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\IBVShead{5585}{3 January 2005}

\IBVStitle{Cataclysmic Variables in Open Clusters: EU C\lowercase{nc}}


\IBVSauth{Nair, P. H.$^1$; Kafka, S.$^1$; Honeycutt, R. K.$^1$; Gilliland, Ronald L.$^2$}
\IBVSinst{Indiana University, 319 SW, Bloomington IN 47405, USA}
\IBVSinst{Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218}

\SIMBADobj{EU Cnc}
\IBVStyp{ CV }
\IBVSkey{photometry}
\IBVSabs{We present a photometric study of the magnetic cataclysmic variable}
\IBVSabs{(MCV) EU Cnc, which is a member of the solar-age/solar-metalicity}
\IBVSabs{open cluster M67. A comparison with field and globular cluster MCVs}
\IBVSabs{reveals that, although the amplitude of the orbital light curve of}
\IBVSabs{EU Cnc corresponds to that of a high $\dot{M}$ polar, the cluster}
\IBVSabs{membership (absolute magnitude) of the system implies a low M-dot}
\IBVSabs{system. Considering that EU Cnc is the only MCV known in an open}
\IBVSabs{cluster, our result suggests that open cluster MCVs may differ from}
\IBVSabs{their field and globular cluster counterparts.}


\begintext

Magnetic CVs (MCVs) or polars are semi-detached binary systems, in 
which a lower main sequence Roche-lobe filling star loses mass to its 
highly magnetic (B$\geq$10$^{7}$G) white dwarf companion.  H-rich
material is channeled to the magnetic poles of the white dwarf, 
emitting most of the observed light of the system.
 When found in star clusters, they provide us with  
the rare opportunity to study their formation environment and extract 
information such as their age, distance and metalicity.
 Globular clusters favor the formation of CVs 
via tidal interactions in their dense environment, although an 
intriguing aspect is that a large fraction of
the globular cluster CVs appear to be magnetic, raising the
possibility that they reflect a new class of objects (Grindlay et
al. 1995).

Only three CVs are known to populate open clusters in spite of relevant 
searches (see for example Kafka et al. 2004). One of them, EU Cnc, is a 
magnetic CV in the central region of the rich and old (4.0$\pm$0.5Gy; Percival \& Salaris 2003) open cluster M67 (Gilliland et al. 1991). Its
discovery  was followed by X-ray studies (e.g. Belloni et al. 1993, 
1998; Van den Berg et al. 2004) confirming the magnetic 
nature of the system. Low-resolution spectra of EU Cnc (Pasquini et al. 
1994) revealed a quite variable spectrum with the He II 4686 line in 
emission and cyclotron humps. Being the only MCV 
in an open cluster, EU Cnc can be the
representative of a unique open cluster CV population, providing valuable 
information on the properties of a system that was likely formed through a 
common envelope process, representing more accurately field MCVs. We recently monitored the system in one filter 
confirming its orbital period and light curve characteristics as presented 
in the discovery paper of Gilliland et al. (1991). In the followings,
we describe our data and data reduction techniques, and present 
a discussion on our findings, placing EU Cnc in the larger framework of 
globular cluster CVs and field polars.

The observations were made on 26-Feb-2004 with the Mini-Mosaic
CCD camera on the 3.5-m WIYN telescope at Kitt Peak.
A total of 32 300-s V-band exposures were obtained under photometric 
 conditions. Data processing 
(bias subtraction and flat fielding) was conducted using the standard 
IRAF routines. An additional step of masking was performed using the 
IRAF routines IMUTIL/IMEXPR, to 
eliminate ghost images due to cross-talk between the amplifiers of the CCD; 
bad pixel masking was also applied. For data reduction, we used 
IRAF/DAOPhot for PSF photometry.  The instrumental magnitudes from 
DAOPhot were then supplied to AstroVar, a custom interactive program based 
on the method of incomplete ensemble photometry (Honeycutt 1992), 
optimized for the detection and study of variables. Secondary stars from Gilliland et al. (1991) were used to fix the zero point of the incomplete ensemble photometry.  The 2.09h period from Gilliland et al. (1991) was verified by both periodogram analysis (Horne \& Baliunas 1986) of the new data and by folding the new data on this period.  Finally, we searched for variability of the  X-ray sources in the field of M67 (Belloni et al. 1998). Although the systems are reported to be variable, no variability was detected, likely due to the short duration of our observations.

\IBVSfig{10cm}{5585-f1.eps}{Folded light curves of EU Cnc. Filled circles with error bars are the 2004 WIYN data, while the open circles are the 1988 data of Gilliland et al. (1991).}
\IBVSfigKey{5585-f1.eps}{EU Cnc}{light curve}

Fig. 1 and Table 1 compare the 1988 and 2004 light curves.  Noticeable similarities of the 1988 light curve of the system with that of VV Pup first suggested its the magnetic nature (Gilliland et al. 1991). The two light curves in Fig.1 have similar shapes, with a 30$\%$ larger amplitude in the 2004 light curve. Such changes are common in polars in the high optical state, and are likely due to changes in the mass accretion rate and/or accretion geometry. On the other hand, Gilliland et al. (1991) used a CuSO$_4$ filter which had an effective bandpass that covers both B and V. This could partially explain the light curve differences between the two epochs, since our 2004 data were obtained using a V filter. For an M67 distance modulus of 9.60$\pm$0.09 (Percival \& Salaris 2003), the M$_{V}$ range of EU Cnc is 12.0 to 10.6, which can be compared to field and globular cluster MCVs.  We searched the literature (e.g Warner 1995, Berriman 1987, Cropper 1990, Harrison et al. 2004) for information on the absolute magnitudes (M$_{V}$) of polars. For field CVs, distance uncertainties result in large (1-2 mag) differences in the calculated  M$_{V}$ values. Parallactic distances are available for only two field polars  (Thorstensen 2003); most of the distances of individual field MCVs we took from the literature used indirect methods.  Fig. 2 shows the distribution of M$_{V}$ for both the high and low states of MCVs, with the mean  M$_{V}$ of EU Cnc marked. 


\IBVSfig{10cm}{5585-f2.eps}{Histogram of M$_{V}$ in the 
 high (top) and low (bottom) photometric state for field polars. The position of EU Cnc is noted with arrows.}
\IBVSfigKey{5585-f2.eps}{EU Cnc}{histogram}

Note that the location of EU Cnc in Fig. 2 suggests that it is in a low optical state. However its large, 1-mag orbital modulation is characteristic of polars in the high state (low-state polars usually have variability of only a few tenths of a magnitude).


Fig. 3 shows a histogram of the absolute magnitude of globular cluster CVs taken from the literature, the majority of which appear to be magnetic, based on their X-ray properties. On the other hand, globular clusters systems are not observed well enough to assess high/low state magnitudes; therefore, Fig. 3 presents the mean ``snapshot'' magnitude of the systems. It is interesting that EU Cnc is about a magnitude fainter than globular cluster CVs, again suggesting a low state behavior.

\IBVSfig{10cm}{5585-f3.eps}{Histogram of M$_{V}$ of the known globular cluster CVs. The position of EU Cnc in this graph is noted with an arrow}
\IBVSfigKey{5585-f3.eps}{EU Cnc}{histogram}

%\vskip 1cm

\centerline{Table 1: Parameters of EU Cnc}
\vskip 3mm
\begin{center}
\begin{tabular}{cccc}
\hline
Epoch  & m$_{V}$  & M$_{V}$ & Amplitude\\
\hline
1988  & 21.4-20.5  & 11.8-10.9   & 0.9   \\
2004  & 21.6-20.3  & 12.0-10.7  & 1.3   \\

\hline
\end{tabular}
\end{center}

\vskip 1cm


More data are needed for further exploration of the properties of this  understudied CV, including the  long-term variations in its optical light curve and its spectroscopic properties. Considering that open clusters more accurately represent the galactic disk population, EU Cnc might shed light on the environment and timescales for CV formation and evolution. Alternatively, it may turn out to be a part of a different CV population, affected by the metal rich environment of open clusters.

\vskip 1cm

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\endreferences


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