RV Tauri stars -I. A long-term photometric survey

Abstract

A programme of photometric and spectroscopic observations has been undertaken in order to investigate the physical characteristics of the R V Tauri variables. This paper presents and discusses the results from the photometric survey. Long time baseline BVRI photometry has been obtained at the Mt John University Observatory (MJUO) over intervals of between 750 and 1300 d for eleven of the RV Tauri variables. RV Tauri stars from both the RVa (constant mean magnitude) and RVb (varying mean magnitude) photometric subclasses and from all three spectroscopic Preston subtypes (A, Band C types) were included in the programme. Fourier and least­ squares analyses of the light and colour variations reveal the dominant periodicities and the stability of the pulsations in these stars. A harmonic (f + 2f) fit is found to be a good representation of the 'deep-shallow' nature of both the light and colour curves in most stars. The derived photometric characteristics have been used to examine a number of theoretical models that attempt to describe the pulsation mechanisms and physical parameters of the RV Tauri stars. The non-linear, hydrodynamic calculations by Fokin show that low-mass, low-metallicity and high-luminosity models can reproduce the correct R V Tauri periods, amplitudes and alternating character. The Fourier spectra from the analysis of the MJUO photometry (which indicate two dominant peaks with a frequency ratio close to 2:1) and the theoretical RV Tauri models of Fokin and Tuchman et al. lend s~pport to the earlier proposal by Takeuti & Petersen that the alternating light curve in these stars may arise from a 2:1 resonance between the fundamental and first overtone modes.