Spectrum of the "invisible" companion of Z Canis Majoris revealed in polarized light

Barbara A. Whitney, Harvard-Smithsonian Center for Astrophysics
Geoffrey C. Clayton, University of Colorado Boulder
Regina E. Schulte-Ladbeck, University of Pittsburgh
Nuria Calvet, Centro de Investigaciones de Astronomía
Lee Hartmann, Harvard-Smithsonian Center for Astrophysics
Scott J. Kenyon, Harvard-Smithsonian Center for Astrophysics


Optical spectropolarimetry of the FU Orionis variable Z CMa in 1991-1992 shows larger polarization in the emission lines than in the continuum. The intensity spectrum at this time has absorption lines with some narrow weak emission. The polarized flux spectrum appears similar to an intensity spectrum of Z CMa obtained in 1987 when it was ∼0.9 mag brighter at V and showed strong emission lines. We argue that the primary component of the Z CMa binary is an emission line source, perhaps an Ae/Be star, that varies at V by 1-3 mag and was responsible for the 1987 outburst. The primary is mostly obscured from view by an asymmetrical distribution of dust which polarizes the light scattered into our line of sight. The secondary, a normal FU Orionis variable, is seen more directly, and therefore contributes a large amount of unpolarized flux, about 80% of the total optical flux. Most of the polarization is intrinsic to the system and oriented perpendicular to the jet axis. We propose that independent variations in the brightness of both sources are responsible for the appearance and disappearance of a narrow emission line spectrum on the broader FU Orionis absorption line spectrum.