Detection of a strong transient blueshifted absorption component in the β Pictoris disc

We present high-resolution spectra (1.0 km s^?1 FWHM) of the circumstellar Ca K line towards β Pictoris obtained on 1997 June 19 and 20. On the former date a strong absorption component was found at a heliocentric velocity of v _helio = +8 km s^?1, that is blueshifted by 14 km s^?1 with respect to the main, 'stable', circumstellar component at v _helio = +22 km s^?1. To our knowledge, this is the first detection of a blueshifted Ca  ii component with a strength comparable to the more frequently observed redshifted events. On the following night a blueshifted component was still present, but its strength had decreased significantly; in addition, a strong redshifted component had appeared at v _helio = +54 km s^?1 which was absent on the previous night. The implications of these observations for the evaporating 'comet' model of spectral variations in the β Pictoris disc are discussed.