1998AJ....115.1610V

We observed AB Doradus, a young and active late-type star (K0-K2 IV-V, P = 0.514 days), with the Goddard High Resolution Spectrograph of the post-COSTAR Hubble Space Telescope with time and spectral resolutions of 27 s and 15 km.s^–1, respectively (1994 November 14.08-14.30 UT). The wavelength band (1531-1565 Å) included the strong C IV doublet (1548.202 and 1550.774 Å, formed in the transition region at 10⁵ K), the chromospheric Si II 1533.432 Å line, and the blend of Si I, C I, and Fe II lines at 1561 Å. The mean quiescent C IV flux state was characterized by F_C IV_ = (7.80±0.34)x10⁵ ergs.cm^–2.s^–1, close to the saturated value and 100 times the solar one. The line profile (after removing the rotational and instrumental profiles) is bimodal, consisting of two Gaussians, one narrow (FWHM = 70 km.s^–1) and the other broad (FWHM = 330 km.s^–1). This bimodality is probably due to two separate broadening mechanisms and velocity fields at the coronal base. It is possible that transition-region transient events (random multiple velocities), with large surface coverage, give rise to the broadening of the narrow component while true microflaring is responsible for the broad one, as suggested by Wood, Linsky, & Ayres.

The transition region was observed to flare frequently with different timescales and magnitudes. The largest impulsive flare seen in the C IV λ1549 emission, at day 14.22, reached in less than 1 minute a peak differential emission measure N²_e V (10^4.85-10^5.15 K) = 10^51.2 cm^–3 and returned exponentially in 5 minutes to the 7 times lower quiescent level. The 3 minute average line profile of the flare was blueshifted (-190 km.s^–1) and broadened (FWHM = 800 km.s^–1). This impulsive flare could have been due to a chromospheric heating and subsequent evaporation by an electron beam, accelerated (by reconnection) at the apex of a coronal loop.