10,527 research outputs found
Indications of a Large Fraction of Spectroscopic Binaries Among Nuclei of Planetary Nebulae
Previous work indicates that about 10% of planetary-nebula nuclei (PNNi) are
photometrically variable short-period binaries with periods of hours to a few
days. These systems have most likely descended from common-envelope (CE)
interactions in initially much wider binaries. Population-synthesis studies
suggest that these very close pairs could be the short-period tail of a much
larger post-CE binary population with periods of up to a few months. We have
initiated a radial-velocity (RV) survey of PNNi with the WIYN 3.5-m telescope
and Hydra spectrograph, which is aimed at discovering these intermediate-period
binaries. We present initial results showing that 10 out of 11 well-observed
PNNi have variable RVs, suggesting that a significant binary population may be
present. However, further observations are required because we have as yet been
unable to fit our sparse measurements with definite orbital periods, and
because some of the RV variability might be due to variations in the stellar
winds of some of our PNNi.Comment: 11 pages, 1 table, no figures. Accepted by the Astrophysical Journal
Letter
No evidence of dark matter in the solar neighborhood
We measured the surface mass density of the Galactic disk at the solar
position, up to 4 kpc from the plane,by means of the kinematics of ~400 thick
disk stars. The results match the expectations for the visible mass only, and
no dark matter is detected in the volume under analysis. The current models of
dark matter halo are excluded with a significance higher than 5sigma, unless a
highly prolate halo is assumed, very atypical in cold dark matter simulations.
The resulting lack of dark matter at the solar position challenges the current
models.Comment: Proceeding of the first binational Sochias-AAA meeting, held in San
Juan, Argentin
Security in Automatic Data Processing Systems
Security in automatic data processing systems is concerned with the protection of equipment and data against unauthorized modifications or destruction, and accidental or intentional disclosure of information. This report consists of two parts: 1. An overview of current literature in the field. In this part, basic problems and solutions, terminology and concepts found in the theoretical aspects of data processing systems security are described. 2. A survey of the state of art in a sample area. During the survey, information about actual applications of security methods in the Orlando, Florida area was collected. This information was gathered by means of a questionnaire and it will permit a comparative analysis between the usage of security measures in large and small firms. It is hoped that this study will provide valuable information to both the computer practitioner and management and in addition, will create an awareness of the important role that the security function plays in an organization
No evidence for a dark matter disk within 4 kpc from the Galactic plane
We estimated the dynamical surface mass density (Sigma) at the solar
Galactocentric distance between 2 and 4 kpc from the Galactic plane, as
inferred from the observed kinematics of the thick disk. We find Sigma(z=2
kpc)=57.6+-5.8 Mo pc^-2, and it shows only a tiny increase in the z-range
considered by our investigation. We compared our results with the expectations
for the visible mass, adopting the most recent estimates in the literature for
contributions of the Galactic stellar disk and interstellar medium, and
proposed models of the dark matter distribution. Our results match the
expectation for the visible mass alone, never differing from it by more than
0.8 $Mo pc^-2 at any z, and thus we find little evidence for any dark
component. We assume that the dark halo could be undetectable with our method,
but the dark disk, recently proposed as a natural expectation of the LambdaCDM
models, should be detected. Given the good agreement with the visible mass
alone, models including a dark disk are less likely, but within errors its
existence cannot be excluded. In any case, these results put constraints on its
properties: thinner models (scale height lower than 4 kpc) reconcile better
with our results and, for any scale height, the lower-density models are
preferred. We believe that successfully predicting the stellar thick disk
properties and a dark disk in agreement with our observations could be a
challenging theoretical task.Comment: Accepted for publication in ApJ Letter
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