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 (ν rot = 0.3756 d −1 ; P rot = 2.66 d) and present both distorted dipole and distorted quadrupole mode models. We show that the pulsation frequency, 150.9898 d −1 (P puls = 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 (P orb = 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.

Discovery of multiple p-mode pulsation frequencies in the roAp star, HD 86181

“…The pulsation phase as a function of rotation does not show a π-rad phase reversal expected at the times of amplitude minima as would be the case for an undistorted mode, although the pulsation phase shows bumps at those times. This then argues for a distorted quadrupole mode, and also is similar to what is observed in other roAp stars with well-studied quadrupole modes (Holdsworth, Saio & Kurtz 2019;Holdsworth et al 2018bHoldsworth et al ,c,a, 2014Kurtz et al 1996b;Holdsworth et al 2016).…”

“…The dipole mode frequencies just above and below the quadrupole mode of the best fitting model are 235.51 and 229.92 d −1 , respectively, at Bp = 7.0 kG, which yield a large frequency spacing of 5.59 d −1 (or 64.7 µHz), which agrees with the observed large frequency spacing, ν3 − ν2 = 5.63 d −1 (or 65.2 µHz). 2 For HD 42659, another roAp star pulsating in a distorted mode (Holdsworth, Saio & Kurtz 2019), the distorted model predicted the polar magnetic field strength to be 0.8 kG by assuming that star pulsates in a quadrupole mode. That result was consistent with the measured mean longitudinal magnetic field, B l = 0.4 kG (Kochukhov & Bagnulo 2006;Hubrig et al 2006).…”

Discovery of multiple p-mode pulsation frequencies in the roAp star, HD 86181

“…However, log T eff = 3.859 (T eff = 7230 K) of the most approriate fit model is somewhat lower than 7750 K listed in Table 1. This T eff value is closer to T eff = 7320 K obtained by McDonald, Zijlstra & Boyer (2012) from a comparison of the SED with model atmospheres, and to T eff = 7205 K obtained by Masana, Jordi & Ribas (2006) For HD 42659, another roAp star pulsating in a distorted mode (Holdsworth, Saio & Kurtz 2019), the distorted model predicted the polar magnetic field strength to be 0.8 kG by assuming that star pulsates in a quadrupole mode. That result was consistent with the measured mean longitudinal magnetic field, B l = 0.4 kG (Kochukhov & Bagnulo 2006;Hubrig et al 2006).…”

TESS cycle 1 observations of roAp stars with 2-min cadence data

“…This star was discovered to be an roAp star by Martinez et al (1993b) with later observations providing more detailed results by . The TESS sector 6 observations have been previously analysed by Balona et al (2019), with a more detailed analysis presented by Holdsworth et al (2019), analysing the dipole triplet. We do not present a re-analysis of the same data here, but refer the reader to the previous literature.…”

“…We do not present a re-analysis of the same data here, but refer the reader to the previous literature. It is worthy of note, however, that TIC 33601621 is the only roAp star known to be a member of a spectroscopic binary (SB1) system (Hartmann & Hatzes 2015;Holdsworth et al 2019).…”