A search for binary candidates among the fundamental mode RR Lyrae stars observed by Kepler
aa r X i v : . [ a s t r o - ph . S R ] N ov A search for binary candidates among the fundamental modeRR Lyrae stars observed by Kepler
Elisabeth Guggenberger , , a and Jakob Steixner Max Planck Institute for Solar System Research, G¨ottingen, Germany Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Denmark Department of German Studies, University of Vienna
Abstract.
Although roughly half of all stars are considered to be part of binary or mul-tiple systems, there are only two confirmed cases of RR Lyrae pulsators with companions.One of them is TU Uma [1] - a classical RR Lyrae star in a very eccentric orbit - and theother is OGLE-BLG-RRLYR-02792 [2]. Considering the wealth of well-studied RR Lyraestars, this number is astoundingly low. Having more RR Lyrae stars in binary systems athand would be extremely valuable to get independent measurements of the masses. Thedata from the
Kepler mission with their unprecedented precision and the long time spanof about four years o ff er a unique possibility to systematically search for the signatures ofbinarity in RR Lyrae stars. Using the pulsation as a clock, we studied the variations in thetiming of maximum light to hunt for possible binary systems in the sample. For our search we made use of the full time span of data provided by the
Kepler mission, i.e., about4 years (Q0-Q17). All the stars in our sample have been observed in long cadence (LC, ∼
29 minsampling) during the whole operational time of the mission. Short cadence (SC, ∼ ff ect. This phenomenon can lead to periodic phase or periodchanges which would be hard to disentangle from any changes due to binarity. Even though this wouldin principle be possible, we here restricted our search to the 18 non-Blazhko stars in the sample. Adetailed analysis of the Q0-Q5 data of these stars has been published by [3], but no search for a lighttime e ff ect or period fluctuations had been performed.For our study we used the classical O-C technique, where O stands for “observed”, and C for“calculated”. The time of maximum light is measured for each pulsation cycle, yielding the ”O” value.On the other hand, an expected time of maximum, ”C”, is predicted assuming a constant pulsationperiod. The di ff erence between the two values is of great diagnostic value, as period changes causedby binarity, continuous period changes due to evolution, and abrupt changes all cause di ff erent patterns.To measure the exact time of maximum light from the light curves, a polynomial fit was computedfor each pulsation cycle. As most of the data were available in LC only, and RRab stars have sharpmaxima, a fit with free parameters showed large scatter. It turned out to be very sensitive to the locationof the data points with respect to the light maximum. Fitting a template (i.e., a polynomial with fixedparameters) is a more robust method to accurately determine the time of maximum light even withsparse sampling. We created templates for each star based on the SC data. These templates were thencross-correlated with the LC light curves. The resulting scatter in the O-C diagram is about 1 minute.Using phase diagrams of three periods decreased the scatter to below one minute in most of the cases. a e-mail: [email protected] PJ Web of Conferences O - C ( d ) KIC7030715 55000 55500 56000truncated JD KIC95915035 55000 55500 56000 KIC7176080
Fig. 1.
The three stars with a possible periodic O-C variation that might be caused by binarity. Long cadence dataare shown in black, short cadence data in red.
Variations in the O-C diagrams were detected for several stars in the sample. Three stars (KIC 07176080,KIC 07030715 and KIC 09591503, see Fig. 1) show an O-C variation that could be interpreted as pe-riodic and which might be indicative of binarity. However, even though only bona-fide non-modulatedstars were used for this study, it is possible that an intrinsic stellar modulation is present that manifestsitself as pure phase modulation. As the mechanism behind the Blazhko e ff ect is still unknown it cannotbe excluded that purely phase-modulated stars exist and mimic the O-C variations of binary stars.Additionally, RR Lyrae stars can show irregular fluctuations and sudden jumps in their periods.These phenomena could possibly also be mistaken for periodic, when observed only over a limitedtime span. In our sample we find one star with a sudden period jump (KIC 11802860) and severalcases of variation that seem irregular (for example KIC 8344381). [4] suggested that mixing eventsmight be held responsible for such period jumps in RR Lyrae stars, as well as for period variations thatare too fast for evolution. [5] have also reported erratic O-C variations for some of their targets in theirstudy on evolutionary period changes in RR Lyrae stars.Several stars in our sample show the typical horizontal flat line which indicates that the period hasremained constant (for example KIC 05299596 and KIC 6100702). The star is neither a binary normodulated (or the variation is too small to be detected).We note that the amplitude of the O-C fluctuations are very small (a few minutes) while the peri-ods of the candidates are long (in the order of years), indicating that if they are indeed binaries, theinclination would be very low. Any attempt to follow up the targets spectroscopically would thereforebe very challenging, especially given the faintness of the stars (13.3 - 17.4 mag). Acknowledgements.
The research leading to the presented results has received funding from the European Re-search Council under the European Community’s Seventh Framework Programme (FP7 / / ERC grantagreement no 338251 (StellarAges).
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