HD 42477: coupled r modes, g modes and a p mode in an A0Vnne star

Abstract

Several studies have shown that a number of stars pulsating in p modes lie between the ${\hat{I}}^2$ Cep and $\hat{I}´$ Sct instability strips in the Hertzsprung-Russell (HR) Diagram. At present, there is no certain understanding of how p~modes can be excited in this $T_{\mathrm{eff}}$ range. The goal of this work is to disprove the conjecture that all stars pulsating in p modes and lying in this $T_{\mathrm{eff}}$ range are the result of incorrect measurements of $T_{\mathrm{eff}}$, contamination, or the presence of unseen cooler companions lying in the $\hat{I}´$ Sct instability strip (given the high binary fraction of stars in this region of the HR Diagram). Using TESS data, we show that the A0Vnne star HD 42477 has a single p mode coupled to several r modes and/or g modes. We rule out a contaminating background star with a pixel-by-pixel examination, and we essentially rule out the possibility of a companion $\hat{I}´$ Sct star in a binary. We model the pulsations in HD 42477 and suggest that the g modes are excited by overstable convective core modes. We also conjecture that the single p mode is driven by coupling with the g modes, or that the oblateness of this rapidly-rotating star permits driving by He II ionization in the equatorial region.

11 pages, 6 figures, 2 tables, accepted to MNRAS