312 research outputs found
Morphology of Hydrodynamic Winds: A Study of Planetary Winds in Stellar Environments
Bathed in intense ionizing radiation, close-in gaseous planets undergo
hydrodynamic atmospheric escape, which ejects the upper extent of their
atmospheres into the interplanetary medium. Ultraviolet detections of escaping
gas around transiting planets corroborate such a framework. Exposed to the
stellar environment, the outflow is shaped by its interaction with the stellar
wind and by the planet's orbit. We model these effects using Athena to perform
3-D radiative-hydrodynamic simulations of tidally-locked hydrogen atmospheres
receiving large amounts of ionizing extreme-ultraviolet flux in various stellar
environments for the low-magnetic-field case. Through a step-by-step
exploration of orbital and stellar wind effects on the planetary outflow, we
find three structurally distinct stellar wind regimes: weak, intermediate, and
strong. We perform synthetic Lyman- observations and find unique
observational signatures for each regime. A weak stellar
windwhich cannot confine the planetary outflow, leading to a
torus of material around the starhas a pre-transit, red-shifted
dayside arm and a slightly redward-skewed spectrum during transit. The
intermediate regime truncates the dayside outflow at large distances from the
planet and causes periodic disruptions of the outflow, producing observational
signatures that mimic a double transit. The first of these dips is blue-shifted
and precedes the optical transit. Finally, strong stellar winds completely
confine the outflow into a cometary tail and accelerate the outflow outwards,
producing large blue-shifted signals post-transit. Across all three regimes,
large signals occur far outside of transit, offering motivation to continue
ultraviolet observations outside of direct transit.Comment: 33 pages, 21 figures (7 of which have embedded movies viewable with
Adobe Acrobat Pro), Submitted to Ap
Using Lyman- transits to constrain models of atmospheric escape
Lyman- transits provide an opportunity to test models of atmospheric
escape directly. However, translating observations into constraints on the
properties of the escaping atmosphere is challenging. The major reason for this
is that the observable parts of the outflow often comes from material outside
the planet's Hill sphere, where the interaction between the planetary outflow
and circumstellar environment is important. As a result, 3D models are required
to match observations. Whilst 3D hydrodynamic simulations are able to match
observational features qualitatively, they are too computationally expensive to
perform a statistical retrieval of properties of the outflow. Here, we develop
a model that determines the trajectory, ionization state and 3D geometry of the
outflow as a function of its properties and system parameters. We then couple
this model to a ray tracing routine in order to produce synthetic transits. We
demonstrate the validity of this approach, reproducing the trajectory of the
outflows seen in 3D simulations. We illustrate the use of this model by
performing a retrieval on the transit spectrum of GJ 436 b. Our model
constrains the sound speed of the outflow ,
indicating that we can rule out core-powered mass loss as the mechanism driving
the outflow for this planet. The bound on planetary outflow velocity and mass
loss rates are consistent with a photoevaporative wind
A Brief History of Trans-Neptunian Space
The Edgeworth-Kuiper belt encodes the dynamical history of the outer solar
system. Kuiper belt objects (KBOs) bear witness to coagulation physics, the
evolution of planetary orbits, and external perturbations from the solar
neighborhood. We critically review the present-day belt's observed properties
and the theories designed to explain them. Theories are organized according to
a possible time-line of events. In chronological order, epochs described
include (1) coagulation of KBOs in a dynamically cold disk, (2) formation of
binary KBOs by fragmentary collisions and gravitational captures, (3) stirring
of KBOs by Neptune-mass planets (``oligarchs''), (4) eviction of excess
oligarchs, (5) continued stirring of KBOs by remaining planets whose orbits
circularize by dynamical friction, (6) planetary migration and capture of
Resonant KBOs, (7) creation of the inner Oort cloud by passing stars in an open
stellar cluster, and (8) collisional comminution of the smallest KBOs. Recent
work underscores how small, collisional, primordial planetesimals having low
velocity dispersion permit the rapid assembly of ~5 Neptune-mass oligarchs at
distances of 15-25 AU. We explore the consequences of such a picture. We
propose that Neptune-mass planets whose orbits cross into the Kuiper belt for
up to ~20 Myr help generate the high-perihelion members of the hot Classical
disk and Scattered belt. By contrast, raising perihelia by sweeping secular
resonances during Neptune's migration might fill these reservoirs too
inefficiently when account is made of how little primordial mass might reside
in bodies having sizes of order 100 km. These and other frontier issues in
trans-Neptunian space are discussed quantitatively.Comment: Final proofed version for Protostars and Planets V; some numbers
adjusted by factors of 2; references update
Overexpression of Protein Kinase C βII Induces Colonic Hyperproliferation and Increased Sensitivity to Colon Carcinogenesis
Protein kinase C βII (PKC βII) has been implicated in proliferation of the intestinal epithelium. To investigate PKC βII function in vivo, we generated transgenic mice that overexpress PKC βII in the intestinal epithelium. Transgenic PKC βII mice exhibit hyperproliferation of the colonic epithelium and an increased susceptibility to azoxymethane-induced aberrant crypt foci, preneoplastic lesions in the colon. Furthermore, transgenic PKC βII mice exhibit elevated colonic β-catenin levels and decreased glycogen synthase kinase 3β activity, indicating that PKC βII stimulates the Wnt/adenomatous polyposis coli (APC)/β-catenin proliferative signaling pathway in vivo. These data demonstrate a direct role for PKC βII in colonic epithelial cell proliferation and colon carcinogenesis, possibly through activation of the APC/β-catenin signaling pathway
Physics of the Galactic Center Cloud G2, on its Way towards the Super-Massive Black Hole
The origin, structure and evolution of the small gas cloud, G2, is
investigated, that is on an orbit almost straight into the Galactic central
supermassive black hole (SMBH). G2 is a sensitive probe of the hot accretion
zone of Sgr A*, requiring gas temperatures and densities that agree well with
models of captured shock-heated stellar winds. Its mass is equal to the
critical mass below which cold clumps would be destroyed quickly by
evaporation. Its mass is also constrained by the fact that at apocenter its
sound crossing timescale was equal to its orbital timescale. Our numerical
simulations show that the observed structure and evolution of G2 can be well
reproduced if it formed in pressure equilibrium with the surrounding in 1995 at
a distance from the SMBH of 7.6e16 cm. If the cloud would have formed at
apocenter in the 'clockwise' stellar disk as expected from its orbit, it would
be torn into a very elongated spaghetti-like filament by 2011 which is not
observed. This problem can be solved if G2 is the head of a larger, shell-like
structure that formed at apocenter. Our numerical simulations show that this
scenario explains not only G2's observed kinematical and geometrical properties
but also the Br_gamma observations of a low surface brightness gas tail that
trails the cloud. In 2013, while passing the SMBH G2 will break up into a
string of droplets that within the next 30 years mix with the surrounding hot
gas and trigger cycles of AGN activity.Comment: 22 pages, 13 figures, submitted to Ap
Simulations of the Origin and Fate of the Galactic Center Cloud G2
We investigate the origin and fate of the recently discovered gas cloud G2
close to the Galactic Center. Our hydrodynamical simulations focussing on the
dynamical evolution of the cloud in combination with currently available
observations favor two scenarios: a Compact Cloud which started around the year
1995 and a Spherical Shell of gas, with an apocenter distance within the
disk(s) of young stars and a radius of a few times the size of the Compact
Cloud. The former is able to explain the detected signal of G2 in the
position-velocity diagram of the Br gamma emission of the year 2008.5 and
2011.5 data. The latter can account for both, G2's signal as well as the
fainter extended tail-like structure G2t seen at larger distances from the
black hole and smaller velocities. In contrast, gas stripped from a compact
cloud by hydrodynamical interactions is not able to explain the location of the
detected G2t emission in the observed position-velocity diagrams. This favors
the Spherical Shell Scenario and might be a severe problem for the Compact
Cloud as well as the so-called Compact Source Scenario. From these first
idealized simulations we expect a roughly constant feeding of the supermassive
black hole through a nozzle-like structure over a long period, starting shortly
after the closest approach in 2013.51 for the Compact Cloud. If the matter
accretes in the hot accretion mode, we do not expect a significant boost of the
current activity of Sgr A* for the Compact Cloud model, but a boost of the
average infrared and X-ray luminosity by roughly a factor of 80 for the
Spherical Shell scenario with order of magnitude variations on a timescale of a
few months. The near-future evolution of the cloud will be a sensitive probe of
the conditions of the gas distribution in the milli-parsec environment of the
massive black hole in the Galactic Center.Comment: 16 pages, 16 figures, accepted by Ap
On the observability of resonant structures in planetesimal disks due to planetary migration
We present a thorough study of the impact of a migrating planet on a
planetesimal disk, by exploring a broad range of masses and eccentricities for
the planet. We discuss the sensitivity of the structures generated in debris
disks to the basic planet parameters. We perform many N-body numerical
simulations, using the symplectic integrator SWIFT, taking into account the
gravitational influence of the star and the planet on massless test particles.
A constant migration rate is assumed for the planet. The effect of planetary
migration on the trapping of particles in mean motion resonances is found to be
very sensitive to the initial eccentricity of the planet and of the
planetesimals. A planetary eccentricity as low as 0.05 is enough to smear out
all the resonant structures, except for the most massive planets. The
planetesimals also initially have to be on orbits with a mean eccentricity of
less than than 0.1 in order to keep the resonant clumps visible. This numerical
work extends previous analytical studies and provides a collection of disk
images that may help in interpreting the observations of structures in debris
disks. Overall, it shows that stringent conditions must be fulfilled to obtain
observable resonant structures in debris disks. Theoretical models of the
origin of planetary migration will therefore have to explain how planetary
systems remain in a suitable configuration to reproduce the observed
structures.Comment: 16 pages, 13 figures. Accepted for publication in A&
- …