Testing pulsation diagnostics in the rapidly oscillating magnetic Ap star γ Equ using near-infrared CRIRES+ observations

Abstract

Context. Pulsations of rapidly oscillating Ap stars and their interaction with the stellar magnetic field have not been studied in the near-infrared (near-IR) region despite the benefits these observations offer compared to visual wavelengths. The main advantage of the near-IR is the quadratic dependence of the Zeeman effect on the wavelength, as opposed to the linear dependence of the Doppler effect. Aims. To test pulsation diagnostics of roAp stars in the near-IR, we aim to investigate the pulsation behaviour of one of the brightest magnetic roAp stars, γ Equ, which possesses a strong surface magnetic field of the order of several kilogauss and exhibits magnetically split spectral lines in its spectra. Methods. Two magnetically split spectral lines belonging to different elements, the triplet Fe I at 1563.63 nm and the pseudo-doublet Ce III at 1629.2 nm, were recorded with CRIRES+ over about one hour in the H band with the aim of understanding the character of the line profile variability and the pulsation behaviour of the magnetic field modulus. Results. The profile shapes of both studied magnetically split spectral lines vary in a rather complex manner probably due to a significant decrease in the strength of the longitudinal field component and an increase in the strength of the transverse field components over the last decade. A mean magnetic field modulus of 3.9 kG was determined for the Zeeman triplet Fe I at 1563.63 nm, whereas for the pseudo-doublet Ce III at 1629.2 nm we observe a much lower value of only about 2.9 kG. For comparison, a mean field modulus of 3.4kG was determined using the Zeeman doublet Fe II at 6249.25 in optical PEPSI spectra recorded just about two weeks before the CRIRES+ observations. Different effects that may lead to the differences in the field modulus values are discussed. Our measurements of the mean magnetic field modulus using the line profiles recorded in different pulsational phase bins suggest a field modulus variability of 32 G for the Zeeman triplet Fe I at 1563.63 nm and 102 G for the pseudo-doublet Ce III at 1629.2 nm.