6,013 research outputs found
Rotating solenoidal perfect fluids of Petrov type D
We prove that aligned Petrov type D perfect fluids for which the vorticity
vector is not orthogonal to the plane of repeated principal null directions and
for which the magnetic part of the Weyl tensor with respect to the fluid
velocity has vanishing divergence, are necessarily purely electric or locally
rotationally symmetric. The LRS metrics are presented explicitly.Comment: 6 pages, no figure
Purely radiative irrotational dust spacetimes
We consider irrotational dust spacetimes in the full non-linear regime which
are "purely radiative" in the sense that the gravitational field satisfies the
covariant transverse conditions div(H) = div(E) = 0. Within this family we show
that the Bianchi class A spatially homogeneous dust models are uniquely
characterised by the condition that is diagonal in the shear-eigenframe.Comment: 6 pages, ERE 2006 conference, minor correction
Detection of CO in Triton's atmosphere and the nature of surface-atmosphere interactions
Triton possesses a thin atmosphere, primarily composed of nitrogen, sustained
by the sublimation of surface ices. The goal is to determine the composition of
Triton's atmosphere and to constrain the nature of surface-atmosphere
interactions. We perform high-resolution spectroscopic observations in the
2.32-2.37 m range, using CRIRES at the VLT. From this first spectroscopic
detection of Triton's atmosphere in the infrared, we report (i) the first
observation of gaseous methane since its discovery in the ultraviolet by
Voyager in 1989 and (ii) the first ever detection of gaseous CO in the
satellite. The CO atmospheric abundance is remarkably similar to its surface
abundance, and appears to be controlled by a thin, CO-enriched, surface veneer
resulting from seasonal transport and/or atmospheric escape. The CH partial
pressure is several times larger than inferred from Voyager. This confirms that
Triton's atmosphere is seasonally variable and is best interpreted by the
warming of CH-rich icy grains as Triton passed southern summer solstice in
2000. The presence of CO in Triton's atmosphere also affects its temperature,
photochemistry and ionospheric composition. An improved upper limit on CO in
Pluto's atmosphere is also reported.Comment: 11 pages, including 4 figures and 2 on-line figures Astronomy and
Astrophysics, in press (accepted March 13, 2010
The deformation complex is a homotopy invariant of a homotopy algebra
To a homotopy algebra one may associate its deformation complex, which is
naturally a differential graded Lie algebra. We show that infinity
quasi-isomorphic homotopy algebras have L-infinity quasi-isomorphic deformation
complexes by an explicit construction.Comment: A revised version. The final version will appear in the volume
"Current Developments and Retrospectives in Lie Theory
Shearfree perfect fluids with solenoidal magnetic curvature and a gamma-law equation of state
We show that shearfree perfect fluids obeying an equation of state p=(gamma
-1) mu are non-rotating or non-expanding under the assumption that the spatial
divergence of the magnetic part of the Weyl tensor is zero.Comment: 11 page
Globular Clusters and Dwarf Spheroidal Galaxies
Traditionally globular clusters and dwarf spheroidal galaxies have been
distinguished by using one or more of the following criteria: (1) mass, (2)
luminosity, (3) size, (4) mass-to-light ratio and (5) spread in metallicity.
However, a few recently discovered objects show some overlap between the
domains in parameter space that are occupied by galaxies and clusters. In the
present note it is shown that ellipticity can, in some cases, be used to help
distinguish between globular clusters and dwarf spheroidal galaxies.Comment: MNRAS (Letters), in pres
Purely radiative perfect fluids
We study `purely radiative' (div E = div H = 0) and geodesic perfect fluids
with non-constant pressure and show that the Bianchi class A perfect fluids can
be uniquely characterized --modulo the class of purely electric and
(pseudo-)spherically symmetric universes-- as those models for which the
magnetic and electric part of the Weyl tensor and the shear are simultaneously
diagonalizable. For the case of constant pressure the same conclusion holds
provided one also assumes that the fluid is irrotational.Comment: 12 pages, minor grammatical change
Exploring the spatial, temporal, and vertical distribution of methane in Pluto's atmosphere
High-resolution spectra of Pluto in the 1.66 um region, recorded with the
VLT/CRIRES instrument in 2008 (2 spectra) and 2012 (5 spectra), are analyzed to
constrain the spatial and vertical distribution of methane in Pluto's
atmosphere and to search for mid-term (4 year) variability. A sensitivity study
to model assumptions (temperature structure, surface pressure, Pluto's radius)
is performed. Results indicate that (i) no variation of the CH4 atmospheric
content (column-density or mixing ratio) with Pluto rotational phase is present
in excess of 20 % (ii) CH4 column densities show at most marginal variations
between 2008 and 2012, with a best guess estimate of a ~20 % decrease over this
time frame. As stellar occultations indicate that Pluto's surface pressure has
continued to increase over this period, this implies a concomitant decrease of
the methane mixing ratio (iii) the data do not show evidence for an
altitude-varying methane distribution; in particular, they imply a roughly
uniform mixing ratio in at least the first 22-27 km of the atmosphere, and high
concentrations of low-temperature methane near the surface can be ruled out.
Our results are also best consistent with a relatively large (> 1180 km) Pluto
radius. Comparison with predictions from a recently developed global climate
model GCM indicates that these features are best explained if the source of
methane occurs in regional-scale CH4 ice deposits, including both low latitudes
and high Northern latitudes, evidence for which is present from the rotational
and secular evolution of the near-IR features due to CH4 ice. Our "best guess"
predictions for the New Horizons encounter in 2015 are: a 1184 km radius, a 17
ubar surface pressure, and a 0.44 % CH4 mixing ratio with negligible
longitudinal variations.Comment: 21 pages, 6 figure
Shear-free perfect fluids with a solenoidal electric curvature
We prove that the vorticity or the expansion vanishes for any shear-free
perfect fluid solution of the Einstein field equations where the pressure
satisfies a barotropic equation of state and the spatial divergence of the
electric part of the Weyl tensor is zero.Comment: 9 page
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