The Rise and Fall of MU Velorum

Mu Velorum (HD 93497) is a close visual pair consisting of a G5 III primary and a fainter companion, currently 2'' apart, with an orbital period of about 140 yr. The distance to the system is 36 pc. Mu Vel A is a 3 M_sun giant, in the Hertzsprung gap beyond the ``rapid braking zone'' just redward of G0 III. Mu Vel recently was the source of a giant EUV flare, caught during a 12-day pointing in March 1998 by the Deep Survey telescope of the Extreme Ultraviolet Explorer. The outburst rose in less than half a day, and decayed with an e-folding time of several days. The peak flux, ~ 0.3 cnts s(-1) in the DS band 80--180 Angstroms, was approximately twice the quiescent level recorded during the previous ten days of observation. The size and long duration of the event are very unusual for a mid-G giant; in fact, more typical of the extremes seen among the hyperactive short-period RS Canum Venaticorum binaries. Although the secondary star is classified as a G2 V, published visual magnitude differences, and the enhanced 1900 Angstroms continuum (mu Vel was detected by IUE, but AB were not resolved), suggest that it falls earlier on the MS (perhaps F5 V) and indeed itself might be a pair. If mu Vel B is a short-period double, then arguably it could be the source of the giant flare. Unfortunately, little is known about the secondary owing to the small separation of the visual components of mu Vel, and their large difference in brightness. We discuss the flare event, EUV spectra obtained in quiescence and during the flare decay, and the nature of the puzzling secondary. [2mm] This work was supported by NASA grant NAG5-3226.