1,207 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
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&
HD 41641: A classical Sct-type pulsator with chemical signatures of an Ap star
Among the known groups of pulsating stars, Sct stars are one of the
least understood. Theoretical models do not predict the oscillation frequencies
that observations reveal. Complete asteroseismic studies are necessary to
improve these models and better understand the internal structure of these
targets. We study the Sct star HD 41641 with the ultimate goal of
understanding its oscillation pattern. The target was simultaneously observed
by the CoRoT space telescope and the HARPS high-resolution spectrograph. The
photometric data set was analyzed with the software package PERIOD04, while
FAMIAS was used to analyze the line profile variations. The method of spectrum
synthesis was used for spectroscopically determining the fundamental
atmospheric parameters and individual chemical abundances. A total of 90
different frequencies was identified and analyzed. An unambiguous
identification of the azimuthal order of the surface geometry could only be
provided for the dominant p-mode, which was found to be a nonradial prograde
mode with m = +1. Using and , we estimated the mass,
radius, and evolutionary stage of HD 41641. We find HD 41641 to be a moderately
rotating, slightly evolved Sct star with subsolar overall atmospheric
metal content and unexpected chemical peculiarities. HD 41641 is a pure
Sct pulsator with p-mode frequencies in the range from 10 d to
20 d. This pulsating star presents chemical signatures of an Ap star and
rotational modulation due to surface inhomogeneities, which we consider
indirect evidence of the presence of a magnetic field.Comment: 11 pages, 11 figures, accepted for publication in A&
Regular modes in rotating stars
Despite more and more observational data, stellar acoustic oscillation modes
are not well understood as soon as rotation cannot be treated perturbatively.
In a way similar to semiclassical theory in quantum physics, we use acoustic
ray dynamics to build an asymptotic theory for the subset of regular modes
which are the easiest to observe and identify. Comparisons with 2D numerical
simulations of oscillations in polytropic stars show that both the frequency
and amplitude distributions of these modes can accurately be described by an
asymptotic theory for almost all rotation rates. The spectra are mainly
characterized by two quantum numbers; their extraction from observed spectra
should enable one to obtain information about stellar interiors.Comment: 5 pages, 4 figures, discussion adde
An asteroseismic study of the O9V star HD 46202 from CoRoT space-based photometry
The O9V star HD 46202, which is a member of the young open cluster NGC 2244,
was observed by the CoRoT satellite in October/November 2008 during a short run
of 34 days. From the very high-precision light curve, we clearly detect beta
Cep-like pulsation frequencies with amplitudes of ~0.1 mmag and below. A
comparison with stellar models was performed using a chi^2 as a measure for the
goodness-of-fit between the observed and theoretically computed frequencies.
The physical parameters of our best-fitting models are compatible with the ones
deduced spectroscopically. A core overshooting parameter alpha_ov = 0.10 +-
0.05 pressure scale height is required. None of the observed frequencies are
theoretically excited with the input physics used in our study. More
theoretical work is thus needed to overcome this shortcoming in how we
understand the excitation mechanism of pulsation modes in such a massive star.
A similar excitation problem has also been encountered for certain pulsation
modes in beta Cep stars recently modelled asteroseismically.Comment: Accepted for publication in Astronomy and Astrophysics on 17/12/2010,
9 pages, 7 figures, 4 table
Modelling J/psi production and absorption in a microscopic nonequilibrium approach
Charmonium production and absorption in heavy ion collisions is studied with
the Ultrarelativisitic Quantum Molecular Dynamics model. We compare the
scenario of universal and time independent color-octet dissociation cross
sections with one of distinct color-singlet J/psi, psi' and chi_c states,
evolving from small, color transparent configurations to their asymptotic
sizes. The measured J/psi production cross sections in pA and AB collisions at
SPS energies are consistent with both - purely hadronic - scenarios. The
predicted rapidity dependence of J/psi suppression can be used to discriminate
between the two experimentally. The importance of interactions with secondary
hadrons and the applicability of thermal reaction kinetics to J/psi absorption
are investigated. We discuss the effect of nuclear stopping and the role of
leading hadrons. The dependence of the psi' to J/psi ratio on the model
assumptions and the possible influence of refeeding processes is also studied.Comment: 35 pages, 16 figure
The CoRoT B-type binary HD50230: a prototypical hybrid pulsator with g-mode period and p-mode frequency spacings
B-type stars are promising targets for asteroseismic modelling, since their
frequency spectrum is relatively simple.
We deduce and summarise observational constraints for the hybrid pulsator,
HD50230, earlier reported to have deviations from a uniform period spacing of
its gravity modes. The combination of spectra and a high-quality light curve
measured by the CoRoT satellite allow a combined approach to fix the position
of HD50230 in the HR diagram.
To describe the observed pulsations, classical Fourier analysis was combined
with short-time Fourier transformations and frequency spacing analysis
techniques. Visual spectra were used to constrain the projected rotation rate
of the star and the fundamental parameters of the target. In a first
approximation, the combined information was used to interpret multiplets and
spacings to infer the true surface rotation rate and a rough estimate of the
inclination angle.
We identify HD50230 as a spectroscopic binary and characterise the two
components. We detect the simultaneous presence of high-order g modes and
low-order p and g-modes in the CoRoT light curve, but were unable to link them
to line profile variations in the spectroscopic time series. We extract the
relevant information from the frequency spectrum, which can be used for seismic
modelling, and explore possible interpretations of the pressure mode spectrum.Comment: 26 pages, 12+6 figures, accepted for publication in Astronomy and
Astrophysic
Periodic mass loss episodes due to an oscillation mode with variable amplitude in the hot supergiant HD50064
We aim to interpret the photometric and spectroscopic variability of the
luminous blue variable supergiant HD\,50064 ().CoRoT space photometry
and follow-up high-resolution spectroscopy, with a time base of 137\,d and
169\,d, respectively, was gathered, analysed and interpreted using standard
time series analysis and light curve modelling methods as well as spectral line
diagnostics.The space photometry reveals one period of 37\,d, which undergoes a
sudden amplitude change with a factor 1.6. The pulsation period is confirmed in
the spectroscopy, which additionally reveals metal line radial velocity values
differing by km\,s depending on the spectral line and on the
epoch. We estimate \teff13\,500\,K, \logg1.5 from the equivalent
width of Si lines. The Balmer lines reveal that the star undergoes episodes of
changing mass loss on a time scale similar to the changes in the photometric
and spectroscopic variability, with an average value of (in M\,yr). We tentatively interpret the 37\,d
period as due to a strange mode oscillation.Comment: 4 pages, accepted for publication in Astronomy & Astrophysics Letter
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