852 research outputs found
Understanding tidal dissipation in gaseous giant planets from their core to their surface
Tidal dissipation in planetary interiors is one of the key physical
mechanisms that drive the evolution of star-planet and planet-moon systems.
Tidal dissipation in planets is intrinsically related to their internal
structure. In particular, fluid and solid layers behave differently under tidal
forcing. Therefore, their respective dissipation reservoirs have to be
compared. In this work, we compute separately the contributions of the
potential dense rocky/icy core and of the convective fluid envelope of gaseous
giant planets, as a function of core size and mass. We demonstrate that in
general both mechanisms must be taken into account.Comment: 2 pages, 2 figures, CoRoT Symposium 3 / Kepler KASC-7 joint meeting,
Toulouse, July 2014; To be published by EPJ Web of Conference
Justification of the two-bulge method in the theory of bodily tides
Mathematical modeling of bodily tides can be carried out in various ways.
Most straightforward is the method of complex amplitudes, which is often used
in the planetary science. Another method, employed both in planetary science
and astrophysics, is based on decomposition of each harmonic of the tide into
two bulges oriented orthogonally to one another.
We prove that the two methods are equivalent. Specifically, we demonstrate
that the two-bulge method is not a separate approximation, but ensues directly
from the Fourier expansion of a linear tidal theory equipped with an arbitrary
rheological model involving a departure from elasticity. To this end, we use
the most general mathematical formalism applicable to linear bodily tides. To
express the tidal amendment to the potential of the perturbed primary, we act
on the tide-raising potential of the perturbing secondary with a convolution
operator. This enables us to interconnect a complex Fourier component of the
tidally generated potential of the perturbed primary with the appropriate
complex Fourier component of the tide-raising potential of the secondary. Then
we demonstrate how this interrelation entails the two-bulge description.
While less economical mathematically, the two-bulge approach has a good
illustrative power, and may be employed on a par with a more concise method of
complex amplitudes. At the same time, there exist situations where the
two-bulge method becomes more practical for technical calculations.Comment: Astronomy & Astrophysics, in pres
A refined sub-grid model for black hole accretion and AGN feedback in large cosmological simulations
In large scale cosmological hydrodynamic simulations simplified sub-grid
models for gas accretion onto black holes and AGN feedback are commonly used.
Such models typically depend on various free parameters, which are not well
constrained. We present a new advanced model containing a more detailed
description of AGN feedback, where those parameters reflect the results of
recent observations. The model takes the dependency of these parameters on the
black hole properties into account and describes a continuous transition
between the feedback processes acting in the so-called radio-mode and
quasar-mode. In addition, we implement a more detailed description of the
accretion of gas onto black holes by distinguishing between hot and cold gas
accretion. Our new implementations prevent black holes from gaining too much
mass, particularly at low redshifts so that our simulations are now very
successful in reproducing the observed present-day black hole mass function.
Our new model also suppresses star formation in massive galaxies slightly more
efficiently than many state-of-the-art models. Therefore, the simulations that
include our new implementations produce a more realistic population of
quiescent and star-forming galaxies compared to recent observations, even if
some discrepancies remain. In addition, the baryon conversion efficiencies in
our simulation are - except for the high mass end - consistent with
observations presented in literature over the mass range resolved by our
simulations. Finally, we discuss the significant impact of the feedback model
on the low-luminous end of the AGN luminosity function.Comment: 25 pages, 19 figures. MNRAS accepted. Magneticum website:
http://www.magneticum.or
Unravelling tidal dissipation in gaseous giant planets
International audienceContext. Tidal dissipation in planetary interiors is one of the key physical mechanisms that drive the evolution of star-planet and planet-moon systems. New constraints on this dissipation are now obtained both in the solar and exo-planetary systems.Aims. Tidal dissipation in planets is intrinsically related to their internal structure. Indeed, the dissipation behaves very differently when we compare its properties in solid and fluid planetary layers. Since planetary interiors consist of both types of regions, it is necessary to be able to assess and compare the respective intensity of the reservoir of dissipation in each type of layers. Therefore, in the case of giant planets, the respective contribution of the potential central dense rocky/icy core and of the deep convective fluid envelope must be computed as a function of the mass and the radius of the core. This will allow us to obtain their respective strengths.Methods. Using a method that evaluates the reservoir of dissipation associated to each region, which is a frequency-average of complex tidal Love numbers, we compared the respective contributions of the central core and of the fluid envelope.Results. For Jupiter- and Saturn-like planets, we show that the viscoelastic dissipation in the core could dominate the turbulent friction acting on tidal inertial waves in the envelope. However, the fluid dissipation would not be negligible. This demonstrates that it is necessary to build complete models of tidal dissipation in planetary interiors from their deep interior to their surface without any arbitrary assumptions.Conclusions. We demonstrate how important it is to carefully evaluate the respective strength of each type of dissipation mechanism in planetary interiors and to go beyond the usually adopted ad-hoc models. We confirm the significance of tidal dissipation in the potential dense core of gaseous giant planets
Origin and properties of dual and offset active galactic nuclei in a cosmological simulation at z=2
In the last few years, it became possible to observationally resolve galaxies
with two distinct nuclei in their centre. For separations smaller than 10kpc,
dual and offset active galactic nuclei (AGN) are distinguished: in dual AGN,
both nuclei are active, whereas in offset AGN only one nucleus is active. To
study the origin of such AGN pairs, we employ a cosmological, hydrodynamic
simulation with a large volume of (182Mpc)^3 from the set of Magneticum
Pathfinder Simulations. The simulation self-consistently produces 35 resolved
black hole (BH) pairs at redshift z=2, with a comoving distance smaller than
10kpc. 14 of them are offset AGN and nine are dual AGN, resulting in a fraction
of (1.2 \pm 0.3)% AGN pairs with respect to the total number of AGN. In this
paper, we discuss fundamental differences between the BH and galaxy properties
of dual AGN, offset AGN and inactive BH pairs and investigate their different
triggering mechanisms. We find that in dual AGN the BHs have similar masses and
the corresponding BH from the less massive progenitor galaxy always accretes
with a higher Eddington ratio. In contrast, in offset AGN the active BH is
typically more massive than its non-active counterpart. Furthermore, dual AGN
in general accrete more gas from the intergalactic medium than offset AGN and
non-active BH pairs. This highlights that merger events, particularly minor
mergers, do not necessarily lead to strong gas inflows and thus, do not always
drive strong nuclear activity.Comment: 17 pages, 18 figures, accepted for publication in MNRAS, website:
http://www.magneticum.or
Lifetime investigations of a lithium iron phosphate (LFP) battery system connected to a wind turbine for forecast improvement and output power gradient reduction
Lithium Ion batteries and especially Lithium Iron Phosphate (LFP) batteries can be characterized by high power densities, relatively long life-time, no maintenance and a lot of research currently being done on increasing their performance. Therefore, they seem to be a good choice for integration with wind turbines. However, the current Lithium Ion energy storage price and its limited lifetime are the main obstacles to the integration of storage technologies with wind turbines. That is why proper lifetime estimation of the lithium ion batteries, working with mission-profile characteristics for specific services, is very important, not only for optimisation of the revenues but also for correct scheduling of the maintenance jobs. This paper analyzes the connection of LFP battery storage systems into wind turbines, with a high focus on the lifetime of LFP batteries for selected services.Postprint (published version
Metabolic resource overlap impacts competition among phyllosphere bacteria
The phyllosphere is densely colonised by microbial communities, despite sparse and heterogeneously distributed resources. The limitation of resources is expected to drive bacterial competition resulting in exclusion or coexistence based on fitness differences and resource overlap between individual colonisers. We studied the impact of resource competition by determining the effects of different bacterial colonisers on the growth of the model epiphyte Pantoea eucalypti 299R (Pe299R). Resource overlap was predicted based on genome-scale metabolic modelling. By combining results of metabolic modelling and pairwise competitions in the Arabidopsis thaliana phyllosphere and in vitro, we found that ten resources sufficed to explain fitness of Pe299R. An effect of both resource overlap and phylogenetic relationships was found on competition outcomes in vitro as well as in the phyllosphere. However, effects of resource competition were much weaker in the phyllosphere when compared to in vitro experiments. When investigating growth dynamics and reproductive success at the single-cell resolution, resource overlap and phylogenetic relationships are only weakly correlated with epiphytic Pe299R reproductive success, indicating that the leaf’s spatial heterogeneity mitigates resource competition. Although the correlation is weak, the presence of competitors led to the development of Pe299R subpopulations that experienced different life histories and cell divisions. In some in planta competitions, Pe299R benefitted from the presence of epiphytes despite high resource overlap to the competitor strain suggesting other factors having stronger effects than resource competition. This study provides fundamental insights into how bacterial communities are shaped in heterogeneous environments and a framework to predict competition outcomes
Hierarchical Control of Droop-Controlled DC and AC Microgrids - A General Approach Towards Standardization
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