181 research outputs found
Spiral arm triggering of star formation
We present numerical simulations of the passage of clumpy gas through a
galactic spiral shock, the subsequent formation of giant molecular clouds
(GMCs) and the triggering of star formation. The spiral shock forms dense
clouds while dissipating kinetic energy, producing regions that are locally
gravitationally bound and collapse to form stars. In addition to triggering the
star formation process, the clumpy gas passing through the shock naturally
generates the observed velocity dispersion size relation of molecular clouds.
In this scenario, the internal motions of GMCs need not be turbulent in nature.
The coupling of the clouds' internal kinematics to their externally triggered
formation removes the need for the clouds to be self-gravitating. Globally
unbound molecular clouds provides a simple explanation of the low efficiency of
star formation. While dense regions in the shock become bound and collapse to
form stars, the majority of the gas disperses as it leaves the spiral arm.Comment: 6 pages, 4 figures: IAU 237, Triggering of star formation in
turbulent molecular clouds, eds B. Elmegreen and J. Palou
Long term evolution of planetary systems with a terrestrial planet and a giant planet
We study the long term orbital evolution of a terrestrial planet under the
gravitational perturbations of a giant planet. In particular, we are interested
in situations where the two planets are in the same plane and are relatively
close. We examine both possible configurations: the giant planet orbit being
either outside or inside the orbit of the smaller planet. The perturbing
potential is expanded to high orders and an analytical solution of the
terrestrial planetary orbit is derived. The analytical estimates are then
compared against results from the numerical integration of the full equations
of motion and we find that the analytical solution works reasonably well. An
interesting finding is that the new analytical estimates improve greatly the
predictions for the timescales of the orbital evolution of the terrestrial
planet compared to an octupole order expansion. Finally, we briefly discuss
possible applications of the analytical estimates in astrophysical problems.Comment: Accepted for publication in MNRA
Radiative Hydrodynamic Simulations of HD209458b: Temporal Variability
We present a new approach for simulating the atmospheric dynamics of the
close-in giant planet HD209458b that allows for the decoupling of radiative and
thermal energies, direct stellar heating of the interior, and the solution of
the full 3D Navier Stokes equations. Simulations reveal two distinct
temperature inversions (increasing temperature with decreasing pressure) at the
sub-stellar point due to the combined effects of opacity and dynamical flow
structure and exhibit instabilities leading to changing velocities and
temperatures on the nightside for a range of viscosities. Imposed on the
quasi-static background, temperature variations of up to 15% are seen near the
terminators and the location of the coldest spot is seen to vary by more than
20 degrees, occasionally appearing west of the anti-solar point. Our new
approach introduces four major improvements to our previous methods including
simultaneously solving both the thermal energy and radiative equations in both
the optical and infrared, incorporating updated opacities, including a more
accurate treatment of stellar energy deposition that incorporates the opacity
relevant for higher energy stellar photons, and the addition of explicit
turbulent viscosity.Comment: Accepted for publication in Ap
Atmospheric Dynamics of Short-period Extra Solar Gas Giant Planets I: Dependence of Night-Side Temperature on Opacity
More than two dozen short-period Jupiter-mass gas giant planets have been
discovered around nearby solar-type stars in recent years, several of which
undergo transits, making them ideal for the detection and characterization of
their atmospheres. Here we adopt a three-dimensional radiative hydrodynamical
numerical scheme to simulate atmospheric circulation on close-in gas giant
planets. In contrast to the conventional GCM and shallow water algorithms, this
method does not assume quasi hydrostatic equilibrium and it approximates
radiation transfer from optically thin to thick regions with flux-limited
diffusion. In the first paper of this series, we consider
synchronously-spinning gas giants. We show that a full three-dimensional
treatment, coupled with rotationally modified flows and an accurate treatment
of radiation, yields a clear temperature transition at the terminator. Based on
a series of numerical simulations with varying opacities, we show that the
night-side temperature is a strong indicator of the opacity of the planetary
atmosphere. Planetary atmospheres that maintain large, interstellar opacities
will exhibit large day-night temperature differences, while planets with
reduced atmospheric opacities due to extensive grain growth and sedimentation
will exhibit much more uniform temperatures throughout their photosphere's. In
addition to numerical results, we present a four-zone analytic approximation to
explain this dependence.Comment: 35 Pages, 13 Figure
Shocks, cooling and the origin of star formation rates in spiral galaxies
Understanding star formation is problematic as it originates in the large
scale dynamics of a galaxy but occurs on the small scale of an individual star
forming event. This paper presents the first numerical simulations to resolve
the star formation process on sub-parsec scales, whilst also following the
dynamics of the interstellar medium (ISM) on galactic scales. In these models,
the warm low density ISM gas flows into the spiral arms where orbit crowding
produces the shock formation of dense clouds, held together temporarily by
their external pressure. Cooling allows the gas to be compressed to
sufficiently high densities that local regions collapse under their own gravity
and form stars. The star formation rates follow a Schmidt-Kennicutt
\Sigma_{SFR} ~ \Sigma_{gas}^{1.4} type relation with the local surface density
of gas while following a linear relation with the cold and dense gas. Cooling
is the primary driver of star formation and the star formation rates as it
determines the amount of cold gas available for gravitational collapse. The
star formation rates found in the simulations are offset to higher values
relative to the extragalactic values, implying a constant reduction, such as
from feedback or magnetic fields, is likely to be required. Intriguingly, it
appears that a spiral or other convergent shock and the accompanying thermal
instability can explain how star formation is triggered, generate the physical
conditions of molecular clouds and explain why star formation rates are tightly
correlated to the gas properties of galaxies.Comment: 13 pages, 12 figures. MNRAS in pres
Radiative Hydrodynamical Studies of Irradiated Atmospheres
Transiting planets provide a unique opportunity to study the atmospheres of
extra-solar planets. Radiative hydrodynamical models of the atmosphere provide
a crucial link between the physical characteristics of the atmosphere and the
observed properties. Here I present results from 3D simulations which solve the
full Navier-Stokes equations coupled to a flux-limited diffusion treatment of
radiation transfer for planets with 1, 3, and 7 day periods. Variations in
opacity amongst models leads to a variation in the temperature differential
across the planet, while atmospheric dynamics becomes much more variable at
longer orbital periods. I also present 3D radiative simulations illustrating
the importance of distinguishing between optical and infrared opacities.Comment: To appear in the Proceedings of IAU Symposium 253, "Transiting
Planets", May 2008, Cambridge, M
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