Vetting and False Positive Analysis of TOI 864.01: Evidence for a Likely Hierarchical Eclipsing Binary Masked by Dilution

Abstract

We present a detailed vetting analysis of the TESS candidate TOI 864.01, initially identified as a potential ultra-short-period (P ~ 0.52 d) Earth-sized planet orbiting an M-dwarf. Using 12 sectors of TESS photometry spanning a multi-year baseline, we recover a robust periodic transit-like signal. While the recovered transit depth is attenuated by detrending (~ 158 ppm), the SPOC pipeline reports an undiluted depth of ~ 640 ppm. Stellar characterization based on Gaia DR3 astrometry yields a nominally single-star solution (RUWE = 1.18), highlighting the limitations of astrometric vetting for tight companions. We performed a statistical validation analysis using TRICERATOPS, aggregating data from all 12 available sectors. The analysis yields a False Positive Probability (FPP) of 0.25 and a Nearby False Positive Probability (NFPP) of < 10^-4. While these metrics ostensibly classify the target as a viable planetary candidate based on Gaia-resolved sources, they fail to account for sub-pixel companions. Crucially, archival high-resolution imaging from the TESS Follow-up Observing Program (TFOP SG1) reveals a stellar companion at a separation of 0.04", unresolved by both Gaia and TESS. When this companion is considered, the signal is best interpreted as a Hierarchical Eclipsing Binary (HEB) on the companion. Bayesian model comparison yields an inconclusive result (Delta ln Z ~ 0.25), consistent with the degeneracy introduced by unresolved blending. Ground-based follow-up photometry further supports significant dilution, with a measured transit depth (~ 0.37 ppt) shallower than predicted (~ 0.64 ppt) and a timing offset of 6.3 minutes. Taken together, we classify TOI 864.01 as a probable False Positive and recommend its retirement from planetary candidate lists.