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\IBVShead{6027}{28 June 2012}

\IBVStitletl{Call for observations of the AZ C\lowercase{as} eclipse}{ and periastron passage of 2012-2014}

\IBVSauth{Ga{\l}an, C.$^1$; Tomov, T.$^1$; Miko{\l}ajewski, M.$^1$; \'Swierczy\'nski, E.$^1$; Wychudzki, P.$^2$; Bondar, A.$^3$; Kolev, D.$^4$; Bro\.zek, T.$^1$; Drozd, K.$^1$; I{\l}kiewicz, K.$^1$}

\IBVSinst{Toru\'n Centre for Astronomy, Nicolaus Copernicus University, ul. Gagarina 11, 87-100 Toru\'n, Poland, e-mail: cgalan@astri.uni.torun.pl}
\IBVSinst{Olsztyn Planetarium and Astronomical Observatory, Al. Marsza{\l}ka J. Pi{\l}sudskiego 38, 10-450 Olsztyn, Poland}
\IBVSinst{International Centre for Astronomical and Medico-Ecological Research, Terskol, Russia}
\IBVSinst{Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, 72 Tsarigradsko Shose blvd., BG-1784 Sofia, Bulgaria}

\SIMBADobjAlias{AZ Cas}{BD+60 310}
\GCVSobj{AZ Cas}
\IBVStyp{  EA/GS  }
\IBVSkey{eclipsing binary, observational campaign, photometry, spectroscopy}
\IBVSabs{AZ Cas is a unique long period eclipsing binary (P_orb approx. 9.3 yr).}
\IBVSabs{It shows long eclipses lasting for more than 4 months which are very}
\IBVSabs{deep in the blue wavebands. The spatial configuration of highly}
\IBVSabs{eccentric orbit, the huge diameter of the supergiant and its extended}
\IBVSabs{envelope causes that the eclipses occurring close to the periastron}
\IBVSabs{passage are accompanied by ellipsoidal, scattering and absorption}
\IBVSabs{effects. The next eclipse in the optical BVRI region will begin at}
\IBVSabs{the end of 2012. We appeal for intensive photometric and}
\IBVSabs{spectroscopic observations during 2012-2014 in the framework of an}
\IBVSabs{international campaign.}

\begintext

AZ\,Cas ($m_{\mathrm V}$=9.3) was discovered as a variable star by
Beljawsky~(1931).  Ashbrook (1956) has found it to be an eclipsing binary of
Algol type with very long orbital period
($P_{\mathrm{orb}}{\approx}$9.3yr).  There are yet only a few
eclipsing binary systems known to contain giant/supergiant stars, which have
very long orbital periods, lasting decades.  Eclipses in
these systems are rare but if they occur they are very long and accompanied
by interesting phenomena. A good example is VV\,Cep
($P_{\mathrm{orb}}\,\approx$\,20.3\,yr), in which the eclipses last for nearly
2 years.  Its most recent eclipse in 1997 was delayed by 1$\%$ of the
orbital period relative to the predictions (Graczyk\,et\,al.~1999).  This system
is the prototype of the group of variables consisting of an M or late K type
supergiant and an early B type star, in whose spectra strong Balmer and
[Fe] emission lines are present (Cowley~1969).  AZ\,Cas belongs to this
group.  Among dozens of known VV\,Cep type binaries, only these two show
eclipses.  

The next spectacular event is about to begin in the AZ\,Cas system, 
which consists of two components with masses as high as about 30\,$M_{\odot}$.
The more massive component of the system is a supergiant of
late\,K or early\,M spectral type ($T_{\mathrm{eff}}\,\sim$\,4000\,K), of which
linear diameter may reach up to 1000\,$R_{\odot}$.  Its companion is a hot
B star ($T_{\mathrm{eff}}\,\sim$\,21000\,K) with a diameter several tens of
times smaller ($D\,\sim30\,R_{\odot}$).  Its total amount of emitted
radiation equals to the supergiant and exceeds it many times in the
ultraviolet.  Due to the great difference in size and surface brightness 
between the components, photometrically only the primary eclipses can be 
observed, when the hot component is obscured.  The depth of the eclipses 
differ strongly in different bands: reaches up to $\sim2\fmm1$ in $U$ and 
decreases rapidly towards the red, being only $\sim0\fmm23$ in $V$ (Fig.\,1\,a). 
The most interesting light variations appear during and near to the eclipses. 
In the $BV(RI)_{\mathrm C}$ photometric bands a gradual increase in the
brightness of the system happens, reaching a maximum around phase 0.09, and then
follows a relatively rapid decline.  This is best seen in the $V$ light
curve (where the most numerous and accurate photometric data is available).
There we can see the brightness
drift starting already a bit before phase $-0.05$ (about half a year
before the photometric eclipse) and the eclipse appears to be imposed on this
(Fig.\,1\,b).  The totality in the whole observed range has a convex bottom
with an increasing maximum towards the shorter wavelengths (Fig.\,1\,b\,$\&$\,c). 
A\,slow increase in brightness by about 0\fmm4 over about 7-8 month
after the egress in $U$ band has been observed, for the first time with
the 60\,cm telescope at Piwnice Observatory (Fig.\,1\,c).

\IBVSfig{8cm}{6027-f1.eps}{Left (panel\,a): $U$ and $V$ light curves
obtained with the 0.6\,m telescope at Piwnice Observatory since the last
eclipse during 0.9 of orbital phase (points) are shown together with data
obtained during the previous eclipses: Suhora and Krak\'ow measurements
(Mikolajewski\,et\,al.~2004) (diamonds) and older Larsson-Leander~(1960) and
Tempesti~(1980) data (crosses).  An expanded view of the curves around the
eclipse and the periastron are shown on the right (panels:\,b\,$\&$\,c): the
$V$ light curve with some dates of characteristic stages, predicted
according to the ephemeris (Eq.\,1) (top - panel\,b) and $U$ light curve
(bottom - panel\,c).  The convex bottom and the long atmospheric eclipse are
highlighted with dashed lines.}
\IBVSfigKey{6027-f1.eps}{AZ Cas}{light curve}

The unusual photometric behavior shown above is caused by the specificity of
the orbit and the characteristics of the components.  The orbit is highly
eccentric (e\,=\,0.55) and has a very special (close to zero) value of
periastron longitude ($\omega=4^{\circ}$) (Cowley\,et\,al.~1977), so that
the phase of occultation (primary eclipse) and transit are very close to the
periastron (see Fig.\,2).  The supergiant has a huge diameter and
furthermore it is surrounded by a very extended gaseous envelope formed as
a result of an efficient mass loss process.  When the components approach each
other during the periastron passage then the equipotential surface is shrinking
and the tidal forces lead to great deformation of the supergiant from the
spherical symmetry.  This phenomenon is responsible for the increase in
brightness with maximum around the periastron.  The maximum observed during
the totality phase, seen as a convex bottom, could indicate the appearance
of some additional, third light in the system.  The increasing amplitude of
this phenomenon towards the short wavelengths suggests that scattering
effect could be responsible for that.  In AZ\,Cas we observe probably for
the first time such a strong scattering of the bright B stars radiation off
particles in the extended envelope.  More detailed studies of the
brightening in the bottom shows that its maximum is shifted somewhat towards
the end of the eclipse in agreement with the change in brightness profile
generated by orbiting scattering particles
(Galan~2009).  The effect connected with scattering is strong during the
occultation, while it seems absent or very weak during the transit, which
suggests that the Mie scattering dominates on particles of significant size in 
proportion to the wavelength of radiation. Additional support for the scattering
nature of the phenomenon is the behavior of the color changes during the
eclipses.  The supergiant is significantly less reddened during the eclipse
than its companion what indicates the existence of radiation excess in the
short wavelengths (Galan\,et\,al.~2011).

\IBVSfig{8cm}{6027-f2.eps}{Schematic representation the
supergiant and its extended envelope orbiting the B
star.  The special cases of mid-eclipse, periastron phase and transit are
denoted.  At the mid-eclipse position the scattering effect of the B star
radiation off particles in the envelope is shown with blue arrows.  During
the periastron phase the supergiant is strongly distorted from the spherical
symmetry as a result of ellipsoidal effect.  The changes in the H${\alpha}$
profile ($FWZI \approx 13$\AA) observed in the selected spectra obtained at
Rozhen, Asiago, Piwnice and Terskol observatories at orbital phases: 0.007,
0.118, 0.17 and 0.917, respectively are shown.}
\IBVSfigKey{6027-f2.eps}{AZ Cas}{other}

The envelope causing the scattering can be responsible for the brightness
increase after the egress in the $U$ band. We interpret this as a result of the
absorption by the envelope (extensive atmosphere) of the
supergiant - a phenomenon analogous to that occurring in $\zeta$\,Aurigae
type eclipsing binaries - a class related to the VV\,Cep stars.  If the
proposed interpretation is correct, we should expect a similar ``wing''
caused by absorption which precedes the eclipse, and it should start in
a short time.  The last spectrum obtained in April~2012 at phase 0.917
shows a strong absorption component in the H$\alpha$ line profile (Fig\,2). 
This could indicate that the spectroscopic eclipse is already in progress.

An opportunity to observe and study in details the phenomena in AZ\,Cas
system happens once in every 10 years.  To exploit this opportunity, 
a dense coverage with
photometric and spectroscopic observations is needed.  The long time scales
of changes in AZ Cas - the whole phenomenon will take more than 2 years - demands
the involvement of a large number of observatories at different locations to
reduce the dependency on the weather conditions and guarantee success during the
important phases of the eclipse, which will be relatively short (e.g. 
ingress and egress).  In addition an engagement of a large number of
instruments in the observations should increase the accuracy of the obtained
light curves.  This is very important when some changes have an amplitude 
as small as few hundredths of a magnitude.  To achieve these goals
we organize an international campaign for observations of the 2012/13
AZ\,Cas eclipse similar to the very fruitful observational campaigns for
another, unique eclipsing binary EE\,Cep
(Mikolajewski\,et\,al.~2005\,a $\&$\,b, Galan\,et\,al.~2010,
Galan\,et\,al.~2012\,a).  To support the campaign coordination and to help
the observers, we have prepared a special webpage:
{\tt{http://www.astri.uni.torun.pl/$\sim$cgalan/AZCas}}.  All willing to
participate in the observational campaign please contact the authors at
{\tt{cgalan@astri.uni.torun.pl}}.  %email
We invite both professional 
and amateur astronomers around the world.  Multicolor photometry is especially 
desirable obtained in bands close to the standard Johnson or
Johnson-Cousins $UBV(RI)_{\mathrm C}$ and spectra with low and high
resolution covering the whole wavelength ranges from $UV$ to $IR$. 
Nevertheless, photometric observations obtained with photographic filters
RGB (owned by many amateurs) could also be very valuable.  Although they
differ from the standard Johnson BVR filters, it is possible to transform
them to standard bands.  Unfiltered CCD observations should be useful as
well for timing purposes, i.e.  to determine the precise moments of minimum. 
However, because of the strong supergiant domination in the infrared, it will 
be better to use at least an $UV$-$IR$ blocking filter.

\IBVSfig{7.5cm}{6027-f3.eps}{The sky area (30$^\prime$ $\times$ 30$^\prime$) around
AZ\,Cas ($\alpha_{\mathrm{2000}}$\,=\,1$^{\mathrm h}$\,42$^{\mathrm
m}$\,17$^{\mathrm s}$, $\delta_{\mathrm{2000}}$\,=\,+61$^{\circ}$\,25\farcs3)
(left) with expanded view of closer area (15$^\prime$ $\times$ 15$^\prime$) shown on the
right. The comparison stars recommended for the CCD photometry are marked with
green color (see Table\,1 for details).  Star ``d'' (TYC\,4031-791-1) is
variable (see the text).}
\IBVSfigKey{6027-f3.eps}{AZ Cas}{finding chart}

BD+60$^{\circ}$306 and BD+60$^{\circ}$317 could be used as comparison stars. 
Both checked to be constant in $V$ band within 0\fmm01 (Tempesti~1980).  We
have used these objects for multicolor $UBV(RI)_{\mathrm C}$ photometry of
AZ\,Cas carried out by a single-channel diaphragm photometer with cooled
Burle\,C31034 photomultiplier attached to the 60\,cm Cassegrain telescope at
Piwnice Observatory (53.0943\,$^{\circ}$N,\,18.5532\,$^{\circ}$E) during the
period August~2003 -- November~2004.  We have found these stars to be
constant within $<0\fmm01$ in $V(RI)_{\mathrm C}$ bands and within
$<0\fmm02$ in $UB$.  However the angular
distance between AZ\,Cas and these stars (see Table\,1) is too large.  For the
purpose of our CCD observations of AZ\,Cas carried out during the last 7
years using the SBIG-STL\,11000 and SBIG-STL\,1001 CCD cameras we
established a new sequence of comparison stars: BD+60$^{\circ}$\,303,
TYC\,4031-437-1 and TYC\,4031-125-1 (see Fig\,3 and Table\,1).  We
discovered that one of the brightest stars in our photometric field -
TYC\,4031-791-1 - is a previously unknown eclipsing binary of Algol type
(Galan\,et\,al.~2012b).  Using photometric Johnsons magnitudes published for
BD+60$^{\circ}$317 by Bern\,$\&$\,Virdefors~(1972), we have obtained $UBV$
magnitudes for the stars mentioned above.  They are shown in Table\,1
together with the coordinates.  We suggest to use star ``a''
(BD+60$^{\circ}$\,303) as main comparison star for the photometric
measurements during the campaign.  The simultaneous use of
other comparisons (stars: ``b'', ``c'', ``E'', ``F'' in Table 1) is recommended. 
The variable star ``d'' is not suitable to be used as comparison, but campaign
will provide a unique opportunity to obtain an accurate multicolor light
curves for this object.

\vskip 0.5cm

\centerline{Table 1. The list of stars selected among those observed at Piwnice Observatory}
\centerline{to establish the sequence of comparison stars.}
\vskip 3mm
{\small{
\begin{center}
\begin{tabular}{llccrccc}
\hline
Des. & Identifier                     & $\alpha_{\mathrm{2000}}$    & $\delta_{\mathrm{2000}}$      & Dist.$^{\star}$ & \multicolumn{3}{c}{Brightness [mag]} \\ %SpT
     &                                & [h\,m\,s]   &[$^{\circ}$\,']& [']  & $U$ ($3\sigma_U$) & $B$ ($3\sigma_B$) & $V$ ($3\sigma_V$)  \\
\hline
 a   & BD+60$^{\circ}$\,303           & 1\,41\,24.9 & 61\,25.76     &  6.2 & 11.30 (0.05) & 11.09 (0.04) & 10.60 (0.03)\\ %A7
 b   & TYC\,4031-437-1                & 1\,41\,11.2 & 61\,18.65     & 10.2 & 12.39 (0.05) & 11.75 (0.04) & 10.93 (0.03)\\ %G2
 c   & TYC\,4031-125-1                & 1\,40\,58.5 & 61\,24.79     &  9.3 & 12.67 (0.05) & 12.41 (0.04) & 11.80 (0.03)\\ %...
 d$^{\star\star}$ & TYC\,4031-791-1   & 1\,41\,16.4 & 61\,21.31     &  8.2 & 11.66 (0.06) & 11.71 (0.04) & 11.16 (0.03)\\ %B5
 E   & BD+60$^{\circ}$\,306           & 1\,41\,44.8 & 61\,15.25     & 10.7 & 11.68 (0.04) & 10.95 (0.03) &  9.90 (0.02)\\ %G5III
 F$^{\star\star\star}$   & BD+60$^{\circ}$\,317           & 1\,43\,31.0 & 61\,32.62     & 12.0 & 10.24 (0.03) & 10.02 (0.02) &  9.59 (0.01)\\ %F1V
\hline
\multicolumn{8}{l}{${\star}$ The angular distance from AZ\,Cas in arcminutes.}\\
\multicolumn{8}{l}{${\star\star}$ The eclipsing variable of Algol type discovered at Piwnice Observatory. The $UBV$ magnitudes mean}\\
\multicolumn{8}{l}{the average brightness outside eclipses in this case.}\\
\multicolumn{8}{l}{${\star\star\star}$ $UBV$ magnitudes by Bern $\&$ Virdefors (1972)}
\end{tabular}
\end{center}
}}

The most important moments during the campaign are marked in Fig.\,1\,b.  To
cover with observations the time interval in which we are interested, it
is necessary to observe from June~2012 to August~2014.  The complete lack of 
multicolor observations is particularly unfortunate 
in orbital phases preceding the eclipse.  We emphasize the particular
importance of filling this gap.  It can contain the initial phase of the drift
in brightness mentioned above and the atmospheric eclipse in $U$.  The
mid-eclipse, according to the ephemeris (Galan\,et\,al.~2011):

\begin{equation}
JD_{\mathrm{mid-ecl}} = 2432477.8 + 3403.85 \times E,
\end{equation}

\noindent should occur on Jan\,12,~2013. It will be important to intensify
the observations by about one month around mid-eclipse because of the
expected maximum in the convex bottom as well as during and around the rapid
brightness changes occurring in ingress and egress.  The mid-ingress is
expected on Nov. \,22,~2012 and mid-egress on Mar.\,3,~2013.  Because of the
disparities in the components sizes, both of these stages are amazingly short. 
The expected duration of the ingress is 11 days while the egress should be
about 4 days shorter due to the increasing orbital velocity of the system
components when they approach the periastron.  It would be also valuable to
intensify observations around the periastron passage in November and
December~2013 due to the presence of the ellipsoidal effect maximum, and the
intriguing local minimum visible in the $V$ light curve.

\vskip 0.5cm
\newpage
\references

Ashbrook, J., 1956, {\it Harvard Obs. Announcement Card}, No. 1340 %\BIBCODE{}

Beljawsky, S., 1931, {\it AN}, {\bf 243}, 115 %\BIBCODE{1931AN....243..115B}

Bern, K., $\&$ Virdefors, B., 1972, {\it A$\&$AS}, {\bf 6}, 117 %\BIBCODE{1972A&AS....6..117B}

Cowley, A.\,P., 1969, {\it PASP}, {\bf 81}, 297 %\BIBCODE{1969PASP...81..297C}

Cowley, A.\,P., Hutchings, J.\,B., Popper, D.\,M., 1977, {\it PASP}, {\bf 89}, 882 %\BIBCODE{1977PASP...89..882C}

Galan, C., 2009, {\it Doctoral Thesis (in Polish)}, UMK (Library of the Centre for Astronomy), Torun

Galan, C., Mikolajewski, M., Tomov, T., et al., 2010, {\it ASPC}, {\bf 435}, 423 %\BIBCODE{2010ASPC..435..423G}

Galan, C., Mikolajewski, M., Tomov, T., et al., 2011, {\it arXiv:1102.5198}  %\BIBCODE{2011arXiv1102.5198G}

Galan, C., Mikolajewski, M., Tomov, T., et al. 2012\,a, {\it A$\&$A}, (in press) arXiv:1205.0028 %\BIBCODE{2012arXiv1205.0028G}

Galan, C., Tomov, T., \'Swierczy\'nski, E., et al., 2012\,b, {\it IBVS}, No. 6028 

Graczyk, D., Mikolajewski, M., Janowski, J.\,L, 1999, {\it IBVS}, No. 4679 %\BIBCODE{1999IBVS.4679....1G}

Larsson-Leander, G. 1960, {\it Arkiv f\"or Astron.}, {\bf 2}, 347 %\BIBCODE{1960ArA.....2..347L}

Mikolajewski, M., Zola, S., Kurpinska-Winiarska, M., et al., 2004, {\it ASPC}, {\bf 318}, 378 %\BIBCODE{2004ASPC..318..378M}

Mikolajewski, M., Galan, C., Gazeas, K., et al., 2005\,a, {\it Ap$\&$SS}, {\bf 296}, 445 %\BIBCODE{2005Ap&SS.296..445M}

Mikolajewski, M., Tomov, T., Hajduk, M., et al., 2005\,b, {\it Ap$\&$SS}, {\bf 296}, 451 %\BIBCODE{2005Ap&SS.296..451M}

Tempesti, P. 1980, A$\&$AS, {\bf 39}, 115 %\BIBCODE{1980A&AS...39..115T}

\endreferences

\IBVSedata{6027-t2.txt}
\IBVSdataKey{6027-t2.txt}{AZ Cas}{photometry}
\IBVSedata{6027-t3.txt}
\IBVSdataKey{6027-t3.txt}{AZ Cas}{photometry}


\end{document}
