THE HIGH STATE OF THE FU ORIONIS VARIABLE Z-CANIS MAJORIS

We present photometry and high-resolution spectroscopy of the pre-main-sequence object Z CMa. During the first part of 1987, this luminous FU Orionis variable had an eruption during which the absorption-line spectrum from an accretion disk seen in the normal state was replaced by a nearly featureless and bluer continuum and a large number of narrow emission lines from neutral and singly ionized metals. Furthermore, the Balmer and Fe II P Cygni lines became much more prominent. The significant weakening of the disk absorption lines, despite the relatively small change in the visual brightness (from V approximately 9.4 to approximately 8.7 mag), shows that the accretion disk is at least partially responsible for the change in the spectral appearance. Though the strength of the Balmer P Cygni profiles changed considerably from the normal to the high state, the velocity range present in the blue absorption component did not, suggesting that the mass-loss rate but not the speed of the wind increased dramatically between the two states. During 1985, a slight increase in the brightness was observed, corresponding both photometrically and spectroscopically to an intermediate state. The profiles of the P Cygni H-alpha line in this middle state is considerably different from either the normal or the high state, with narrower absorption wings and a much less abrupt change from the absorption to the emission components. Since the 1987 eruption, Z CMa has become slightly fainter and redder than normal but shows the same disk spectrum as that seen in the normal state. We discuss the possible explanations for this complex behavior within the context of viscous accretion disk models. The high-state spectrum of Z CMa resembles that of other luminous, pre-main-sequence objects, and we suggest that such objects are also pre-main-sequence disks with large mass accretion rates.