Achernar: Rapid polarization variability as evidence of photospheric and circumstellar activity

We present the results of a high-accuracy (sigma approximate to 0.005%) polarization monitoring of the Be star Achernar that was carried out between 2006 July 7 and November 5. Our results indicate that after a near-quiescent phase from 1998 to 2002, Achernar is currently in an active phase and has built a circumstellar disk. We detect variations both in the polarization level and position angle on timescales as short as 1 hr and as long as several weeks. Detailed modeling of the observed polarization strongly suggests that the short-term variations originate from discrete mass ejection events which produce transient inhomogeneities in the inner disk. Long-term variations, on the other hand, can be explained by the formation of an inner ring following one or several mass ejection events.