Three case studies in spectroscopic mode identification of non-radially pulsating stars

Abstract

Gravity modes present in gamma 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. The methods used in this thesis are also applied to a low degree pressure mode pulsator as a check for their validity. A frequency analysis followed by a mode identification were done based on the high resolution spectroscopic data of one β Cephei star, PT Puppis, and two γ Doradus stars, HD 189631 and AC Lepus. Extensive spectroscopic data sets are obtained by three instruments: HARPS, FEROS and HERCULES. We obtained 161 spectra for PT Puppis, 422 spectra for HD 189631 and 248 spectra for AC Lepus. The pulsational frequencies were determined by four methods: analysis of the variations in equivalent width, radial velocity, asymmetry of the line profile and by using 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 two pulsational modes in PT Puppis: (l = 0 ; m = 0) at f₁ = 6.07 d⁻¹ and (2 ; 0) or (1 ; +1) at f₂ = 5.99 d⁻¹, four modes in HD 189631: (1 ; +1) at f₁ = 1.67 d⁻¹, (3 ; -2) at f₂ = 1.42 d⁻¹, (2 ; -2) at f₃ = 0.07 d⁻¹ and (4 ; +1) at f₄ = 1.82 d⁻¹ and two modes in AC Lepus: (2 ; -1) at f₁ = 0.75 d⁻¹ and (3 ; -3) at f₂ = 1.09 d⁻¹. This study provides the first pulsational analysis based on spectroscopy of PT Puppis, HD 189631 and AC Lepus. We discuss the performance of current methods of analysis, outline the difficulties presented by γ Doradus stars, and compare our results with other published pulsational mode identifications.