LCO observations of a super-critical distorted pulsation in the roAp star J0855 (TYC 2488-1241-1)

We report the results of a 60-h photometric campaign of a rapidly oscillating Ap star, J0855 (TYC 2488-1241-1). We have utilized the multi-site Las Cumbres Observatory's (LCO) 0.4-m telescopes to obtain short cadence B-band observations of an roAp star previously lacking detailed study. Our observations confirm the rotation period presented in the discovery paper of this star (P-rot = 3.0918 d), and reveal the star to be pulsating in a distorted mode. The B data show this star to be among the highest amplitude roAp stars, with a peak-to-peak amplitude of 24 mmag. Modelling of the pulsation frequency at 197.2714 d( )(-1)(2283 mu Hz; P= 7.30 min) shows that this star belongs to the subgroup of super-critical pulsators, where the observed frequencies are above the theoretical acoustic cutoff frequency. From the modelling, we deduce that the star's rotation axis is inclination angle of about 30 degrees to the line of sight, with an angle of obliquity of the magnetic axis to the rotation axis of either 40 degrees or 24 degrees depending on whether the pulsation mode is dipole or quadrupole, respectively.