Detection and CCD photometry of eclipsing binaries in the Large and Small Magellanic Clouds (2003)
AuthorsBayne, Glennshow all
Eclipsing binary stars allow the physics of the component stars to be studied. These systems can provide a wealth of knowledge ranging from fundamental parameters such as mass, radius, and luminosity through to tests of stellar evolution and distances to the systems. A search for eclipsing binaries in the SMC using the MOA photometric time series database yielded 169 detections. These detections were cross-referenced with the OGLE catalogue of eclipsing binaries in the SMC. A total of 35 systems were new detections with most of these lying in the outer, less dense regions of the SMC, and some outside the OGLE fields. The remainder were within the overlap region of the fields of the two surveys. In the overlap region of the two MOA survey fields, 21 systems were detected but only 2 detections were common to both fields. This implies the MOA survey is far from complete. Additionally, 14 systems (~10%) were not detected by OGLE in the MOA-OGLE overlap region implying the OGLE survey is also not complete, though to a lesser extent. The unequivocal determination of whether all systems were real eclipsing binaries was not able to be made based on the photometric light curves alone. Objects for which there was some uncertainty concerning their true nature were indicated as such. Two of the new detections in the MOA SMC catalogue were selected for follow-up photometry, MOA J005018.4-723855 and MOA J005623.5-722123. An LMC target was selected from the MACHO LMC catalogue, MACHO*05:36:48.7-69:17:00. Observations were made from MJUO in Strömgren u, Johnson VJ and Cousins IC filters between 1999 April and 2002 July. The McLellan 1-m telescope and CCD photometer head were used. Differential fluxes for the target stars were extracted using the ISIS-2.1 difference imaging package. To obtain flux changes with respect to a level of zero flux, DAOPHOT II was used to establish the targets' flux levels in the reference images relative to which ISIS-2.1 works. The light curves were analysed using the Wilson synthetic light-curve code. It was not possible to make a definitive analysis of the physical nature of the three targets based on differential photometry alone. Nevertheless, it was possible to make rough estimates of the systems' apparent magnitudes and hence, via the known distance moduli and reddenings to the Magellanic Clouds, their absolute magnitudes. Coupled with additional constraints derived from Geneva models of stellar evolution for stars of reduced metallicity, it was possible to obtain astrophysically plausible parameters for the component stars comprising each target. The LMC target MACHO*05:36:48.7-69:17:00 is located near the Tarantula nebula and very close to SN1987A. It is a well-detached, eccentric system, (e = 0.2) with a sidereal period of 3.853529 ± 0.000005d and an apsidal period of 100 ± 5 years. The difference in mean epochs for the VJ and u/IC observations resulted in an averaging of the solution parameters. The most astrophysically plausible solution was obtained by matching the derived components to a pair of Geneva LMC models of identical age. This solution indicates a primary of ~20 Mסּ and secondary of ~14 Mסּ with a common age of 5 x 106 yr. Other parameters are mean effective temperatures Teff,1=33500K, Teff,2=29500K, semi-major axis a=33.3Rסּ , mean radii R1 /a = 0.21, R2/a = 0.15 and inclination i = 85.8º. MOA J005018.4-723855 is a semi-detached system, period 1.839870 ± 0.000005d, with the secondary filling its Roche lobe and with the O'Connell effect evident in the light curve. Of the three systems, this is the most affected by blending. Third light was necessary to obtain satisfactory light-curve fits in all bandpasses. Light-curve solutions fix the mass ratio and suggest the system is undergoing case A mass transfer from the present secondary. Since the mass receptor in such a system may mimic a normal main-sequence star to within a factor of two in luminosity for a given mass and temperature, an estimate of the physical nature of this system was found by requiring the parameters to match those of a Geneva evolutionary model. This yielded M1 =13.5Mסּ), Teff,1=29000K, R1=5.9Rסּ), M2 =20.5 Mסּ), Teff,2=20000K, R2=8.5Rסּ), a=20.5 Rסּ) and i=77.6°. MOA J005623.5-722123 is a detached system, period 2.32005 ± 0.00002d. Third light was necessary in the analysis of this star. Analysis of the derived solutions and comparison to the Geneva evolutionary models indicates a system 10 x 106 yr old. The most astrophysically plausible parameters for this system are M1 = 13.5 Mסּ), Teff,1 = 29 000K, R1 = 5.8 Rסּ), M2 = 15.5 Mסּ), Teff,2=29350K, R1 =7.8 Rסּ), a=22.65 Rסּ) and i=87.8°. The preferred light curve solution gives a secondary marginally hotter than the primary, yet with shallower eclipse depths. The high inclination as well as the gravity brightening in B-stars are the cause of this. During the secondary eclipse, the primary's path passes across the cooler central region the secondary, thereby leaving the hotter poles exposed. This accounts for the higher hemisphere-averaged temperature in the solution.