SHORT-TERM CHROMOSPHERIC VARIABILITY IN ALPHA-TAURI (K5-III) - RESULTS FROM IUE TIME-SERIES OBSERVATIONS

We evaluate time series observations of chromospheric lines (Mg II, Mg I, and C II) for the K giant alpha Tau obtained using the IUE LWP cam era at high dispersion. These observations cover a time span of about 2 weeks in 1994 February-March and were designed to resolve variations occurring within hours, days, and weeks. We consider the observationa l results in relation to theoretical acoustic heating models, motivate d by the fact that alpha Tau may exhibit a basal (i.e., minimum) level of chromospheric activity. The data reveal flux variations between th e extremes of 8% in Mg II h+k and 15% in each emission component. Thes e variations occur on timescales as short as 8 hr but not on timescale s longer than similar to 3 days. For the h and k components, flux vari ations occurring on a timescale as short as 1.5 hr are also found. The se changes are often not correlated (and are sometimes even anticorrel ated), leading to remarkable differences in the h/k ratios. We argue t hat these results are consistent with the presence of strong acoustic shocks, which can lead to variable Mg II line emission when only a sma ll number of strong shocks are propagating through the atmosphere. We deduce the electron density in the C II] lambda 2325 line formation re gion to be log N-e similar or equal to 9.0, in agreement with previous studies. Our data provide evidence that the Mg II basal flux limit fo r K giants might be a factor of 4 higher than suggested by Rutten et a l.