The nature of the recent extreme outburst of the Herbig Be/FU Orionis binary Z Canis Majoris

Context. Z CMa is a binary system which consists of two young stars: a Herbig AeBe component Z CMa NW embedded in a dust cocoon and a less massive component Z CMa SE, which is classified as a FU Orionis type star. Associated to the binary system is a giant parsec-size jet. Past spectropolarimetric observations showed that the position angle of the linear optical polarization is perpendicular to the jet axis, indicating that the visual light escapes the cocoon via cavities aligned with the jet axis and is then scattered back into the line of sight of the observer. Recently the system showed the largest outburst reported during the almost 90 years of available observations. Aims. We present new spectrophotometric and spectropolarimetric data obtained in 2008 during the recent outburst phase. Methods. The data obtained in the visual spectral range at medium spectral resolution were used to study the geometry of the system from the linear polarization spectra as well as its magnetic field from the circular polarization spectra. Results. During the recent outburst we detected that the Z CMa system is polarized by 2.6% in the continuum and emission line spectrum, with a position angle still perpendicular to the jet. From the high level of polarization we concluded that the outburst is associated with the dust-embedded Herbig AeBe NW component. The deep absorption components of the Balmer lines in the velocity frame which extend from zero velocity and reach a wind velocity of ~700 km s-1, together with the absence of a red-shifted broad emission at similar velocities, indicate a bi-polar wind. We did not detect a significant mean longitudinal magnetic field during the outburst, but in the data obtained in 2004 we detected the possible presence of a rather strong magnetic field of the order of ~1 kG. However, we critically review the applied method of magnetic field measurements in the presence of a strong stellar wind. The main result of our studies is that the bolometric luminosity of Z CMa remained surprisingly constant during the recent outburst. We conclude that either the geometry of the cavity through which the light escapes from the cocoon has opened a new path or that the screen of dust, which reflects the light toward the observer became more efficient, causing the observed increase of the visual brightness by about 2$\fm$5