444 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
The surface signature of the tidal dissipation of the core in a two-layer planet
Tidal dissipation, which is directly linked to internal structure, is one of
the key physical mechanisms that drive systems evolution and govern their
architecture. A robust evaluation of its amplitude is thus needed to predict
evolution time for spins and orbits and their final states. The purpose of this
paper is to refine recent model of the anelastic tidal dissipation in the
central dense region of giant planets, commonly assumed to retain a large
amount of heavy elements, which constitute an important source of dissipation.
The previous paper evaluated the impact of the presence of the static fluid
envelope on the tidal deformation of the core and on the associated anelastic
tidal dissipation, through the tidal quality factor Qc. We examine here its
impact on the corresponding effective anelastic tidal dissipation, through the
effective tidal quality factor Qp. We show that the strength of this mechanism
mainly depends on mass concentration. In the case of Jupiter- and Saturn-like
planets, it can increase their effective tidal dissipation by, around, a factor
2.4 and 2 respectively. In particular, the range of the rheologies compatible
with the observations is enlarged compared to the results issued from previous
formulations. We derive here an improved expression of the tidal effective
factor Qp in terms of the tidal dissipation factor of the core Qc, without
assuming the commonly used assumptions. When applied to giant planets, the
formulation obtained here allows a better match between the an elastic core's
tidal dissipation of a two-layer model and the observations.Comment: 5 pages, 2 figures, Accepted for publication in Astronomy &
Astrophysic
The Morphology-Density-Relation: Impact on the Satellite Fraction
In the past years several authors studied the abundance of satellites around
galaxies in order to better estimate the halo masses of host galaxies. To
investigate this connection, we analyze galaxies with
from the hydrodynamical cosmological
simulation Magneticum. We find that the satellite fraction of centrals is
independent of their morphology. With the exception of very massive galaxies at
low redshift, our results do not support the assumption that the dark matter
(DM) haloes of spheroidal galaxies are significantly more massive than those of
disc galaxies at fixed . We show that the
density-morphology-relation starts to build up at and is independent
of the star-formation properties of central galaxies. We conclude that
environmental quenching is more important for satellites than for centrals. Our
simulations indicate that conformity is already in place at , where
formation redshift and current star-formation rate (SFR) of central and
satellite galaxies correlate. Centrals with low SFRs have formed earlier (at
fixed ) while centrals with high SFR formed later, with
typical formation redshifts well in agreement with observations. However, we
confirm the recent observations that the apparent number of satellites of
spheroidal galaxies is significantly larger than for disc galaxies. This
difference completely originates from the inclusion of companion galaxies, i.e.
galaxies that do not sit in the potential minimum of a DM halo. Thus, due to
the density-morphological-relation the number of satellites is not a good
tracer for the halo mass, unless samples are restricted to the central galaxies
of DM haloes.Comment: 17 pages, submitted to MNRAS, 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
Declining rotation curves at in CDM galaxy formation simulations
Selecting disk galaxies from the cosmological, hydrodynamical simulation
Magneticum Pathfinder we show that almost half of our poster child disk
galaxies at show significantly declining rotation curves and low dark
matter fractions, very similar to recently reported observations. These
galaxies do not show any anomalous behavior, reside in standard dark matter
halos and typically grow significantly in mass until , where they span
all morphological classes, including disk galaxies matching present day
rotation curves and observed dark matter fractions. Our findings demonstrate
that declining rotation curves and low dark matter fractions in rotation
dominated galaxies at appear naturally within the CDM paradigm
and reflect the complex baryonic physics, which plays a role at the peak epoch
of star-formation. In addition, we find some dispersion dominated galaxies at
which host a significant gas disk and exhibit similar shaped rotation
curves as the disk galaxy population, rendering it difficult to differentiate
between these two populations with currently available observation techniques.Comment: 6 pages, 4 figures, ApJ Letters in press, www.magneticum.or
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
Connecting Angular Momentum and Galactic Dynamics: The complex Interplay between Spin, Mass, and Morphology
The evolution and distribution of the angular momentum of dark matter (DM)
halos have been discussed in several studies over the past decades. In
particular, the idea arose that angular momentum conservation should allow to
infer the total angular momentum of the entire DM halo from measuring the
angular momentum of the baryonic component, which is populating the center of
the halo, especially for disk galaxies. To test this idea and to understand the
connection between the angular momentum of the DM halo and its galaxy, we use
the Magneticum simulations. We successfully produce populations of spheroidal
and disk galaxies self-consistently. Thus, we are able to study the dependence
of galactic properties on their morphology. We find that (1) the specific
angular momentum of stars in disk and spheroidal galaxies as a function of
their stellar mass compares well with observational results; (2) the specific
angular momentum of the stars in disk galaxies is slightly smaller compared to
the specific angular momentum of the cold gas, in good agreement with
observations; (3) simulations including the baryonic component show a dichotomy
in the specific stellar angular momentum distribution when splitting the
galaxies according to their morphological type (this dichotomy can also be seen
in the spin parameter, where disk galaxies populate halos with slightly larger
spin compared to spheroidal galaxies); (4) disk galaxies preferentially
populate halos in which the angular momentum vector of the DM component in the
central part shows a better alignment to the angular momentum vector of the
entire halo; and (5) the specific angular momentum of the cold gas in disk
galaxies is approximately 40 percent smaller than the specific angular momentum
of the total DM halo and shows a significant scatter.Comment: 25 pages, accepted by ApJ, www.magneticum.or
Contributions to the Bohr topology by W.W. Comfort
The important rôle that W. W. Comfort played in the study of the Bohr topology is described
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