610 research outputs found
Slovac Republic repository of radioactive waste
The Slovac Republic Repository of Radioactive Waste (radwaste) in place Mochovce presents a multi-barrier repository of the surface type designed as an ultimate storage of treated solid and fixed, low-and very low-level radwaste generated during the operation and decommissioning of the nuclear power plants, in research institutes, laboratories and hospitals in the Slovak Republic. The isolation of the radwaste and retardation of the radionuclides are provided by the barrier system of the repository. To assess the complete system and parts of one of the most important barriers â the multi-barrier ultimate shielding of the repository â the model of the ultimate shielding of the repository was designed. The monitoring results of the model â in situâ will be applicable for projecting the ultimate shielding of the repository
Gamma-Ray Emission from PWNe Interacting with Molecular Clouds
We consider a situation in which a pulsar (and its nebula) is formed inside
or close to a high density regions of a molecular cloud. We apply a recent
model for the gamma radiation of pulsar wind nebulae (PWN), which includes not
only radiation processes due to injected leptons but also processes due to
injection of relativistic hadrons, in order to calculate the expected gamma-ray
emission from such interacting PWNe. The example calculations have been
performed for two objects of this type from which directions TeV gamma-ray
sources have recently been observed (IC443 and W41). We show that the gamma-ray
emission below a few TeV can be produced by leptons accelerated in the past in
the vicinity of the pulsars. gamma-rays with energies above ~10 TeV can be
produced by hadrons interacting with the matter inside the supernova remnant
and surrounding dense clouds. In contrary to the low energy TeV emission, this
high energy TeV emission should be correlated with the location of dense clouds
able to capture hadrons due to their strong magnetic fields.Comment: Proceedings of the 30th ICRC, Merida, Mexico, 2007, in pres
Dynamical constraints on the origin of the young B-stars in the Galactic center
Regular star formation is thought to be inhibited close to the massive black
hole (MBH) in the Galactic center. Nevertheless, tens of young main sequence B
stars have been observed in an isotropic distribution close to it. Various
models have been suggested for the formation of the B-stars closest to the MBH
(<0.05 pc; the S-stars), typically involving the migration of these stars from
their original birthplace to their currently observed position. Here we explore
the orbital phase space distribution of the B-stars throughout the central pc
expected from the various suggested models for the origin of the B-stars. We
find that most of these models have difficulties in explaining, by themselves,
both the population of the S-stars (<0.05 pc), and the population of the young
B-stars further away (up to 0.5 pc). Most models grossly over-predict the
number of B-stars up to 0.5 pc, given the observed number of S-stars. Such
models include the intermediate-mass black hole assisted cluster inspiral
scenario, Kozai-like perturbations by two disks, spiral density waves migration
in a gaseous disk, and some of the eccentric disk instability models. We focus
on one of the other models, the massive perturber induced binary disruption,
which is consistent with both the S-stars and the extended population of
B-stars further away. For this model we use analytical arguments and N-body
simulations to provide further observational predictions. These could be
compared with future observations to further support this model, constrain it
or refute it. These predictions include the radial distribution of the young
B-stars, their eccentricity distribution and its dependence on distance from
the MBH (higher eccentricities at larger distances from the MBH), as well as
less specific expectations regarding their mass function.Comment: Comments are welcome
Slovac Republic repository of radioactive waste
The Slovac Republic Repository of Radioactive Waste (radwaste) in place Mochovce presents a multi-barrier repository of the surface type designed as an ultimate storage of treated solid and fixed, low-and very low-level radwaste generated during the operation and decommissioning of the nuclear power plants, in research institutes, laboratories and hospitals in the Slovak Republic. The isolation of the radwaste and retardation of the radionuclides are provided by the barrier system of the repository. To assess the complete system and parts of one of the most important barriers â the multi-barrier ultimate shielding of the repository â the model of the ultimate shielding of the repository was designed. The monitoring results of the model â in situâ will be applicable for projecting the ultimate shielding of the repository
NACO/SAM observations of sources at the Galactic Center
Sparse aperture masking (SAM) interferometry combined with Adaptive Optics
(AO) is a technique that is uniquely suited to investigate structures near the
diffraction limit of large telescopes. The strengths of the technique are a
robust calibration of the Point Spread Function (PSF) while maintaining a
relatively high dynamic range. We used SAM+AO observations to investigate the
circumstellar environment of several bright sources with infrared excess in the
central parsec of the Galaxy. For our observations, unstable atmospheric
conditions as well as significant residuals after the background subtraction
presented serious problems for the standard approach of calibrating SAM data
via interspersed observations of reference stars. We circumvented these
difficulties by constructing a synthesized calibrator directly from sources
within the field-of-view. When observing crowded fields, this novel method can
boost the efficiency of SAM observations because it renders interspersed
calibrator observations unnecessary. Here, we presented the first NaCo/SAM
images reconstructed using this method.Comment: 8 pages, 10 figures, proceedings of the conference "Astrophysics at
High Angular Resolution" (AHAR-2011
Dynamical evolution of the young stars in the Galactic center: N-body simulations of the S-stars
We use N-body simulations to study the evolution of the orbital
eccentricities of stars deposited near (<0.05 pc) the Milky Way massive black
hole (MBH), starting from initial conditions motivated by two competing models
for their origin: formation in a disk followed by inward migration; and
exchange interactions involving a binary star. The first model predicts modest
eccentricities, lower than those observed in the S-star cluster, while the
second model predicts higher eccentricities than observed. The N-body
simulations include a dense cluster of 10 M_sun stellar black holes (SBHs),
expected to accumulate near the MBH by mass segregation. Perturbations from the
SBHs tend to randomize the stellar orbits, partially erasing the dynamical
signatures of their origin. The eccentricities of the initially highly
eccentric stars evolve, in 20 Myr (the S-star lifespan), to a distribution that
is consistent at the ~95 % level with the observed eccentricity distribution.
In contrast, the eccentricities of the initially more circular orbits fail to
evolve to the observed values in 20 Myr, arguing against the disk migration
scenario. We find that 20 % - 30 % of the S-stars are tidally disrupted by the
MBH over their lifetimes, and that the S-stars are not likely to be ejected as
hypervelocity stars outside the central 0.05 pc by close encounters with
stellar black holes.Comment: 6 pages, 2 figures. Minor corrections, Sumitted to Ap
Secular evolution of compact binaries near massive black holes: gravitational wave sources and other exotica
The environment near super massive black holes (SMBHs) in galactic nuclei
contain a large number of stars and compact objects. A fraction of these are
likely to be members of binaries. Here we discuss the binary population of
stellar black holes and neutron stars near SMBHs and focus on the secular
evolution of such binaries, due to the perturbation by the SMBH. Binaries with
highly inclined orbits in respect to their orbit around the SMBH are strongly
affected by secular Kozai processes, which periodically change their
eccentricities and inclinations (Kozai-cycles). During periapsis approach, at
the highest eccentricities during the Kozai-cycles, gravitational wave emission
becomes highly efficient. Some binaries in this environment can inspiral and
coalesce at timescales much shorter than a Hubble time and much shorter than
similar binaries which do not reside near a SMBH. The close environment of
SMBHs could therefore serve as catalyst for the inspiral and coalescence of
binaries, and strongly affect their orbital properties. Such compact binaries
would be detectable as gravitational wave (GW) sources by the next generation
of GW detectors (e.g. advanced- LIGO). About 0.5% of such nuclear merging
binaries will enter the LIGO observational window while on orbit that are still
very eccentric (e>~0.5). The efficient gravitational wave analysis for such
systems would therefore require the use of eccentric templates. We also find
that binaries very close to the MBH could evolve through a complex dynamical
(non-secular) evolution leading to emission of several GW pulses during only a
few yrs (though these are likely to be rare). Finally, we note that the
formation of close stellar binaries, X-ray binaries and their merger products
could be induced by similar secular processes, combined with tidal friction
rather than GW emission as in the case of compact object binaries.Comment: 15 pages, 7 Figures. ApJ accepte
The S-Star Cluster at the Center of the Milky Way: On the nature of diffuse NIR emission in the inner tenth of a parsec
Sagittarius A*, the super-massive black hole at the center of the Milky Way,
is surrounded by a small cluster of high velocity stars, known as the S-stars.
We aim to constrain the amount and nature of stellar and dark mass associated
with the cluster in the immediate vicinity of Sagittarius A*. We use
near-infrared imaging to determine the -band luminosity function
of the S-star cluster members, and the distribution of the diffuse background
emission and the stellar number density counts around the central black hole.
This allows us to determine the stellar light and mass contribution expected
from the faint members of the cluster. We then use post-Newtonian N-body
techniques to investigate the effect of stellar perturbations on the motion of
S2, as a means of detecting the number and masses of the perturbers. We find
that the stellar mass derived from the -band luminosity
extrapolation is much smaller than the amount of mass that might be present
considering the uncertainties in the orbital motion of the star S2. Also the
amount of light from the fainter S-cluster members is below the amount of
residual light at the position of the S-star cluster after removing the bright
cluster members. If the distribution of stars and stellar remnants is strongly
enough peaked near Sagittarius A*, observed changes in the orbital elements of
S2 can be used to constrain both their masses and numbers. Based on simulations
of the cluster of high velocity stars we find that at a wavelength of 2.2
m close to the confusion level for 8 m class telescopes blend stars will
occur (preferentially near the position of Sagittarius A*) that last for
typically 3 years before they dissolve due to proper motions.Comment: 14 pages, 11 figures, minor changes to match the published version in
Astronomy & Astrophysic
Advanced Camera for Surveys Observations of Young Star Clusters in the Interacting Galaxy UGC 10214
We present the first Advanced Camera for Surveys (ACS) observations of young
star clusters in the colliding/merging galaxy UGC 10214. The observations were
made as part of the Early Release Observation (ERO) program for the newly
installed ACS during service mission SM3B for the Hubble Space Telescope (HST).
Many young star clusters can be identified in the tails of UGC 10214, with ages
ranging from ~3 Myr to 10 Myr. The extreme blue V-I (F606W-F814W) colors of the
star clusters found in the tail of UGC 10214 can only be explained if strong
emission lines are included with a young stellar population. This has been
confirmed by our Keck spectroscopy of some of these bright blue stellar knots.
The most luminous and largest of these blue knots has an absolute magnitude of
M_V = -14.45, with a half-light radius of 161 pc, and if it is a single star
cluster, would qualify as a super star cluster (SSC). Alternatively, it could
be a superposition of multiple scaled OB associations or clusters. With an
estimated age of ~ 4-5 Myr, its derived mass is < 1.3 x 10^6 solar masses. Thus
the young stellar knot is unbound and will not evolve into a normal globular
cluster. The bright blue clusters and associations are much younger than the
dynamical age of the tail, providing strong evidence that star formation occurs
in the tail long after it was ejected. UGC 10214 provides a nearby example of
processes that contributed to the formation of halos and intra-cluster media in
the distant and younger Universe.Comment: 6 pages with embedded figures, ApJ in pres
The Luminosity Function of Early-Type Galaxies at z~0.75
We measure the luminosity function of morphologically selected E/S0 galaxies
from to using deep high resolution Advanced Camera for Surveys
imaging data. Our analysis covers an area of 48\Box\arcmin (8 the
area of the HDF-N) and extends 2 magnitudes deeper ( mag) than was
possible in the Deep Groth Strip Survey (DGSS). At , we find
and , and at
, we find . These luminosity
functions are similar in both shape and number density to the luminosity
function using morphological selection (e.g., DGSS), but are much steeper than
the luminosity functions of samples selected using morphological proxies like
the color or spectral energy distribution (e.g., CFRS, CADIS, or COMBO-17). The
difference is due to the `blue', , E/S0 galaxies, which make up to
of the sample at all magnitudes and an increasing proportion of faint
galaxies. We thereby demonstrate the need for {\it both morphological and
structural information} to constrain the evolution of galaxies.
We find that the `blue' E/S0 galaxies have the same average sizes and Sersic
parameters as the `red', , E/S0 galaxies at brighter luminosities
(), but are increasingly different at fainter magnitudes where
`blue' galaxies are both smaller and have lower Sersic parameters. Fits of the
colors to stellar population models suggest that most E/S0 galaxies have short
star-formation time scales ( Gyr), and that galaxies have formed at an
increasing rate from until after which there has been a
gradual decline.Comment: 39 pages, 21 figures, accepted in A
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