1,632 research outputs found
Population of the lower part of the instability strip: Delta Scuti stars and dwarf Cepheids (or AI Velorum)
Some of the properties of the atmospheric variations in delta Scuti stars were investigated with emphasis on the amplitude and the shape of both light curves and radial velocity curves. It is shown that these curves are small and rapidly variable in the case of dwarf Scuti stars; for the evolved stars the situation is more complex. The relation between variables and nonvariables, and also the results on abundances in the atmospheres of these stars were surveyed with respect to the hydrodynamics of their envelopes. The abundance anomalies of Am stars were qualitatively examined. The coexistence of abundance anomalies and variability among giants were also studied. Attempts were made to relate the variability to the hydrogen ionization zone in an envelope deprived of helium. Specific results are reported
J/Psi production in proton induced collisions at FAIR
We have examined the production of J/ mesons in high energy
proton-proton and proton-nucleus collisions at beam energies in the range from
158 GeV to 920 GeV, available from different fixed target experiments. In the
employed model J/ production in hadronic collisions is assumed to be a
factorisable two step process: (i) production of a pair which can be
reliably described by perturbative QCD, and (ii) formation of J/
resonance from the pair, which can be conveniently parameterized
incorporating different existing physical mechanisms of color neutralization.
We show that, for lower collision energies, J/ production through
quark-anti-quark annihilation gives larger contribution at higher , while
gluon-gluon fusion dominates the production at smaller . For
proton-nucleus collisions the model takes into account both the initial state
modification of parton distributions in nuclei and the final state interaction
of the produced pairs with the target nucleons. The model is found
to give reasonable description of data on J/ production in protonic and
proton-nucleus collisions, for different existing fixed target experiments. In
case of proton-nucleus collisions, our calculations show a non-negligible
dependence of the final state nuclear dissociation of J/ mesons on the
energy of the incident proton beam. The model has been applied to predict the
J/ production and suppression expected in proton-nucleus collisions at
energies relevant to FAIR, the upcoming accelerator facility at Darmstadt,
Germany. The amount of suppressions, for different mechanisms of J/
hadronization has been found to be distinguishably different which might help
an experimental settlement of the much controversial issue of color
neutralization.Comment: 17 pages, 37 figures. To appear in Physical Review
Colour Screening, Quark Propagation in Nuclear Matter and the Broadening of the Momentum Distribution of Drell-Yan Pairs
We calculate the broadening of the transverse momentum distribution of a
quark propagating through nuclear matter. Colour screening plays a fundamental
role in that it cuts off quark-nucleon interactions with soft gluons. The mean
transverse momentum of the quark acquired along its trajectory, observed via
Drell-Yan pairs, is related to it the ratio of the total inelastic
meson-nucleon cross section it to the meson mean squared radius. Parameter-free
calculations agree with the data.Comment: LaTex file, 7 pages + 2 figs (not included, available by fax)
Heidelberg , HD-TVP-93-
Wave chaos in rapidly rotating stars
Effects of rapid stellar rotation on acoustic oscillation modes are poorly
understood. We study the dynamics of acoustic rays in rotating polytropic stars
and show using quantum chaos concepts that the eigenfrequency spectrum is a
superposition of regular frequency patterns and an irregular frequency subset
respectively associated with near-integrable and chaotic phase space regions.
This opens new perspectives for rapidly rotating star seismology and also
provides a new and potentially observable manifestation of wave chaos in a
large scale natural system.Comment: 5 pages, 3 figures; accepted for publication in Phys. Rev.
Asymptotic and measured large frequency separations
With the space-borne missions CoRoT and Kepler, a large amount of
asteroseismic data is now available. So-called global oscillation parameters
are inferred to characterize the large sets of stars, to perform ensemble
asteroseismology, and to derive scaling relations. The mean large separation is
such a key parameter. It is therefore crucial to measure it with the highest
accuracy. As the conditions of measurement of the large separation do not
coincide with its theoretical definition, we revisit the asymptotic expressions
used for analysing the observed oscillation spectra. Then, we examine the
consequence of the difference between the observed and asymptotic values of the
mean large separation. The analysis is focused on radial modes. We use series
of radial-mode frequencies to compare the asymptotic and observational values
of the large separation. We propose a simple formulation to correct the
observed value of the large separation and then derive its asymptotic
counterpart. We prove that, apart from glitches due to stellar structure
discontinuities, the asymptotic expansion is valid from main-sequence stars to
red giants. Our model shows that the asymptotic offset is close to 1/4, as in
the theoretical development. High-quality solar-like oscillation spectra
derived from precise photometric measurements are definitely better described
with the second-order asymptotic expansion. The second-order term is
responsible for the curvature observed in the \'echelle diagrams used for
analysing the oscillation spectra and this curvature is responsible for the
difference between the observed and asymptotic values of the large separation.
Taking it into account yields a revision of the scaling relations providing
more accurate asteroseismic estimates of the stellar mass and radius.Comment: accepted in A&
Charmonium Suppression by Comover Scattering in Pb+Pb Collisions
The first reports of and production from experiment NA50 at
the CERN SPS are compared to predictions based on a hadronic model of
charmonium suppression. Data on centrality dependence and total cross sections
are in good accord with these predictions.Comment: 9 pages, latex, 6 figures, epsf, figure added and text modified to
clarify result
Evaluating learning theory-based methods for improving the learning outcomes of introductory statistics courses
Modern introductory statistics courses have undergone significant change in recent times. The move towards emphasising more data and concepts has seen a dramatic increase in the use of technology and innovative learning methods. However, while many of these changes have been informed by learning theories, their efficacy has not been thoroughly evaluated. This thesis reports the findings of three major projects that have evaluated theory-based interventions aimed at improving key learning outcomes in introductory statistics courses. With the growing importance of technological skills required by modern notions of statistical literacy, Part I assesses the use of active-exploratory training approaches for the development of statistical package skills using experimental and qualitative methodologies. In Part II, the use of short lecture-based conceptual change activities for correcting common statistical misconceptions are evaluated in a prospective cohort study. Part III explores the use of the online virtual environment, known as the Island, for project-based learning aimed at developing students’ statistical thinking using both survey and experimental studies. The outcomes of these major parts provide valuable insight into the importance of evaluation research in statistics education and the challenges it presents to researchers. The findings discussed build upon statistics education research and suggest promising directions for future inquiry
Revisiting CoRoT RR Lyrae stars: detection of period doubling and temporal variation of additional frequencies
We search for signs of period doubling in CoRoT RR Lyrae stars. The
occurrence of this dynamical effect in modulated RR Lyrae stars might help us
to gain more information about the mysterious Blazhko effect. The temporal
variability of the additional frequencies in representatives of all subtypes of
RR Lyrae stars is also investigated. We pre-process CoRoT light curves by
applying trend and jump correction and outlier removal. Standard Fourier
technique is used to analyze the frequency content of our targets and follow
the time dependent phenomena. The most comprehensive collection of CoRoT RR
Lyrae stars, including new discoveries is presented and analyzed. We found
alternating maxima and in some cases half-integer frequencies in four CoRoT
Blazhko RR Lyrae stars, as clear signs of the presence of period doubling. This
reinforces that period doubling is an important ingredient to understand the
Blazhko effect - a premise we derived previously from the Kepler RR Lyrae
sample. As expected, period doubling is detectable only for short time
intervals in most modulated RRab stars. Our results show that the temporal
variability of the additional frequencies in all RR Lyrae sub-types is
ubiquitous. The ephemeral nature and the highly variable amplitude of these
variations suggest a complex underlying dynamics of and an intricate interplay
between radial and possibly nonradial modes in RR Lyrae stars. The omnipresence
of additional modes in all types of RR Lyrae - except in non-modulated RRab
stars - implies that asteroseismology of these objects should be feasible in
the near future (Abridged).Comment: 20 pages, 13 figures, accepted for publication in A&
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