Hierarchical triple stars as astrophysical contaminants in planetary transit surveys

Kozhura, Oleg (2023). Hierarchical triple stars as astrophysical contaminants in planetary transit surveys. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.00016b5c


The transit method is by far the most effective method of detecting exoplanets, as it is responsible for approximately 78% of exoplanetary detections. Since it involves detecting small deviations in flux received from the target star, several factors could cause false positives (FPs). One such factor, which I explore in detail in this work, is caused by eclipsing multiple systems in the background or foreground of the target star. Under certain circumstances, the eclipse in such a multiple system could mimic a planetary transit, resulting in an FP planetary transit detection.
The current study is done in the context of the Planetary Transits and Oscillations of stars (PLATO) satellite mission which is planned to launch in 2026 by the European Space Agency (ESA) and will search for thousands of terrestrial planets by monitoring up to a million stars. To achieve this goal, it will have an unprecedently large field of view which, due to operational constraints, results in a large pixel scale. Therefore, a target star’s point-spread function (PSF) will be contaminated by other stars in the background or foreground, some of which may be eclipsing multiples.
I estimate the expected rate of FPs by producing and studying synthetic populations of stars. The population synthesis code BiSEPS (Willems and Kolb 2002, Farmer, Kolb, and Norton 2013, Rowden 2019) is a tool we use to create such synthetic populations, containing single and binary stars, as well as planets orbiting around single stars. In my work, I added triple stars to BiSEPS and used it to explore the impact of higher-order multiples on contamination and to estimate the false positive planetary transit rate of the PLATO fields, using a realistic Galactic extinction model.
I found that the number of FPs rapidly increases with decreasing galactic latitude, while no strong variation with galactic longitude was observed. Most FPs are caused by multiples with combined apparent V magnitudes between 14 and 20. While an average triple star is slightly less likely to be an FP than an average binary, bright (combined mV ⩽ 14) triples contribute a similar number of FPs as binaries. The most common apparent FP radius is much larger than that of the terrestrial exoplanets, with apparent radius r =log(R/R⊕) between 0.4 and 0.6. The highest density of true planets (TPs) is at the centre of the synthetic Long-duration Observation Phase South (synLOPS) field, as all PLATO camera groups overlap in that area. The fraction of FPs (%FPs) in the total synLOPS field is the highest (73 ± 13%) for the planets in the range r =[1.2, 1.4], while terrestrial exoplanets (r =[0.0, 0.2]) have %FPs much lower (4±1%). I also studied the dependence of %FPs on |b| and saw that large planets (e.g. r =[0.6, 0.8]) are affected by changes in |b| the most with %FPs changing from 5 ± 12% in the area furthest from the Galactic plane to 71 ± 8% as we approach the Galactic plane. My work shows that some unresolved multiple systems could be mistaken for single stars and potentially be selected as targets for the PLATO mission. Eclipses within those systems could mimic planetary transits in the range of interest, however, they would have very short periods of about two days. Therefore, it is unlikely that these FPs would mimic planets in Earth-like orbits.

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