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Pulsating stars in NGC 6231 Frequency analysis and photometric mode identification near the main sequence

We used Johnson UBV photometric CCD observations to identify pulsating and other variable stars in the young open cluster NGC 6231. The multi-color information was used to classify pulsating variables, perform frequency analysis, and - where possible - to compare observed to theoretical amplitude ratios for mode identification. The data reduction was performed with standard IRAF tools. Differential light curves have been obtained by identifying a set of suitable comparison stars and the frequency analysis was then conducted on the basis of Fourier methods. Our classification of pulsating stars was based on the time scales and amplitudes of the variability with respect to the different filters and stellar parameters as calculated from published Strömgren and Geneva photometry. We identified 32 variable stars in the field of the cluster out of which 21 are confirmed members and twelve are newly detected variable stars. Ten stars were classified as Slowly Pulsating B (SPB) stars in NGC 6231 out of which seven are new discoveries. We also analyzed six previously reported β Cephei variables in more detail. One of them may be a hybrid β Cephei/SPB pulsator. In addition, we investigated five more previously suspected pulsators of this group which we cannot convincingly confirm. The remaining eleven variable stars are either not members of NGC 6231 or the membership status is questionable. Among them are three previously known δ Scuti stars, two newly detected pulsators of this class, one new and two already known eclipsing binaries, one new SPB variable, one possible Pre-Main-Sequence (PMS) pulsator and another new variable star for which we cannot present a classification. With more than 20 main sequence pulsators of spectral type B, NGC 6231 becomes the open cluster with the largest population of such pulsating stars known.