Rotational modulation and flares on RS Canum Venaticorum and BY Draconis.stars. X. The 1981 October 3 flare on V711 Tauri (= HR 1099).

We present a unique set of high resolution spectra of V71 1 Tauri = HR 1099 (G5 V + K1 IV) obtained with both the SWP and LWR cameras of lUE, together with simultaneous 6.4 GHz microwave emission and optical photometry, during a bright flare on 3 October 1981. The electron density of the flaring plasma at 6 10⁴K was about 110¹¹ cm^-3, 15 times higher than quiescent, and the radiating volume was about 210³⁰ cm^-3, 200 times smaller than quiescent. A constrained multigaussian fit to the Mg II k line profile shows that the flare component profile was very broad (66 km s^-1 FWHM), indicating significant turbulence, and redshifted by 90 ± 30 km s^-1 relative to the center of mass of the K 1 IV star. We interpret this redshift as due to downflowing material probably located above a large starspot known from optical photometry and Doppler imaging to be near disk center of the K 1 IV star during the flare. The flux of kinetic energy at flare peak due to the downflow and turbulence was <1 10³² erg s^-1, essentially equal to the flare radiative luminosity. The microwave emission was probably gyrosynchrotron emission from mildly relativistic electrons (typical energy 1.7 MeV) trapped in magnetic flux tubes emerging from the whole area of this spot.