English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Emission lines formed in decretion disks of Be stars often undergo long-termcyclic variations, especially in the violet-to-red (V/R) ratio of their primarycomponents. From observations of the bright Be-shell star zeta Tau, thepossibly broadest and longest data set illustrating the prototype of thisbehaviour was compiled from our own and archival observations. It comprisesoptical and infrared spectra, broad-band polarimetry, and interferometricobservations. From 3 V/R cycles between 1997 and 2008, a mean cycle length in Halpha of 1400-1430 days was derived. After each minimum in V/R, the shellabsorption weakens and splits into two components, leading to 3 emission peaks.This phase makes the strongest contribution to the variability in cycle length.V/R curves of different lines are shifted in phase. Lines formed on averagecloser to the central star are ahead of the others. The shell absorption linesfall into 2 categories differing in line width, ionization/excitationpotential, and variability of the equivalent width. The interferometry hasresolved the continuum and the line emission in Br gamma and He I 2.06. Thephasing of the Br gamma emission shows that the photocenter of theline-emitting region lies within the plane of the disk but is offset from thecontinuum source. The plane of the disk is constant throughout the observed V/Rcycles. The self-consistent, one-armed, disk-oscillation model is developed inPaper II.

Cyclic variability of the circumstellar disk of the Be star zetaTau - I. Long-term monitoring observations