4,608 research outputs found
Physics of the ALICE Experiment
A short description of the ALICE detector at CERN is given. The experiment is
aiming to study the properties of the quark-gluon plasma by means of a whole
set of probes that can be subdivided into three classes: soft, heavy-flavour
and high-Pt probes. Each of the classes is illustrated by a few typical
examples.Comment: Proceedings of the 41st Rencontres de Moriond, QCD and high energy
hadronic interactions, La Thuile, Italy, 18-25 March 200
Extragalactic Cepheid database
We present in this paper an exhaustive compilation of all published data of
extragalactic Cepheids. We have checked every light curve in order to
characterize the different types of Cepheid and detect potential overtone
pulsators, or to estimate the quality of the data. This compilation of about
3000 photometric measurements will constitute a very useful tool for
astronomers involved for instance in the extragalactic distance scale.Comment: Updated version of this database is now available through WWW at
http://www-obs.univ-lyon1.fr/~planoix/ECD/ . 1321 Cepheids located in 39
galaxies make up the base at the moment. One can also plot PL-relations and
compute distance moduli based on Hipparcos PL-relation
Equivalence Principle Violation in Weakly Vainshtein-Screened Systems
Massive gravity, galileon and braneworld models that modify gravity to
explain cosmic acceleration utilize the nonlinear field interactions of the
Vainshtein mechanism to screen fifth forces in high density regimes. These
source-dependent interactions cause apparent equivalence principle violations.
In the weakly-screened regime violations can be especially prominent since the
fifth forces are at near full strength. Since they can also be calculated
perturbatively, we derive analytic solutions for illustrative cases: the motion
of massive objects in compensated shells and voids and infall toward halos that
are spherically symmetric. Using numerical techniques we show that these
solutions are valid until the characteristic scale becomes comparable to the
Vainshtein radius. We find a relative acceleration of more massive objects
toward the center of a void and a reduction of the infall acceleration that
increases with the mass ratio of the halos which can in principle be used to
test the Vainshtein screening mechanism.Comment: 7 pages, 4 figure
Simulation of a method to directly image exoplanets around multiple stars systems
Direct imaging of extra-solar planets has now become a reality, especially
with the deployment and commissioning of the first generation of specialized
ground-based instruments such as the GPI, SPHERE, P1640 and SCExAO. These
systems will allow detection of planets 1e7 times fainter than their host star.
For space-based missions, such as EXCEDE, EXO-C, EXO-S, WFIRST-AFTA,
different teams have shown in laboratories contrasts reaching 1e-10 within a
few diffraction limits from the star using a combination of a coronagraph to
suppress light coming from the host star and a wavefront control system. These
demonstrations use a deformable mirror (DM) to remove residual starlight
(speckles) created by the imperfections of telescope. However, all these
current and future systems focus on detecting faint planets around a single
host star or unresolved binaries/multiples, while several targets or planet
candidates are located around nearby binary stars such as our neighbor star
Alpha Centauri.
Until now, it has been thought that removing the light of a companion star is
impossible with the current technology, excluding binary star systems from
target lists of direct imaging missions. Direct imaging around binaries or
multiples systems at a level of contrast allowing Earth-like planets detection
is challenging because the region of interest, where a dark zone is essential,
is contaminated by the light coming from the host star's companion. We propose
a method to simultaneously correct aberration sand diffraction of light coming
from the target star. This method works even if the companion star is outside
the control region of the DM (beyond its half-Nyquist frequency), by taking
advantage of aliasing effects.Comment: 8 pages, 13 figures, SPIE Astronomical Telescope and Instrumentation
conferenc
Searching For Dark Matter Subhalos In the Fermi-LAT Second Source Catalog
The dark matter halo of the Milky Way is expected to contain an abundance of
smaller subhalos. These subhalos can be dense and produce potentially
observable fluxes of gamma rays. In this paper, we search for dark matter
subhalo candidates among the sources in the Fermi-LAT Second Source Catalog
which are not currently identified or associated with counterparts at other
wavelengths. Of the nine high-significance, high-latitude (|b|>60 degrees),
non-variable, unidentified sources contained in this catalog, only one or two
are compatible with the spectrum of a dark matter particle heavier than
approximately 50-100 GeV. The majority of these nine sources, however, feature
a spectrum that is compatible with that predicted from a lighter (~5-40 GeV)
dark matter particle. This population is consistent with the number of
observable subhalos predicted for a dark matter candidate in this mass range
and with an annihilation cross section of a simple thermal relic (sigma
v~3x10^{-26} cm^3/s). Observations in the direction of these sources at other
wavelengths will be necessary to either reveal their astrophysical nature (as
blazars or other active galactic nuclei, for example), or to further support
the possibility that they are dark matter subhalos by failing to detect any
non-gamma ray counterpart.Comment: 8 pages, 4 figure
Pre-Main sequence Turn-On as a chronometer for young clusters: NGC346 as a benchmark
We present a novel approach to derive the age of very young star clusters, by
using the Turn-On (TOn). The TOn is the point in the color-magnitude diagram
(CMD) where the pre-main sequence (PMS) joins the main sequence (MS). In the MS
luminosity function (LF) of the cluster, the TOn is identified as a peak
followed by a dip. We propose that by combining the CMD analysis with the
monitoring of the spatial distribution of MS stars it is possible to reliably
identify the TOn in extragalactic star forming regions. Compared to alternative
methods, this technique is complementary to the turn-off dating and avoids the
systematic biases affecting the PMS phase. We describe the method and its
uncertainties, and apply it to the star forming region NGC346, which has been
extensively imaged with the Hubble Space Telescope (HST). This study extends
the LF approach in crowded extragalactic regions and opens the way for future
studies with HST/WFC3, JWST and from the ground with adaptive optics.Comment: 6 pages, 4 figures. Accepted for publication in ApJ Letter
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