1,641 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
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
gamma Doradus pulsation in two pre-main sequence stars discovered by CoRoT
Pulsations in pre-main sequence stars have been discovered several times
within the last years. But nearly all of these pulsators are of delta
Scuti-type. gamma Doradus-type pulsation in young stars has been predicted by
theory, but lack observational evidence. We present the investigation of
variability caused by rotation and (gammaDoradus-type) pulsation in two
pre-main sequence members of the young open cluster NGC2264 using
high-precision time series photometry from the CoRoT satellite and dedicated
high-resolution spectroscopy. Time series photometry of NGC2264VAS20 and NGC
2264VAS87 was obtained by the CoRoT satellite during the dedicated short run
SRa01 in March 2008. NGC2264VAS87 was re-observed by CoRoT during the short run
SRa05 in December 2011 and January 2012. Frequency analysis was conducted using
Period04 and SigSpec. The spectral analysis was performed using equivalent
widths and spectral synthesis. The frequency analysis yielded 10 and 14
intrinsic frequencies for NGC2264VAS20 and NGC2264VAS 87, respectively, in the
range from 0 to 1.5c/d which are attributed to be caused by a combination of
rotation and pulsation. The effective temperatures were derived to be
6380150K for NGC2264VAS20 and 6220150K for NGC2264VAS87. Membership
of the two stars to the cluster is confirmed independently using X-ray fluxes,
radial velocity measurements and proper motions available in the literature.
The derived Li abundances of log n(Li)=3.34 and 3.54 for NGC2264VAS20 and
NGC2264VAS87, respectively, are in agreement with the Li abundance for other
stars in NGC2264 of similar Teff reported in the literature. We conclude that
the two objects are members of NGC2264 and therefore are in their pre-main
sequence evolutionary stage. Assuming that part of their variability is caused
by pulsation, these two stars might be the first pre-main sequence gamma
Doradus candidates.Comment: 11 pages, 10 figures, A&A accepte
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&
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.
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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