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

TL;DR

This work addresses false positives in PLATO transit photometry by compensating for limited onboard centroid measurements with a double-aperture photometry approach. It develops extended and secondary mask strategies, derives flux and centroid-shift metrics, and tests their FP-detection efficiency using Gaia DR3-based targets and Kepler EB distributions. The main result is that secondary flux provides the highest FP-detection efficiency (≈$92 ext{%}$), followed by extended centroid shifts (≈$87 ext{%}$) and nominal centroids (≈$84 ext{%}$), with extended flux offering complementary gains. The proposed strategy leverages on-board resources to discard a large fraction of FP signals, informing mission design and future transit surveys by combining flux- and centroid-based diagnostics under practical constraints.

Abstract

PLATO will discover exoplanets around Sun-like stars through transit photometry and characterize their host stars using asteroseismology. Since photometry for most PLATO targets will be extracted on board, an efficient strategy to detect false positives (FPs), defined as transit-like signals not caused by planets, is required. Centroid shifts are a standard FP diagnostic, but only 5% to 20% of PLATO's largest stellar sample (P5) will have centroids computed on board, motivating the need for an alternative strategy. We propose a double-aperture photometry approach to detect FPs, testing two mask types: extended masks, which enlarge the nominal aperture, and secondary masks, centered on the main contaminant. For each mask type, we derive flux and centroid-shift metrics and evaluate their ability to discriminate FPs. Using Gaia DR3, we define P5 targets and their background stars, which are assumed to be eclipsing binaries with transit depths and durations drawn from observed distributions. From simulated photometry and centroid shifts, we compute extended and secondary fluxes as well as extended, secondary, and nominal centroids, and compare their FP detection efficiency. Under these assumptions, approximately 35% of P5 targets have a single FP-producing contaminant and about 22% have two or more. Secondary flux achieves the highest detection efficiency (92%), followed by extended centroid shifts (87%) and nominal centroids (84%). Owing to its lower computational and telemetry cost, double-aperture photometry provides an efficient solution for rejecting a large fraction of FP signals caused by eclipsing binaries.

Figures (9)

• Figure 1: Schematic view (from left to right) of example nominal, extended and secondary masks. For the extended and secondary masks, the respective nominal mask is represented with dashed lines. In this case the nominal mask consists in 5 pixels, the extended mask in 19 pixels and the secondary mask in 3 pixels. The red circle represents the target star and the cyan triangle represents a nearby contaminant star that could be an EB. Nominal masks are used to extract target photometry and are built to have the lowest NSR. Extended masks are extensions of nominal masks, such as each nominal mask is surrounded by a ring of usually one pixel. Secondary masks are typically smaller and are centered on the most problematic contaminant star in the window.
• Figure 2: Simulated PLATO PSFs (1/64 pixel resolution) at different angular positions, $\rm \alpha$, of a flux source in the FoV of a PLATO camera. When $\rm \alpha = 0^{\circ}$ the source is in the center of the FoV and when $\rm \alpha = 18^{\circ}$ the source is at the edge of the FoV. The top row shows the PSFs without charge diffusion while on the middle row a Gaussian kernel was applied to simulate charge diffusion in the CCD. The bottom row shows the PSF integrated over the pixels of a 6x6 window. The color scheme goes from the highest value (red) to the smallest values (blue).
• Figure 3: Distribution of the number of contaminants with $\rm \eta_{k}^{nom} >\rm \eta_{\rm min}$ for the targets in the magnitude range $10 \leq \rm P \leq 13$ from the catalog described in Sect. \ref{['sec:methods']} and observed with 24 cameras.
• Figure 4: Cumulative count of unique mask shapes as a function of target P magnitude. The plot shows the unique number of each mask, i.e. extended (blue triangles), secondary (red crosses) and nominal (black circles) and also the three types of masks together (orange squares).
• Figure 5: Comparison of centroid shifts and flux measurements with double-aperture photometry for detecting FPs. The results are produced for the case of variable transit parameters for the EBs. On the left we see the efficiency for centroid shift measurements using the three types of masks mentioned in this work so far. On the right, the efficiency for both secondary and extended masks. Both figures were obtained using 24 and 6 cameras for each mask. The Earth-like planet detection region is colored in light blue. The vertical red, dot-dashed line corresponds to the P = 10.7 magnitude threshold. The pink colored region is is the region where on-board light curves will be produced on-board for the P5 sample. The threshold of P = 11.7 is the vertical green, dashed line.