16,015 research outputs found
Modulation of galactic protons in the heliosphere during the unusual solar minimum of 2006 to 2009
The last solar minimum activity period, and the consequent minimum modulation
conditions for cosmic rays, was unusual. The highest levels of galactic protons
were recorded at Earth in late 2009 in contrast to expectations. Proton spectra
observed for 2006 to 2009 from the PAMELA cosmic ray detector on-board the
Resurs-DK1 satellite are presented together with the solutions of a
comprehensive numerical model for the solar modulation of cosmic rays. The
model is used to determine what mechanisms were mainly responsible for the
modulation of protons during this period, and why the observed spectrum for
2009 was the highest ever recorded. From mid-2006 until December 2009 we find
that the spectra became significantly softer because increasingly more low
energy protons had reached Earth. To simulate this effect, the rigidity
dependence of the diffusion coefficients had to decrease significantly below ~3
GeV. The modulation minimum period of 2009 can thus be described as relatively
more "diffusion dominated" than previous solar minima. However, we illustrate
that drifts still had played a significant role but that the observable
modulation effects were not as well correlated with the waviness of the
heliospheric current sheet as before. Protons still experienced global gradient
and curvature drifts as the heliospheric magnetic field had decreased
significantly until the end of 2009, in contrast to the moderate decreases
observed during previous minimum periods. We conclude that all modulation
processes contributed to the observed increases in the proton spectra for this
period, exhibiting an intriguing interplay of these major mechanisms
Massive young clusters in the disc of M31
We have studied the properties of a sample of 67 very blue and likely young
massive clusters in M31 extracted from the Bologna Revised Catalog of globular
clusters, selected according to their color [(B-V) < 0.45] and/or to the
strength of their Hbeta spectral index (Hbeta > 3.5 A). Their existence in M31
has been noted by several authors in the past; we show here that these Blue
Luminous Compact Clusters (BLCCs) are a significant fraction (>~ 15%) of the
whole globular cluster system of M31. Compared to the global properties of the
M31 globular cluster system, they appear to be intrinsically fainter,
morphologically less concentrated, and with a shallower Balmer jump and
enhanced absorption in their spectra.
Empirical comparison with integrated properties of clusters with known age as
well as with theoretical SSP models consistently indicate that their typical
age is less than ~2 Gyr, while they probably are not so metal-poor as deduced
if considered to be old. Either selecting BLCCs by their (B-V) colors or by the
strength of their Hbeta index the cluster sample turns out to be distributed
onto the outskirts of M31 disc, sharing the kinematical properties of the thin,
rapidly rotating disc component.
If confirmed to be young and not metal-poor, these clusters indicate the
occurrence of a significant recent star formation in the thin disc of M31,
although they do not set constraints on the epoch of its early formation.Comment: Submitted for publication in the Astronomical Journal. Aastex Latex
file of 22 pages, 12 figures and 3 table
Kaon physics with the KLOE detector
In this paper we discuss the recent finalized analyses by the KLOE experiment
at DANE: the CPT and Lorentz invariance test with entangled pairs, and the precision measurement of the branching fraction of
the decay . We also present the
status of an ongoing analysis aiming to precisely measure the mass
Theory of continuum percolation II. Mean field theory
I use a previously introduced mapping between the continuum percolation model
and the Potts fluid to derive a mean field theory of continuum percolation
systems. This is done by introducing a new variational principle, the basis of
which has to be taken, for now, as heuristic. The critical exponents obtained
are , and , which are identical with the mean
field exponents of lattice percolation. The critical density in this
approximation is \rho_c = 1/\ve where \ve = \int d \x \, p(\x) \{ \exp [-
v(\x)/kT] - 1 \}. p(\x) is the binding probability of two particles
separated by \x and v(\x) is their interaction potential.Comment: 25 pages, Late
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