56 research outputs found
Unexpected series of regular frequency spacing of delta Scuti stars in the non-asymptotic regime -- I. The methodology
A sequence search method was developed to search regular frequency spacing in
delta Scuti stars by visual inspection and algorithmic search. We searched for
sequences of quasi-equally spaced frequencies, containing at least four members
per sequence, in 90 delta Scuti stars observed by CoRoT. We found an
unexpectedly large number of independent series of regular frequency spacing in
77 delta Scuti stars (from 1 to 8 sequences) in the non-asymptotic regime. We
introduce the sequence search method presenting the sequences and echelle
diagram of CoRoT 102675756 and the structure of the algorithmic search. Four
sequences (echelle ridges) were found in the 5-21 d^{-1} region, where the
pairs of the sequences are shifted (between 0.5-0.59 d^{-1}) by twice the value
of the estimated rotational splitting frequency (0.269 d^{-1}). The general
conclusions for the whole sample are also presented in this paper. The
statistics of the spacings derived by the sequence search method, by FT and
that of the shifts are also compared. In many stars, more than one almost
equally valid spacing appeared. The model frequencies of FG Vir and their
rotationally split components were used to reveal a possible explanation that
one spacing is the large separation, while the other is a sum of the large
separation and the rotational frequency. In CoRoT 102675756, the two spacings
(2.249 and 1.977 d^{-1}) agree better with the sum of a possible 1.710 d^{-1}
large separation and two or one times, respectively, the value of the
rotational frequency.Comment: 12 pages, 10 figures, accepted for publication in Ap
Unexpected series of regular frequency spacing of delta Scuti stars in the non-asymptotic regime -- II. Sample -- echelle diagrams and rotation
A sequence search method was developed for searching for regular frequency
spacing in delta Scuti stars by visual inspection and algorithmic search. The
sample contains 90 delta Scuti stars observed by CoRoT. An example is given to
represent the visual inspection. The algorithm (SSA) is described in detail.
The data treatment of the CoRoT light curves, the criteria for frequency
filtering and the spacings derived by two methods (three approaches: VI, SSA
and FT) are given for each target. Echelle diagrams are presented for 77
targets, for which at least one sequence of regular spacing was identified.
Comparing the spacing and the shifts between pairs of echelle ridges revealed
that at least one pair of echelle ridges is shifted to midway between the
spacing for 22 stars. The estimated rotational frequencies compared to the
shifts revealed rotationally split doublets, triplets and multiplets not only
for single frequencies, but for the complete echelle ridges in 31 delta Scuti
stars. Using several possible assumptions for the origin of the spacings, we
derived the large separation (), which are distributed along the
mean density versus large separations relation derived from stellar models
Suarez 2014.Comment: 25 pages, 16 figures, 9 tables. Accepted for publication in ApJ
Atmospheric parameters and chemical properties of red giants in the CoRoT asteroseismology fields
A precise characterisation of the red giants in the seismology fields of the
CoRoT satellite is a prerequisite for further in-depth seismic modelling.
High-resolution FEROS and HARPS spectra were obtained as part of the
ground-based follow-up campaigns for 19 targets holding great asteroseismic
potential. These data are used to accurately estimate their fundamental
parameters and the abundances of 16 chemical species in a self-consistent
manner. Some powerful probes of mixing are investigated (the Li and CNO
abundances, as well as the carbon isotopic ratio in a few cases). The
information provided by the spectroscopic and seismic data is combined to
provide more accurate physical parameters and abundances. The stars in our
sample follow the general abundance trends as a function of the metallicity
observed in stars of the Galactic disk. After an allowance is made for the
chemical evolution of the interstellar medium, the observational signature of
internal mixing phenomena is revealed through the detection at the stellar
surface of the products of the CN cycle. A contamination by NeNa-cycled
material in the most massive stars is also discussed. With the asteroseismic
constraints, these data will pave the way for a detailed theoretical
investigation of the physical processes responsible for the transport of
chemical elements in evolved, low- and intermediate-mass stars.Comment: Accepted for publication in A&A, 25 pages, 13 colour figures (revised
version after language editing
Photometric variability of the Herbig Ae star HD 37806
The more massive counterparts of T Tauri stars, Herbig Ae/Be stars, are known
to vary in a complex way with no variability mechanism clearly identified. We
attempt to characterize the optical variability of HD~37806 (MWC 120) on time
scales ranging between minutes and several years. A continuous, one-minute
resolution, 21 day-long sequence of MOST (Microvariability & Oscillations of
STars) satellite observations has been analyzed using wavelet, scalegram and
dispersion analysis tools. The MOST data have been augmented by sparse
observations over 9 seasons from ASAS (All Sky Automated Survey), by previously
non-analyzed ESO (European Southern Observatory) data partly covering 3 seasons
and by archival measurements dating back half a century ago. Mutually
superimposed flares or accretion instabilities grow in size from about 0.0003
of the mean flux on a time scale of minutes to a peak-to-peak range of <~0.05
on a time scale of a few years. The resulting variability has properties of
stochastic "red" noise, whose self-similar characteristics are very similar to
those observed in cataclysmic binary stars, but with much longer characteristic
time scales of hours to days (rather than minutes) and with amplitudes which
appear to cease growing in size on time scales of tens of years. In addition to
chaotic brightness variations combined with stochastic noise, the MOST data
show a weakly defined cyclic signal with a period of about 1.5 days, which may
correspond to the rotation of the star.Comment: Accepted for publication by Astron. & Astroph. 8 pages, 9 figures.
For some reason Fig.5 incorrectly shows in arXiv: Contours OK, gray scale no
Gamma Dor and Gamma Dor - Delta Sct Hybrid Stars In The CoRoT LRa01
A systematic search for gamma Dor and gamma Dor - delta Scuti hybrid
pulsators was conducted on the CoRoT LRa01 Exo-archive yielding a total of 418
gamma Dor and 274 hybrid candidates. After an automatic jump correction 194 and
167 respectively, show no more obvious jumps and were investigated in more
detail. For about 25\% of these candidates classification spectra from the
Anglo-Australian Observatory (AAO) are available. The detailed frequency
analysis and a check for combination frequencies together with spectroscopic
information allowed us to identify I) 34 gamma Dor stars which show very
different pulsation spectra where mostly two modes dominate. Furthermore, a
search for regularities in their oscillation spectra allowed to derive
recurrent period spacings for 5 of these gamma Dor stars. II) 25 clear hybrid
pulsators showing frequencies in the gamma Dor and delta Sct domain and are of
A-F spectral type.Comment: Proceedings of the 4th HELAS International Conference held in
Lanzarote, 201
Models of red giants in the CoRoT asteroseismology fields combining asteroseismic and spectroscopic constraints
Context. The availability of asteroseismic constraints for a large sample of
red giant stars from the CoRoT and Kepler missions paves the way for various
statistical studies of the seismic properties of stellar populations.
Aims. We use the first detailed spectroscopic study of 19 CoRoT red-giant
stars (Morel et al 2014) to compare theoretical stellar evolution models to
observations of the open cluster NGC 6633 and field stars.
Methods. In order to explore the effects of rotation-induced mixing and
thermohaline instability, we compare surface abundances of carbon isotopic
ratio and lithium with stellar evolution predictions. These chemicals are
sensitive to extra-mixing on the red-giant branch.
Results. We estimate mass, radius, and distance for each star using the
seismic constraints. We note that the Hipparcos and seismic distances are
different. However, the uncertainties are such that this may not be
significant. Although the seismic distances for the cluster members are self
consistent they are somewhat larger than the Hipparcos distance. This is an
issue that should be considered elsewhere. Models including thermohaline
instability and rotation-induced mixing, together with the seismically
determined masses can explain the chemical properties of red-giants targets.
However, with this sample of stars we cannot perform stringent tests of the
current stellar models. Tighter constraints on the physics of the models would
require the measurement of the core and surface rotation rates, and of the
period spacing of gravity-dominated mixed modes. A larger number of stars with
longer times series, as provided by Kepler or expected with Plato, would help
for ensemble asteroseismology.Comment: Accepted 03/05/201
CoRoT 102749568: mode identification in a delta Scuti star based on regular spacings
The high accuracy of space data increased the number of the periodicities
determined for pulsating variable stars, but the mode identification is still a
critical point in the non-asymptotic regime. We use regularities in frequency
spacings for identifying the pulsation modes of the recently discovered delta
Sct star ID 102749568. In addition to analysing CoRoT light curves (15252
datapoints spanning 131 days), we obtained and analysed both spectroscopic and
extended multi-colour photometric data. We applied standard tools (MUFRAN,
Period04, SigSpec, and FAMIAS) for time-series analysis. A satisfactory
light-curve fit was obtaining by means of 52 independent modes and 15
combination terms. The frequency spacing revealed distinct peaks around large
(25.55-31.43 microHz), intermediate (9.80, 7.66 microHz), and low (2.35
microHz) separations. We directly identified 9 modes, and the l and n values of
other three modes were extrapolated. The combined application of spectroscopy,
multi-colour photometry, and modelling yielded the precise physical parameters
and confirmed the observational mode identification. The large separation
constrained the log g and related quantities. The dominant mode is the radial
first overtone.Comment: 13 pages, 8 figures, accepted for publication in A&
CoRoT observations of O stars: diverse origins of variability
Six O-type stars were observed continuously by the CoRoT satellite during a
34.3-day run. The unprecedented quality of the data allows us to detect even
low-amplitude stellar pulsations in some of these stars (HD 46202 and the
binaries HD 46149 and Plaskett's star). These cover both opacity-driven modes
and solar-like stochastic oscillations, both of importance to the
asteroseismological modelling of O stars. Additional effects can be seen in the
CoRoT light curves, such as binarity and rotational modulation. Some of the
hottest O-type stars (HD 46223, HD 46150 and HD 46966) are dominated by the
presence of red-noise: we speculate that this is related to a sub-surface
convection zone.Comment: 5 pages, 3 figures, conference paper. To be published in "Four
decades of Research on Massive Stars", Astronomical Society of the Pacific.
Eds. C. Robert, N. St-Louis and L. Drisse
gamma Doradus stars in the COROT exoplanets fields: first inspection
We present here preliminary results concerning 32 stars identified as main
gamma Doradus candidates by the COROT Variable Classifier (CVC) among the 4
first fields of the exoplanet CCDs.Comment: To appear in "Stellar Pulsation: Challenges for Theory and
Observation", AI
MOST discovers a multimode delta Scuti star in a triple system: HD 61199
A field star, HD 61199 (V ~ 8), simultaneously observed with Procyon by the
MOST (Microvariability & Oscillations of STars) satellite in continuous runs of
34, 17, and 34 days in 2004, 2005, and 2007, was found to pulsate in 11
frequencies in the delta Scuti range with amplitudes from 1.7 down to 0.09
mmag. The photometry also showed variations with a period of about four days.
To investigate the nature of the longer period, 45 days of time-resolved
spectroscopy was obtained at the Thueringer Landessternwarte Tautenburg in
2004. The radial velocity measurements indicate that HD 61199 is a triple
system. A delta Scuti pulsator with a rich eigenspectrum in a multiple system
is promising for asteroseismology. Our objectives were to identify which of the
stars in the system is the delta Scuti variable and to obtain the orbital
elements of the system and the fundamental parameters of the individual
components, which are constrained by the pulsation frequencies of the delta
Scuti star. Classical Fourier techniques and least-squares multi-sinusoidal
fits were applied to the MOST photometry to identify the pulsation frequencies.
The groundbased spectroscopy was analysed with least-squares-deconvolution
(LSD) techniques, and the orbital elements derived with the KOREL and ORBITX
routines. Asteroseismic models were also generated. The photometric and
spectroscopic data are compatible with a triple system consisting of a close
binary with an orbital period of 3.57 days and a delta Scuti companion (HD
61199,A) as the most luminous component. The delta Scuti star is a rapid
rotator with about vsin i = 130 km/s and an upper mass limit of about 2.1 Msun.
For the close binary components, we find they are of nearly equal mass, with
lower mass limits of about 0.7 Msun.Comment: 11 pages, 14 figures, accepted by A&
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