1,912 research outputs found
HELIOS-K: An Ultrafast, Open-source Opacity Calculator for Radiative Transfer
We present an ultrafast opacity calculator that we name HELIOS-K. It takes a
line list as an input, computes the shape of each spectral line and provides an
option for grouping an enormous number of lines into a manageable number of
bins. We implement a combination of Algorithm 916 and Gauss-Hermite quadrature
to compute the Voigt profile, write the code in CUDA and optimise the
computation for graphics processing units (GPUs). We restate the theory of the
k-distribution method and use it to reduce to lines to to wavenumber bins, which may then be used for radiative transfer,
atmospheric retrieval and general circulation models. The choice of line-wing
cutoff for the Voigt profile is a significant source of error and affects the
value of the computed flux by . This is an outstanding physical
(rather than computational) problem, due to our incomplete knowledge of
pressure broadening of spectral lines in the far line wings. We emphasize that
this problem remains regardless of whether one performs line-by-line
calculations or uses the k-distribution method and affects all calculations of
exoplanetary atmospheres requiring the use of wavelength-dependent opacities.
We elucidate the correlated-k approximation and demonstrate that it applies
equally to inhomogeneous atmospheres with a single atomic/molecular species or
homogeneous atmospheres with multiple species. Using a NVIDIA K20 GPU, HELIOS-K
is capable of computing an opacity function with spectral lines in
second and is publicly available as part of the Exoclimes Simulation
Platform (ESP; www.exoclime.org).Comment: Accepted by ApJ. 8 pages, 5 figure
The GENGA Code: Gravitational Encounters in N-body simulations with GPU Acceleration
We describe an open source GPU implementation of a hybrid symplectic N-body
integrator, GENGA (Gravitational ENcounters with Gpu Acceleration), designed to
integrate planet and planetesimal dynamics in the late stage of planet
formation and stability analyses of planetary systems. GENGA uses a hybrid
symplectic integrator to handle close encounters with very good energy
conservation, which is essential in long-term planetary system integration. We
extended the second order hybrid integration scheme to higher orders. The GENGA
code supports three simulation modes: Integration of up to 2048 massive bodies,
integration with up to a million test particles, or parallel integration of a
large number of individual planetary systems. We compare the results of GENGA
to Mercury and pkdgrav2 in respect of energy conservation and performance, and
find that the energy conservation of GENGA is comparable to Mercury and around
two orders of magnitude better than pkdgrav2. GENGA runs up to 30 times faster
than Mercury and up to eight times faster than pkdgrav2. GENGA is written in
CUDA C and runs on all NVIDIA GPUs with compute capability of at least 2.0.Comment: Accepted by ApJ. 18 pages, 17 figures, 4 table
Creation of ultracold Sr2 molecules in the electronic ground state
We report on the creation of ultracold 84Sr2 molecules in the electronic
ground state. The molecules are formed from atom pairs on sites of an optical
lattice using stimulated Raman adiabatic passage (STIRAP). We achieve a
transfer efficiency of 30% and obtain 4x10^4 molecules with full control over
the external and internal quantum state. STIRAP is performed near the narrow
1S0-3P1 intercombination transition, using a vibrational level of the 0u
potential as intermediate state. In preparation of our molecule association
scheme, we have determined the binding energies of the last vibrational levels
of the 0u, 1u excited-state, and the 1\Sigma_g^+ ground-state potentials. Our
work overcomes the previous limitation of STIRAP schemes to systems with
Feshbach resonances, thereby establishing a route that is applicable to many
systems beyond bi-alkalis.Comment: 7 pages, 7 figures, 3 table
Stochasticity & Predictability in Terrestrial Planet Formation
Terrestrial planets are thought to be the result of a vast number of
gravitational interactions and collisions between smaller bodies. We use
numerical simulations to show that practically identical initial conditions
result in a wide array of final planetary configurations. This is a result of
the chaotic evolution of trajectories which are highly sensitive to minuscule
displacements. We determine that differences between systems evolved from
virtually identical initial conditions can be larger than the differences
between systems evolved from very different initial conditions. This implies
that individual simulations lack predictive power. For example, there is not a
reproducible mapping between the initial and final surface density profiles.
However, some key global properties can still be extracted if the statistical
spread across many simulations is considered. Based on these spreads, we
explore the collisional growth and orbital properties of terrestrial planets
which assemble from different initial conditions (we vary the initial
planetesimal distribution, planetesimal masses, and giant planet orbits).
Confirming past work, we find that the resulting planetary systems are sculpted
by sweeping secular resonances. Configurations with giant planets on eccentric
orbits produce fewer and more massive terrestrial planets on tighter orbits
than those with giants on circular orbits. This is further enhanced if the
initial mass distribution is biased to the inner regions. In all cases, the
outer edge of the system is set by the final location of the resonance
and we find that the mass distribution peaks at the resonance. Using
existing observations, we find that extrasolar systems follow similar trends.
Although differences between our numerical modelling and exoplanetary systems
remain, we suggest that CoRoT-7, HD 20003, and HD 20781 may host undetected
giant planets.Comment: replaced to match published version, 20 pages, 11 figures, published
in MNRAS, simulation outputs available at https://cheleb.net/astro/sp15
Bose-Einstein condensation of 86Sr
We report on the attainment of Bose-Einstein condensation of 86Sr. This
isotope has a scattering length of about +800 a0 and thus suffers from fast
three-body losses. To avoid detrimental atom loss, evaporative cooling is
performed at low densities around 3x10^12 cm^-3 in a large volume optical
dipole trap. We obtain almost pure condensates of 5x10^3 atoms.Comment: 4 pages, 3 figure
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