The nature of the recent extreme outburst of the Herbig Be/FU Ori binary Z CMa

TL;DR

This paper investigates the recent extreme outburst of the binary system Z CMa, revealing polarization data, the likely source of the outburst, and the surprising constancy of its bolometric luminosity, suggesting changes in dust geometry or reflectivity.

Contribution

It provides new polarization measurements and analysis linking the outburst to the Herbig AeBe component, with insights into the outburst mechanism and dust environment.

Findings

Polarization of 2.6% detected during outburst

Outburst likely associated with the Herbig AeBe NW component

Bolometric luminosity remained constant despite brightness increase

Abstract

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. Recently, the system showed the largest outburst reported during the almost 90 years of available observations. During the recent outburst we detect 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 conclude that the outburst is associated with the dust embedded Herbig AeBe NW component. 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 2.5 magnitudes.