91 research outputs found
What Fraction of Sun-like Stars have Planets?
The radial velocities of ~1800 nearby Sun-like stars are currently being
monitored by eight high-sensitivity Doppler exoplanet surveys. Approximately 90
of these stars have been found to host exoplanets massive enough to be
detectable. Thus at least ~5% of target stars possess planets. If we limit our
analysis to target stars that have been monitored the longest (~15 years), ~11%
possess planets. If we limit our analysis to stars monitored the longest and
whose low surface activity allow the most precise velocity measurements, ~25%
possess planets. By identifying trends of the exoplanet mass and period
distributions in a sub-sample of exoplanets less-biased by selection effects,
and linearly extrapolating these trends into regions of parameter space that
have not yet been completely sampled, we find at least ~9% of Sun-like stars
have planets in the mass and orbital period ranges Msin(i) > 0.3 M_Jupiter and
P 0.1
M_Jupiter and P < 60 years. Even this larger area of the mass-period plane is
less than 20% of the area occupied by our planetary system, suggesting that
this estimate is still a lower limit to the true fraction of Sun-like stars
with planets, which may be as large as ~100%.Comment: Conforms to version accepted by ApJ. Color version and movie
available at http://bat.phys.unsw.edu.au/~charley/download/whatfrac
Workshop on Physics of Accretion Disks Around Compact and Young Stars
The purpose of the two-day Workshop on Physics of Accretion Disks Around Compact and Young Stars was to bring together workers on accretion disks in the western Gulf region (Texas and Louisiana). Part 2 presents the workshop program, a list of poster presentations, and a list of workshop participants. Accretion disks are believed to surround many stars. Some of these disks form around compact stars, such as white dwarfs, neutron stars, or black holes that are members of binary systems and reveal themselves as a power source, especially in the x-ray and gamma regions of the spectrum. On the other hand, protostellar disks are believed to be accretion disks associated with young, pre-main-sequence stars and manifest themselves mostly in infrared and radio observations. These disks are considered to be a natural outcome of the star formation process. The focus of this workshop included theory and observations relevant to accretion disks around compact objects and newly forming stars, with the primary purpose of bringing the two communities together for intellectual cross-fertilization. The nature of the workshop was exploratory, to see how much interaction is possible between distinct communities and to better realize the local potential in this subject. A critical workshop activity was identification and documentation of key issues that are of mutual interest to both communities
Global Evolution of Solid Matter in Turbulent Protoplanetry Disks
The problem of planetary system formation and its subsequent character can only be addressed by studying the global evolution of solid material entrained in gaseous protoplanetary disks. We start to investigate this problem by considering the space-time development of aerodynamic forces that cause solid particles to decouple from the gas. The aim of this work is to demonstrate that only the smallest particles are attached to the gas, or that the radial distribution of the solid matter has no momentary relation to the radial distribution of the gas. We present the illustrative example wherein a gaseous disk of 0.245 solar mass and angular momentum of 5.6 x 10(exp 52) g/sq cm/s is allowed to evolve due to turbulent viscosity characterized by either alpha = 10(exp -2) or alpha = 10(exp -3). The motion of solid particles suspended in a viscously evolving gaseous disk is calculated numerically for particles of different sizes. In addition we calculate the global evolution of single-sized, noncoagulating particles. We find that particles smaller than 0.1 cm move with the gas; larger particles have significant radial velocities relative to the gas. Particles larger than 0.1 cm but smaller than 10(exp 3) cm have inward radial velocities much larger than the gas, whereas particles larger than 10(exp 4) cm have inward velocities much smaller than the gas. A significant difference in the form of the radial distribution of solids and the gas develops with time. It is the radial distribution of solids, rather than the gas, that determines the character of an emerging planetary system
Hybrid Mechanisms for Gas/Ice Giant Planet Formation
The effects of gas pressure gradients on the motion of solid grains in the
solar nebula substantially enhances the efficiency of forming protoplanetary
cores in the standard core accretion model in 'hybrid' scenarios for gas/ice
giant planet formation. Such a scenario is enhanced core accretion which
results from Epstein-drag induced inward radial migration of mm-sized grains
and subsequent particle subdisk gravitational instability needed to build up a
population of 1 km planetesimals. Solid/gas ratios can be enhanced by nearly
over those in Minimum Mass Solar Nebula (MMSN) in the outer
solar nebula (a 20 AU), increasing the oligarchic core masses and
decreasing formation timescales for protoplanetary cores. A 10
core can form on year timescales at 15 - 25 AU compared to
years in the standard model,alleviating the major problem
plaguing the core accretion model for gas/ice giant planet formation.Comment: 8 pages, 3 figures, 12pt preprint, emulateapj style; two sections
added addressing shear-dominated accretion & disk conditions necessary for
GI; Accepted for publication in the Astrophysical Journa
An alternative look at the snowline in protoplanetary disks
We have calculated an evolution of protoplanetary disk from an extensive set
of initial conditions using a time-dependent model capable of simultaneously
keeping track of the global evolution of gas and water-ice. A number of
simplifications and idealizations allows for an embodiment of gas-particle
coupling, coagulation, sedimentation, and evaporation/condensation processes.
We have shown that, when the evolution of ice is explicitly included, the
location of the snowline has to be calculated directly as the inner edge of the
region where ice is present and not as the radius where disk's temperature
equals the evaporation temperature of water-ice. The final location of the
snowline is set by an interplay between all involved processes and is farther
from the star than implied by the location of the evaporation temperature
radius. The evolution process naturally leads to an order of magnitude
enhancement in surface density of icy material.Comment: Accepted for publication in A&A. 8 pages, 4 figure
Planetary migration in evolving planetesimals discs
In the current paper, we further improved the model for the migration of
planets introduced in Del Popolo et al. (2001) and extended to time-dependent
planetesimal accretion disks in Del Popolo and Eksi (2002). In the current
study, the assumption of Del Popolo and Eksi (2002), that the surface density
in planetesimals is proportional to that of gas, is released. In order to
obtain the evolution of planetesimal density, we use a method developed in
Stepinski and Valageas (1997) which is able to simultaneously follow the
evolution of gas and solid particles for up to 10^7 yrs. Then, the disk model
is coupled to migration model introduced in Del Popolo et al. (2001) in order
to obtain the migration rate of the planet in the planetesimal. We find that
the properties of solids known to exist in protoplanetary systems, together
with reasonable density profiles for the disk, lead to a characteristic radius
in the range 0.03-0.2 AU for the final semi-major axis of the giant planet.Comment: IJMP A in prin
Protoplanetary Disk Turbulence Driven by the Streaming Instability: Non-Linear Saturation and Particle Concentration
We present simulations of the non-linear evolution of streaming instabilities
in protoplanetary disks. The two components of the disk, gas treated with grid
hydrodynamics and solids treated as superparticles, are mutually coupled by
drag forces. We find that the initially laminar equilibrium flow spontaneously
develops into turbulence in our unstratified local model. Marginally coupled
solids (that couple to the gas on a Keplerian time-scale) trigger an upward
cascade to large particle clumps with peak overdensities above 100. The clumps
evolve dynamically by losing material downstream to the radial drift flow while
receiving recycled material from upstream. Smaller, more tightly coupled solids
produce weaker turbulence with more transient overdensities on smaller length
scales. The net inward radial drift is decreased for marginally coupled
particles, whereas the tightly coupled particles migrate faster in the
saturated turbulent state. The turbulent diffusion of solid particles, measured
by their random walk, depends strongly on their stopping time and on the
solids-to-gas ratio of the background state, but diffusion is generally modest,
particularly for tightly coupled solids. Angular momentum transport is too weak
and of the wrong sign to influence stellar accretion. Self-gravity and
collisions will be needed to determine the relevance of particle overdensities
for planetesimal formation.Comment: Accepted for publication in ApJ (17 pages). Movies of the simulations
can be downloaded at http://www.mpia.de/~johansen/research_en.ph
Formation of giant planets in disks with different metallicities
We present the first results from simulations of processes leading to planet
formation in protoplanetary disks with different metallicities. For a given
metallicity, we construct a two-dimensional grid of disk models with different
initial masses and radii (, ). For each disk, we follow the evolution
of gas and solids from an early evolutionary stage, when all solids are in the
form of small dust grains, to the stage when most solids have condensed into
planetesimals. Then, based on the core accretion - gas capture scenario, we
estimate the planet-bearing capability of the environment defined by the final
planetesimal swarm and the still evolving gaseous component of the disk. We
define the probability of planet-formation, , as the normalized fractional
area in the (, ) plane populated by disks that have formed
planets inside 5 AU. With such a definition, and under the assumption that the
population of planets discovered at 5 AU is not significantly
contaminated by planets that have migrated from 5 AU, our results agree
fairly well with the observed dependence between the probability that a star
harbors a planet and the star's metal content. The agreement holds for the disk
viscosity parameter ranging from to , and it
becomes much poorer when the redistribution of solids relative to the gas is
not allowed for during the evolution of model disks.Comment: Accepted for publication in A&A. 6 pages, 6 figure
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