Ultraviolet Spectroscopy of AB Doradus with the Hubble Space Telescope: Impulsive Flares and Bimodal Profiles of C IV lambda1549 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 doublet (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 blend 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) 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 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 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.