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

Several studies have shown that a number of stars pulsating in p modes lie between the b Cep and d 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-eff range. The goal of this work is to disprove the conjecture that all stars pulsating in p modes and lying in this T-eff range are the result of incorrect measurements of T-eff, contamination, or the presence of unseen cooler companions lying in the d 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 d 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.