Detection of Coronal Mass Ejections in V471 Tauri with the Hubble Space Telescope

V471 Tauri, an eclipsing system consisting of a hot DA white dwarf (WD) and a dK2 companion in a 12.5 hr orbit, is the prototype of the precataclysmic binaries. The late-type component is magnetically active, due to its being constrained to rotate synchronously with the short orbital period. During a program of ultraviolet spectroscopy of V471 Tau, carried out with the Goddard High Resolution Spectrograph (GHRS) on board the Hubble Space Telescope, we serendipitously detected two episodes in which transient absorptions in the Si III 1206 Å resonance line appeared suddenly, on a timescale of ≲2 minutes. The observations were taken in a narrow spectral region around Lyα and were all obtained near the two quadratures of the binary orbit, i.e., at maximum projected separation (~3.3 R☉) of the WD and K star. We suggest that these transient features arise when coronal mass ejections (CMEs) from the K2 dwarf pass across the line of sight to the WD. Estimates of the velocities, densities, and masses of the events in V471 Tau are generally consistent with the properties of solar CMEs. Given our detection of two events during 6.8 hr of GHRS observing, along with a consideration of the restricted range of latitudes and longitudes on the K star's surface that can give rise to trajectories passing in front of the WD as seen from Earth, we estimate that the active V471 Tau dK star emits some 100-500 CMEs day-1, as compared to ~1-3 day-1 for the Sun. The K dwarf's mass-loss rate associated with CMEs is at least (5-25) × 10-14 M☉ yr-1, but it may well be orders of magnitude higher if most of the silicon is in ionization states other than Si III.