Author: Michael Freed
Institution: Northern University Arizona
Date: March 2007
Instrumental magnitude photometry of the eclipsing binary star DE CVn in the U color band was carried out using the Lowell Observatory 31-inch telescope with the NURO CCD photometer. Deep and flat primary minima with steep ingress and egress confirm the hypothesis that the primary component of this binary system is a hot white dwarf. New, more precise ephemeris of the system based on these and other observations is presented.
Most of the stars visible in the night sky are actually binary star systems, or two stars that revolve around a common center of gravity. Many astronomical phenomena, such as Type IA Supernovae, are thought to arise from binary systems. These systems are very important to astronomers because the interaction between the stars provides far more information about the components than the observation of a solitary star possibly can. Furthermore, when the system is oriented so that one companion passes in front of the other, even more data is available. Therefore the study of eclipsing binary stars is very important area of research in the field of stellar evolution.
DE CVn is an eclipsing binary star system associated with the X-ray source RX J1326.9+4532. Robb & Greimel (1997) carried out photometry of this star in V (visual) and R (red) color bands and came to the conclusion that the system contains a cool red dwarf (70% as massive and 63% as large as our Sun) and a hot white dwarf (47% as massive and 1.4% as large as our Sun), and that the orbital plane of the system is inclined 76° to the "plane" of the sky. Because of the huge size difference of the components and the large inclination of the orbit (90° inclination means that the orbit is edge-on to the observer), the white dwarf should disappear entirely behind its companion star. However, at long wavelengths the light from this binary system is overpowered by the cool red companion, and the eclipses do not produce large effects in the light curve of the system. The shallowness of the minima in V and R color bands did not allow Robb & Greimel to determine the ephemeris (schedule) of the future minima with high precision. Since white dwarfs are very hot, one could surmise that at shorter wavelength, in particular, in the color band U (ultra-violet), the minima caused by the eclipses of the white dwarf would be much deeper and the timing of disappearance (ingress) and reappearance (egress) of this very small star could be measured more precisely. An additional check of the white dwarf nature of the primary would be the form of the light curve in the minimum; for a completely eclipsed star providing the bulk of the radiation in a color band, the bottom of the minimum must be practically horizontal.
Observations and Reductions
Observations of DE CVn in U color band were carried out in May 2005, using the 31" Lowell Observatory telescope in Flagstaff, AZ, and the CCD photometer of the National Undergraduate Research Observatory (NURO). The only comparison star available in our 4' field of view was too weak in U, but the high stability of the photometer and weather allowed for reliable direct photometry in instrumental magnitudes. The data were flat-fielded, biased, and measured for instrumental magnitudes using the IRAF software package. Corrections for atmospheric extinction were made using the registered air mass values and the standard average value of the coefficient of extinction in the U band.
The first night of observation showed the published ephemeris to be incorrect when the primary minimum was not observed when predicted, but roughly one hour later. This scheduling problem along with a few cloudy nights resulted in three observations of the minimum. However, the constancy of the U magnitude of the system between the minima, after the introduction of the correction for the atmospheric extinction, is a strong confirmation that the reduction was quantitatively meaningful. Indeed, because of the strong dominance of the white dwarf in this color band, one can anticipate that between its eclipses by the cool star the system will keep its brightness at a practically constant value.
The U light curve of DE CVn we obtained, reduced to one period, is shown (Figure 1). It demonstrates the deepest (about 1 magnitude) minima observed in this system so far, with very steep ingress and egress and a virtually flat bottom, as anticipated for the eclipses of a white dwarf. The steepness of ingress and egress and a good coverage of the minimum with the data points allowed us to significantly improve the ephemeris of the system.
An O-C (Observed minus Calculated) curve constructed using these new data, along with other published (Tas et al. 2004) and unpublished (Robb, private communication) photometric data from a time interval of eight years, revealed a large error of 0.000044 days in the period of the system (Figure 2). Wilson and Devinney's (1971) lcdc code was used for the final adjustment of the ephemeris (Equation 1). This new ephemeris (Figure 3) practically eliminates the discrepancy between the observed and calculated moments of minimum.
Discussion and Conclusion
The photometry of DE CVn in the U color band provided strong confirmation to Robb and Greimel's hypothethis that the primary component of this binary system is a white dwarf. We confirm the predicted depth of the minima, very short duration of ingress and egress and the flatness of the bottom of the light curve during the eclipse. These properties of the light curve allowed for a more accurate determination of the elements of the system's ephemeris. This more accurate ephemeris will allow for further observations of this very interesting binary system; radial velocity curves in particular would allow for a more thorough understanding and more accurate modeling of this system.
Robb, R.M. and Greimel R. (1997) The Eclipsing Binary RX J1326.9+4532, Information Bulletin on Variable Stars 4486, 1-4.
Tas, G.; Sipahi, E.; Dal, H. A.; Göker, Ü. D.; Tigrak, E.; Yigen, S.; Özdarcan, O.; Topçu, A. T; Güngör, C.; Çelik, S.; Evren, S. (2004) Times of Minima for Some Eclipsing Binaries, Information Bulletin on Variable Stars 5548, 1-2.
Wilson R.E. and Devinney E.J. (1971) Realization of Accurate Close-Binary Light Curves: Application to MR Cygni, The Astrophysical Journal 166, 605-619.