35,901 research outputs found
On the kinematic evolution of young local associations and the Sco-Cen complex
Context: Over the last decade, several groups of young (mainly low-mass)
stars have been discovered in the solar neighbourhood (closer than ~100 pc),
thanks to cross-correlation between X-ray, optical spectroscopy and kinematic
data. These young local associations -- including an important fraction whose
members are Hipparcos stars -- offer insights into the star formation process
in low-density environments, shed light on the substellar domain, and could
have played an important role in the recent history of the local interstellar
medium. Aims: To study the kinematic evolution of young local associations and
their relation to other young stellar groups and structures in the local
interstellar medium, thus casting new light on recent star formation processes
in the solar neighbourhood. Methods: We compiled the data published in the
literature for young local associations. Using a realistic Galactic potential
we integrated the orbits for these associations and the Sco-Cen complex back in
time. Results: Combining these data with the spatial structure of the Local
Bubble and the spiral structure of the Galaxy, we propose a recent history of
star formation in the solar neighbourhood. We suggest that both the Sco-Cen
complex and young local associations originated as a result of the impact of
the inner spiral arm shock wave against a giant molecular cloud. The core of
the giant molecular cloud formed the Sco-Cen complex, and some small cloudlets
in a halo around the giant molecular cloud formed young local associations
several million years later. We also propose a supernova in young local
associations a few million years ago as the most likely candidate to have
reheated the Local Bubble to its present temperature.Comment: 27 pages, 13 figures, 14 tables. Accepted for publication in
Astronomy & Astrophysic
Puzzles in quarkonium hadronic transitions with two pion emission
The anomalously large rates of some hadronic transitions from quarkonium are
studied using QCD multipole expansion (QCDME) in the framework of a constituent
quark model which has been successful in describing hadronic phenomenology. The
hybrid intermediate states needed in the QCDME method are calculated in a
natural extension of our constituent quark model based on the Quark Confining
String (QCS) scheme. Some of the anomalies are explained due to the presence of
an hybrid state with a mass near the mass of the decaying resonance whereas
other are justified by the presence of molecular components in the wave
function. Some unexpected results are pointed out.Comment: Conference proceedings of the XI Quark Confinement and the Hadron
Spectrum (CONFINEMENT XI). Saint Petersburg (Russia) from 8 to 12 September
201
Nonleptonic decays and the nature of the orbitally excited charmed-strange mesons
The Belle Collaboration has recently reported a study of the decays and has given also estimates of relevant
ratios between branching fractions of decays
providing important information to check the structure of the
, and mesons. The
disagreement between experimental data and Heavy Quark Symmetry has been used
as an indication that and mesons could
have a more complex structure than the canonical one. We analyze
these ratios within the framework of a constituent quark model, which allows us
to incorporate the effects given by finite -quark mass corrections. Our
findings are that while the meson could have a sizable
non- component, the and mesons
seem to be well described by a pure structure.Comment: 13 pages, 1 figur
Critical behavior of long straight rigid rods on two-dimensional lattices: Theory and Monte Carlo simulations
The critical behavior of long straight rigid rods of length (-mers) on
square and triangular lattices at intermediate density has been studied. A
nematic phase, characterized by a big domain of parallel -mers, was found.
This ordered phase is separated from the isotropic state by a continuous
transition occurring at a intermediate density . Two analytical
techniques were combined with Monte Carlo simulations to predict the dependence
of on , being . The first involves
simple geometrical arguments, while the second is based on entropy
considerations. Our analysis allowed us also to determine the minimum value of
(), which allows the formation of a nematic phase on a
triangular lattice.Comment: 23 pages, 5 figures, to appear in The Journal of Chemical Physic
Status report of the baseline collimation system of CLIC. Part II
Important efforts have recently been dedicated to the characterisation and
improvement of the design of the post-linac collimation system of the Compact
Linear Collider (CLIC). This system consists of two sections: one dedicated to
the collimation of off-energy particles and another one for betatron
collimation. The energy collimation system is further conceived as protection
system against damage by errant beams. In this respect, special attention is
paid to the optimisation of the energy collimator design. The material and the
physical parameters of the energy collimators are selected to withstand the
impact of an entire bunch train. Concerning the betatron collimation section,
different aspects of the design have been optimised: the transverse collimation
depths have been recalculated in order to reduce the collimator wakefield
effects while maintaining a good efficiency in cleaning the undesired beam
halo; the geometric design of the spoilers has been reviewed to minimise
wakefields; in addition, the optics design has been optimised to improve the
collimation efficiency. This report presents the current status of the the
post-linac collimation system of CLIC. Part II is mainly dedicated to the study
of the betatron collimation system and collimator wakefield effects.Comment: 25 pages, 13 figure
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