38,563 research outputs found
Probing the Atmospheres of Planets Orbiting Microlensed Stars via Polarization Variability
We present a new method to identify and probe planetary companions of stars
in the Galactic Bulge and Magellanic Clouds using gravitational microlensing.
While spectroscopic studies of these planets is well beyond current
observational techniques, monitoring polarization fluctuations during high
magnification events induced by binary microlensing events will probe the
composition of the planetary atmospheres, an observation which otherwise is
currently unattainable even for nearby planetary systems.Comment: 7 pages, 2 figures. To appear in Astrophysical Journal Letter
A census of massive stars in NGC 346. Stellar parameters and rotational velocities
Spectroscopy for 247 stars towards the young cluster NGC 346 in the Small
Magellanic Cloud has been combined with that for 116 targets from the
VLT-FLAMES Survey of Massive Stars. Spectral classification yields a sample of
47 O-type and 287 B-type spectra, while radial-velocity variations and/or
spectral multiplicity have been used to identify 45 candidate single-lined
systems, 17 double-lined systems, and one triple-lined system. Atmospheric
parameters (T and log) and projected rotational velocities
(sin) have been estimated using TLUSTY model atmospheres; independent
estimates of sin were also obtained using a Fourier Transform method.
Luminosities have been inferred from stellar apparent magnitudes and used in
conjunction with the T and sin estimates to constrain stellar
masses and ages using the BONNSAI package. We find that targets towards the
inner region of NGC 346 have higher median masses and projected rotational
velocities, together with smaller median ages than the rest of the sample.
There appears to be a population of very young targets with ages of less than 2
Myr, which have presumably all formed within the cluster. The more massive
targets are found to have lower sin consistent with previous studies.
No significant evidence is found for differences with metallicity in the
stellar rotational velocities of early-type stars, although the targets in the
SMC may rotate faster than those in young Galactic clusters. The rotational
velocity distribution for single non-supergiant B-type stars is inferred and
implies that a significant number have low rotational velocity (10\%
with <40 km/s), together with a peak in the probability distribution at
300 km/s. Larger projected rotational velocity estimates have been
found for our Be-type sample and imply that most have rotational velocities
between 200-450 km/s.Comment: Accepted by A&
Qualitative Theory for Lensed QSOs
We show that some characteristics of multiply-imaged QSO systems are very
model-independent and can be deduced accurately by simply scrutinizing the
relative positions of images and galaxy-lens center. These include the
time-ordering of the images, the orientation of the lens potential, and the
rough morphology of any ring. Other features can differ considerably between
specific models; H_0 is an example. Surprisingly, properties inherited from a
circularly symmetric lens system are model-dependent, whereas features that
arise from the breaking of circular symmetry are model-independent. We first
develop these results from some abstract geometrical ideas, then illustrate
them for some well-known systems (the quads Q2237+030, H1413+117,
HST14113+5211, PG1115+080, MG0414+0534, B1608+656, B1422+231, and RXJ0911+0551,
and the ten-image system B1933+507), and finally remark on two systems
(B1359+154 and PMN J0134-0931) where the lens properties are more complex. We
also introduce a Java applet which produces simple lens systems, and helps
further illustrate the concepts.Comment: 26 pages, incl. 15 figs; accepted to AJ; java applet available at
http://ankh-morpork.maths.qmw.ac.uk/~saha/astron/lens
Lensing effect on the relative orientation between the Cosmic Microwave Background ellipticities and the distant galaxies
The low redshift structures of the Universe act as lenses in a similar way on
the Cosmic Microwave Background light and on the distant galaxies (say at
redshift about unity). As a consequence, the CMB temperature distortions are
expected to be statistically correlated with the galaxy shear, exhibiting a
non-uniform distribution of the relative angle between the CMB and the galactic
ellipticities. Investigating this effect we find that its amplitude is as high
as a 10% excess of alignement between CMB and the galactic ellipticities
relative to the uniform distribution. The relatively high signal-to-noise ratio
we found should makes possible a detection with the planned CMB data sets,
provided that a galaxy survey follow up can be done on a sufficiently large
area. It would provide a complementary bias-independent constraint on the
cosmological parameters.Comment: 7 pages, 3 figures; uses emulateapj.sty; submitted to Ap
Mass-detection of a matter concentration projected near the cluster Abell 1942: Dark clump or high-redshift cluster?
A weak-lensing analysis of wide-field - and -band images centered on
the cluster Abell 1942 has uncovered a mass concentration arcminutes
South of the cluster center. A statistical analysis shows that the detections
are highly significant. No strong concentration of bright galaxies is seen at
the position of the mass concentration, though a slight galaxy number
overdensity and a weak extended X-ray source are present about 1' away from its
center.
From the spatial dependence of the tangential alignment around the center of
the mass concentration, we inferred a lower bound on the mass inside a sphere
of radius \ts Mpc of , much higher
than crude mass estimates based on X-ray data. No firm conclusion can be
inferred about the nature of the clump. If it were a high-redshift cluster, the
weak X-ray flux would indicate that it had an untypically low X-ray luminosity
for its mass; if the X-ray emission were physically unrelated to the mass
concentration, this conclusion would be even stronger.
The search for massive halos by weak lensing enables us for the first time to
select halos based on their mass properties only and to detect new types of
objects, e.g., dark halos. The mass concentration in the field of A1942 may be
the first example of such a halo.Comment: Sumitted to A&A Main Journal. 15 pages, 11 figures. 75 Kb gzipped tar
file. Figures with images not included, but available on ftp.iap.fr
/pub/from_users/mellier/A1942: a1942darkclump.ps.gz (2.1 Mb
The non-Gaussian tail of cosmic-shear statistics
Due to gravitational instability, an initially Gaussian density field
develops non-Gaussian features as the Universe evolves. The most prominent
non-Gaussian features are massive haloes, visible as clusters of galaxies. The
distortion of high-redshift galaxy images due to the tidal gravitational field
of the large-scale matter distribution, called cosmic shear, can be used to
investigate the statistical properties of the LSS. In particular, non-Gaussian
properties of the LSS will lead to a non-Gaussian distribution of cosmic-shear
statistics. The aperture mass () statistics, recently introduced as
a measure for cosmic shear, is particularly well suited for measuring these
non-Gaussian properties. In this paper we calculate the highly non-Gaussian
tail of the aperture mass probability distribution, assuming Press-Schechter
theory for the halo abundance and the `universal' density profile of haloes as
obtained from numerical simulations. We find that for values of
much larger than its dispersion, this probability distribution is closely
approximated by an exponential, rather than a Gaussian. We determine the
amplitude and shape of this exponential for various cosmological models and
aperture sizes, and show that wide-field imaging surveys can be used to
distinguish between some of the currently most popular cosmogonies. Our study
here is complementary to earlier cosmic-shear investigations which focussed
more on two-point statistical properties.Comment: 9 pages, 5 figures, submitted to MNRA
Query processing of spatial objects: Complexity versus Redundancy
The management of complex spatial objects in applications, such as geography and cartography,
imposes stringent new requirements on spatial database systems, in particular on efficient
query processing. As shown before, the performance of spatial query processing can be improved
by decomposing complex spatial objects into simple components. Up to now, only decomposition
techniques generating a linear number of very simple components, e.g. triangles or trapezoids, have
been considered. In this paper, we will investigate the natural trade-off between the complexity of
the components and the redundancy, i.e. the number of components, with respect to its effect on
efficient query processing. In particular, we present two new decomposition methods generating
a better balance between the complexity and the number of components than previously known
techniques. We compare these new decomposition methods to the traditional undecomposed representation
as well as to the well-known decomposition into convex polygons with respect to their
performance in spatial query processing. This comparison points out that for a wide range of query
selectivity the new decomposition techniques clearly outperform both the undecomposed representation
and the convex decomposition method. More important than the absolute gain in performance
by a factor of up to an order of magnitude is the robust performance of our new decomposition
techniques over the whole range of query selectivity
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