20,822 research outputs found
Turbulent Disks are Never Stable: Fragmentation and Turbulence-Promoted Planet Formation
A fundamental assumption in our understanding of disks is that when the
Toomre Q>>1, the disk is stable against fragmentation into self-gravitating
objects (and so cannot form planets via direct collapse). But if disks are
turbulent, this neglects a spectrum of stochastic density fluctuations that can
produce rare, high-density mass concentrations. Here, we use a
recently-developed analytic framework to predict the statistics of these
fluctuations, i.e. the rate of fragmentation and mass spectrum of fragments
formed in a turbulent Keplerian disk. Turbulent disks are never completely
stable: we calculate the (always finite) probability of forming
self-gravitating structures via stochastic turbulent density fluctuations in
such disks. Modest sub-sonic turbulence above Mach number ~0.1 can produce a
few stochastic fragmentation or 'direct collapse' events over ~Myr timescales,
even if Q>>1 and cooling is slow (t_cool>>t_orbit). In trans-sonic turbulence
this extends to Q~100. We derive the true Q-criterion needed to suppress such
events, which scales exponentially with Mach number. We specify to turbulence
driven by MRI, convection, or spiral waves, and derive equivalent criteria in
terms of Q and the cooling time. Cooling times >~50*t_dyn may be required to
completely suppress fragmentation. These gravoturbulent events produce mass
spectra peaked near ~M_disk*(Q*M_disk/M_star)^2 (rocky-to-giant planet masses,
increasing with distance from the star). We apply this to protoplanetary disk
models and show that even minimum mass solar nebulae could experience
stochastic collapse events, provided a source of turbulence.Comment: 15 pages, 5 figures (+appendix), accepted to ApJ (added
clarifications and discussion to match accepted version
The Phoenix survey: the pairing fraction of faint radio sources
The significance of tidal interactions in the evolution of the faint radio
population (sub-mJy) is studied using a deep and homogeneous radio survey (1.4
GHz), covering an area of 3.14 deg and complete to a flux density of 0.4
mJy. Optical photometric and spectroscopic data are also available for this
sample. A statistical approach is employed to identify candidate physical
associations between radio sources and optically selected `field' galaxies. We
find an excess of close pairs around optically identified faint radio sources,
albeit at a low significance level, implying that the pairing fraction of the
sub-mJy radio sources is similar to that of `field' galaxies (at the same
magnitude limit) but higher than that of local galaxies.Comment: 5 pages, 4 figures. Accepted for publication in MNRAS Letter
General Rotorcraft Aeromechanical Stability Program (GRASP) version 1.03: User's manual
The Rotorcraft Dynamics Division, Aeroflightdynamics Directorate, U.S. Army Aviation Research and Technology Activity has developed the General Rotorcraft Aeromechanical Stability Program (GRASP) to perform calculations that will assess the stability of rotorcraft in hovering flight and ground contact conditions. The program is designed to be state-of-the-art, hybrid, finite-element/multibody code that can be applied to all existing and future helicopter configurations. While GRASP was specifically designed to solve rotorcraft stability problems, its innovative structure and formulation allow for application to a wide range of structures. This manual describes the preparation of the input file required by Version 1.03 of GRASP, the procedures used to invoke GRASP on the NASA Ames Research Center CRAY X-MP 48 computer, and the interpretation of the output produced by GRASP. The parameters used by the input file are defined, and summaries of the input file and the job control language are included
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