Asteroseismology of HADS stars: V974 Oph, a radial pulsator flavoured by nonradial components

The analysis of a dense time-series on V974 Oph disclosed the rich pulsational content (at least five independent terms) of this high-amplitude (0.60 mag in B-light) Delta Sct star. A mode with a frequency very close to the main one (probably the fundamental radial mode) has been detected: such a doublet is not a common feature in stars of the same class. Also another term can be considered a radial one, but the high ratio (0.786) raises some problems that can be solved only by admitting very low metallicity. It is quite evident that some undetectable terms are again hidden in the noise, as the least--squares fit leaves a rms residual much higher than the observational noise. All that considered, nonradial modes seem to play a key role in the light variability of V974 Oph. Revealing an unsuspected asteroseismic interest, V974 Oph provides a link between low- and high-amplitude Delta Sct stars. By discussing other high-amplitude Delta Sct stars showing unusual period ratios, it is evident that the asteroseismic approach is possible also for this class of pulsators.Comment: 6 pages, 5 figures. Accepted for publication in Astronomy & Astrophysic

Fourier decomposition and frequency analysis of the pulsating stars with P<1 d in the OGLE database. I. Monoperiodic Delta Scuti, RRc and RRab variables. Separation criteria and particularities

The OGLE database is revisited to investigate in more detail the properties of the Fourier parameters. Methodological improvements led us to identify a clear separation among High-Amplitude Delta Scuti (HADS), RRc and RRab stars. The bimodal distribution of the R21 parameter in HADS stars is explained as a contamination effect from RRc stars: there is evidence that all stars with 0.20<P<0.25 d are RRc variables. The previously claimed existence of a subclass of unusual HADS is demonstrated to be a spurious result. Candidate overtone pulsators are found among HADS and RRc variables. The properties of the Fourier parameters are discussed as a function of the physical conditions in the stars involved. Among the field RRab stars we detected different light-curve groups producing distinct "tails" in the Fourier plots for P>0.55 d; evolutionary phases or the combination of different physical conditions (not only metallicity) are suggested to explain this separation, observed also in the cluster RRab stars. The stellar parameters of RRc stars in a given globular cluster show different tendencies than those of RRc stars from different clusters.Comment: 12 pages (in A&A style), 14 eps figures. Accepted for A&A Main Journal. Table 3, 4 and 5 are also included as ascii files. The atlas of the light curves and least-squares fits can be requested from the autho