\documentstyle[epsf]{article}

\def\mwc{MWC\,560}
\def\kms{km\,s$^{-1}$}

\input ibvs2.sty

\begin{document}
\IBVShead{4598}{8 June 1998}

\IBVStitle{Does V694 M\lowercase{on} enter an  inactive phase?}

\IBVSauth{M.~Miko{\L}ajewski$^1$, J.{\L}.~Janowski$^1$, T.~Tomov$^{1,2}$, U.~Munari$^3$, A.~Antov$^4$, A.~Jones$^5$}

\IBVSinst{Centre for Astronomy, Nicolaus 
Copernicus University, Toru\'n, Poland}

\IBVSinst{NAO Rozhen, Smolyan, Bulgaria} 

\IBVSinst{Osservatorio Astronomico di Padova, Sede di Asiago, Italy}

\IBVSinst{Institute of Astronomy, Sofia, Bulgaria}
 
\IBVSinst{Carter Observatory, Wellington, New Zealand} 

\IBVSabs{New photometric and spectral observations show a significant}
\IBVSabs{change in a behaviour of the \mwc\, one of the peculiar members of }
\IBVSabs{symbiotic binaries.}

\SIMBADobj{V694 Mon}
\IBVStyp{ * }
\IBVSkey{photometry, spectroscopy}


\begintext


MWC 560 $\equiv$V694 Mon is a very peculiar member of the symbiotic
variables. The system consists of an M4.5 red giant and probably a magnetic
white dwarf. The lack of evidence of orbital motion suggests a very low orbital
inclination: the {\it face-on} orientation is quite probable. The same is
suggested by the most intriguing feature of \mwc\, fast (1000--6000\,\kms)
and collimated jet along the line of sight, seen in the spectrum as a
very broad and variable blue-shifted absorption of Balmer and singly ionized
metal lines (Tomov et al. 1990, Shore et al. 1994). The star is supposed to
be one of the prototypes of a possible new subclass of the symbiotic
binaries, {\it propellers} (Miko{\l}ajewski et al. 1996). The luminosity of
the hot component in these systems is partially derived from the stellar
wind accretion and partially from the rotation of a white dwarf transferred
into the accreted matter {\it via} the magnetic field (oblique rotator).

In this paper we present spectroscopic and photometric results for the
last year. The spectra were obtained with the echelle spectrograph
mounted on the 1.82m telescope of the Asiago Observatory.
Photometric data were collected with the single channel UBVRI 
photometer mounted on 60\,cm telescope at Toru\'n Centre for Astronomy
(Poland) and UBV photometers at Belogradchik Observatory and Rozhen National
Astronomical Observatory (Bulgaria), also attached to 60\,cm reflectors. 

In an earlier paper about \mwc\ (Miko{\l}ajewski et al. 1997) we presented
the photometric behaviour of the star after its highest maximum in 1990,
when it reached
$B\approx$9\fmm4. Since then we have observed a systematic decrease until
April 1997 when B dropped to about 12\mm. During this period, brightness in
UBVR bands varied in a very similar way. The changes of the I magnitudes
were different and probably reflected variations of the red giant. In 1995
the system had a weak maximum that is consistent with 1930\day\ orbital
period (Doroshenko et al. 1993) and may correspond to the periastron
passage. The spectra showed wide, variable rectangular-shaped, jet-origin
blue-shifted absorption components during the whole 1990--97 period (see
also Tomov \& Kolev 1997).

The significant drop in brightness in April 1997 (Miko{\l}ajewski et al.
1997) appeared to be only a temporary change because at the beginning of
1997/98 observational season (from October 1997 to April 1998) \mwc\ was
slightly brighter again. However, there is a substantial difference in the
character of the light curve. The scatter of colours has\break 

\IBVSfig{10cm}{4598-f1.ps}{The colour light curves of MWC 560 during last
three seasons (Oct 1995 -- Apr 1998): 
Toru\'n UBVRI data ({\it filled squares}), Rozhen and Belogradchik
UBV data ({\it filled circles}) and visual estimates by A.J.
({\it crosses}). The visual magnitudes are corrected by $-$0\fmm22 as in
Miko{\l}ajewski et al. (1997) and vertical lines mark the dates of spectroscopic
observations shown in Fig. 3} 

\IBVSfig{6cm}{4598-f2.ps}{Flickering in U band on 1st January 1998.
Crosses show the U magnitude of the comparison star (HD 59380) during the
observation run shifted by a constant}

\vskip 3mm
\noindent significantly
increased on a time scale of days to weeks (Fig. 1.) and more rapid decrease of
the hot continuum, especially since January 1998, has been observed in BV
bands. In April 1998 the V--R and V--I indices reached values typical for
an M--type giant.


\IBVSfig{8.5cm}{4598-f3.ps}{Left: the $H_{\alpha}$ spectral region.
The $H_{\alpha}$ emission peaks are truncated and the February 1997 spectrum
is shifted by 1.5 for plot clarity. Right: enlarged $H_{\alpha}$ profile of
the 10 March 1998. For a better demonstration of the red-shifted jet a
mirror profile with respect to central emission was plotted as a thin
line} 

The decrease of brightness and large scatter of colour indices indicate that
the pseudophotosphere around hot component has become optically thin and
unstable. The A-B continuum is much weaker now or even completely absent.
Probably only HII emission in the B band with a prominent Balmer jump in U
are noticeable in March--April 1998. Flickering in the U band in January
1998 reached the largest amplitude ever observed, 0\fmm7 (Fig. 2.). From the
analysis of the \mwc\ flickering given by Tomov et al. (1996, see their
Fig. 6.) we conclude, that there is a possible correlation between the
peak-to-valley amplitude of the \mwc\ U band flickering and the U
brightness: when the star is fainter, the amplitude of flickering is
greater. January results follow that rule. An amplitude of\, 0\fmm7
magnitude was observed when U brightness was about 11\fmm1. If this
correlation is true, it means that the rapidly variable source of the
flickering remains independent from the A--B pseudophotosphere brightness
and varies with almost constant amplitude in flux units $\Delta\, F_{u}
\approx 10^{-13}\, erg\,s^{-1}\,cm^{-2}\,\AA^{-1}$. The only visible source
of the hot continuum now seems to be HII emission, so it would be identified
with the flickering source.

All the photometric changes are confirmed by visual estimates made by A.J. in
New Zealand with an outstanding time resolution. Only the last three
photoelectric points lie \,$\sim$\,0\fmm3--0\fmm5 below the visual light
curve.  We observed similar effect at the end of the previous season.  The
reason may be the very low position of the star above the horizon in Europe
at this time, but independent UBV observations from Toru\'n and Belogradchik
give similar results. Also, our spectral observations confirm that the
significant fading of the veiling hot continuum in the optical and TiO bands
of the M giant are stronger than before (Fig. 3. - left).


Generally, during 1997--98 \mwc\ was still ejecting matter in a
quasi-stationary regime (see Tomov \& Kolev 1997 for details). However, some
changes are visible. In the last year's spectra the main blue-shifted
rectangular absorption component was always visible. Occasionally an additional
faster component appeared, but both were weaker than during the previous seven
years (Fig. 3. - left). The typical velocity of the strongest absorption
components was in the range from $-$1500 to
$-$1800\,\kms \,till November 1997. At the end of 1997 the main absorption
appeared slower (about $-$1300\,\kms). In the spectrum from March 1998 the
main, trapezoidal absorption of the H$_{\alpha}$ is significantly weaker and
slower ($-$1000\,\kms) than in 1997 and the shape of
$H_{\alpha}$ looks like a P-Cyg profile. However, the emission and absorption
are not physically connected. The emission probably arises close to the hot
companion in a disk and/or envelope, while both absorptions originate in two
separate jet-ejections. Additional faster ($\sim$2000 \kms) absorption
and emission (!) components are clearly visible. The red-shifted (+2000
\kms) emission with the same shape and width as the blue-shifted absorption
indicates the presence of a {\it counter}, receding jet (Fig. 3. - right). In
the red wing of H$\alpha$, the emission equivalent of the main, trapezoidal
absorption is probably also visible.

The photographic $m_{pg}$ light curve from Sonneberg Sky Patrol plates which covered
the period 1930--1990 (Luthardt, 1991) shows that \mwc\ spent most of that time
varying between 12\mm\, and 11\mm. Only three times (1943--1954,
1958--61 and 1969--71) did the star drop to about
$m_{pg}\approx$12\fmm5 and remain relatively stable at this level for
several years. This level seems to be the lowest $m_{pg}$/B
brightness and probably indicates the non-active phase of the system, when the hot component remains inactive.
Our data show that B brightness has fallen below 12\mm, so we may expect a
similar non-active phase that may last for a few years unless the behaviour
of the star changes during its next possible periastron passage (in a year $\sim2000$).
During such a quiescence phase we have a unique possibility to take a look at the
close neighbourhood of a white dwarf, so additional optical, UV and X-ray observations 
are crucial. 

{\it Acknowledgments:} We are very much indebted to K. Rumi{\'n}ski for
carrying out some photometric observations and Bryan Jacoby for English
correction of the text. This research was supported by the Polish KBN Grant No. 2 P03D
011 13 , UMK Grant No. 362-a and Bulgarian NFSR Grant No. F--574/1995.

\references

Doroshenko, V.T., Goranskij, V.P, Efimov, Yu.S., 1993, {\it IBVS}, No.\,3824

Luthardt, R., 1991, {\it IBVS}, No.\,3563

Miko{\l}ajewski, M., Miko{\l}ajewska, J., Tomov, T., 1996, {\it Compact Stars
in Binaries}, eds. J. van Paradijs et al., Kluwer, Dordrecht, p.451

Miko{\l}ajewski, M., Tomov, T., Georgiev, L., Jones, A., 1997, {\it IBVS},
No.\,4519

Shore, S.N., Aufdenberg, J.P., Michalitsianos, A.G., 1994, {\it AJ}, {\bf 108}, 671

Tomov, T., Kolev, D., Georgiev, L., Zamanov, R., Antov, A., Bellas, Y., 1990,
{\it Nature}, {\bf 346}, 637

Tomov, T., Kolev, D., 1997, {\it A\&A Suppl.}, {\bf 122}, 43

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