1,240 research outputs found
The Dynamical Origin of the Multi-Planetary System HD45364
The recently discovered planetary system HD45364 which consists of a Jupiter
and Saturn mass planet is very likely in a 3:2 mean motion resonance. The
standard scenario to form planetary commensurabilities is convergent migration
of two planets embedded in a protoplanetary disc. When the planets are
initially separated by a period ratio larger than two, convergent migration
will most likely lead to a very stable 2:1 resonance for moderate migration
rates. To avoid this fate, formation of the planets close enough to prevent
this resonance may be proposed. However, such a simultaneous formation of the
planets within a small annulus, seems to be very unlikely.
Rapid type III migration of the outer planet crossing the 2:1 resonance is
one possible way around this problem. In this paper, we investigate this idea
in detail. We present an estimate for the required convergent migration rate
and confirm this with N-body and hydrodynamical simulations. If the dynamical
history of the planetary system had a phase of rapid inward migration that
forms a resonant configuration, we predict that the orbital parameters of the
two planets are always very similar and hence should show evidence of that.
We use the orbital parameters from our simulation to calculate a radial
velocity curve and compare it to observations. Our model can explain the
observational data as good as the previously reported fit. The eccentricities
of both planets are considerably smaller and the libration pattern is
different. Within a few years, it will be possible to observe the planet-planet
interaction directly and thus distinguish between these different dynamical
states.Comment: 9 pages, 9 figures - accepted for publication in Astronomy and
Astrophysic
Modeling the resonant planetary system GJ876
The two planets about the star GJ 876 appear to have undergone extensive
migration from their point of origin in the protoplanetary disk -- both because
of their close proximity to the star (30 and 60 day orbital periods) and
because of their occupying three stable orbital resonances at the 2:1
mean-motion commensurability. The resonances were most likely established by
converging differential migration of the planets leading to capture into the
resonances. A problem with this scenario is that continued migration of the
system while it is trapped in the resonances leads to orbital eccentricities
that rapidly exceed the observational upper limits of e_1 = 0.31 and e_2 =
0.05. As seen in forced 3-body simulations, lower eccentricities would persist
during migration only for an applied eccentricity damping.
Here we explore the evolution of the GJ 876 system using two-dimensional
hydrodynamical simulations that include viscous heating and radiative effects.
We find that a hydrodynamic evolution within the resonance, where only the
outer planet interacts with the disk, always rapidly leads to large values of
eccentricities that exceed those observed.
Only if mass is removed from the disk on a time scale of the order of the
migration time scale (before there has been extensive migration after capture),
as might occur for photoevaporation in the late phases of planet formation, can
we end up with eccentricities that are consistent with the observations.Comment: Paper accepted by A&A, 17 Pages, 17 Figure
Magnetized Accretion and Dead Zones in Protostellar Disks
The edges of magnetically-dead zones in protostellar disks have been proposed
as locations where density bumps may arise, trapping planetesimals and helping
form planets. Magneto-rotational turbulence in magnetically-active zones
provides both accretion of gas on the star and transport of mass to the dead
zone. We investigate the location of the magnetically-active regions in a
protostellar disk around a solar-type star, varying the disk temperature,
surface density profile, and dust-to-gas ratio. We also consider stellar masses
between 0.4 and 2 , with corresponding adjustments in the disk mass
and temperature. The dead zone's size and shape are found using the Elsasser
number criterion with conductivities including the contributions from ions,
electrons, and charged fractal dust aggregates. The charged species' abundances
are found using the approach proposed by S. Okuzumi. The dead zone is in most
cases defined by the ambipolar diffusion. In our maps, the dead zone takes a
variety of shapes, including a fish-tail pointing away from the star and
islands located on and off the midplane. The corresponding accretion rates vary
with radius, indicating locations where the surface density will increase over
time, and others where it will decrease. We show that density bumps do not
readily grow near the dead zone's outer edge, independently of the disk
parameters and the dust properties. Instead, the accretion rate peaks at the
radius where the gas-phase metals freeze out. This could lead to clearing a
valley in the surface density, and to a trap for pebbles located just outside
the metal freeze-out line.Comment: 58 pages, 25 figures, 2 tables, accepted to Ap
High dispersive and monolithic 100% efficiency grisms
We present a type of grism, a series combination of transmission grating and
prism, in which we reduce the number of diffraction orders and achieve a
configuration with very high angular dispersion. The grism can be fabricated
from a single dielectric material and requires no metallic or dielectric film
layers for high transmission diffraction efficiency. One can reach 100% in the
-1st transmission diffraction order and the equal damage threshold as the
dielectric bulk material. We realized such an element in fused silica with an
efficiency of more then 99%. The bevel backside reflection is reduced by a
statistical antireflective structure, so we measured an efficiency of the
entire grism of 95% at a single wavelength
The GSFC Mark-2 three band hand-held radiometer
A self-contained, portable, hand-radiometer designed for field usage was constructed and tested. The device, consisting of a hand-held probe containing three sensors and a strap supported electronic module, weighs 4 1/2 kilograms. It is powered by flashlight and transistor radio batteries, utilizes two silicon and one lead sulfide detectors, has three liquid crystal displays, sample and hold radiometric sampling, and its spectral configuration corresponds to LANDSAT-D's thematic mapper bands. The device was designed to support thematic mapper ground-truth data collection efforts and to facilitate 'in situ' ground-based remote sensing studies of natural materials. Prototype instruments were extensively tested under laboratory and field conditions with excellent results
On disc driven inward migration of resonantly coupled planets with application to the system around GJ876
We consider two protoplanets gravitationally interacting with each other and
a protoplanetary disc. The two planets orbit interior to a tidally maintained
disc cavity while the disc interaction indices inward migration. When the
migration is slow enough, the more rapidly migrating outer protoplanet
approaches and becomes locked in a 2:1 commensurability with the inner one.
This is maintained in subsequent evolution. We study this evolution using a
simple anaytic model, full hydrodynamic 2D simulations of the disc planet
system and longer time N body integrations incorporating simple prescriptions
for the effect of the disc on the planet orbits. The eccentricity of the
protoplanets are found to be determined by the migration rate induced in the
outer planet orbit by the external disc. We apply our results to the recently
discovered resonant planets around GJ876. Simulation shows that a disc with
parameters expected for protoplanetary discs causes trapping in the 2:1
commensurability when the planets orbit in an inner cavity and that
eccentricities in the observed range may be obtained.Comment: 8 pages, 5 figures, submitted to A&A on 30/03/200
On the observability of bow shocks of Galactic runaway OB stars
Massive stars that have been ejected from their parent cluster and
supersonically sailing away through the interstellar medium (ISM) are
classified as exiled. They generate circumstellar bow shock nebulae that can be
observed. We present two-dimensional, axisymmetric hydrodynamical simulations
of a representative sample of stellar wind bow shocks from Galactic OB stars in
an ambient medium of densities ranging from n_ISM=0.01 up to 10.0/cm3.
Independently of their location in the Galaxy, we confirm that the infrared is
the most appropriated waveband to search for bow shocks from massive stars.
Their spectral energy distribution is the convenient tool to analyze them since
their emission does not depend on the temporary effects which could affect
unstable, thin-shelled bow shocks. Our numerical models of Galactic bow shocks
generated by high-mass (~40 Mo) runaway stars yield H fluxes which
could be observed by facilities such as the SuperCOSMOS H-Alpha Survey. The
brightest bow shock nebulae are produced in the denser regions of the ISM. We
predict that bow shocks in the field observed at Ha by means of
Rayleigh-sensitive facilities are formed around stars of initial mass larger
than about 20 Mo. Our models of bow shocks from OB stars have the emission
maximum in the wavelength range 3 <= lambda <= 50 micrometer which can be up to
several orders of magnitude brighter than the runaway stars themselves,
particularly for stars of initial mass larger than 20 Mo.Comment: 13 pages, 12 figures. Accepted to MNRAS (2016
A torque formula for non-isothermal Type I planetary migration - II. Effects of diffusion
We study the effects of diffusion on the non-linear corotation torque, or
horseshoe drag, in the two-dimensional limit, focusing on low-mass planets for
which the width of the horseshoe region is much smaller than the scale height
of the disc. In the absence of diffusion, the non-linear corotation torque
saturates, leaving only the Lindblad torque. Diffusion of heat and momentum can
act to sustain the corotation torque. In the limit of very strong diffusion,
the linear corotation torque is recovered. For the case of thermal diffusion,
this limit corresponds to having a locally isothermal equation of state. We
present some simple models that are able to capture the dependence of the
torque on diffusive processes to within 20% of the numerical simulations.Comment: 12 pages, 8 figures, accepted for publication in MNRA
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