Eclipsing Binaries in the MOA-II Database

Abstract

The present evolution of the Microlensing Observations in Astrophysics (MOA-II) project began in 2006. MOA-II monitors millions of stars simultaneously towards the densely populated regions of the Galactic bulge (GB) using a 1.8m telescope with a wide-field CCD camera and has resulted in about 100TBs of image data spanning over nine years to date. Such a large amount of data is a valuable resource for variable star research. This thesis is focused on the aspects of eclipsing binary (EB) study using the MOA-II data including identifying EBs in the MOA-II database and searching for tertiary companions in EBs via eclipse time variation (ETV) analysis. We first present the first catalogue of EBs in two MOA fields towards the GB, i.e., GB9 and GB10, in which 8733 EB candidates, mostly contact and semi-detached binaries of periods< 1 day, were identified using two MOA observational seasons’ worth of data. At this stage, we also identified three triple candidates among these 8733 EB candidates by detecting the light travel time effect (LTTE) signals in their ETV curves. A sample of 542 EBs with periods less than two days in two subfields in GB9 and GB10 - GB9-9 and G10-1 - were selected for further ETV analysis. For this sample we were able to obtain the full time series from MOA-II that spans 9.5 years. We discovered 91 EBs among these 542 EBs with detected LTTE signals suggesting the presence of tertiary companions of orbiting periods from 250 days to 28 years. The frequency of EBs with tertiary companions in our sample increases as the period decreases and reaches the value of 0.65 for contact binaries of periods < 0.3 days. If only the contact binaries of periods < 0.26 days are considered, the frequency even goes to the unit. Our results suggest that contact binaries with periods close to the 0.22-day contact binary limit are commonly accompanied by relatively close tertiary companions.