16,975 research outputs found
Multi-object spectroscopy of low redshift EIS clusters. I
We report the results of the first multi-object spectroscopic observations at
the Danish 1.54m telescope at La Silla, Chile. Observations of five cluster
candidates from the ESO Imaging Survey Cluster Candidate Catalog are described.
From these observations we confirm the reality of the five clusters with
measured redshifts of 0.11<=z<=0.35. We estimate velocity dispersions in the
range 294-621km/s indicating rather poor clusters. This, and the measured
cluster redshifts are consistent with the results of the matched filter
procedure applied to produce the Cluster Candidate Catalog.Comment: 7pages, accepted by Astronomy and Astrophysic
Outer boundary conditions for evolving cool white dwarfs
White dwarf evolution is essentially a gravothermal cooling process,
which,for cool white dwarfs, sensitively depends on the treatment of the outer
boundary conditions. We provide detailed outer boundary conditions appropriate
for computing the evolution of cool white dwarfs employing detailed non-gray
model atmospheres for pure H composition. We also explore the impact on the
white dwarf cooling times of different assumptions for energy transfer in the
atmosphere of cool white dwarfs. Detailed non-gray model atmospheres are
computed taken into account non-ideal effects in the gas equation of state and
chemical equilibrium, collision-induced absorption from molecules, and the
Lyman alpha quasi-molecular opacity. Our results show that the use of detailed
outer boundary conditions becomes relevant for effective temperatures lower
than 5800 and 6100K for sequences with 0.60 and 0.90 M_sun, respectively.
Detailed model atmospheres predict ages that are up to approx 10% shorter at
log L/L_sun=-4 when compared with the ages derived using Eddington-like
approximations at tau_Ross=2/3. We also analyze the effects of various
assumptions and physical processes of relevance in the calculation of outer
boundary conditions. In particular, we find that the Ly_alpha red wing
absorption does not affect substantially the evolution of white dwarfs. White
dwarf cooling timescales are sensitive to the surface boundary conditions for
T_eff < 6000K. Interestingly enough, non-gray effects have little consequences
on these cooling times at observable luminosities. In fact, collision-induced
absorption processes, which significantly affect the spectra and colors of old
white dwarfs with hydrogen-rich atmospheres, have not noticeable effects in
their cooling rates, except throughout the Rosseland mean opacity.Comment: 6 pages, 9 figures, to be published in Astronomy and Astrophysic
Properties of patchy colloidal particles close to a surface: a Monte Carlo and density functional study
We investigate the behavior of a patchy particle model close to a hard-wall
via Monte Carlo simulation and density functional theory (DFT). Two DFT
approaches, based on the homogeneous and inhomogeneous versions of Wertheim's
first order perturbation theory for the association free energy are used. We
evaluate, by simulation and theory, the equilibrium bulk phase diagram of the
fluid and analyze the surface properties for two isochores, one of which is
close to the liquid side of the gas-liquid coexistence curve. We find that the
density profile near the wall crosses over from a typical high-temperature
adsorption profile to a low-temperature desorption one, for the isochore close
to coexistence. We relate this behavior to the properties of the bulk network
liquid and find that the theoretical descriptions are reasonably accurate in
this regime. At very low temperatures, however, an almost fully bonded network
is formed, and the simulations reveal a second adsorption regime which is not
captured by DFT. We trace this failure to the neglect of orientational
correlations of the particles, which are found to exhibit surface induced
orientational order in this regime
The white dwarf cooling sequence of NGC 6791: a unique tool for stellar evolution
NGC 6791 is a well-studied, metal-rich open cluster that is so close to us
that can be imaged down to luminosities fainter than that of the termination of
its white dwarf cooling sequence, thus allowing for an in-depth study of its
white dwarf population. We use a Monte Carlo simulator that employs up-to-date
evolutionary cooling sequences for white dwarfs with hydrogen-rich and
hydrogen-deficient atmospheres, with carbon-oxygen and helium cores. The
cooling sequences for carbon-oxygen cores account for the delays introduced by
both Ne^22 sedimentation in the liquid phase and by carbon-oxygen phase
separation upon crystallization. We do not find evidence for a substantial
fraction of helium-core white dwarfs, and hence our results support the
suggestion that the origin of the bright peak of the white dwarf luminosity
function can only be attributed to a population of unresolved binary white
dwarfs. Moreover, our results indicate that the number distribution of
secondary masses of the population of unresolved binaries has to increase with
increasing mass ratio between the secondary and primary components of the
progenitor system. We also find that the observed cooling sequence appears to
be able to constrain the presence of progenitor sub-populations with different
chemical compositions and the fraction of non-DA white dwarfs. Our simulations
place interesting constraints on important characteristics of the stellar
populations of NGC 6791. In particular, we find that the fraction of single
helium-core white dwarfs must be smaller than 5%, that a sub-population of
stars with zero metallicity must be <12%, while if the adopted metallicity of
the sub-population is solar the upper limit is ~8%. Finally, we also find that
the fraction of non-DA white dwarfs in this particular cluster is surprinsingly
small <6%.Comment: 9 pages, 14 figures, accepted for publication in Astronomy &
Astrophysic
Thermally Pulsing Asymptotic Giant Branch Star Models and Globular Cluster Planetary Nebulae I: The Model
Thermally pulsing asymptotic giant branch models of globular cluster stars
are calculated using a synthetic model with the goal of reproducing the
chemical composition, core masses and other observational parameters of the
four known globular cluster planetary nebulae as well as roughly matching the
overall cluster properties. The evolution of stars with an enhanced helium
abundance () and blue stragglers are modeled. New pre-thermally pulsing
asymptotic giant branch mass-losses for red giant branch and early asymptotic
giant branch stars are calculated from the Padova stellar evolution models
\citep{berta,bertb}. The new mass-losses are calculated to get the relative
differences in mass-losses due to enhanced helium abundances.
The global properties of the globular cluster planetary nebula are reproduced
with these models. The metallicity, mass of the central star, overall
metallicities, helium abundance and the nebular mass are matched to the
observational values. Globular cluster planetary nebulae JaFu 1 and JaFu 2 are
reproduced {\it by assuming progenitor stars} with masses near the typical main
sequence turn-offs of globular clusters and with enhanced helium abundances
very similar to the enhancements inferred from fitting isochrones to globular
cluster colour-magnitude diagrams. The globular cluster PN GJJC-1 can be
roughly fit by a progenitor star with very extreme helium enhancement
() near the turn-off producing a central star with the same mass
as inferred by observations and a very low nebular mass. The abundances and
core mass of planetary nebula Ps 1 and its central star (K648) are reproduced
by a blue straggler model. However, it turned out to be impossible to reproduce
its nebular mass and it is concluded some kind of binary scenario may be needed
to explain K648.Comment: 14 pages, 8 figures, accepted for publication in MNRA
Globular Clusters around Galaxies in Groups
We have obtained deep photometry of NGC 1199 (in HCG 22) and NGC 6868 (in the
Telescopium group). Both galaxies are the optically brightest galaxies of their
groups. Our analysis of B and R images taken with the Keck II and the VLT/ESO
telescopes, detected a population of globular clusters around both galaxies,
with total specific frequencies S_N=1.7\pm0.6 for NGC 1199 and S_N = 1.3\pm0.6
for NGC 6868. The color distributions of the globular cluster systems shows
bimodal peaks centered at (B-R)_0 = 1.13\pm0.10 and 1.42\pm0.10 (NGC 1199) and
(B-R)_0=1.12\pm0.10 and 1.42\pm0.10 (NGC 6868).Comment: 3 pages, 1 figure. To appear in the proceedings of IAU Symp. 207,
"Extragalactic Star Clusters", eds. E. Grebel, D. Geisler, D. Minnit
Dynamic critical behavior of failure and plastic deformation in the random fiber bundle model
The random fiber bundle (RFB) model, with the strength of the fibers
distributed uniformly within a finite interval, is studied under the assumption
of global load sharing among all unbroken fibers of the bundle. At any fixed
value of the applied stress (load per fiber initially present in the bundle),
the fraction of fibers that remain unbroken at successive time steps is shown
to follow simple recurrence relations. The model is found to have stable fixed
point for applied stress in the range 0 and 1; beyond which total failure of
the bundle takes place discontinuously. The dynamic critical behavior near this
failure point has been studied for this model analysing the recurrence
relations. We also investigated the finite size scaling behavior. At the
critical point one finds strict power law decay (with time t) of the fraction
of unbroken fibers. The avalanche size distribution for this mean-field
dynamics of failure has been studied. The elastic response of the RFB model has
also been studied analytically for a specific probability distribution of fiber
strengths, where the bundle shows plastic behavior before complete failure,
following an initial linear response.Comment: 13 pages, 5 figures, extensively revised and accepted for publication
in Phys. Rev.
Avoiding deontic explosion by contextually restricting aggregation
In this paper, we present an adaptive logic for deontic conflicts, called P2.1(r), that is based on Goble's logic SDLaPe-a bimodal extension of Goble's logic P that invalidates aggregation for all prima facie obligations. The logic P2.1(r) has several advantages with respect to SDLaPe. For consistent sets of obligations it yields the same results as Standard Deontic Logic and for inconsistent sets of obligations, it validates aggregation "as much as possible". It thus leads to a richer consequence set than SDLaPe. The logic P2.1(r) avoids Goble's criticisms against other non-adjunctive systems of deontic logic. Moreover, it can handle all the 'toy examples' from the literature as well as more complex ones
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