10,061 research outputs found
Notes on planar semimodular lattices. I. Construction
We construct all planar semimodular lattices in three simple steps from the
direct product of two chains.Comment: 13 pages with 9 diagram
Circumstellar shells and mass loss rates: Clues to the evolution of S stars
It is the purpose of this paper to rediscuss the circumstellar properties of
S stars and to put these properties in perspective with our current
understanding of the evolutionary status of S stars, in particular the
intrinsic/extrinsic dichotomy. Accordingly, an extensive data set probing the
circumstellar environment of S stars (IRAS flux densities, maser emission, CO
rotational lines) has been collected and critically evaluated. This data set
combines new observations (9 stars have been observed in the CO J=2-1 line and
3 in the CO J=3-2 line, with four new detections) with existing material (all
CO and maser observations of S stars published in the literature). The IRAS
flux densities of S stars have been re-evaluated by co-adding the individual
scans, in order to better handle the intrinsic variability of these stars in
the IRAS bands, and possible contamination by Galactic cirrus.
Mass loss rates or upper limits have been derived for all S stars observed in
the CO rotational lines, and range from < 2 10^{-8} Msun y^{-1} for extrinsic S
stars to 10^{-5} Msun y^{-1}. These mass-loss rates correlate well with the K -
[12] color index, which probes the dust loss rate, provided that the mass loss
rate be larger than 10^{-8} Msun~y^{-1}. Small mass-loss rates are found for
extrinsic S stars, consistent with their not being so evolved (RGB or
Early-AGB) as the Tc-rich S stars. This result does not support the claim often
made in relation with symbiotic stars that binarity strongly enhances the
mass-loss rate.Comment: Astronomy & Astrophysics Suppl., 40 pages, 22 figures, 6 tables
(LaTeX). Also available at: http://astro.ulb.ac.be/Htm/ps.ht
Measurement of the sizes of circumstellar dust shells around evolved stars with high mass loss rates
The research supported by the NASA ADP contract NAG5-1153 has been completed. The attached paper, which will be submitted for publication in the Astrophysical Journal in January 1992, presents the results of this work. Here is a summary of the project and its results. A set of computer programs was developed to process the raw 60 micron and 100 micron IRAS survey data. The programs were designed to detect faint extended emission surrounding a bright unresolved source. Candidate objects were chosen from a list of red giant stars and young planetary nebulae which have been detected in millimeter/submillimeter lines of CO. Of the 279 stars examined, 55 were resolved at 60 microns. The principle results of the study are given. The average age for the shells surrounding the 9 Mira-type stars which are extended is 6 x 10(exp 4) yr. This suggests that the period during which these stars lose mass lasts for approx 10(exp 5) yr. The oldest shell found surrounds U Ori, and the youngest surrounds Mira itself. Some shells appear to be detached from the central star. This phenomenon is more common among older stars, suggesting that the mass loss becomes more episodic as the star sheds its envelope. Although all 8 stars less distant than 200 pc are resolved in the IRAS 60 micron data, 29 stars within 500 pc were not. These stars probably have younger circumstellar shells than those which were resolved. Almost all the carbon stars with distances of 500 pc or less have resolved shells, while only 1/2 of the oxygen-rich stars do. The resolved carbon star shells also are older on average than the oxygen-rich ones. These facts imply that carbon stars have been losing mass for a longer period, on average, than oxygen-rich red giants. Large circumstellar shells tend to be found at large distances from the galactic plane, confirming that the ISM density limits the size to which a dust shell can grow. Surprisingly, even very large shells seem to be nearly spherical, and do not appear to be distorted by ram-pressure caused by the star's motion with respect to the ISM. Radiative transfer models and the value of I sub 60 microns/I sub 100 microns allow the average dust temperature in the outer regions of a circumstellar shell to be estimated. The typical value obtained in about 35 K
CO observations of the expanding envelope of IRC plus 10216
High-sensitivity emission profiles were observed for the transition of C12O16 and C13O16 towards IRC + or - 10216. It appears that the spherically symmetric uniform mass-outflow model proposed by Morris is necessary to describe the line profiles. The outflow appears to be slightly accelerated, having a velocity of 15 km/sec at the edges of the CO cloud, compared with 12 km/sec for the more centrally confined molecules
Cosmic-Ray Events as Background in Imaging Atmospheric Cherenkov Telescopes
The dominant background for observations of gamma-rays in the energy region
above 50 GeV with Imaging Atmospheric Cherenkov telescopes are cosmic-ray
events. The images of most of the cosmic ray showers look significantly
different from those of gamma-rays and are therefore easily discriminated.
However, a small fraction of events seems to be indistinguishable from
gamma-rays. This constitutes an irreducible background to the observation of
high-energy gamma-ray sources, and limits the sensitivity achievable with a
given instrument. Here, a Monte Carlo study of gamma-like cosmic-ray events is
presented. The nature of gamma-like cosmic-ray events, the shower particles
that are responsible for the gamma-like appearance, and the dependence of these
results on the choice of the hadronic interaction model are investigated. Most
of the gamma-like cosmic ray events are characterised by the production of
high-energy pi0's early in the shower development which dump most of the shower
energy into electromagnetic sub-showers. Also Cherenkov light from single muons
can mimic gamma-rays in close-by pairs of telescopes. Differences of up to 25%
in the collection area for gamma-like proton showers between QGSJet/FLUKA and
Sibyll/FLUKA simulations have been found.Comment: Accepted by Journal of Astroparticle Physic
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