ULTRAVIOLET SPECTROSCOPY OF AB DORADUS WITH THE HUBBLE-SPACE-TELESCOPE - IMPULSIVE FLARES AND BIMODAL PROFILES OF C-IV LAMBDA-1549 IN A YOUNG STAR

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 Angstrom) included the strong C Iv doub let (1548.202 and 1550.774 Angstrom formed in the transition region at 10(5) K), the chromospheric Si Ir 1533.432 Angstrom line, and the ble nd of Si I, C I, and Fe It lines at 1561 Angstrom, The mean quiescent C Iv flux state was characterized by F-CIV = (7.80 +/- 0.34) x 10(5) e rgs 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 bimodali ty is probably due to two separate broadening mechanisms and velocity fields at the coronal base. It is possible that transition-region tran sient events (random multiple velocities), with large surface coverage , give rise to the broadening of the narrow component while true micro flaring 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 lambda 1549 emission, at day 14.22, reached in less than 1 minute a peak differential emission measure N-e(2) 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 t he flare was blueshifted(-190 km s(-1)) and broadened (FWHM = 800 km s (-1)). This impulsive flare could have been due to a chromospheric hea ting and subsequent evaporation by an electron beam, accelerated (by r econnection) at the apex of a coronal loop.