HD 42659: The only known roAp star in a spectroscopic binary observed with B photometry, TESS, and SALT

Abstract

We present a multi-instrument analysis of the rapidly oscillating Ap (roAp) star HD 42659. We have obtained B photometric data for this star and use these data, in conjunction with TESS observations, to analyse the high-frequency pulsation in detail. We find a triplet which is split by the rotation frequency of the star (νrot =
0.3756 d−1; Prot = 2.66 d) and present both distorted dipole and distorted quadrupole mode models. We show
that the pulsation frequency, 150.9898 d−1 (Ppuls = 9.54 min) is greater than the acoustic cutoff frequency. We utilise 27 high-resolution (R ≃ 65 000), high signal-to-noise (∼ 120) spectra to provide new orbital parameters for this, the only known roAp star to be in a short period binary (Porb = 93.266 d). We find the system to be more eccentric than previously thought, with e = 0.317, and suggest the companion is a mid-F to early-K star. We find no significant trend in the average pulsation mode amplitude with time, as measured
by TESS, implying that the companion does not have an affect on the pulsation in this roAp star. We suggest further photometric observations of this star, and further studies to find more roAp stars in close binaries to characterise how binarity may affect the detection of roAp pulsations.