49 research outputs found
Dust Coagulation and Settling in Layered Protoplanetary Disks
Previous models of dust growth in protoplanetary disks considered either
uniformly laminar or turbulent disks. This Letter explores how dust growth
occurs in a layered protoplanetary disk in which the magnetorotational
instability generates turbulence only in the surface layers of a disk. Two
cases are considered: a completely laminar dead zone and a dead zone in which
turbulence is ``stirred up'' from the MRI acting above. It is found that dust
is depleted from high altitudes in layered disks faster than in those cases of
a uniformly laminar or turblent disks. This is a result of the accelerated
growth of particles in the turbulent regions and their storage in the lower
levels where they escape energetic collisions which would result in disruption.
Thus the regions of a protoplanetary disk above a dead zone would become
rapidly depleted in small dust grains, whereas the outer regions, where the MRI
is active throughout, will maintain a small dust poplulation at all heights due
to the disruptive collisions and vertical mixing from turbulence. This
structure is similar to that which has been inferred for disks around TW Hydra,
GM Auriga, and CoKu Tau/4, which are depleted in dust close to the star, but
are optically thick at larger heliocentric distances.Comment: 4 pages, 3 figures, accepted to ApJ Letter
The Evolution of the Water Distribution in a Viscous Protoplanetary Disk
(Abridged) Astronomical observations have shown that protoplanetary disks are
dynamic objects through which mass is transported and accreted by the central
star. Age dating of meteorite constituents shows that their creation,
evolution, and accumulation occupied several Myr, and over this time disk
properties would evolve significantly. Moreover, on this timescale, solid
particles decouple from the gas in the disk and their evolution follows a
different path. Here we present a model which tracks how the distribution of
water changes in an evolving disk as the water-bearing species experience
condensation, accretion, transport, collisional destruction, and vaporization.
Because solids are transported in a disk at different rates depending on their
sizes, the motions will lead to water being concentrated in some regions of a
disk and depleted in others. These enhancements and depletions are consistent
with the conditions needed to explain some aspects of the chemistry of
chondritic meteorites and formation of giant planets. The levels of
concentration and depletion, as well as their locations, depend strongly on the
combined effects of the gaseous disk evolution, the formation of rapidly
migrating rubble, and the growth of immobile planetesimals. We present examples
of evolution under a range of plausible assumptions and demonstrate how the
chemical evolution of the inner region of a protoplanetary disk is intimately
connected to the physical processes which occur in the outer regions.Comment: 45 pages, 7 figures, revised for publication in Icaru
The Exoplanet Population Observation Simulator. I - The Inner Edges of Planetary Systems
The Kepler survey provides a statistical census of planetary systems out to
the habitable zone. Because most planets are non-transiting, orbital
architectures are best estimated using simulated observations of ensemble
populations. Here, we introduce EPOS, the Exoplanet Population Observation
Simulator, to estimate the prevalence and orbital architectures of multi-planet
systems based on the latest Kepler data release, DR25. We estimate that at
least 42% of sun-like stars have nearly coplanar planetary systems with 7 or
more exoplanets. The fraction of stars with at least one planet within 1 au
could be as high as 100% depending on assumptions about the distribution of
single transiting planets. We estimate an occurrence rate of planets in the
habitable zone around sun-like stars of eta_earth=36+-14%. The innermost
planets in multi-planet systems are clustered around an orbital period of 10
days (0.1 au), reminiscent of the protoplanetary disk inner edge or could be
explained by a planet trap at that location. Only a small fraction of planetary
systems have the innermost planet at long orbital periods, with fewer than ~8%
and ~3% having no planet interior to the orbit of Mercury and Venus,
respectively. These results reinforce the view that the solar system is not a
typical planetary system, but an outlier among the distribution of known
exoplanetary systems. We predict that at least half of the habitable zone
exoplanets are accompanied by (non-transiting) planets at shorter orbital
periods, hence knowledge of a close-in exoplanet could be used as a way to
optimize the search for Earth-size planets in the Habitable Zone with future
direct imaging missions.Comment: Accepted in AAS journals, code available on githu
Organic Synthesis via Irradiation and Warming of Ice Grains in the Solar Nebula
Complex organic compounds, including many important to life on Earth, are commonly found in meteoritic and cometary samples, though their origins remain a mystery. We examined whether such molecules could be produced within the solar nebula by tracking the dynamical evolution of ice grains in the nebula and recording the environments they were exposed to. We found that icy grains originating in the outer disk, where temperatures were less than 30 K, experienced UV irradiation exposures and thermal warming similar to that which has been shown to produce complex organics in laboratory experiments. These results imply that organic compounds are natural byproducts of protoplanetary disk evolution and should be important ingredients in the formation of all planetary systems, including our own