disks as superplanets. For HD202206, the radial
velocities show an additional drift revealing a further outer companion, the
nature of which is still unknown. Finally, the stellar-host and orbital
properties of massive planets are examined in comparison to lighter exoplanets.
Observed trends include the need of metal-rich stars to form massive exoplanets
and the lack of short periods for massive planets. If confirmed with improved
statistics, these features may provide constraints for the migration scenario.Comment: 14 pages including figures, accepted for publication in A&
The CORALIE survey for southern extrasolar planets V: 3 new extrasolar planets
We report the detection of 3 new planetary companions orbiting the solar-type
stars GJ 3021, HD 52265 and HD 169830 using radial-velocity measurements taken
with the CORALIE echelle spectrograph. All these planetary companions have
longer orbital periods than the 51 Peg-like objects. The orbits are fairly
eccentric. The minimum masses of these planets range from 1 to 3.3 M_Jup. The
stars have spectral types from F8V to G6V. They are metal-rich. We also present
our radial-velocity measurements for three solar-type stars known to host
planetary companions iota Hor (HD 17051), HD 210277 and HD 217107.Comment: 14 pages, 5 figures (included), A&A in press, a1sini unit corrected
in table 6, Fig.3 correcte
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