2004MNRAS.349.1516V

We report on a recent event in which, after more than a decade of slow fading, the visual brightness of the massive young binary Z CMa suddenly started to rise by about 1 mag in 1999 December, followed by a rapid decline to its previous brightness over the next six months. This behaviour is similar to that exhibited by this system around its eruption in 1987 February. A comparison of the intrinsic luminosities of the system with recent evolutionary calculations shows that Z CMa may consist of a 16-M_☉B0 IIIe primary star and a ∼3-M_☉FUor secondary with a common age of ∼3x10⁵ yr. We also compare new high-resolution spectra obtained in 2000 January and February, during the recent rise in brightness, with archive data from 1991 and 1996. The spectra are rich in emission lines, which originate from the envelope of the early B-type primary star. The strength of these emission lines increased strongly with the brightness of Z CMa. We interpret the collected spectral data in terms of an accretion disc with atmosphere around the Herbig B0e component of Z CMa, which has expanded during the outbursts of 1987 and 2000. A high-resolution profile of the 6300 Å[Oi] emission line, obtained by us in 2002 March, shows an increase in flux and a prominent blue shoulder to the feature extending to ∼-700 km/s, which was much fainter in the pre-outburst spectra. We propose that this change in profile is a result of a strong change in the collimation of a jet, as a result of the outburst at the start of this century.