UBV photometry (Table 3 (click here)), carried out in January 1996,
shows that Z CMa is presently in a low state at 
.
This value is in good agreement with the decreasing luminosity trend shown in
the AAVSO database regarding Z CMa (Mattei 1996). We derived
the following colors: B-V = + 1.00 and
U-B = + 0.54.
| Filter | J.D. | Mag | Exp. |
| (s) | |||
| U | 2450100.4780 | 11.78 .04 | 600 |
| B | 2450100.4910 | 11.24.04 | 300 |
| V | 2450100.4976 | 10.24.01 | 120 |
|
|
In the period March-April 1985 the star was in an
active state, confirmed by the measured value V = 9.0, in
comparison with V = 9.4 measured in April 1984 (Hessman et al.
1991). The variability trends of H
, 
, 
and Fe II lines are shown
in Fig. 1 (click here).
Figure 1: Variation of the Z CMa spectrum from the 1984 quiescent state
to the 1985 active state (IDS spectra). Transition of from pure
absorption to P Cygni morphology, enhancement of ,
and the appearance of FeII emission are seen
The main variations occurred from April 1984 to March-April 1985 are the following:
of the emission line and
an increase of 100 kms-1 of the velocity of its blue-shifted absorption component
(Fig. 1 (click here), Tables 4 (click here) and 5 (click here)),
emission from 6
to 8.6 Å, corresponding to an expansion velocity increase of about 120
kms-1 (Fig. 1 (click here), Table 5 (click here)).
line from a pure absorption profile to a
strong P Cygni profile and an increase of 160 kms-1 of its
blue-shifted absorption component (Fig. 1 (click here), Table 5 (click here)).
No significant variations are present in the NaID doublet.
The spectrum obtained on January 17 1996 shows a very prominent variation
of the state of the star, characterized by a drastic change of the
profile and by a 20 increase of emission EW
in comparison with the 1985 spectrum (Fig. 1 (click here), Table 4 (click here)).
The presence of some slight P Cygni effects both in and in H
seems to indicate a residual activity. A much more detailed analysis of the
unusual profile and of other peculiar characteristics was performed
on the medium dispersion BFOSC spectrum, the results of which are discussed in
Sect. 3.3.
No clear evidence of variability of EW and profile on a monthly or daily time scale is present in any spectrum.
A study of very short-term variability (seconds to minutes), carried out with a
time-sequence of 15 spectra on April 16, 1984 and with a time-sequence of 7
spectra on April 17, 1984, gave a negative result: the most important lines
( and ) did not show any EW or morphological variation.
This result can be expected because of the quiescent
state of the star in this period, in which friction-driven fast variations in
the gas of the accretion disk are unlikely to be observed using
low-dispersion spectroscopy. A study of the short-term
variability could give more significant results in the active states of the
star. Unfortunately we could not carry out this monitoring in 1985 due to the
limited available observing time.
Our CASPEC spectrum on April 11, 1985 supplied the most detailed
line analysis. As it is possible to see in Figs. 2 (click here), 3 (click here) and
Table 7 (click here), the 15 reduced echelle orders present a great number of
emission lines. The strongest P Cygni effect is present in
(Fig. 4 (click here)), where we measured a heliocentric velocity 
from the absorption component. The velocity difference
between the emission and absorption components is on the order
of 
, while the blue wing of the absorption component
is characterized by a terminal velocity of about 
. The
doublet NaID presents many absorption components
(Fig. 5 (click here)): the deepest one has 
,
the bluest one has 
, extending up to a
terminal velocity of (see also Table 7 (click here)).
Similar values can be deduced from the CES spectrum.
Other P Cygni features can be seen in all FeII lines but their
absorption components are too weak to allow accurate equivalent width (EW)
and radial velocity (RV) measurements.
Much stronger P Cygni effects, as easily shown in the 1985 low-dispersion
spectra, should have appeared in the range 4900-5200ÅÅ in FeII 4923Å, 5018Å and 5169-71ÅÅ lines, but unfortunately
this range was not covered by CASPEC spectra.
The average velocity of the FeII absorption components is - 90
kms-1 while the average velocity of the FeII emissions is about
38 kms-1, comparable with the average velocity of the
emission component (
).
Figure 2: CASPEC overall spectrum (5700-6250 Å)
obtained during the 1985 active state.
Identified lines are indicated
Figure 3: CASPEC overall spectrum (6250-6750 Å)
obtained during the 1985 active state. Identified lines are indicated
Figure 4: line in 1985 CASPEC spectra (short dash), 1989 REOSC
spectra (long dash) and 1996 BFOSC spectra (solid).
Radial velocity (
) is heliocentric
Figure 5: Absorption components of doublet NaID (CASPEC spectrum
acquired in 1985 April 11).
Heliocentric velocities () of absorption components of
NaI 5889.9 Å are indicated
In addition to emission and P Cygni features, CASPEC spectra show clear
evidence of double-peaked absorption lines at 
5915Å,
6142Å, 6192Å, 6440Å, 6451Å, 6496Å, 6644Å, 6678Å,
6664Å, 6709Å and 6719Å: we measured an average velocity of
their peak separation 
.
The absorption line HeI 6678.15 Å, blended with the FeI
disk doubled absorption line at 6678 Å (Fig. 6 (click here)), apparently is
the only photospheric feature of interest in our CASPEC spectra. A
blue-shift corresponding to 
was derived
for the HeI 6678.15 Å absorption line.
Our CES spectra (Fig. 7 (click here), Table 8 (click here)) were mostly concentrated
on .
No relevant monthly EW variation of the emission is detected between
the spectra taken on April 8-9, 1985 and the spectrum taken on March 15, 1985.
The EW's may be affected by an erroneous determination of the continuum level due to
the small spectral range covered by CES spectra. At the same time, some
differences are evident in absorption components between the March and April
spectra.
Figure 6: CASPEC spectrum (1985 April 11)
showing HeI 6678.15 deep absorption line.
Double-peaked absorption lines at 6644, 6679, 6709 and 6719 Å\
are also indicated
Figure 7: CES spectra, obtained during the 1985 active state, showing variation
of the absorption component within a period of
approximately a month
From CES spectra we determine an average velocity of - 470 kms-1 for
the absorption component and an average velocity of +44 kms-1
for the emission component. There is a velocity decrease of about 20
kms-1 from 15 March to 8-9 April, which we interpretate as a consequence
of wind deceleration. Furthermore, CES spectra show also very strong
CaII 8498Å and 8542Å emission, whose radial velocities
seem to be in good agreement with the emission measurement.
CaII lines are present
with a (more or less) pronounced P Cygni profile. The derived velocities from
absorption components are - 116 and - 196 kms-1 respectively. This
difference can be interpreted as the effect of two equally dense and hot
shells ejected at different velocities.
The small difference between the velocities measured with CES and the ones measured with CASPEC is probably due to the fact we used two different instruments. Because of this, we focus attention on measurements made with a single instrument and consequently to ratios obtainable between values of a given line parameter present in a specific spectrum.
From the medium-dispersion spectra taken with the RES spectrograph, we
obtained (Fig. 4 (click here)) and profiles.
The emissiony (EW = 15.48 Å) and absorption (EW = 6.42 Å)
components of show a velocity difference of 656 kms-1, while
the blue wing of the absorption component extends out to - 1150
kms-1. is
present strongly in absorption (EW = 7.00 Å) with a weak emission
component (EW = 0.10 Å), absorption and emission components show a velocity difference of 270
kms-1.
BFOSC spectra (Table 9 (click here) and Figs. 8 (click here) and 9 (click here)) obtained with a medium-dispersion of 22 Å/mm show the most interesting characteristics both in terms of variability and in terms of peculiar morphology. The following fundamental features are evident:
Figure 8: BFOSC overall spectrum (5000-7500 Å) obtained in 1996
January 17. Identified lines are indicated
Figure 9: BFOSC overall spectrum (7500-9800 Å) obtained in 1996
January 17. Identified lines are indicated
1. The profile is completely changed in comparison to
the profile shown in the 1984-1985 spectra (Fig. 4 (click here)).
In addition to the main emission a second blue shifted emission is present.
The central absorption, shifted by - 516 kms-1 with respect to
the main emission, corresponds perfectly to the absorption detected in the
ESO spectra. The central wavelengths of the two emission components
are separated by a velocity of over 800 kms-1.
2. The spectrum shows the usual FeII emissions which are normally present in the spectrum of Z CMa during the active states, but only the FeII 5169Å line shows a sharp P Cygni effect. All the other FeII lines do not show P Cygni features. The average velocity of the FeII emissions is + 77 kms-1.
3. The spectrum shows the forbidden [OI] 6300Å emission line (EW = 1.5 Å), much stronger than in the 1985 CASPEC spectrum, and the forbidden [SII] 6717Å (EW = 0.1 Å), [SII] 6731Å (EW = 0.5 Å) and [FeII] 7155Å (EW = 0.4 Å) emission lines. In particular, [OI] 6300Å and [SII] 6731Å\ present both an asymmetric profile with the red component much stronger than the blue, and with barycenters strongly blue-shifted.
4. The blue wing of the NaI 5889.9 Å absorption line corresponds to a velocity of over - 470 kms-1, about 80 kms-1 less than the values derived from April 1985 CASPEC and CES spectra.
5. In five cases, at 6192 Å, 6345 Å, 6496 Å, 6644 Å and 6664 Å,
we identified double-peaked "disk absorption lines''. An average
velocity of 
kms-1 was obtained from their peak separation.
The shallow HeI 6678Å absorption line, present in CASPEC 1985
spectrum, is absent. At the same wavelength the BFOSC spectrum shows a strong
double peaked "disk absorption line''.
As shown in Table 10 (click here), JHKLM photometry shows decreases of up to 0.2-0.3 mag from April 1984 to March-April 1985. In the same period, color indexes J-K, H-K and K-L are subject to a blueing effect consistent with a decrease of up to 0.2, 0.08 and 0.08 mag respectively. We notice that such IR luminosity increases and color blueing can be correlated with analogous variations detected in the optical range (Hessman et al. 1991). Similar variations in the IR range can be found in the literature (Kenyon & Hartmann 1991; Berrilli et al. 1992; Hamann & Persson 1992; Molinari et al. 1993; Noguchi et al. 1993). No luminosity or blue increases larger than 0.02-0.06 mag are recorded on a daily or monthly timescale.
The energy distributions in the range 0.4-5 
m obtained in 1984 and 1985,
are presented in Fig. 10 (click here).
The optical components of these distributions are represented
by low-dispersion
spectra taken on March 17, 1985 and on April 17 1984.
The IR components are given by an average of spectrophotometric data taken
in March and April 1985, overlapped with IR photometric data
taken in April 1984 and April 1985.
Figure 10: Energy distribution of Z CMa in the range 
constructed from April 1984 (thick line) and March 1985 (dotted line)
spectroscopic data merged with IR March-April 1985
spectrophotometric data (dashed line), IR April 1984
photometric data (filled circles) and IR April 1985
photometric data (open circles)
The following main features can be outlined from the energy distributions:
m.
m.
| Identification | Absorption | Emission | |||
| EW | RV** | EW | RV** | ||
| (Å) | (km s-1) | (Å) | (km s-1) | ||
|
NaI D | 7.0 | - 52 | |||
| '' | - 127 | ||||
| '' | - 172 | ||||
| '' | - 234 | ||||
| '' | + 26* | ||||
| FeII 5991.4 | 0.2 | - 69 | 0.2 | + 40 | |
| FeII 6238.4 | weak | - 66 | 0.5 | + 37 | |
| FeII 6247.6 | weak | - 119 | 0.6 | + 40 | |
| [OI] 6300.2 | 0.2 | - 2.4 | |||
| FeII 6369.5 | weak | - 74 | 0.1 | + 25 | |
| FeII 6416.9 | weak | - 144 | 0.2 | + 38 | |
| FeII 6432.6 | weak | - 105 | 0.4 | + 45 | |
| FeII 6456.4 | weak | - 72 | 0.5 | + 37 | |
| FeII 6516.0 | weak | - 84 | 0.7 | + 38 | |
H | 6.5 | - 510 | 29.1 | + 46 | |
| HeI 6678.1 | 1.0 | - 207 | |||
| FeII 6729.9 | 0.1 | + 20 | |||
|
* Interstellar line | |||||
| ** RV heliocentric. | |||||
|
Date | J.D. | J | H | K | L | M |
|
1984 Apr. 17 | 2445807.558 | 6.11 | 4.84 | 3.68
| 1.85 | 0.93 |
| 1984 Apr. 18 | 2445808.551 | 6.15 | 4.88 | 3.71 | 1.90 | 0.94 |
| 1985 Mar. 17 | 2446141.507 | 5.82 | 4.61 | 3.52 | 1.76 | 0.80 |
| '' | 2446141.666 | 5.80 | 4.65 | 3.52 | 1.73 |
0.77 |
| 1985 Mar. 18 | 2446142.579 | 5.81 | 4.63 | 3.53 | 1.75 | 0.83 |
| 1985 Apr. 9 | 2446164.546 | 5.84 | 4.70 | 3.60 | 1.83 | 0.85 |
| 1985 Apr. 10 | 2446165.499 | 5.78 | 4.66 | 3.56 | 1.81 | 0.87 |
| 1985 Apr. 11 | 2446166.490 | 5.83 | 4.68 | 3.59 | 1.81 | 0.83 |
| '' | 2446166.497 | 5.82 | 4.66 | 3.56 | 1.79 |
0.80 |
| '' | 2446166.516 | 5.82 | 4.67 | 3.56 | 1.79 |
0.80 |
| '' | 2446166.523 | 5.81 | 4.66 | 3.57 | 1.81 |
0.79 |
|
|
The absorption band at 1.9 m, due to the composite
transitions of vibration-rotation bands of water vapor (Sato
et al. 1992), is
the only clear and seemingly constant characteristic of the IR
spectrophotometric energy distribution. This absorption feature is the
result of the heating effect on water-ice grains which are contained in the
external region of the proto-stellar accretion disk. Such heating is caused by
the ionization front developed during the 1985 phase.
Given the variability of Z CMa, different energy distributions are expected from data taken in different epochs. In particular, in our data this difference can be noticed in the general flux increase from 1984 to 1985, in a steep increase in the J and H fluxes and in the basic equality of H and K fluxes of 1985 (in comparison with 1984 IR data).
Previous authors (Berrilli et al. 1992; Natta et al.
1993) demonstrate that the envelope is the source of a strong IR
excess starting at about 2 m.
20%
, error in RV is 
. Four values of
RV are measured for NaID corresponding to different
components as shown in Fig. 
)
spectra acquired on January 17 1996.
Error in EW is 
.
For lines with 
it was not possible to measure
RV because of bad calibration due to echelle order overlapping