Time-variability analysis of the Halpha, He i and Na i D lines in the T Tauri star LkHalpha 264

Abstract

We present a time-series analysis of quasi-simultaneous high-resolution profiles of the Hα, He i (λ5876 Å) and Na i D lines for the highly active T Tauri star LkHα 264. Our data include repeated observations on time-scales ranging from approximately 1 d down to less than 1 h, at two different epochs. For the three lines we have analysed the time variability of the equivalent width and of the profile, using in this case both the normalized variance and correlation matrix analysis. The behaviours of the equivalent widths of the He i and Na i lines on time-scales of the order of 1 d were found to correlate well. The correlation is more conspicuous when the level of variability of the lines is more dramatic. We conclude that there is a common region of formation for these lines, very close to the star. However, the Hα equivalent width follows such behaviour only when the He i and Na i D lines are less variable. In terms of profile variations, He i and Na i differ from Hα. The He i and Na i D variations occur coherently over the entire profile, independently of the time-scales, although there are indications that such behaviour may break down for time-scales shorter than ≈20 min. In contrast, the variability of Hα occurs mainly on the blue side of the line. Variations on short time-scales seem to be associated with a secondary blue emission peak. Variations on time-scales of 1 d are linked to two weak blueshifted absorptions. These absorptions occur in an outer region and their behaviour can be understood in terms of an optical depth effect. We conclude that the Hα line is formed in two distinct regions. The inner region, which is not too distinct from the one producing the He i and Na i D lines, is highly sensitive to the level of activity. The outer and more extended region dominates the contribution to the flux in Hα when the general level of stellar activity is high.