6,281 research outputs found
Massive planet migration: Theoretical predictions and comparison with observations
We quantify the utility of large radial velocity surveys for constraining
theoretical models of Type II migration and protoplanetary disk physics. We
describe a theoretical model for the expected radial distribution of extrasolar
planets that combines an analytic description of migration with an empirically
calibrated disk model. The disk model includes viscous evolution and mass loss
via photoevaporation. Comparing the predicted distribution to a uniformly
selected subsample of planets from the Lick / Keck / AAT planet search
programs, we find that a simple model in which planets form in the outer disk
at a uniform rate, migrate inward according to a standard Type II prescription,
and become stranded when the gas disk is dispersed, is consistent with the
radial distribution of planets for orbital radii 0.1 AU < a < 2.5 AU and planet
masses greater than 1.65 Jupiter masses. Some variant models are disfavored by
existing data, but the significance is limited (~95%) due to the small sample
of planets suitable for statistical analysis. We show that the favored model
predicts that the planetary mass function should be almost independent of
orbital radius at distances where migration dominates the massive planet
population. We also study how the radial distribution of planets depends upon
the adopted disk model. We find that the distribution can constrain not only
changes in the power-law index of the disk viscosity, but also sharp jumps in
the efficiency of angular momentum transport that might occur at small radii.Comment: ApJ, in press. References updated to match published versio
MHD simulations of the collapsar model for GRBs
We present results from axisymmetric, time-dependent magnetohydrodynamic
(MHD) simulations of the collapsar model for gamma-ray bursts. Our main
conclusion is that, within the collapsar model, MHD effects alone are able to
launch, accelerate and sustain a strong polar outflow. We also find that the
outflow is Poynting flux-dominated, and note that this provides favorable
initial conditions for the subsequent production of a baryon-poor fireball.Comment: 4 pages, to appear in proceedings of "2003 GRB Conference" (Santa Fe,
NM, September 8-12, 2003), needs aipprocs LaTeX class, movies are available
at http://rocinante.colorado.edu/~proga
An integrated charge amplifier for a pyroelectric sensor
This paper presents an integrated charge amplifier that measures a small charge. This charge is generated by a pyroelectric detector. The charge amplifier consists of a single-stage c-annon source configuration with a passive feedback network. The charge amplifier has a bandwidth of 700 kHz and an output noise voltage of 20 nV Hz 1/2 at 1 kHz. A 2Ă—2 integrated pyroelectric sensor based on VDF/TrFE copolymer has been realized. The voltage response of this sensor-amplifier is reported
Talking about organisations
Can dialogue reveal hidden truths? This is what prompted Bronwen Rees and
Andrew Armitage to experiment with the dialogical process to explore how
realities and knowledge about our organisations are created
Extrasolar planetary dynamics with a generalized planar Laplace-Lagrange secular theory
The dynamical evolution of nearly half of the known extrasolar planets in
multiple-planet systems may be dominated by secular perturbations. The commonly
high eccentricities of the planetary orbits calls into question the utility of
the traditional Laplace-Lagrange (LL) secular theory in analyses of the motion.
We analytically generalize this theory to fourth-order in the eccentricities,
compare the result with the second-order theory and octupole-level theory, and
apply these theories to the likely secularly-dominated HD 12661, HD 168443, HD
38529 and Ups And multi-planet systems. The fourth-order scheme yields a
multiply-branched criterion for maintaining apsidal libration, and implies that
the apsidal rate of a small body is a function of its initial eccentricity,
dependencies which are absent from the traditional theory. Numerical results
indicate that the primary difference the second and fourth-order theories
reveal is an alteration in secular periodicities, and to a smaller extent
amplitudes of the planetary eccentricity variation. Comparison with numerical
integrations indicates that the improvement afforded by the fourth-order theory
over the second-order theory sometimes dwarfs the improvement needed to
reproduce the actual dynamical evolution. We conclude that LL secular theory,
to any order, generally represents a poor barometer for predicting secular
dynamics in extrasolar planetary systems, but does embody a useful tool for
extracting an accurate long-term dynamical description of systems with small
bodies and/or near-circular orbits.Comment: 14 pages, 12 figures, 1 table, accepted for publication in Ap
Annotated draft genome sequence of the apple scab pathogen Venturia inaequalis
Apple scab is one of the most economically important diseases of ap- ples worldwide. The disease is caused by the haploid ascomycete Venturia inaequalis. We present here an annotated V. inaequalis whole-genome sequence of 72 Mb, assembled into 238 contigs, with 13,761 predicted genes
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