Detecting false positives with PLATO using double-aperture photometry and centroid shifts

Context . PLATO will discover exoplanets and characterize their host stars. Since photometry for most PLATO targets will be extracted on board, an efficient strategy to detect false positives (FPs) – transit-like signals not caused by planets – is needed. Centroid shifts are a standard FP diagnostic; however, only 5–20% of PLATO’s largest stellar sample (P5 sample) will have centroids computed on board. An alternative onboard strategy is required for the remaining targets. Aims . We propose a double-aperture photometry strategy to detect FPs and test two mask types: extended masks, which enlarge the nominal aperture, and secondary masks, centered on the main contaminant. For each mask we derive flux and centroid metrics to assess their ability to discriminate FPs. Methods . Using Gaia Data Release 3, we defined our P5 targets and background stars, which we assumed to be eclipsing binaries with transit depths and durations drawn from observed distributions. From simulated photometry and centroids, we computed extended and secondary fluxes and also extended, secondary, and nominal centroids, and compared the FP detection efficiency of each metric. Results . Under these assumptions, ~35% of P5 targets have a single FP-creating contaminant, and ~22% have two or more. Extended centroid shifts reach an efficiency of 87%, while nominal and secondary centroids reach 84% and 75%, respectively. The secondary flux attains an efficiency of 92%, whereas the extended flux reaches 73%. Conclusions . The secondary flux is the most efficient metric. Since double-aperture photometry is 50% less demanding in CPU and telemetry budgets, secondary and extended fluxes are optimal for most P5 targets. Secondary masks are optimal for targets with one FP-creating contaminant. Extended masks are preferable when extended flux is competitive with centroids. Our results show that double-aperture photometry and centroid shifts will allow PLATO to discard a large fraction of false positives caused by eclipsing binaries.