Pulsation mode switching in the pulsating star HD 81589 and a proposed classification as a field FaRPB star

Extensive multicolour photometry and low-resolution spectroscopy obtained for HD 81589 place the star in the gap between δ Scuti and slowly pulsating B pulsators on the main sequence. A clear episode of pulsation mode switching has apparently occurred in HD 81589, with its primary pulsation frequency switching from 4.57 to 3.71 c d ⁻¹ in less than 6 months. The pulsation frequencies obtained for this star match remarkably well with modelled frequencies of prograde sectoral g-modes for rapidly rotating late B stars on the main sequence. This match, coupled with the rapid rotation rate determined for HD 81589, implies that HD 81589 is one of the first identified fast-rotating pulsating B-type (FaRPB) stars found in the field, complementing the sample of FaRPB stars previously discovered in the open cluster NGC 3766.