19 research outputs found
The census of non-radial pulsation in first-overtone RR Lyrae stars of the OGLE Galactic bulge collection
We analyzed photometry for the up-to-date collection of the first-overtone RR
Lyrae stars (RRc; 11415 stars) and double-mode RR Lyrae stars (RRd; 148 stars)
towards the Galactic bulge from the Optical Gravitational Lensing Experiment.
We analyzed frequency spectra of these stars in search for additional,
low-amplitude signals, beyond the radial modes. We focused on stars from two
groups: and . In the first group, additional
low-amplitude signals have periods shorter than the first-overtone period;
period ratios fall in the 0.60-0.64 range. In the second group, additional
low-amplitude signals have periods longer than the first-overtone period;
period ratios tightly cluster around 0.68. Altogether we have detected 960 and
147 RR Lyrae stars that belong to and groups,
respectively, which yield the incidence rates of 8.3 and 1.3 per cent of the
considered sample. We discuss statistical properties of RR Lyrae stars with
additional periodicities. For group we provide strong arguments
that additional periodicities are connected to non-radial pulsation modes of
degrees and , as proposed by Dziembowski. We have also
detected two double-periodic variables, with two close periodicities, similar
to RR Lyrae variable V37 in NGC 6362. Properties of these peculiar variables,
which may form a new group of double-mode pulsators, are discussed.Comment: 33 pages, 14 figures, 5 table
Modelling turbulent fluxes due to thermal convection in rectilinear shearing flow
We revisit a phenomenological description of turbulent thermal convection
along the lines proposed originally by Gough (1965) in which eddies grow solely
by extracting energy from the unstably stratified mean state and are
subsequently destroyed by internal shear instability. This work is part of an
ongoing investigation for finding a procedure to calculate the turbulent fluxes
of heat and momentum in the presence of a shearing background flow in stars.Comment: 2 pages, 1 figure, accepted for publication in IAU Symposium 271
"Astrophysical Dynamics: From Galaxies to Stars", Nice, 201
Blazhko Effect in the first overtone RR Lyrae stars of the OGLE Galactic bulge collection
We present the analysis of the Blazhko effect - quasi-periodic modulation of
pulsation amplitude and/or phase - in the Galactic bulge first overtone RR
Lyrae stars (RRc). We used the data gathered during the fourth phase of the
Optical Gravitational Lensing Experiment (OGLE). Out of 10 826 analyzed RRc
stars, Blazhko effect was detected in 607 stars which constitute 5.6 percent of
the sample. It is the largest and most homogeneous sample of modulated RRc
stars analyzed so far. Modulation periods cover a wide range, from slightly
above 2 d to nearly 3000 d. Multiperiodic modulation was detected in 47 stars.
The appearance of modulation in the frequency domain was studied in detail.
Modulation manifests either as close doublets or as equidistant triplets and
multiplets centered on radial mode frequency and its harmonics. In a
significant fraction (29 percent) of stars, we have detected the modulation
frequency itself, which corresponds to the modulation of the mean stellar
brightness. Our search for period doubling effect, that was discovered recently
in modulated fundamental mode RR Lyrae stars, and triggered development of new
model behind the Blazhko modulation, yielded negative result. In 104 stars we
detected additional signals that could correspond to both radial and non-radial
modes. Statistical properties of modulated stars were analyzed in detail and
confronted with properties of non-modulated stars and of modulated fundamental
mode RR Lyrae stars. Our analysis provides constraints for the models to
explain the Blazhko phenomenon, which still remains a puzzle more than hundred
years after its discovery.Comment: 35 pages, 22 figures, 4 table