CITA Research September 2002 - August 2003
Dynamics and Planets
The recent discovery of planets orbiting stars other than the sun has
revitalized the fields of dynamics and planet formation. One of the
planets has been observed to transit its star, with the result that we
know the radius and mass of the planet---it is a gas giant similar to
Jupiter. Many of the planets follow small but eccentric orbits; the
former suggests that the planets have migrated inward from their
birthplaces, while the latter suggests that two or more planets have
interacted. In 2002 CITA researchers made some novel suggestions
for how the mutual gravitational perturbations between planets,
and between a companion star and a planet, have influenced the
orbits of some planetary systems. Research has also continued
on the tidal dissipation in exoplanets, on their heated atmospheres,
and on the formation of exoplanets in dusty disks.
Young stellar disk in the galactic center
Andrei Beloborodov and Yuri Levin analyzed the three-dimensional
velocities of young stars in the inner 0.1 pc region of our galaxy and
discovered that 10 out of total 13 stars in the sample have velocities
lying in a single plane. The 10 stars also have common (clockwise)
sense rotation on the sky. Levin and Beloborodov concluded that a thin
stellar disk exists in the galactic center. This sheds light on the
mysterious origin of young stars in this region where normal star
formation is inhibited by the strong tidal field of the central black
hole. Levin and Beloborodov proposed that the stars are remnants of a
dense gaseous disk around the black hole that existed several million
years ago and clumped into stars by gravitational instability.
Mass estimation in gravitating systems
Andrei Beloborodov and Yuri Levin developed a new method of ``cosmic
roulette'' for mass estimation in gravitating systems with known
instantaneous positions and velocities of test bodies. The method is
based on a novel statistical approach to this classical astronomical
problem. It allows one to calculate consistently both mean expectation
(best-fit) value and error bars at a given confidence level, which
was not possible with traditional methods based on the virial theorem.
As a test, the method was applied to many Monte-Carlo realizations of
N random satellites around a known mass, and showed a high performance.
The method was applied to the galactic center and a new independent
estimate was obtained for the mass of the central black hole
Mechanism for halting planet migration
Norm Murray, Isamu Matsuyama and Doug Johnstone (HIA) proposed a
mechanism for halting migration in a gas disk. Photoevaporation of
the disk by irradiation from the central star can produce a gap in the
disk, preventing planets outside the gap from migrating down to the
star. This would result in an excess of systems with planets at or
just outside the photoevaporation radius. It may also halt migration
in the inner disk, a possibility that becomes more likely when a
massive star is located in the vicinity of the star-disk system.
Orbit of the extra-solar planet HD 80606
Norm Murrayand Yaqin Wu (University of Toronto) studied the origin of
the highly eccentic (e=0.93) and tight (a=0.46 AU) orbit of the
extra-solar planet HD 80606. The presence of a stellar companion to
the host star suggests the possibility that the Kozai mechanism and
tidal dissipation combined to draw the planet inward well after it
formed. This implies that the initial planet orbit is highly inclined
(nearly perpendicular) relative to the binary orbit. Wu and Murray
investigated the likelihood of this scenario, as well as its
consequences. Moreover, they showed that it is unlikely that the tide
raised on the star by the planet can account for the velocity residual
(after the motion induced by the planet is removed) observed on the
host star. They also discussed the relevance of such a migration
scenario for other planetary systems.
Detection of extrasolar dust particles
Norm Murray, Joe Weingartner and Chris Capobianco presented estimates
of the minimum detectable particle sizes and collecting areas for
ground based radar at Arecibo and New Zealand. Micron size extrasolar
dust particles have been convincingly detected by satellites and
Larger extrasolar meteoroids (5-35 microns) have most likely been
detected by ground based radar at Arecibo and New Zealand.
Murray and collaborators showed that particles larger than
about 10 micron can propagate for tens of parsecs through the
interstellar medium, opening up the possibility that ground based
radar systems can detect AGB stars, young stellar objects such as T
Tauri stars, and debris disks around Vega-like stars. They provide
analytical and numerical estimates of the ejection velocity in the
case of a debris disk interacting with a Jupiter mass planet. They also
provide rough estimates of the flux of large micrometeoroids from all
three classes of sources. Current radar systems are unlikely to detect
significant numbers of meteors from debris disks such as Beta
Pictoris. However, they suggest improvements to radar systems that
should allow for the detection of multiple examples of all three
classes.
Lithium-6 abundance
Using high-resolution, high quality spectra, Avi Mandell (Penn State),
Jian Ge (Penn State) and Norman Murray investigated the presence of
Lithium 6 in two lithium-poor stars that host extrasolar planetary
systems. They present improved atomic and molecular line lists for the
region in the vicinity of the lithium line at 6707.8 Angstroms, and
produce an excellent fit to the solar spectrum. From line profle
fitting, they find results consistent with no lithium 6 in either of the
lithium-poor planet-bearing stars or in three comparison stars with
and without planets, and 1-sigma upper limits of 0.04 for the isotopic
ratios of the two lithium- poor stars give an upper limit of 0.3
Jupiter masses of material with primordial abundances that could have
been recently deposited in their outer layers. These results suggest
that post-main sequence accretion of planets or planetary material
that is undepleted in lithium is uncommon.
Tidal Dissipation in Extra-Solar Planets
Wu and Arras (now in UCSB) are working on a research project:
explaining the efficiency of tidal dissipation in Jupiter and
extra-solar planets. Tide raised by Io on Jupiter is dissipated much
faster than theorists expected. Interestingly, similar efficiency is
measured in extra-solar planets for tide raised by their host stars.
They focus on a physical mechanism that involves resonant excitation of
inertial waves in these giant planets. These waves may be the common
mechanism shared by Jupiter and exo-planets.
Atmospheric Dynamics of Extra-Solar Jupiters
Wu works on the atmospheric dynamics of Jupiter-like exo-planets.
Turbulent diffusivity in the stratosphere of these planets is
responsible for mixing different elements. She studied the spatial
distribution and magnitude of this diffusivity, and found that sodium
is likely to settle out of the stratosphere, thereby explaining the
observed sodium deficit in the famous transiting planet HD 209458b.
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