1,230 research outputs found
Monte Carlo transition probabilities. II
The macroscopic quantizations of matter into macro-atoms and radiant and
thermal energies into r- and k-energy packets initiated in Paper I is completed
with the definition of transition probabilities governing energy flows to and
from the thermal pool. The resulting Monte Carlo method is then applied to the
problem of computing the hydrogen spectrum of a Type II supernova. This test
problem is used to demonstrate the scheme's consistency as the number of energy
packets N -> infinity, to investigate the accuracy of Monte Carlo estimators of
radiative rates, and to illustrate the convergence characteristics of the
geometry-independent, constrained Lambda-iteration method employed to obtain
the NLTE stratifications of temperature and level populations. In addition, the
method's potential, when combined with analytic ionization and excitation
formulae, for obtaining useful approximate NLTE solutions is emphasized.Comment: 17 pages, 4 figure
First exit times and residence times for discrete random walks on finite lattices
In this paper, we derive explicit formulas for the surface averaged first
exit time of a discrete random walk on a finite lattice. We consider a wide
class of random walks and lattices, including random walks in a non-trivial
potential landscape. We also compute quantities of interest for modelling
surface reactions and other dynamic processes, such as the residence time in a
subvolume, the joint residence time of several particles and the number of hits
on a reflecting surface.Comment: 19 pages, 2 figure
High-latitude artificial aurora using the EISCAT high-gain HF facility
The EISCAT high-frequency (HF) transmitter facility at Ramfjord, Norway, has been used to accelerate F-region electrons sufficiently to excite the oxygen atoms and nitrogen molecules, resulting in optical emissions at 630, 557.7 and 427.8 nm. During O-mode transmissions at 5.423 MHz, using 630 MW effective radiated power, in the hours after sunset on 12 November 2001 several new observations were made, including: (1) The first high-latitude observation of an HF induced optical emission at 427.8 nm and (2) Optical rings being formed at HF on followed by their collapse into a central blob. Both discoveries remain unexplained with current theories
Exact solution for random walks on the triangular lattice with absorbing boundaries
The problem of a random walk on a finite triangular lattice with a single
interior source point and zig-zag absorbing boundaries is solved exactly. This
problem has been previously considered intractable.Comment: 10 pages, Latex, IOP macro
Another Non-segregated Blue Straggler Population in a Globular Cluster: the Case of NGC 2419
We have used a combination of ACS-HST high-resolution and wide-field SUBARU
data in order to study the Blue Straggler Star (BSS) population over the entire
extension of the remote Galactic globular cluster NGC 2419. The BSS population
presented here is among the largest ever observed in any stellar system, with
more than 230 BSS in the brightest portion of the sequence. The radial
distribution of the selected BSS is essentially the same as that of the other
cluster stars. In this sense the BSS radial distribution is similar to that of
omega Centauri and unlike that of all Galactic globular clusters studied to
date, which have highly centrally segregated distributions and, in most cases,
a pronounced upturn in the external regions. As in the case of omega Centauri,
this evidence indicates that NGC 2419 is not yet relaxed even in the central
regions. This observational fact is in agreement with estimated half-mass
relaxation time, which is of the order of the cluster age.Comment: in press in the Ap
The Clusters AgeS Experiment (CASE). II. The Eclipsing Blue Straggler OGLEGC-228 in the Globular Cluster 47 Tuc
We use photometric and spectroscopic observations of the eclipsing binary
OGLEGC-228 (V228) to derive the masses, radii, and luminosities of the
component stars. Based on measured systemic velocity, proper motion and
distance, the system is a blue straggler member of the globular cluster 47 Tuc.
Our analysis shows that V228 is a semi-detached Algol. We obtain M=1.512 +/-
0.022 Msun, R=1.357 +/- 0.019 Rsun, L=7.02 +/- 0.050 Lsun for the hotter and
more luminous primary component and M=0.200 +/- 0.007 Msun, R=1.238 +/- 0.013
Rsun, L=1.57 +/- 0.09 Lsun for the Roche lobe filling secondary.Comment: 19 pages, 5 figures, AJ, in pres
The surprising external upturn of the Blue Straggler radial distribution in M55
By combining high-resolution HST and wide-field ground based observations, in
ultraviolet and optical bands, we study the Blue Straggler Star (BSS)
population of the low density galactic globular cluster M55 (NGC 6809) over its
entire radial extent. The BSS projected radial distribution is found to be
bimodal, with a central peak, a broad minimum at intermediate radii, and an
upturn at large radii. Similar bimodal distributions have been found in other
globular clusters (M3, 47 Tucanae, NGC 6752, M5), but the external upturn in
M55 is the largest found to date. This might indicate a large fraction of
primordial binaries in the outer regions of M55, which seems somehow in
contrast with the relatively low (\sim 10%) binary fraction recently measured
in the core of this cluster.Comment: in press on Ap
Relative Frequencies of Blue Stragglers in Galactic Globular Clusters: Constraints for the Formation Mechanisms
We discuss the main properties of the Galactic globular cluster (GC) blue
straggler stars (BSS), as inferred from our new catalog containing nearly 3000
BSS. The catalog has been extracted from the photometrically homogeneous V vs.
(B-V) color-magnitude diagrams (CMD) of 56 GCs, based on WFPC2 images of their
central cores. In our analysis we used consistent relative distances based on
the same photometry and calibration. The number of BSS has been normalized to
obtain relative frequencies (F_{BSS}) and specific densities (N_S) using
different stellar populations extracted from the CMD. The cluster F_{BSS} is
significantly smaller than the relative frequency of field BSS. We find a
significant anti-correlation between the BSS relative frequency in a cluster
and its total absolute luminosity (mass). There is no statistically significant
trend between the BSS frequency and the expected collision rate. F_{BSS} does
not depend on other cluster parameters, apart from a mild dependence on the
central density. PCC clusters act like normal clusters as far as the BSS
frequency is concerned. We also show that the BSS luminosity function for the
most luminous clusters is significantly different, with a brighter peak and
extending to brighter luminosities than in the less luminous clusters. These
results imply that the efficiency of BSS production mechanisms and their
relative importance vary with the cluster mass.Comment: 12 pages, 3 figures. accepted for publication in ApJ
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