3,555 research outputs found
Understanding the Mass-Radius Relation for Sub-Neptunes: Radius as a Proxy for Composition
Transiting planet surveys like Kepler have provided a wealth of information
on the distribution of planetary radii, particularly for the new populations of
super-Earth and sub-Neptune sized planets. In order to aid in the physical
interpretation of these radii, we compute model radii for low-mass rocky
planets with hydrogen-helium envelopes. We provide model radii for planets 1-20
Earth masses, with envelope fractions from 0.01-20%, levels of irradiation
0.1-1000x Earth's, and ages from 100 Myr to 10 Gyr. In addition we provide
simple analytic fits that summarize how radius depends on each of these
parameters. Most importantly, we show that at fixed composition, radii show
little dependence on mass for planets with more than ~1% of their mass in their
envelope. Consequently, planetary radius is to first order a proxy for
planetary composition for Neptune and sub-Neptune sized planets. We recast the
observed mass-radius relationship as a mass-composition relationship and
discuss it in light of traditional core accretion theory. We discuss the
transition from rocky super-Earths to sub-Neptune planets with large volatile
envelopes. We suggest 1.75 Earth radii as a physically motivated dividing line
between these two populations of planets. Finally, we discuss these results in
light of the observed radius occurrence distribution found by Kepler.Comment: 17 pages, 9 figures, 7 tables, submitted to Ap
Re-inflated Warm Jupiters Around Red Giants
Since the discovery of the first transiting hot Jupiters, models have sought
to explain the anomalously large radii of highly irradiated gas giants. We now
know that the size of hot Jupiter radius anomalies scales strongly with a
planet's level of irradiation and numerous models like tidal heating, ohmic
dissipation, and thermal tides have since been developed to help explain these
inflated radii. In general however, these models can be grouped into two broad
categories: 1) models that directly inflate planetary radii by depositing a
fraction of the incident irradiation into the interior and 2) models that
simply slow a planet's radiative cooling allowing it to retain more heat from
formation and thereby delay contraction. Here we present a new test to
distinguish between these two classes of models. Gas giants orbiting at
moderate orbital periods around post main sequence stars will experience
enormous increases their irradiation as their host stars move up the sub-giant
and red-giant branches. If hot Jupiter inflation works by depositing
irradiation into the planet's deep interiors then planetary radii should
increase in response to the increased irradiation. This means that otherwise
non-inflated gas giants at moderate orbital periods >10 days can re-inflate as
their host stars evolve. Here we explore the circumstances that can lead to the
creation of these "re-inflated" gas giants and examine how the existence or
absence of such planets can be used to place unique constraints of the physics
of the hot Jupiter inflation mechanism. Finally, we explore the prospects for
detecting this potentially important undiscovered population of planets.Comment: Accepted by ApJ. 8 Figures and 8 page
Dynamics of swimming bacteria at complex interfaces
Flagellated bacteria exploiting helical propulsion are known to swim along
circular trajectories near surfaces. Fluid dynamics predicts this circular
motion to be clockwise (CW) above a rigid surface (when viewed from inside the
fluid) and counter-clockwise (CCW) below a free surface. Recent experimental
investigations showed that complex physicochemical processes at the nearby
surface could lead to a change in the direction of rotation, both at solid
surfaces absorbing slip-inducing polymers and interfaces covered with
surfactants. Motivated by these results, we use a far-field hydrodynamic model
to predict the kinematics of swimming near three types of interfaces: clean
fluid-fluid interface, slipping rigid wall, and a fluid interface covered by
incompressible surfactants. Representing the helical swimmer by a superposition
of hydrodynamic singularities, we first show that in all cases the surfaces
reorient the swimmer parallel to the surface and attract it, both of which are
a consequence of the Stokes dipole component of the swimmer flow field. We then
show that circular motion is induced by a higher-order singularity, namely a
rotlet dipole, and that its rotation direction (CW vs. CCW) is strongly
affected by the boundary conditions at the interface and the bacteria shape.
Our results suggest thus that the hydrodynamics of complex interfaces provide a
mechanism to selectively stir bacteria
The Mass-Metallicity Relation for Giant Planets
Exoplanet discoveries of recent years have provided a great deal of new data
for studying the bulk compositions of giant planets. Here we identify 47
transiting giant planets () whose stellar
insolation is low enough (, or roughly ) that they are not affected
by the hot Jupiter radius inflation mechanism(s). We compute a set of new
thermal and structural evolution models and use these models in comparison with
properties of the 47 transiting planets (mass, radius, age) to determine their
heavy element masses. A clear correlation emerges between the planetary heavy
element mass and the total planet mass, approximately of the form . This finding is consistent with the core accretion model of
planet formation. We also study how stellar metallicity [Fe/H] affects
planetary metal-enrichment and find a weaker correlation than has been
previously reported from studies with smaller sample sizes. We confirm a strong
relationship between the planetary metal-enrichment relative to the parent star
and the planetary mass, but see no relation in
with planet orbital properties or stellar mass.
The large heavy element masses of many planets ( ) suggest
significant amounts of heavy elements in H/He envelopes, rather than cores,
such that metal-enriched giant planet atmospheres should be the rule. We also
discuss a model of core-accretion planet formation in a one-dimensional disk
and show that it agrees well with our derived relation between mass and .Comment: Accepted to The Astrophysical Journal. This revision adds a
substantial amount of discussion; the results are the sam
Defining the interaction of the protease CpaA with its type II secretion chaperone CpaB and its contribution to virulence in Acinetobacter species
Removal of Hot Saturns in Mass-Radius Plane by Runaway Mass Loss
The hot Saturn population exhibits a boundary in mass-radius space, such that
no planets are observed at a density less than 0.1 g cm. Yet,
planet interior structure models can readily construct such objects as the
natural result of radius inflation. Here, we investigate the role XUV-driven
mass-loss plays in sculpting the density boundary by constructing interior
structure models that include radius inflation, photoevaporative mass loss and
a simple prescription of Roche lobe overflow. We demonstrate that planets
puffier than 0.1 g cm experience a runaway mass loss caused by
adiabatic radius expansion as the gas layer is stripped away, providing a good
explanation of the observed edge in mass-radius space. The process is also
visible in the radius-period and mass-period spaces, though smaller,
high-bulk-metallicity planets can still survive at short periods, preserving a
partial record of the population distribution at formation.Comment: 10 pages, 5 figures, submitted to ApJ Letter
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