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\IBVShead{5571}{20 October 2004}

\IBVStitle{XX~Ophiuchi in Deep Minimum after 37 Years}

\IBVSauth{Sobotka, P. $^{1,2}$}

\IBVSinsto{$^{1}$Astronomical Institute, Academy of Sciences of the Czech Republic,
251~65 Ond\v{r}ejov, Czech Republic.}
\IBVSinsto{$^{2}$MEDUZA group, N.~Copernicus Observatory, Krav\'{i} hora~2, 616~00 Brno, Czech Republic;\\ e-mail: {\tt petr.sobotka@astro.cz}}
\SIMBADobjJ2000{XX Oph}{17 43 56.50 -06 16 08.7}

\IBVStyp{ unique }

\IBVSkey{photometry - deep minimum}

\IBVSabs{XX Ophiuchi is a unique object, which is very difficult}
\IBVSabs{to classify. This star only slowly varied}
\IBVSabs{around the maximum magnitude (V approx 9 mag) during the past} 
\IBVSabs{37 years. In March 2004,}
\IBVSabs{the brightness started to fade by 0.2 mag per month. I stress the} 
\IBVSabs{urgent need of the spectroscopic observations during this minimum.}


\begintext

The first spectrum of XX~Ophiuchi (= HD~161114, SAO~141834, BD--6.4638), obtained by 
Mrs. Fleming in 1907 (see Fleming and Pickering 1908), showed the well marked emission 
line of H$\beta$. Annie Cannon in Henry Draper Catalogue classified the spectrum as
Bep, so the star seemed to belong to the group of Be stars. However, substantial 
differences in the spectra were found by Merrill (1924), who noted a lot of emission 
lines of ionised iron (Fe II). Merrill described that the enhanced line spectrum is 
presented in greater purity than in any ordinary laboratory source, so since that time 
the star has been called {\em Iron Star}. In 1925, however, the absorption lines of 
the enhanced titanium spectrum were the dominant feature (Merrill 1926).

The optical variability of XX~Oph was noted for the first time by Woods in 1921 
(see Cannon 1922). Most of the time the brightness of XX~Oph slowly varies
around the mean value of $V = 9$~mag. As can be seen from the historical light curve of
Prager (1940), constructed from the measurements on Harvard plates obtained from 1890 to 
1939, XX~Oph undergoes occasional one magnitude deep and one to three years long minima. 
Such an optical behaviour is well described in R~Coronae Borealis stars. In spite of that, 
XX~Oph does not belong to this group as no traces of carbon were observed in the spectra.

Lockwood et al.\ (1975) introduced a cool companion to the spectrum because the spectral 
energy distribution could be represented by the contributions of a B0III star and an M6III
star. XX~Oph has been considered to be a mass-exchanging binary star since Lockwood et al.\ 
(1975). This was confirmed by Evans et al.\ (1993), with a slight change of the spectral 
types of the components~-- B0V and M7III.

Interestingly, the optical variability does not seem to be correlated with the spectral
variability. XX~Oph underwent a deep and wide minimum between 1921 and 1922 but the 
spectrum remained unchanged. On the contrary, the strong absorption lines 
were discovered in 1925 during the constant phase at maximum (Merrill 1932).

Spectra of some O and B supergiants in the Magellanic Clouds show some
similarities to XX~Oph. These supergiants belong to LBV stars (Luminous Blue 
Variables). However, XX~Oph is not so luminous because it is a giant.

Recently, XX~Oph has been observed in a deep minimum after 37~years of the maximum 
brightness. These variations are presented in Figure~1.

Observations presented in this paper are of two kinds, CCD and visual. CCD
observations consist of the HIPPARCOS data (Perryman et al.\ 1997)
from March 1990 (JD~2447960) to March 1993 (JD 2449060), ROTSE
(Wozniak et al.\ 2004) from May 1999 (JD 2451318) to October 1999
(JD 2451482), ASAS--3 (Pojmanski 2002) from February 2001 (JD
2451947) to October 2004 (JD 2453280), and ESA INTEGRAL Optical Monitoring 
Camera (OMC) from April 2003 (JD 2452753) to March 2004 (JD 2453077). Unfiltered ROTSE data were 
shifted by $+0.5$~mag to match the other measurements. Visual data were taken from the 
following organizations: AAVSO (USA), AFOEV (France), MEDUZA (Czech Republic and 
Slovakia) and HAA~VSS (Hungary).

The light curve is plotted in Figure 1. Visual data obviously do
not attain the precision of the CCD observations. However, when
averaged and smoothed, visual observations are a useful tool to
judge the long-term optical changes. In this case, I used 20-day bins.

Seven deep minima have been observed in XX~Oph since 1890. 
Approximate timings derived from the historical light curve are given
in Table~1. The brightness of XX~Oph slowly varied from 8.3 to 9.3~mag 
between 1968 and March 2004, when the gradual fading by 0.2 mag per month 
started. It seems that the fading stopped on 2nd September 2004 and reached 
$V=10.15$~mag. The star has never been in maximum for such a long time (see 
Table~1).

\IBVSfig{9cm}{5571-f1.eps}{Light curve of XX~Oph from 1974 to
2004. Filled circles represent CCD $V$-filtered observations of
ASAS--3, filled squares are CCD unfiltered observations of ROTSE,  
the HIPPARCOS magnitudes are plotted as filled triangles, while INTEGRAL 
OMC $V$-filtered data are plotted as open squares. Averaged
visual observations with 20-day bins of AAVSO, AFOEV, MEDUZA and HAA~VSS 
are shown as open circles.}
\IBVSfigKey{5571-f1.eps}{XX Oph}{light curve}

\IBVSfig{9cm}{5571-f2.eps}{Detailed light curve of the past four years
from 2001 to 2004. The symbols are the same as in Figure~1.}
\IBVSfigKey{5571-f2.eps}{XX Oph}{light curve} 

\begin{table}[!ht]
\centerline{Table~1. Approximate timings of the deep minima of XX~Oph.} 
\begin{center}
\begin{tabular}{cccl}
\hline
$T_{\rm min}$ (year) & $T_{\rm min,i} - T_{\rm min, i-1}$ (years)  & note & data source\\
\hline
1889&       - & &Prager 1940 \\
1901&       12& &Prager 1940 \\
1915&       14& &Prager 1940 \\
1921-22&    06& (double minimum)&Prager 1940 \\
1931&       09& &Prager 1940 \\
1947&       16& &Burnham 1977 \\
1967&       20& &this paper\\
2004&       37& &this paper\\
\hline
\end{tabular}
\end{center}
\end{table}

\vskip-2mm
We are now experiencing the first possibility to observe XX~Oph in
minimum in modern era. Instrumentation and observing techniques
have significantly improved in the last 37~years, thus we have an
unique opportunity to understand XX~Oph. The minima of brightness
are probably caused by sudden weakening of the stellar wind (that
is common in LBV stars, see Conti 1984) and this is our chance to see the stellar
atmosphere. Mainly spectroscopic observations may reveal important 
information.


{\it Acknowledgement.} {Based on observations with INTEGRAL, 
an ESA project with instruments and science data centre funded by 
ESA member states (especially the PI countries: Denmark, France, 
Germany, Italy, Switzerland, Spain), Czech Republic and Poland, 
and with the participation of Russia and the USA. I acknowledge 
the collaboration with the OMC Team, INTA, Madrid, on use 
of INTEGRAL OMC light curves, as well as collaboration within the 
INTEGRAL Cataclysmic Variables Working Group.
This work made use of the SIMBAD database, operated at CDS, 
Strasbourg, France. I acknowledge with
thanks the variable star observers from the AAVSO ({\tt http://www.aavso.org}), AFOEV ({\tt http://astro.u-strasbg.fr/afoev}),
MEDUZA ({\tt http://www.meduza.info}) and HAA VSS ({\tt http://vcssz.mcse.hu}) databases.}


\pagebreak

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