753 research outputs found
Variable stars in the Fornax dSph Galaxy. II. Pulsating stars below the horizontal branch
We have carried out an intensive survey of the northern region of the Fornax
dwarf spheroidal galaxy with the aim of detecting the galaxy's short--period
pulsating stars (P<0.25 days). Observations collected over three consecutive
nights with the Wide Field Imager of the 2.2m MPI telescope at ESO allowed us
to detect 85 high-amplitude (0.20-1.00 mag in B-light) variable stars with
periods in the range from 0.046 to 0.126 days, similar to SX Phoenicis stars in
Galactic metal-poor stellar populations. The plots of the observed periods vs.
the B and V magnitudes show a dispersion largely exceeding the observational
errors. To disentangle the matter, we separated the first-overtone from the
fundamental-mode pulsators and tentatively identified a group of subluminous
variables, about 0.35 mag fainter than the others. Their nature as either
metal-poor intermediate-age stars or stars formed by the merging of close
binary systems is discussed. The rich sample of the Fornax variables also led
us to reconstruct the Period-Luminosity relation for short-period pulsating
stars. An excellent linear fit, M(V)=-1.83(+/-0.08)-3.65(+/-0.07) log P(fund),
was obtained using 153 Delta Scuti and SX Phoenicis stars in a number of
different stellar systems.Comment: 11 pages plus 1 on-line figure and 1 on-line table; accepted for
publication in ApJ. Part of this work has been the subject of the Laurea
thesis of LDA. His supervisor and our colleague, Prof. Laura E. Pasinetti,
suddendly passed away on September 13, 2006. Several astronomers have been
trained under her tutelage and we gratefully honor her memor
Understanding the dynamical structure of pulsating stars: The Baade-Wesselink projection factor of the delta Scuti stars AI Vel and beta Cas
Aims. The Baade-Wesselink method of distance determination is based on the
oscillations of pulsating stars. The key parameter of this method is the
projection factor used to convert the radial velocity into the pulsation
velocity. Our analysis was aimed at deriving for the first time the projection
factor of delta Scuti stars, using high-resolution spectra of the
high-amplitude pulsator AI Vel and of the fast rotator beta Cas. Methods. The
geometric component of the projection factor (i.e. p0) was calculated using a
limb-darkening model of the intensity distribution for AI Vel, and a
fast-rotator model for beta Cas. Then, using SOPHIE/OHP data for beta Cas and
HARPS/ESO data for AI Vel, we compared the radial velocity curves of several
spectral lines forming at different levels in the atmosphere and derived the
velocity gradient associated to the spectral-line-forming regions in the
atmosphere of the star. This velocity gradient was used to derive a dynamical
projection factor p. Results. We find a flat velocity gradient for both stars
and finally p = p0 = 1.44 for AI Vel and p = p0 = 1.41 for beta Cas. By
comparing Cepheids and delta Scuti stars, these results bring valuable insights
into the dynamical structure of pulsating star atmospheres. They suggest that
the period-projection factor relation derived for Cepheids is also applicable
to delta Scuti stars pulsating in a dominant radial mode
A comprehensive asteroseismic modelling of the high-amplitude delta Scuti star RV Arietis
We present a comprehensive asteroseismic study of the double-mode
high-amplitude delta Scuti star HD 187642 (RV Arietis). The modelling includes
some of the most recent techniques: 1) effects of rotation on both equilibrium
models and adiabatic oscillation spectrum, 2) non-adiabatic study of radial and
non-radial modes, 3) relationship between the fundamental radial mode and the
first overtone in the framework of Petersen diagrams. The analysis reveals that
two of the observed frequencies are very probably identified as the fundamental
and first overtone radial modes. Analysis of the colour index variations,
together with theoretical non-adiabatic calculations, points to models in the
range of [7065,7245] K in effective temperature and of [1190, 1270] Myr in
stellar age. These values were found to be compatible with those obtained using
the three other asteroseismic techniques.Comment: accepted for publication in A&
An asteroseismic study of the Delta Scuti star 44 Tau
In this paper we investigate theoretical pulsation models for the delta Scuti
star 44 Tau. The star was monitored during several multisite campaigns which
confirmed the presence of radial and nonradial oscillations. Moreover, its
exceptionally low rotational velocity makes 44 Tau particulary interesting for
an asteroseismic study. Due to the measured log g value of 3.6 +/- 0.1, main
sequence and post-main sequence models have to be considered. We perform mode
identification based on photometric and spectroscopic data. A nonadiabatic
pulsation code is used to compute models that fit the identified modes. The
influence of different opacity tables and element mixtures on the results is
tested. The observed frequencies of 44 Tau can be fitted in both the main
sequence and the post-main sequence evolutionary stage. Post-main sequence
models are preferable as they fulfill almost all observational constraints (fit
of observed frequencies, position in the HRD and instability range). These
models can be obtained with normal chemical composition which is in agreement
with recent spectroscopic measurements. The efficiency of envelope convection
(in the framework of the mixing-length theory) is predicted to be very low in
44 Tau. We show that the results are sensitive to the choice between the OPAL
and OP opacities. While the pulsation models of 44 Tau computed with OP
opacities are considerably too cool and too faint, the use of OPAL opacities
results in models within the expected temperature and luminosity range.Comment: 9 pages, 15 figures, 2 tables, accepted for publication in A&
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&
Measuring mean densities of delta Scuti stars with asteroseismology. Theoretical properties of large separations using TOUCAN
We aim at studying the theoretical properties of the regular spacings found
in the oscillation spectra of delta Scuti stars. We performed a multi-variable
analysis covering a wide range of stellar structure and seismic properties and
model parameters representative of intermediate-mass, main sequence stars. The
work-flow is entirely done using a new Virtual Observatory tool: TOUCAN (the VO
gateway for asteroseismic models), which is presented in this paper. A linear
relation between the large separation and the mean density is predicted to be
found in the low frequency frequency domain (i.e. radial orders spanning from 1
to 8, approximately) of the main-sequence, delta Scuti stars' oscillation
spectrum. We found that such a linear behavior stands whatever the mass,
metallicity, mixing length, and overshooting parameters considered in this
work. The intrinsic error of the method is discussed. This includes the
uncertainty in the large separation determination and the role of rotation. The
validity of the relation found is only guaranteed for stars rotating up to 40
percent of their break-up velocity. Finally, we applied the diagnostic method
presented in this work to five stars for which regular patterns have been
found. Our estimates for the mean density and the frequency of the fundamental
radial mode match with those given in the literature within a 20 percent of
deviation. Asteroseismology has thus revealed an independent direct measure of
the average density of delta Scuti stars, analogous to that of the Sun. This
places tight constraints on the mode identification and hence on the stellar
internal structure and dynamics, and allows a determination the radius of
planets orbiting around delta Scuti stars with unprecedented precision. This
opens the way for studying the evolution of regular patterns in pulsating
stars, and its relation with stellar structure and evolution.Comment: 11 pages, 6 figures, A&A in pres
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