20,331 research outputs found
CCD and photon-counting photometric observations of asteroids carried out at Padova and Catania observatories
We present the results of observational campaigns of asteroids performed at
Asiago Station of Padova Astronomical Observatory and at M.G. Fracastoro
Station of Catania Astrophysical Observatory, as part of the large research
programme on Solar System minor bodies undertaken since 1979 at the Physics and
Astronomy Department of Catania University. Photometric observations of six
Main-Belt asteroids (27 Euterpe, 173 Ino, 182 Elsa, 539 Pamina, 849 Ara, and
984 Gretia), one Hungaria (1727 Mette), and two Near-Earth Objects (3199
Nefertiti and 2004 UE) are reported. The first determination of the synodic
rotational period of 2004 UE was obtained. For 182 Elsa and 1727 Mette the
derived synodic period of 80.23+/-0.08 h and 2.981+/-0.001 h, respectively,
represents a significant improvement on the previously published values. For
182 Elsa the first determination of the H-G magnitude relation is also
presented.Comment: 19 pages, 11 figures, accepted for publication in Planetary and Space
Scienc
Comment on ``Neutrino oscillations in the early universe: how can large lepton asymmetry be generated?"
We comment on the recent paper by A. D. Dolgov, S. H. Hansen, S. Pastor and
D. V. Semikoz (DHPS) [Astropart. Phys. {\bf 14}, 79 (2000)] on the generation
of neutrino asymmetries from active-sterile neutrino oscillations. We
demonstrate that the approximate asymmetry evolution equation obtained therein
is an expansion, up to a minor discrepancy, of the well-established static
approximation equation, valid only when the supposedly new higher order
correction term is small. In the regime where this so-called ``back-reaction''
term is large and artificially terminates the asymmetry growth, their evolution
equation ceases to be a faithful approximation to the Quantum Kinetic Equations
(QKEs) simply because pure Mikheyev-Smirnov-Wolfenstein (MSW) transitions have
been neglected. At low temperatures the MSW effect is the dominant asymmetry
amplifier. Neither the static nor the DHPS approach contains this important
physics. Therefore we conclude that the DHPS results have sufficient veracity
at the onset of explosive asymmetry generation, but are invalid in the ensuing
low temperature epoch where MSW conversions are able to enhance the asymmetry
to values of order . DHPS do claim to find a significant final
asymmetry for very large values. However, for this regime the
effective potential they employed is not valid.Comment: RevTeX, 32 pages, including 4 embedded figures; this version to
appear in Astropart.Phy
Neutrino oscillations in the early universe. Resonant case
Lepton asymmetry generated in the early universe by neutrino oscillations
into sterile partners is calculated. Kinetic equations are analytically reduced
to a simple form that permits an easy numerical treatment. Asymptotic values of
the asymmetry are at the level of 0.2-0.3 and are reasonably close to those
obtained by other groups, though the approach to asymptotics in some cases is
noticeably slower. No chaoticity is observed.Comment: 25 pages, late
Transits and Lensing by Compact Objects in the Kepler Field: Disrupted Stars Orbiting Blue Stragglers
Kepler's first major discoveries are two hot objects orbiting stars in its
field. These may be the cores of stars that have each been eroded or disrupted
by a companion star. The companion, which is the star monitored today, is
likely to have gained mass from its now-defunct partner, and can be considered
to be a blue straggler. KOI-81 is almost certainly the product of stable mass
transfer; KOI-74 may be as well, or it may be the first clear example of a blue
straggler created throughthree-body interactions.
We show that mass transfer binaries are common enough that Kepler should
discover ~1000 white dwarfs orbiting main sequence stars. Most, like KOI-74 and
KOI-81, will be discovered through transits, but many will be discovered
through a combination of gravitational lensing and transits, while lensing will
dominate for a subset. In fact, some events caused by white dwarfs will have
the appearance of "anti-transits" --i.e., short-lived enhancements in the
amount of light received from the monitored star. Lensing and other mass
measurements methods provide a way to distinguish white dwarf binaries from
planetary systems. This is important for the success of Kepler's primary
mission, in light of the fact that white dwarf radii are similar to the radii
of terrestrial planets, and that some white dwarfs will have orbital periods
that place them in the habitable zones of their stellar companions. By
identifying transiting and/or lensing white dwarfs, Kepler will conduct
pioneering studies of white dwarfs and of the end states of mass transfer. It
may also identify orbiting neutron stars or black holes. The calculations
inspired by the discovery of KOI-74 and KOI-81 have implications for
ground-based wide-field surveys as well as for future space-based surveys.Comment: 29 pages, 6 figures, 1 table; submitted to The Astrophysical Journa
Parameters affecting ion intensities in transmission-mode Direct Analysis in Real-Time mass spectrometry
A survey of the effect of temperature, transmission module material and analysis time on ion intensities in transmission mode direct analysis in real time mass spectrometry is presented. Ion intensity profiles obtained for two related compounds are similar when analysed separately but are very different when analysed as a mixture
X-ray Images of Hot Accretion Flows
We consider the X-ray emission due to bremsstrahlung processes from hot, low
radiative-efficiency accretion flows around supermassive and galactic black
holes. We calculate surface brightness profiles and Michelson visibility
functions for a range of density profiles, rho ~ r^(-3/2+p), with 0 < p < 1, to
allow for the presence of outflows. We find that although the 1 keV emitting
region in these flows can always extend up to 10^6 Schwarzschild radii (R_S),
their surface brightness profiles and visibility functions are strongly
affected by the specific density profile. The advection-dominated solutions
with no outflows (p=0) lead to centrally peaked profiles with characteristic
sizes of only a few tens of R_S. Solutions with strong outflows (p~1) lead to
flat intensity profiles with significantly larger characteristic sizes of up to
10^6 R_S. This implies that low luminosity galactic nuclei, such as M87, may
appear as extended X-ray sources when observed with current X-ray imaging
instruments. We show that X-ray brightness profiles and their associated
visibility functions may be powerful probes for determining the relevant mode
of accretion and, in turn, the properties of hot accretion flows. We discuss
the implications of our results for observations with the Chandra X-ray
Observatory and the planned X-ray interferometer MAXIM.Comment: 14 pages, 4 figures, accepted by The Astrophysical Journal, minor
change
Synthetic properties of bright metal-poor variables. II. BL Her stars
We investigate the properties of the so-called BL Her stars, i.e., Population
II Cepheids with periods shorter than 8 days, using updated pulsation models
and evolutionary tracks computed adopting a metal abundance in the range of
Z=0.0001 to Z=0.004. We derive the predicted Period-Magnitude (PM) and
Period-Wesenheit (PW) relations at the various photometric bands and we show
that the slopes of these relations are in good agreement with the slopes
determined by observed variables in Galactic globular clusters, independently
of the adopted M_V(RR)-[Fe/H] relation to get the cluster RR Lyrae-based
distance. Moreover, we show that also the distances provided by the predicted
PM and PW relations for BL Her stars agree within the errors with the RR Lyrae
based values. The use of the predicted relations with W Vir stars, which are
Population II Cepheids with periods longer than 8 days, provides no clear
evidence for or against a change in the PM and PW slopes around P~10 days.Comment: 12 pages, 6 figures, 6 tables, accepted for publication in A&A on
date 16/05/200
Curvature Matrix Models for Dynamical Triangulations and the Itzykson-DiFrancesco Formula
We study the large-N limit of a class of matrix models for dually weighted
triangulated random surfaces using character expansion techniques. We show that
for various choices of the weights of vertices of the dynamical triangulation
the model can be solved by resumming the Itzykson-Di Francesco formula over
congruence classes of Young tableau weights modulo three. From this we show
that the large-N limit implies a non-trivial correspondence with models of
random surfaces weighted with only even coordination number vertices. We
examine the critical behaviour and evaluation of observables and discuss their
interrelationships in all models. We obtain explicit solutions of the model for
simple choices of vertex weightings and use them to show how the matrix model
reproduces features of the random surface sum. We also discuss some general
properties of the large-N character expansion approach as well as potential
physical applications of our results.Comment: 37 pages LaTeX; Some clarifying comments added, last Section
rewritte
Branched polymers, complex spins and the freezing transition
We show that by coupling complex three-state systems to branched-polymer like
ensembles we can obtain models with gamma-string different from one half. It is
also possible to study the interpolation between dynamical and crystalline
graphs for these models; we find that only when geometry fluctuations are
completely forbidden is there a crystalline phase.Comment: 14 pages plain LateX2e, 4 eps figures included using eps
RR Lyrae-based calibration of the Globular Cluster Luminosity Function
We test whether the peak absolute magnitude Mv(TO) of the Globular Cluster
Luminosity Function (GCLF) can be used for reliable extragalactic distance
determinations. Starting with the luminosity function of the Galactic Globular
Clusters listed in Harris catalog, we determine Mv(TO) either using current
calibrations of the absolute magnitude Mv(RR) of RR Lyrae stars as a function
of the cluster metal content [Fe/H] and adopting selected cluster samples. We
show that the peak magnitude is slightly affected by the adopted Mv(RR)-[Fe/H]
relation, while it depends on the criteria to select the cluster sample. As for
the GCLFs in other external galaxies, using Surface Brightness Fluctuations
(SBF) measurements we give evidence that the luminosity functions of the blue
(metal-poor) Globular Clusters peak at the same luminosity within ~0.2 mag,
whereas for the red (metal-rich) samples the agreement is within ~0.5 mag even
accounting for the theoretical metallicity correction expected for clusters
with similar ages and mass distributions. Then, using the SBF absolute
magnitudes provided by a Cepheid distance scale calibrated on a fiducial
distance to LMC (m(LMC)=18.50 mag), we show that the Mv(TO) value of the
metal-poor clusters in external galaxies(-7.67+/-0.23 mag) is in excellent
agreement with the value of both Galactic (-7.66+/-0.11 mag) and M31
(-7.65+/-0.19 mag)ones.Comment: 13 pages, 8 figures, 8 tables, accepted for publication on MNRA
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