5,600 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
FASTER: Fast and Safe Trajectory Planner for Flights in Unknown Environments
High-speed trajectory planning through unknown environments requires
algorithmic techniques that enable fast reaction times while maintaining safety
as new information about the operating environment is obtained. The requirement
of computational tractability typically leads to optimization problems that do
not include the obstacle constraints (collision checks are done on the
solutions) or use a convex decomposition of the free space and then impose an
ad-hoc time allocation scheme for each interval of the trajectory. Moreover,
safety guarantees are usually obtained by having a local planner that plans a
trajectory with a final "stop" condition in the free-known space. However,
these two decisions typically lead to slow and conservative trajectories. We
propose FASTER (Fast and Safe Trajectory Planner) to overcome these issues.
FASTER obtains high-speed trajectories by enabling the local planner to
optimize in both the free-known and unknown spaces. Safety guarantees are
ensured by always having a feasible, safe back-up trajectory in the free-known
space at the start of each replanning step. Furthermore, we present a Mixed
Integer Quadratic Program formulation in which the solver can choose the
trajectory interval allocation, and where a time allocation heuristic is
computed efficiently using the result of the previous replanning iteration.
This proposed algorithm is tested extensively both in simulation and in real
hardware, showing agile flights in unknown cluttered environments with
velocities up to 3.6 m/s.Comment: IROS 201
Robust Adaptive Control Barrier Functions: An Adaptive & Data-Driven Approach to Safety (Extended Version)
A new framework is developed for control of constrained nonlinear systems
with structured parametric uncertainties. Forward invariance of a safe set is
achieved through online parameter adaptation and data-driven model estimation.
The new adaptive data-driven safety paradigm is merged with a recent adaptive
control algorithm for systems nominally contracting in closed-loop. This
unification is more general than other safety controllers as closed-loop
contraction does not require the system be invertible or in a particular form.
Additionally, the approach is less expensive than nonlinear model predictive
control as it does not require a full desired trajectory, but rather only a
desired terminal state. The approach is illustrated on the pitch dynamics of an
aircraft with uncertain nonlinear aerodynamics.Comment: Added aCBF non-Lipschitz example and discussion on approach
implementatio
Evaluating the Efficacy of Programs for Veteran Students
Veteran students are increasingly prominent in U.S. colleges and universities. The transition from military to civilian life is already challenging, and the transition to the academic realm can be even more so, especially when most veteran students are also first-generation college students, transfer students from community colleges to 4-year universities, or both. The specific problem is that the transition from military to student life requires significant adjustment on the part of both veterans and schools, and it was not known how these students’ school leaders could best help these veteran students adjust. The purpose for conducting this quantitative, historical, non-experimental correlational study is to determine whether targeted programs for veteran students are helpful to show how best to allocate resources for their education. The theoretical framework guiding this study is stress processing theory. The existing literature had not yet explored how effective school programs for veteran students are in helping these students. Therefore, the current study furthered the understanding of stress processing theory and might show academic and practical insights into how resources and funding allocation influenced veteran students. Specifically, I investigated the relationships between school funding, VA benefit funding, academic success, and graduation rates. The findings of the study showed no relationship between VA funding, school funding, and veteran graduation rates. The findings warrant future research on how VA funding and school funding are being used and the types of student veteran programs that are effective in increasing veteran graduation rates and academic performances
Simulations of GRB Jets in a Stratified External Medium: Dynamics, Afterglow Lightcurves, Jet Breaks and Radio Calorimetry
The dynamics of GRB jets during the afterglow phase is most reliably and
accurately modelled using hydrodynamic simulations. All published simulations,
however, have considered only a uniform external medium, while a stratified
external medium is expected around long duration GRB progenitors. Here we
present simulations of the dynamics of GRB jets and the resulting afterglow
emission for both uniform and stratified external media with for k = 0, 1, 2. The simulations are performed in 2D using the special
relativistic version of the Mezcal code. The dynamics for stratified external
media are broadly similar to those derived for expansion into a uniform
external medium. The jet half-opening angle start increasing logarithmically
with time once the Lorentz factor drops below 1/theta_0. For larger k values
the lateral expansion is faster at early times and slower at late times with
the jet expansion becoming Newtonian and slowly approaching spherical symmetry
over progressively longer timescales. We find that contrary to analytic
expectations, there is a reasonably sharp jet break in the lightcurve for k = 2
although the shape of the break is affected more by the viewing angle than by
the slope of the external density profile. Steeper density profiles are found
to produce more gradual jet breaks while larger viewing angles cause smoother
and later appearing jet breaks. The counter-jet becomes visible as it becomes
sub-relativistic, and for k=0 this results in a clear bump-like feature in the
light curve. However, for larger k values the jet decelerates more gradually,
causing only a mild flattening in the radio light curve that might be hard to
discern when k=2. Late time radio calorimetry is likely to consistently
over-estimate the true energy by up to a factor of a few for k=2, and either
over-predict or under-predict it by a smaller factor for k = 0,1.Comment: 10 pages, 13 figures, submitted to Ap
- …