11,148 research outputs found
Stellar populations in the dwarf spheroidal galaxy Leo I
We present a detailed study of the color magnitude diagram (CMD) of the dwarf
spheroidal galaxy Leo I, based on archival Hubble Space Telescope data. Our
photometric analysis, confirming previous results on the brighter portion of
the CMD, allow us to obtain an accurate sampling of the stellar populations
also at the faint magnitudes corresponding to the Main Sequence. By adopting a
homogeneous and consistent theoretical scenario for both hydrogen and central
helium-burning evolutionary phases, the various features observed in the CMD
are interpreted and reliable estimations for both the distance modulus and the
age(s) for the main stellar components of Leo I are derived. More in details,
from the upper luminosity of the Red Giant Branch and the lower luminosity of
the Subgiant Branch we simultaneously constrain the galaxy distance and the age
of the oldest stellar population in Leo I. In this way we obtain a distance
modulus (m-M)_V=22.000.15 mag and an age of 10--15 Gyr or 9--13 Gyr,
adopting a metallicity Z=0.0001 and 0.0004, respectively. The reliability of
this distance modulus has been tested by comparing the observed distribution of
the Leo I anomalous Cepheids in the period-magnitude diagram with the predicted
boundaries of the instability strip, as given by convective pulsating models.Comment: 19 pages, 3 tables, 14 figures To be published in A
RR LYRAE VARIABLE STARS: PULSATIONAL CONSTRAINTS RELEVANT TO THE OOSTERHOFF CONTROVERSY
A solution to the old Oosterhoff controversy is proposed on the basis of a
new theoretical pulsational scenario concerning RR Lyrae cluster variables
(Bono and coworkers). We show that the observed constancy of the lowest
pulsation period in both Oosterhoff type I (OoI) and Oosterhoff type II (OoII)
prototypes (M3, M15) can be easily reproduced only by assuming the canonical
evolutionary horizontal-branch luminosity levels of these Galactic globular
clusters and therefore by rejecting the Sandage period shift effect (SPSE).Comment: postscript file of 7 pages and 2 figures; one non postcript figure is
available upon request; for any problem please write to
[email protected]
Detecting the Stimulated Decay of Axions at Radio Frequencies
Assuming axion-like particles account for the entirety of the dark matter in
the Universe, we study the possibility of detecting their decay into photons at
radio frequencies. We discuss different astrophysical targets, such as dwarf
spheroidal galaxies, the Galactic Center and halo, and galaxy clusters. The
presence of an ambient radiation field leads to a stimulated enhancement of the
decay rate; depending on the environment and the mass of the axion, the effect
of stimulated emission may amplify the photon flux by serval orders of
magnitude. For axion-photon couplings allowed by astrophysical and laboratory
constraints(and possibly favored by stellar cooling), we find the signal to be
within the reach of next-generation radio telescopes such as the Square
Kilometer Array.Comment: Minor changes, references added, matches published versio
RR Lyrae variables in Galactic globular clusters: IV. Synthetic HB and RR Lyrae predictions
We present theoretical predictions concerning horizontal branch stars in
globular clusters, including RR Lyrae variables, as derived from synthetic
procedures collating evolutionary and pulsational constraints. On this basis,
we explore the predicted behavior of the pulsators as a function of the
horizontal branch morphology and over the metallicity range Z=0.0001 to 0.006,
revealing an encouraging concordance with the observed distribution of
fundamentalised periods with metallicity. Theoretical relations connecting
periods to K magnitudes and BV or VI Wesenheit functions are presented, both
appearing quite independent of the horizontal branch morphology only with Z
greater or equal than 0.001. Predictions concerning the parameter R are also
discussed and compared under various assumptions about the horizontal branch
reference luminosity level.Comment: 11 pages, 10 figures. Accepted for publication in "Astronomy and
Astrophysics
Fourier mode dynamics for the nonlinear Schroedinger equation in one-dimensional bounded domains
We analyze the 1D focusing nonlinear Schr\"{o}dinger equation in a finite
interval with homogeneous Dirichlet or Neumann boundary conditions. There are
two main dynamics, the collapse which is very fast and a slow cascade of
Fourier modes. For the cubic nonlinearity the calculations show no long term
energy exchange between Fourier modes as opposed to higher nonlinearities. This
slow dynamics is explained by fairly simple amplitude equations for the
resonant Fourier modes. Their solutions are well behaved so filtering high
frequencies prevents collapse. Finally these equations elucidate the unique
role of the zero mode for the Neumann boundary conditions
Developing and evaluating a five minute phishing awareness video
Confidence tricksters have always defrauded the unwary. The computer era has merely extended their range and made it possible for them to target anyone in the world who has an email address. Nowadays, they send phishing messages that are specially crafted to deceive. Improving user awareness has the potential to reduce their effectiveness. We have previously developed and empirically-validated phishing awareness programmes. Our programmes are specifically designed to neutralize common phish-related misconceptions and teach people how to detect phishes. Many companies and individuals are already using our programmes, but a persistent niggle has been the amount of time required to complete the awareness programme. This paper reports on how we responded by developing and evaluating a condensed phishing awareness video that delivered phishing awareness more efficiently. Having watched our video, participants in our evaluation were able to detect phishing messages significantly more reliably right after watching the video (compared to before watching the video). This ability was also demonstrated after a retention period of eight weeks after first watching the video
The distance to galactic globular clusters through RR Lyrae pulsational properties
By adopting the same approach outlined in De Santis & Cassisi (1999), we
evaluate the absolute bolometric magnitude of the Zero Age Horizontal Branch
(ZAHB) at the level of the RR Lyrae variable instability strip in selected
galactic globular clusters. This allows us to estimate the ZAHB absolute visual
magnitude for these clusters and to investigate its dependence on the cluster
metallicity. The derived relation, corrected in order to
account for the luminosity difference between the ZAHB and the mean RR Lyrae
magnitude, has been compared with some of the most recent empirical
determinations in this field, as the one provided by Baade-Wesselink analyses,
RR Lyrae periods, Hipparcos data for field variables and Main Sequence fitting
based on Hipparcos parallaxes for field subdwarfs. As a result, our relation
provides a clear support to the "long" distance scale. We discuss also another
method for measuring the distance to galactic globular clusters. This method is
quite similar to the one adopted for estimating the absolute bolometric
magnitude of the ZAHB but it relies only on the pulsational properties of the
Lyrae variables in each cluster. The reliability and accuracy of this method
has been tested by applying it to a sample of globulars for which, due to the
morphology of their horizontal branch (HB), the use of the commonly adopted
ZAHB fitting is a risky procedure.Comment: 7 pages, 2 postscript figures, accepted for publication on MNRA
On the helium content of Galactic globular clusters via the R parameter
We estimate the empirical R parameter in 26 Galactic Globular Clusters
covering a wide metallicity range, imaged by WFPC2 on board the HST. The
improved spatial resolution permits a large fraction of the evolved stars to be
measured and permits accurate assessment of radial populaton gradients and
completeness corrections. In order to evaluate both the He abundance and the He
to metal enrichment ratio, we construct a large set of evolutionary models by
adopting similar metallicities and different He contents. We find an absolute
He abundance which is lower than that estimated from spectroscopic measurements
in HII regions and from primordial nucleosynthesis models. This discrepancy
could be removed by adopting a C12O16 nuclear cross section about a factor of
two smaller than the canonical value, although also different assumptions for
mixing processes can introduce systematical effects. The trend in the R
parameter toward solar metallicity is consistent with an upper limit to the He
to metal enrichment ratio of the order of 2.5.Comment: accepted for pubblication on Ap
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