Spectroscopic follow-up of Gaia exoplanet candidates: Impostor binary stars invade the Gaia DR3 astrometric exoplanet candidates

In this paper we report on the follow-up of five potential exoplanets detected with Gaia astrometry and provide an overview of what is currently known about the nature of the entire Gaia astrometric exoplanet candidate sample, 72 systems in total. We discuss the primary false-positive scenario for astrometric planet detections: binary systems with alike components that produce small photocenter motions, mimicking exoplanets. These false positives can be identified as double-lined SB2 binaries through analysis of high resolution spectra. Doing so we find that three systems, Gaia DR3 1916454200349735680, Gaia DR3 2052469973468984192, and Gaia DR3 5122670101678217728 are indeed near equal mass double star systems rather than exoplanetary systems. The spectra of the other two analyzed systems, HD 40503 and HIP 66074, are consistent with the exoplanet scenario in that no second set of lines can be found in the time series of publicly available high resolution spectra. However, their Gaia astrometric solutions imply radial-velocity semi-amplitudes $\sim$\,3 (HD 40503) and $\sim$\,15 (HIP 66074) larger than what was observed with ground based spectrographs. The Gaia astrometry orbital solutions and ground-based radial-velocity measurements exhibit inconsistencies in six out of a total of 12 exoplanet candidate systems where such data are available, primarily due to substantial differences between observed ground-based radial-velocity semi-amplitudes and those implied by the Gaia orbits. We investigated various hypotheses as to why this might be the case, and though we found no clear perpetrator, we note that a mismatch in orbital inclination offers the most straightforward explanation.