900 research outputs found
Simultaneous intensive photometry and high resolution spectroscopy of Delta Scuti stars. V. The high--degree modes in the pulsational content of BV Circini
We discuss here the pulsation properties of the Scuti star BV
Circini on the basis of data obtained during a simultaneous photometric and
spectroscopic campaign in 1996 and a spectroscopic one in 1998, and taking also
advantage of the previous photometric observations by Kurtz (1981). Nine
pulsation modes were detected from photometry and thirteen from spectroscopy;
five of them are in common to both techniques. The spectroscopic data give
ample evidence of dramatic amplitude variations in some modes, in particular
the strongest spectroscopic mode in 1998 was not detectable in 1996 data. The
two dominant photometric modes (6.33 and 7.89 c\d) are observed on both
seasons. The typing of the modes was performed by means of a simultaneous model
fit of line profile and light variations. The 6.33 c\d photometric term is
probably the fundamental radial mode, while the 7.89 c\d is a nonradial mode
with mi different than 0. There are six high-degree prograde modes with an
azimuthal order m ranging from -12 to -14, and also a retrograde mode with m~7.
These modes combined with the identification of the 6.33 c\d mode allowed us to
estimate i~60 deg for the value of the inclination of the rotation axis. An
accurate evaluation of the main stellar physical parameters is also proposed as
a result of the pulsational analysis.Comment: 12 pages (in A&A style), 9 ps figures (Fig. 7 in colour) Accepted for
A&A Main Journa
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. HARPS spectroscopy of the delta Scuti stars rho Pup and DX Cet
High-resolution spectroscopy is a powerful tool to study the dynamical
structure of pulsating stars atmosphere. We aim at comparing the line asymmetry
and velocity of the two delta Sct stars rho Pup and DX Cet with previous
spectroscopic data obtained on classical Cepheids and beta Cep stars. We
obtained, analysed and discuss HARPS high-resolution spectra of rho Pup and DX
Cet. We derived the same physical quantities as used in previous studies, which
are the first-moment radial velocities and the bi-Gaussian spectral line
asymmetries. The identification of f=7.098 (1/d) as a fundamental radial mode
and the very accurate Hipparcos parallax promote rho Pup as the best standard
candle to test the period-luminosity relations of delta Sct stars. The action
of small-amplitude nonradial modes can be seen as well-defined cycle-to-cycle
variations in the radial velocity measurements of rho Pup. Using the
spectral-line asymmetry method, we also found the centre-of-mass velocities of
rho Pup and DX Cet, V_gamma = 47.49 +/- 0.07 km/s and V_gamma = 25.75 +/- 0.06
km/s, respectively. By comparing our results with previous HARPS observations
of classical Cepheids and beta Cep stars, we confirm the linear relation
between the atmospheric velocity gradient and the amplitude of the radial
velocity curve, but only for amplitudes larger than 22.5 km/s. For lower values
of the velocity amplitude (i.e., < 22.5 km/s), our data on rho Pup seem to
indicate that the velocity gradient is null, but this result needs to be
confirmed with additional data. We derived the Baade-Wesselink projection
factor p = 1.36 +/- 0.02 for rho Pup and p = 1.39 +/- 0.02 for DX Cet. We
successfully extended the period-projection factor relation from classical
Cepheids to delta Scuti stars.Comment: Accepted for publication in A&A (in press
Stellar evolution through the ages: period variations in galactic RRab stars as derived from the GEOS database and TAROT telescopes
The theory of stellar evolution can be more closely tested if we have the
opportunity to measure new quantities. Nowadays, observations of galactic RR
Lyr stars are available on a time baseline exceeding 100 years. Therefore, we
can exploit the possibility of investigating period changes, continuing the
pioneering work started by V. P. Tsesevich in 1969. We collected the available
times of maximum brightness of the galactic RR Lyr stars in the GEOS RR Lyr
database. Moreover, we also started new observational projects, including
surveys with automated telescopes, to characterise the O-C diagrams better. The
database we built has proved to be a very powerful tool for tracing the period
variations through the ages. We analyzed 123 stars showing a clear O-C pattern
(constant, parabolic or erratic) by means of different least-squares methods.
Clear evidence of period increases or decreases at constant rates has been
found, suggesting evolutionary effects. The median values are beta=+0.14
day/Myr for the 27 stars showing a period increase and beta=-0.20 day/Myr for
the 21 stars showing a period decrease. The large number of RR Lyr stars
showing a period decrease (i.e., blueward evolution) is a new and intriguing
result. There is an excess of RR Lyr stars showing large, positive
values. Moreover, the observed beta values are slightly larger than those
predicted by theoretical models.Comment: 15 pages, 9 figures; to be published in Astronomy and Astrophysics;
full resolution version available at
http://dbrr.ast.obs-mip.fr/tarot/publis/publis.htm
The CoRoT star 105288363: strong cycle to cycle changes of the Blazhko modulation
We present the analysis of the CoRoT star 105288363, a new Blazhko RR Lyrae
star of type RRab (f0 = 1.7623 c/d), observed with the CoRoT space craft during
the second long run in direction of the galactic center (LRc02, time base 145
d). The CoRoT data are characterized by an excellent time sampling and a low
noise amplitude of 0.07 mmag in the 2-12 c/d range and allow us to study not
only the fine details of the variability of the star but also long-term changes
in the pulsation behaviour and the stability of the Blazhko cycle. We use,
among other methods, standard Fourier analysis techniques and O-C diagrams to
investigate the pulsational behavior of the Blazhko star 105288363. In addition
to the frequency pattern expected for a Blazhko RR Lyrae star, we find an
independent mode (f1 = 2.984 c/d) showing a f0/f1 ratio of 0.59 which is
similar to that observed in other Blazhko RR Lyrae stars. The bump and hump
phenomena are also analysed, with their variations over the Blazhko cycle. We
carefully investigated the strong cycle-to-cycle changes in the Blazhko
modulation (PB = 35.6 d), which seem to happen independently and partly
diametrically in the amplitude and the phase modulation. Furthermore, the
phasing between the two types of modulation is found to change during the
course of the observations.Comment: 15 pages, 8 figures, accepted for publication in MNRA
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
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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