Properties and nature of Be stars. 26. Long-term and orbital changes of ζ Tauri

Context: One way to understand the still mysterious Be phenomenon is to study the time variations of particular Be stars with a long observational history. ζ Tau is one obvious candidate.
Aims: Using our rich series of spectral and photometric observations and a critical compilation of available radial velocities, spectrophotometry of Hα, and UB{}V photometry, we characterize the pattern of time variations of ζ Tau over about a century. Our goal is to find the true timescales of its variability and confront them with the existing models related to various aspects of the Be phenomenon.
Methods: Spectral reductions were carried out using the IRAF and SPEFO programs. The HEC22 program was used for both photometric reductions and transformations to UB{}V. Orbital solutions were derived with the latest publicly available version of the program FOTEL, period analyses employed both the PDM and Fourier techniques - programs HEC27 and PERIOD04.
Results: We derived a new orbital ephemeris T_{RV max.}= HJD (2447025.6±1.8) + (132^d.987 ±0^d.050) × E. The analysis of long-term spectral and light variations shows a clear correlation between the RV and V/R changes, and a very complex behaviour of the light changes. The character of the orbital light and V/R changes varies from season to season.

Based on new spectral and photometric observations from the following observatories: Dominion Astrophysical Observatory, Fairborn, Haute Provence, Hvar, Limber, McDonald, Ondřejov, and Ritter.

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