Contributed Talk - Splinter HotStars

Thursday, 15 September 2022, 15:25   (SFG 1030 / virtual HotStars)

Eclipse timing variation of hot subdwarf binaries

Ramon Jaeger and Veronika Schaffenroth
Institute for Physics and Astronomy, University of Potsdam

Only about half of all star systems are truly single stars. Most stars exists in binaries or even higher multiple systems. These binaries can get very close to one another, to a point where they start to interact via mass transfer. The most compact binaries have very short orbital periods down to an hour or less, as they evolved through a common envelope phase. Our focus lies on hot subdwarf B-Type stars (sdB), which are assumed to be the result of such an interaction, originating from a red giant that lost its outer layers to its companion. Many of them are found in eclipsing binaries with cool low-mass companions and are easily found because of their distinct lightcurves. From these lightcurves we can measure the mid eclipse times of the primary and secondary eclipse of the binary. This is important as it allows us to determine the systems period and period changes as well as shifts of the secondary eclipse compared to the primary eclipse. This gives insight into the systems interaction, as well as the Roemer delay, which, among others, contains information of the orbits eccentricity. A common technique is the bisected chords method, which uses the average mid point of a number of horizontal lines connecting two points of equal flux, one on eclipse ingress and one on egress. We have now tried a different approach. By folding the lightcurve of a system over its calculated period and than binning that data, one gets an averaged lightcurve for one period. We shifted this average lightcurve over the data and calculate the chi^2 at each time step. The eclipse time is then given by the point in time with minimal chi^2 for each cycle. From this we can measure the systems period for every cycle to determine the O-C diagram, as well as the mid eclipse times for the determination of the Roemer delay. The procedure was tested on the 1 year long 20 s and 2 year long 120 s exposure TESS light curves of AA Doradus and is still a work in progress. This talk will give an insight into this work, explaining the idea in more detail, presenting the currently available results, as well as some issues with this procedure and show an outlook for further possibilities.