Frequency analysis and pulsational mode identification of two γ Doradus stars: HD 40745 and HD 189631★

Abstract

Gravity modes present in γ Doradus stars probe the deep stellar interiors and are thus of particular interest in asteroseismology. Mode identification will improve the knowledge of these stars considerably and allow an understanding of the issues with current pulsational models. A frequency analysis followed by a mode identification were done based on the high-resolution spectroscopic data of two γ Doradus stars: HD 189631 and HD 40745. Extensive spectroscopic data sets are obtained by three instruments: HARPS. FEROS and HERCULES. We obtained 422 spectra for HD 189631 and 248 spectra for HD 40745. The pulsational frequencies were determined by four methods: analysis of the variation in equivalent width, variation in radial velocity, asymmetry of the line profile and the pixel-by-pixel frequency analysis. The mode identification was done using the recently developed Fourier Parameter Fit method. Without achieving the same degree of confidence for all results, we report the identification of four pulsational modes in HD 189631: (l = 1; m = +1) at f 1 = 1.67 d ―1 ; (3; ―2) at f 2 = 1.42 d ―1 ; (2; ―2) at f 3 = 0.07 d ―1 ; and (4; +1) at f 4 = 1.82 d ―1 ; and two modes in HD 40745: (2; -1) at f 1 = 0.75 d ―1 and (3; -3) at f 2 = 1.09 d ―1 . This study provides the first pulsational analysis based on spectroscopy of HD 189631 and HD 40745. We discuss the performance of current methods of analysis and outline the difficulties presented by γ Doradus stars.