20,456 research outputs found
An optical view of the filament region of Abell 85
We compare the distribution of optically and Halpha (Ha) selected galaxies in
the Southern half of the nearby Abell 85 (A85) cluster with the recently
discovered X-ray filament (XRF). We search for galaxies where star formation
(SF) may have been triggered by interactions with intracluster gas or tidal
pressure due to the cluster potential when entering the cluster. Our analysis
is based on images obtained with CFHT MegaPrime/MegaCam (1x1 deg2 field) in
four bands (ugri) and ESO 2.2mWFI (38'x36' field) in a narrow band filter
corresponding to the redshifted Halpha (Ha) line and in a broad R-band filter.
The LFs are estimated by statistically subtracting a reference field.
Background contamination is minimized by cutting out galaxies redder than the
observed red sequence in the g-i vs. i colour-magnitude diagram. The galaxy
distribution shows a significantly flattened cluster, whose principal axis is
slightly offset from the XRF. The analysis of the broad band LFs shows that the
filament region is well populated. The filament is also independently detected
as a gravitationally bound structure by the Serna & Gerbal hierarchical method.
101 galaxies are detected in Ha, among which 23 have spectroscopic redshifts in
the cluster, 2 have spectroscopic redshifts higher than the cluster and 58 have
photometric redshifts that tend to indicate that they are background
objects.The 23 galaxies with spectroscopic redshifts in the cluster are mostly
concentrated in the South part of the cluster and along the filament. We find a
number of galaxies showing evidence for SF in the XRF, and all our results are
consistent with the previous hypothesis that the XRF in A85 is a
gravitationally bound structure made of groups falling on to the main cluster.Comment: Accepted in A&A. 39 pages, 107 figures. Full resolution images
available at ftp://ftp.iap.fr/pub/from_users/gam/A85
A collisionless scenario for Uranus tilting
The origin of the high inclination of Uranus' spin-axis (Uranus' obliquity)
is one of the great unanswered questions about the Solar system. Giant planets
are believed to form with nearly zero obliquity, and it has been shown that the
present behaviour of Uranus' spin is essentially stable. Several attempts were
made in order to solve this problem. Here we report numerical simulations
showing that Uranus' axis can be tilted during the planetary migration, without
the need of a giant impact, provided that the planet had an additional
satellite and a temporary large inclination. This might have happened during
the giant planet instability phase described in the Nice model. In our
scenario, the satellite is ejected after the tilt by a close encounter at the
end of the migration. This model can both explain Uranus' large obliquity and
bring new constraints on the planet orbital evolution.Comment: 5 pages, 5 figures, ApJL in pres
Tilting Jupiter (a bit) and Saturn (a lot) During Planetary Migration
We study the effects of planetary late migration on the gas giants
obliquities. We consider the planetary instability models from Nesvorny &
Morbidelli (2012), in which the obliquities of Jupiter and Saturn can be
excited when the spin-orbit resonances occur. The most notable resonances occur
when the and frequencies, changing as a result of planetary
migration, become commensurate with the precession frequencies of Jupiter's and
Saturn's spin vectors. We show that Jupiter may have obtained its present
obliquity by crossing of the resonance. This would set strict constrains
on the character of migration during the early stage. Additional effects on
Jupiter's obliquity are expected during the last gasp of migration when the
resonance was approached. The magnitude of these effects depends on the
precise value of the Jupiter's precession constant. Saturn's large obliquity
was likely excited by capture into the resonance. This probably happened
during the late stage of planetary migration when the evolution of the
frequency was very slow, and the conditions for capture into the spin-orbit
resonance with were satisfied. However, whether or not Saturn is in the
spin-orbit resonance with at the present time is not clear, because the
existing observations of Saturn's spin precession and internal structure models
have significant uncertainties.Comment: 29 pages, 8 figures, accepted for publication in The Astrophysical
Journa
Kinematic deprojection and mass inversion of spherical systems of known velocity anisotropy
Traditionally, the mass / velocity anisotropy degeneracy (MAD) inherent in
the spherical, stationary, non-streaming Jeans equation has been handled by
assuming a mass profile and fitting models to the observed kinematical data.
Here, the opposite approach is considered: the equation of anisotropic
kinematic projection is inverted for known arbitrary anisotropy to yield the
space radial velocity dispersion profile in terms of an integral involving the
radial profiles of anisotropy and isotropic dynamical pressure. Then, through
the Jeans equation, the mass profile is derived in terms of double integrals of
observable quantities. Single integral formulas for both deprojection and mass
inversion are provided for several simple anisotropy models (isotropic, radial,
circular, general constant, Osipkov-Merritt, Mamon-Lokas and
Diemand-Moore-Stadel). Tests of the mass inversion on NFW models with these
anisotropy models yield accurate results in the case of perfect observational
data, and typically better than 70% (in 4 cases out of 5) accurate mass
profiles for the sampling errors expected from current observational data on
clusters of galaxies. For the NFW model with mildly increasing radial
anisotropy, the mass is found to be insensitive to the adopted anisotropy
profile at 7 scale radii and to the adopted anisotropy radius at 3 scale radii.
This anisotropic mass inversion method is a useful complementary tool to
analyze the mass and anisotropy profiles of spherical systems. It provides the
practical means to lift the MAD in quasi-spherical systems such as globular
clusters, round dwarf spheroidal and elliptical galaxies, as well as groups and
clusters of galaxies, when the anisotropy of the tracer is expected to be
linearly related to the slope of its density.Comment: Accepted in MNRAS. 19 pages. Minor changes from previous version:
Table 1 of nomenclature, some math simplifications, paragraph in Discussion
on alternative deprojection method by deconvolution. 19 pages. 6 figure
AMD-stability in presence of first order Mean Motion Resonances
The AMD-stability criterion allows to discriminate between a-priori stable
planetary systems and systems for which the stability is not granted and needs
further investigations. AMD-stability is based on the conservation of the
Angular Momentum Deficit (AMD) in the averaged system at all orders of
averaging. While the AMD criterion is rigorous, the conservation of the AMD is
only granted in absence of mean-motion resonances (MMR). Here we extend the
AMD-stability criterion to take into account mean-motion resonances, and more
specifically the overlap of first order MMR. If the MMR islands overlap, the
system will experience generalized chaos leading to instability. The
Hamiltonian of two massive planets on coplanar quasi-circular orbits can be
reduced to an integrable one degree of freedom problem for period ratios close
to a first order MMR. We use the reduced Hamiltonian to derive a new overlap
criterion for first order MMR. This stability criterion unifies the previous
criteria proposed in the literature and admits the criteria obtained for
initially circular and eccentric orbits as limit cases. We then improve the
definition of AMD-stability to take into account the short term chaos generated
by MMR overlap. We analyze the outcome of this improved definition of
AMD-stability on selected multi-planet systems from the Extrasolar Planets
Encyclopeadia.Comment: Accepted by A and A 07/10/1
High inclination orbits in the secular quadrupolar three-body problem
The Lidov-Kozai mechanism allows a body to periodically exchange its
eccentricity with inclination. It was first discussed in the framework of the
quadrupolar secular restricted three-body problem, where the massless particle
is the inner body, and later extended to the quadrupolar secular nonrestricted
three body problem. In this paper, we propose a different point of view on the
problem by looking first at the restricted problem where the massless particle
is the outer body. In this situation, equilibria at high mutual inclination
appear, which correspond to the population of stable particles that Verrier &
Evans (2008,2009) find in stable, high inclination circumbinary orbits around
one of the components of the quadruple star HD 98800. We provide a simple
analytical framework using a vectorial formalism for these situations. We also
look at the evolution of these high inclination equilibria in the non
restricted case.Comment: 11 pages, 6 figures. Accepted by MNRAS 2009 September 1
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