On the source of the flaring activity in AB Doradus: the UV spectral signatures

Abstract

AB Dor is a young, 30-Myr-old, low-mass star with a short rotation period and strong flaring activity. In this work, a detailed analysis of the flaring activity is carried out based on ultraviolet spectral tracers of hot (T similar to10(5) ) plasma, namely HST /GHRS profiles of the C iv[uv1] and Si iv[uv1] lines. The quiescent component has been subtracted out and the spectral signature of the flares is analysed in detail. A total of nine events are detected in 10.63 h. The e-folding time for flares has been derived when possible, yielding values between 300 and 1200 s. It is shown that the C iv[uv1] and Si iv[uv1] profiles associated with the flaring activity can be classified into three main groups: narrow redshifted profiles, 'broad' profiles and double-peaked profiles. Therefore, the spectroscopic signature of flares in hot (T similar to10(5) K) plasma is not always the same. As a consequence, it is highly likely that different mechanisms are involved in the flaring activity of AB Dor. The most frequently observed profiles are the narrow redshifted profiles; six out of the nine events display this type of profile. An analysis of the velocity field at the stellar surface shows up that they are most likely associated with matter infall. Henceforth, it is feasible that this type of flares are associated with downward flowing material in post-flare magnetic loops , as frequently observed in the Sun. The broad profiles have a FWHMsimilar or equal to300 km s(-1) and are very asymmetric with an extended blue wing reaching 400 km s(-1) . These profiles have been observed during the strongest flare in the monitoring when the line flux rose up to three times the quiescent value. The large width and asymmetry of the profiles are best explained if this type of flares are associated with gas flows in curved magnetic structures. Three possible mechanisms are analysed: (1) the development of thermal instabilities in large magnetic loops, (2) gas infall and (3) the interaction between the fast and slow components of the stellar wind. The last mechanism seems to be the most feasible given the short duration of the flares. Finally, a possible absorption caused by the hot component of 'sling-shot' prominences has been searched for in the C iv[uv1] and Si iv[uv1] profiles. Two of the three strongest Halpha prominences (A and H) seem to have been detected. The most compelling evidence corresponds to A, when an absorption component is observed moving from blue to redshifted velocities from phi =10440.978 to phi =10441.010. The strength of the absorption at phi =10441.010 is 0.2x10(-12) erg s(-1) cm(-2) and has a FWHM=86 km s(-1) .