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

Abstract

We report the results of a 60-hr photometric campaign of a rapidly oscillating Ap star, J0855 (TYC 2488-1241-1). We have utilised 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 (Prot = 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 µHz; 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◦ to the line-of-sight, with an angle of obliquity of the magnetic axis to the rotation axis of either 40◦ or 24◦ depending on whether the pulsation mode is dipole or quadrupole, respectively.