Our observations show three well distinguished states of Z CMa.
In April 1984 the star was in a quiescent state mainly characterized by
m V = 9.7 (Mattei 1996) and by the
absence of emission components in the 
and FeII line profiles.
In October 1989 Z CMa was in a semiquiescent state characterized by
m V
= 9.6 (Mattei 1996) and by the presence of very weak and FeII 4923.9 emission components. The extension out to
-1150 kms-1 of the blue wing of the 
absorption
component still indicates the presence of a strong wind with a terminal
velocity comparable to the post-eruption one (Hessman et al.
1991).
In March-April 1985 we record an increase of 0.2- 0.3 mag in the
JHKLM bands and
our low-dispersion spectra show the transient occurrence of FeII line
emissions (in particular, the 5169Å line with a strong P Cygni profile), a
strong enhancement of the line and a sharp transition of
from a pure absorption to a strong P Cygni profile, and
finally the increase of over 100 kms-1 of the blue shifted absorption
component of the most prominent P Cygni lines.
Our data allow us to compare the behaviour of Z CMa
in the quiescent-active transition state (April 1984 - March 1985)
to the active-quiescent transition
state (February 1987 - December 1988) described by Hessman et al.
(1991). Thus we are able to confirm that the active state of the
star is characterized by a strong enhancement of the outflow power, as
strongly indicated by the increase
of both the EW and FWHM of the emission component and the
RV of the
absorption component.
The blue-wing extensions of the absorption components of the most prominent
P Cygni lines present in the April 1985 spectra have velocities of
less than the values measured by Hessman et al.
(1991) during the 1987 outburst. For this reason, and taking into
account also the coeval low-dispersion data, we find that the state of Z CMa
in April 1985 was characterized by a small eruption.
Moreover, the presence of HeI 6678Å absorption line, mixed with double-peaked "disk absorption lines'', further confirms that, in 1985, Z CMa was in a much less active state than the one recorded in 1987, in which absorption disk-lines were almost absent in the region 6610-6740ÅÅ, while a veiling effect, together with a strong HeI 6678Å line, was observed (Hessman et al. 1991). Finally, from the peak separation of the few disk-lines at our disposal, we measured a velocity lower than that measured in 1987. Therefore, in 1985 the accretion disk of Z CMa was crossing an intermediate stage of the shrinkage process and of the consequent keplerian rotation rate.
In general, the appearance of a photospheric line such as HeI 6678Å demonstrates that, during the active states of Z CMa, a stellar wind driven by the central star is overlapped with an outflow, as evidenced by P Cygni lines, driven by the accretion disk.
Considering the "mini-eruption phenomenology'' as a whole, we notice that the
object Z CMa, capable of long-duration and very low-frequency outbursts up
to 3 magnitudes in amplitude, possesses additional and important explosive
characteristics, consisting in small-amplitude and short-duration eruptions
which coexist with the big outbursts. For this reason we suggest that Z CMa has
properties of both a FUOr object and an EXOr object. EXOrs are more evolved
proto-stars which are characterized by small-duration (
1 yr), small-amplitude
(0.4-1.0 mag) and high-frequency (few years) eruptions (Hartmann et al.
1993): this seems to indicate that accretion disk thermal instability
processes decrease in intensity and increase in frequency as the object
approaches the ZAMS. Perhaps Z CMa could be a serendipitous example of a
link between the FUOr phase and the EXOr phase. For this reason, we stress
the importance of developing further models regarding both stellar evolution
and accretion disk thermodynamics.
The most relevant aspects of Z CMa's behaviour in 1996 are the following:
1. The star is at a low-level luminosity (V = 10.24), with a red
U-B color typical of post-outburst states but with a B-V color bluer
than typically measured in post-outburst states (B-V = 1.15-1.20).
The blueing of B-V could be due to a decreased extinction of the central
star due to a strong decrease of the circumstellar envelope density. A
B-V color change due to a shrinkage process of the accretion disk could
be an alternative possibility as well, but we consider it unlikely as it
would be inconsistent with the low value of the velocity of double-peaked
disk absorption lines and with the missing transition of the spectrum toward an
earlier type (absence of HeI 6678Å absorption line). The value
(
)
inferred from our double-peaked absorption lines, compared with the value
(
) typical of the post-eruption low state (Hessman et
al. 1991), together with the total absence of HeI 6678Å
\
absorption line, proves in fact the existence of a semi-low state of Z CMa
in January 1996.
2. The profile is completely changed from a pure P Cygni profile to
a composite profile which seems to be a mix of a P Cygni and of
a double emission profile. We think that
this combination, never encountered before in the
relevant literature on Z CMa, might be the consequence of
a significant and possibly asymmetrical
increase of the
opening angle of the outflow. If this is the case,
it could be expected that the blue-shifted small emission is
produced, via mechanical gas heating, in the optically thin
approaching part of the outflow
where the line emission is restricted to the volume outside the surface of
an asymmetric hollow cone or paraboloid. If
one assumes that such an increase of the outflow opening angle really occurs,
one can expect a general gas rarefaction as well.
Such an effect could produce evidence of an optically thin
and very extended outflow where forbidden lines can
be found.
In fact we record [OI] 6300 Å and [SII] 6731 Å forbidden lines.
Finally, the red region of the spectrum is characterized by the presence of strong emissions due to OI and CaII.
3. The star shows a residual activity, which can be inferred from the high
value of the emission EW, from the presence of FeII
emissions, of faint P Cygni profiles at H
,
(Fig. 2 (click here)), and at CaII 8542-8662Å lines. The
reason for such residual activity is, at the present time, not clearly
understood.