7,411 research outputs found
Parameters of the crystalline undulator and its radiation for particular experimental conditions
We report the results of theoretical and numerical analysis of the
crystalline undulators planned to be used in the experiments which are the part
of the ongoing PECU project [1]. The goal of such an analysis was to define the
parameters (different from those pre-set by the experimental setup) of the
undulators which ensure the highest yield of photons of specified energies. The
calculations were performed for 0.6 and 10 GeV positrons channeling through
periodically bent Si and SiGe crystals.Comment: 13 pages, 8 figures, submitted to SPI
Close stars and an inactive accretion disk in Sgr A*: Eclipses and flares
A cold neutral and extremely dim accretion disk may be present as a remnant
of a past vigorous activity around the black hole in our Galactic Center (GC).
Here we discuss ways to detect such a disk through its interaction with
numerous stars present in the central ~0.1 parsec of the Galaxy. The first
major effect expected is X-ray and near infrared (NIR) flares arising when
stars pass through the disk. The second is eclipses of the stars by the disk.
We point out conditions under which the properties of the expected X-ray flares
are similar to those recently discovered by Chandra. Since orbits of bright
stars are now being precisely measured, the combination of the expected flares
and eclipses offers an invaluable tool for constraining the disk density, size,
plane and even direction of rotation. The winds of the O-type stars are
optically thick to free-free absorption in radio frequencies. If present near
Sgr A* core, such powerful stellar winds can modulate and even occult the radio
source.Comment: typo in eq. 3 correcte
Dynamical interpretation of chemical freeze-out in heavy ion collisions
It is demonstrated that there exists a direct correlation between chemical
freeze-out point and the softest point of the equation of state where the
pressure divided by the energy density, , has a minimum.
A dynamical model is given as an example where the passage of the softest point
coincides with the condition for chemical freeze-out, namely an average energy
per hadron 1 GeV. The sensitivity of the result to the equation of
state used is discussed.Comment: 10 pages, 2 figure
A Catalogue of Be Stars in the Direction of the Galactic Bulge
Detailed studies of Be stars in environments with different metallicities
like the Magellanic Clouds or the Galactic bulge are necessary to understand
the formation and evolution mechanisms of the circumstellar disks. However, a
detailed study of Be stars in the direction of the bulge of our own galaxy has
not been performed until now. We report the first systematic search for Be star
candidates in the direction of the Galactic Bulge. We present the catalogue,
give a brief description of the stellar variability seen, and show some light
curve examples. We searched for stars matching specific criteria of magnitude,
color and variability in the I band. Our search was conducted on the 48 OGLE II
fields of the Galactic Bulge.This search has resulted in 29053 Be star
candidates, 198 of them showing periodic light variations. Nearly 1500 stars in
this final sample are almost certainly Be stars, providing an ideal sample for
spectroscopic multiobject follow-up studies.Comment: Accepted for publication in A &
The evolution of the stellar populations in low surface brightness galaxies
We investigate the star formation history and chemical evolution of low
surface brightness (LSB) disk galaxies by modelling their observed
spectro-photometric and chemical properties using a galactic chemical and
photometric evolution model incorporating a detailed metallicity depen dent set
of stellar input data. For a large fraction of the LSB galaxies in our sample,
observed properties are best explained by models incorporating an exponentially
decreasing global star formation rate (SFR) ending at a present-day gas
fraction (M_{gas}/(M_{gas}+M_{stars}) = 0.5 for a galaxy age of 14 Gyr. For
some galaxies small amplitude star formation bursts are required to explain the
contribution of the young (5-50 Myr old) stellar population to the galaxy
integrated luminosity. This suggests that star formation has proceeded in a
stochastic manner.
The presence of an old stellar population in many late-type LSB galaxies
suggests that LSB galaxies roughly follow the same evolutionary history as HSB
galaxies, except at a much lower rate. In particular, our results imply that
LSB galaxies do not form late, nor have a delayed onset of star formation, but
simply evolve slowly.Comment: To be published in A&
LBT/MODS spectroscopy of globular clusters in the irregular galaxy NGC 4449
We present intermediate-resolution (R1000) spectra in the
3500-10,000 A range of 14 globular clusters in the magellanic irregular
galaxy NGC 4449 acquired with the Multi Object Double Spectrograph on the Large
Binocular Telescope. We derived Lick indices in the optical and the
CaII-triplet index in the near-infrared in order to infer the clusters' stellar
population properties. The inferred cluster ages are typically older than
9 Gyr, although ages are derived with large uncertainties. The clusters
exhibit intermediate metallicities, in the range
[Fe/H], and typically sub-solar []
ratios, with a peak at . These properties suggest that i) during the
first few Gyrs NGC 4449 formed stars slowly and inefficiently, with galactic
winds having possibly contributed to the expulsion of the -elements,
and ii) globular clusters in NGC 4449 formed relatively "late", from a medium
already enriched in the products of type Ia supernovae. The majority of
clusters appear also under-abundant in CN compared to Milky Way halo globular
clusters, perhaps because of the lack of a conspicuous N-enriched,
second-generation of stars like that observed in Galactic globular clusters.
Using the cluster velocities, we infer the dynamical mass of NGC 4449 inside
2.88 kpc to be M(2.88 kpc)=. We
also report the serendipitous discovery of a planetary nebula within one of the
targeted clusters, a rather rare event.Comment: Accepted for publication in MNRAS; corrected typo in author lis
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