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CQ Cephei--from robotic observations to computer modelling.

CQ Cephei (= WR155= HD214419) is a magnitude 8.6 to 9.1 eclipsing binary system with a period of ~1.64 days. Its binary nature was discovered over 70 years ago by McLaughlin & Hiltner (1) and it is now understood to comprise a Wolf-Rayet (WR) primary star of subclass WN5.5 and a massive early O7 type companion.

Described in the nineteenth century by the French astronomers Charles Wolf and Georges Rayet, WR stars are characterised by broad emission bands on an otherwise continuous spectrum. They are evolved, highly luminous, massive stars which are losing mass rapidly by means of very strong stellar winds. Most WR stars are believed to finally progress to become Type Ib or Type Ic core-collapse supernovae (SNe).

CQ Cep is one of a number of objects that can be requested to be observed by the 6 inch [152mm] telescopes of the NASA-funded MicroObservatory Robotic Telescope Network developed by the Harvard-Smithsonian Center for Astrophysics (CfA) and accessible over the Internet through the Observing With NASA (OWN) website (http://mo-www.harvard.edu/ OWN/). Whilst the utility of the MicroObservatory for quantitative astronomy has been demonstrated by the author's observations of the SNe 2011dh and 2011fe, (2) the single nightly observations available through OWN, together with a tendency of the unfiltered CCD images to be over-exposed, precludes the effective study of this star using the telescopes. However, following successful use of the MicroObservatory to observe the two SNe in 2011, the CfA has kindly provided the author with access to the Legacy MicroObservatory which allows the telescopes to be requested to acquire BVRI and unfiltered images from any location in the sky. The opportunity has therefore been taken to observe CQ Cep in order to investigate the utility of the telescopes to study variable stars and to see whether the observations can be used together with eclipsing binary star modelling software to investigate their physical attributes.

Between 2012 July 23 and Sept 30, CCD observations of CQ Cep were successfully made on 44 nights giving a total of 258 usable Johnson B passband images (see Figure 1a for a representative image). Each 60s exposure image was dark-field subtracted using the relevant nightly dark-field image, and the magnitude of CQ Cep determined by differential photometry using AstroArt 5.0 and a 10.08 V magnitude comparison star (TYC 3991-1480-1) together with a 10.62 V mag check star (TYC 3991-2054-1).

The resulting lightcurve is illustrated in Figure 1b and shows the star varying in brightness by up to ~0.5 magnitudes. Period analysis using the Analysis of Variance (ANOVA) algorithm in Peranso (http://www.peranso.com/) gives a period of 1.6411 [+ or -] 0.0036 days (Figure 1c) and is comparable with the value of 1.641249 days included in the International Variable Star Index (http://www.aavso.org/vsx/index.php?view= results.get&ident=000-BCQ-573). Using the observed periodicity, the resulting phase diagram (Figure 1d) is comparable with previously published observations and is typical of the Beta Lyrae (EB) type. The lightcurve is asymmetric with the primary and secondary maxima occurring at phases ~0.25 and ~0.78 respectively and has a primary eclipse slightly narrower and ~0.1 magnitudes deeper than the secondary.

Despite a number of investigations over the past 30 years, there remains some uncertainty about the physical nature of the CQ Cep binary system. Using the freely available eclipsing binary star model StarLight Pro (http://www. midnightkite.com/index.aspx?URL=Binary) that takes into account various properties of a binary system including inclination, stellar size, mass ratio, star shape, star temperature and limb darkening, the nature of the CQ Cep system can be investigated by comparing the observed lightcurve with synthetic lightcurves predicted by the model. By representing the various models that have been proposed for CQ Cep, a variation of the model proposed by Harries & Hilditch (3) was found to have a good degree of fit with the observed data (Figure 2). In this model the WR primary star is slightly cooler than that proposed by Harries & Hilditch, having a surface temperature of 36,500K rather than 39,200K, but is otherwise similar with a mass of the order of 30 solar masses and a radius of 8.8 solar radii. The O-type companion is similarly massive at some 24 solar masses and has a radius of 8 solar radii and a surface temperature of 35,000K. The binary system is at an inclination of 72[degrees] and is in a close overcontact configuration in which both stars fill their Roche lobes and share a common envelope.

From this short study it is apparent that the relatively modest robotic telescopes of the MicroObservatory Robotic Telescope Network can be used to undertake quantitative studies of variable stars. Modelling using an eclipsing binary model such as StarLight Pro moves one step beyond the lightcurve and provides an insight into the possible physical nature of binary star systems. Within the 1[degrees] field of view of CQ Cep lie a number of variable stars including the classical Cepheid Z Lac, the eclipsing binary AO Cep and a further possible eclipsing binary OT Lac, for which lightcurves have been successfully produced from the images acquired in this study. Having shown that it is possible to model the observations of CQ Cep with a reasonable degree of accuracy, it is now intended to use this approach to investigate the physical nature of the less well known AO Cep and OT Lac eclipsing binary systems.

Martin J. F. Fowler [danebury216@hotmail.co.uk]

(1) McLaughlin D. B. & Hiltner W. A., PASP, 53(316), 328330 (1941)

(2) Fowler M. J. F., J. Brit. Astron. Assoc., 121(5), 313 (2011) & 122(2), 121 (2012)

(3) Harries T. J. & Hilditch R. W., MNRAS 291, 544 (1997)
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Title Annotation:Observer's Forum
Author:Fowler, Martin J.F.
Publication:Journal of the British Astronomical Association
Geographic Code:4EUUK
Date:Feb 1, 2013
Words:966
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