23,520 research outputs found
Magnetically controlled mass loss from extrasolar planets in close orbits
We consider the role magnetic fields play in guiding and controlling
mass-loss via evaporative outflows from exoplanets that experience UV
irradiation. First we present analytic results that account for planetary and
stellar magnetic fields, along with mass-loss from both the star and planet. We
then conduct series of numerical simulations for gas giant planets, and vary
the planetary field strength, background stellar field strength, UV heating
flux, and planet mass. These simulations show that the flow is magnetically
controlled for moderate field strengths and even the highest UV fluxes, i.e.,
planetary surface fields gauss and fluxes
erg s. We thus conclude that outflows from all hot Jupiters with
moderate surface fields are magnetically controlled. The inclusion of magnetic
fields highly suppresses outflow from the night-side of the planet. Only the
magnetic field lines near the pole are open and allow outflow to occur. The
fraction of open field lines depends sensitively on the strength (and geometry)
of the background magnetic field from the star, along with the UV heating rate.
The net effect of the magnetic field is to suppress the mass loss rate by
(approximately) an order of magnitude. Finally, some open field lines do not
allow the flow to pass smoothly through the sonic point; flow along these
streamlines does not reach steady-state, resulting in time-variable mass-loss.Comment: Accepted for publication in MNRAS, 20 pages, 13 figure
A direct-execution parallel architecture for the Advanced Continuous Simulation Language (ACSL)
A direct-execution parallel architecture for the Advanced Continuous Simulation Language (ACSL) is presented which overcomes the traditional disadvantages of simulations executed on a digital computer. The incorporation of parallel processing allows the mapping of simulations into a digital computer to be done in the same inherently parallel manner as they are currently mapped onto an analog computer. The direct-execution format maximizes the efficiency of the executed code since the need for a high level language compiler is eliminated. Resolution is greatly increased over that which is available with an analog computer without the sacrifice in execution speed normally expected with digitial computer simulations. Although this report covers all aspects of the new architecture, key emphasis is placed on the processing element configuration and the microprogramming of the ACLS constructs. The execution times for all ACLS constructs are computed using a model of a processing element based on the AMD 29000 CPU and the AMD 29027 FPU. The increase in execution speed provided by parallel processing is exemplified by comparing the derived execution times of two ACSL programs with the execution times for the same programs executed on a similar sequential architecture
Inside-Out Planet Formation. V. Structure of the Inner Disk as Implied by the MRI
The large population of Earth to super-Earth sized planets found very close
to their host stars has motivated consideration of formation
models. In particular, Inside-Out Planet Formation is a scenario in which
planets coalesce sequentially in the disk, at the local gas pressure maximum
near the inner boundary of the dead zone. The pressure maximum arises from a
decline in viscosity, going from the active innermost disk (where thermal
ionization of alkalis yields high viscosities via the magneto-rotational
instability (MRI)) to the adjacent dead zone (where the MRI is quenched).
Previous studies of the pressure maximum, based on -disk models, have
assumed ad hoc values for the viscosity parameter in the active zone,
ignoring the detailed physics of the MRI. Here we explicitly couple the MRI
criteria to the -disk equations, to find steady-state (constant
accretion rate) solutions for the disk structure. We consider the effects of
both Ohmic and ambipolar resistivities, and find solutions for a range of disk
accretion rates ( = - /yr),
stellar masses ( = 0.1 - 1 ), and fiducial values of
the -MRI -viscosity in the dead zone ( - ). We find that: (1) A midplane pressure maximum forms
radially the inner boundary of the dead zone; (2) Hall resistivity
dominates near the midplane in the inner disk, which may explain why close-in
planets do form in 50% of systems; (3) X-ray ionization can be
competitive with thermal ionization in the inner disk, because of the low
surface density there in steady-state; and (4) our inner disk solutions are
viscously unstable to surface density perturbations.Comment: 34 pages, 28 figures, 3 appendices. Accepted by the Astrophysical
Journa
Ion-tracer anemometer
Gas velocity measuring instrument measures transport time of ion-trace traveling fixed distance between ionization probe and detector probe. Electric field superimposes drift velocity onto flow velocity so travel times can be reduced to minimize ion diffusion effects
- …