Z CMa is a variable luminous pre-main-sequence object. Although Z CMa is normally classified as an Herbig Ae/Be star, it has many features in common with FU Ori variables (Hartmann et al. 1989). The FU Ori variables are pre-main-sequence objects associated with curved reflection nebulosity that have undergone large, decade-long photometric outbursts. Their spectra vary from F or G supergiant spectral type in the optical region to M in the near-infrared and show large infrared excesses and signs of strong mass-outflow. Doubled absorption line profiles are observed in many FU Ori objects. Z CMa is associated with a curved reflection nebulosity (Bhatt & Sagar 1992) and a high-velocity optical jet-like outflow with a linear extent of 3.6 pc (Poetzel et al. 1989). In the past, Z CMa showed irregular optical variability from a quiescent state at V = 11.5 to an active state at V = 8.5 (Covino et al. 1984; Hessman et al. 1991). A 0.4 mag eruption and a 0.7 mag outburst occurred in 1985 and 1987 respectively, were observed and well studied (Hessman et al. 1991). More recently Z CMa reached in 1992 a low state at V = 10.50 (Miroshinenko et al. 1993). Moreover, it is possible that irregular variations mask regular variations produced by the modulation of an inhomogeneous spot-like photosphere due to axial rotation of the star (Herbst et al. 1987). Photometric variations of the order of 0.1- 0.2 mag on a scale of 2-3 days are also present (Covino et al. 1984) and it seems that they are strictly connected with the active state of the star.
The spectrum of Z CMa is a superposition of a spectrum from a rotating middle F star with that of a late B star with P Cygni Balmer lines (Strom et al. 1972) but, with the exception of the HeI 6678.15 Å line, seldom detected (Herbig 1960; Covino et al. 1984; Hartmann et al. 1989; Hessman et al. 1991), there is no convincing evidence of any HeI absorption which should be expected from a B-type photosphere.
The most prominent spectroscopic features and their variability are explained
by the presence of a circumstellar disk with a high accretion rate.
The disk axis was
recently estimated to have an inclination 
(Poetzel et
al. 1989; Hessman et al. 1991). In particular, three
very distinguished spectroscopic effects were observed: accretion-driven
disk absorption lines, accretion-driven disk emission lines and wind-driven
strong P Cygni lines.
The disk-driven absorption line signature (Hartmann et al.
1989; Welty et al. 1992) comes from the presence of
double-peaked absorption-line profiles with a velocity difference of 100
kms-1 or more, most common longward of 5800Å, as expected from
a disk rotating at Keplerian velocities and whose vertical gravity produces
a spectrum consistent with that of a low-gravity atmosphere. This relevant
occurrence has been strongly suggested by the comparison of the observed
lines with model disk spectra synthesized from supergiant spectra of
standard stars (Welty et al. 1992). It is also possible to
confirm the disk-driven absorption line doubling effect by
cross-correlating Z CMa spectra with spectra of giants as templates
(Hartmann et al. 1989). Moreover observations by Welty et al.
(1992) confirm a theoretically predicted correlation between
rotational velocity 
and wavelength, in the framework of a disk
model in which the temperature rises toward the internal region of a disk
in the Keplerian regime of motion. All these effects, relative to
absorption lines, are proof of an internally heated object, consistent with
the inner zone of an optically thick accretion disk where most of the
energy is generated near the midplane.
The observed emission lines, mostly from neutral and singly ionized metals, are particularly strong longward of 7000Å. These lines cannot be formed in the stellar atmosphere, as any line emission from the central star should not be detectable against the background of the disk emission. They can arise from the outer region of the accretion disk at disk radii at least 2-4 times larger than those at which double absorption profiles are produced, as their rotational width in all states is a factor 1.5-2.0 smaller than the peak-to-peak separation of the double absorption lines (Hessman et al. 1991; Welty et al. 1992). A very low opacity in the external region of the disk can be the origin of emission line formation. Strong emission features in the spectrum of Z CMa are particularly present in the high states of the star.
The observed P Cygni effects (Covino et al. 1984; Welty et al.
1992; Hartmann & Calvet 1995), particularly prominent
shortward of 7000Å and sometimes with doubled emission, are clear
evidence of very strong winds, with terminal velocities of over 1000
kms-1. These winds cannot form in the star photosphere, but may be
triggered directly from the inner rapidly rotating region of the accretion
disk and probably carry away large amounts of angular momentum from the
system. The strong P Cygni effect often present in the spectrum of Z CMa is
consistent with mass outflow with a rate of 
(Croswell et al. 1987). P Cygni Balmer lines are an
almost constant feature (Pickering 1913; Merrill
1927), both in the active and in the quiescent state of the star.
During the active states, a P Cygni effect is also present in CaII,
TiII, NaI and FeII lines and becomes much stronger
in the Balmer lines (Hessman et al. 1991) with a sharp
increase of both the equivalent width of the emission component and the
radial velocity of the absorption component.
There is clear evidence that the inner, absorption-line region and the outer, emission-line region are dynamically, directly or indirectly, coupled by a not yet understood physical process (Hessman et al. 1991). This can be clearly noticed in the high photometric states of Z CMa (V = 8.5) in which there is an increase of both the emission line widths and the separation of the double absorption lines, thus suggesting that the maximum luminosity is related to a shrinkage process of both the optically thick and the optically thin disk regions. This process, which is also accompanied by a strong increase of wind-driven P Cygni lines and which culminates in strong outbursts with an increase of over 2 mag and a 60-100 days duration, can be reasonably due to the occurrence of extremely rapid accretion events, in analogy with other accreting systems like dwarf novae.
Z CMa is also characterized by a strong infrared excess (Oudmaijer et al. 1992; Berrilli et al. 1992), larger than the one predicted by steady accretion disk models and possibly caused by a large, non-spherically symmetric dust cloud surrounding the star, which absorbs light generated from interior regions and reemits this energy in the infrared. A possible correlation of infrared excess with emission lines is suggested by Hamann & Persson (1992), Natta et al. (1993), Torres et al. (1995).
Near-infrared speckle observations of Z CMa reveal it to be a double star with
separation 0
1 at 
. The north-west component is
an infrared object whose broadband spectrum is reminiscent of the infrared
companions to several T Tauri stars. The south-east component has the
spectral energy distribution expected for a circumstellar disk whose luminosity
is dominated by gravitational accretion (Koresko et al. 1991).
In this paper we give the results of the analysis of our photometric
observations, of our low, medium and high dispersion spectroscopic observations
and of our IR spectrophotometric and photometric observations of Z CMa. We
present the mean features of three well distinguished states of this star:
a semi-quiescent state in 1984 with much weaker emission features in
FeII lines and with no P Cygni effect in 
and H
, a
semi-active state in 1985 with the presence of strong emissions and P Cygni
effects in Balmer and FeII lines, a similar semi-quiescent state in 1989
and finally a new state in 1996 characterized by the presence of [OI],
[SII] emission lines and of a double-emission
feature at 
.