Astrometric Accelerations of Provisional Targets for the Habitable Worlds Observatory

Abstract

NASA's Habitable Worlds Observatory (HWO) will be the first space telescope capable of directly imaging Earth-like planets in the habitable zones of Sun-like stars to probe their atmospheres for signs of life. Now in its early stages of design, a list of the 164 most promising targets for HWO has been released to the community to carry out precursor science. Massive companions in these systems--stars, brown dwarfs, or giant planets--could preclude the existence of Earth-sized planets in the habitable zone by impacting their long-term dynamical stability. Here, we use astrometry from Hipparcos and Gaia EDR3 to identify stars in the HWO preliminary target list that exhibit astrometric accelerations and determine joint constraints on the expected mass and separation of these companions. We find that 54 HWO targets have significant astrometric accelerations, 37 of which are accounted for by known giant planets and stellar companions. Follow-up efforts are required to clarify the specific nature of the suspected companions around the remaining 17 accelerating stars. Stars without significant accelerations are used to rule out large regions of companion mass and separation down to planetary masses. We find that with Hipparcos and Gaia EDR3 we are $\sim$85$\%$ sensitive to 2 $M_\mathrm{Jup}$ planets between 4 and 10 AU. Future Gaia releases will provide sensitivity to sub-Jovian mass planets on Solar System scales for provisional HWO targets. Finally, using analytical estimates of dynamical stability, we find that 13 HWO targets have known stellar or planetary companions that are likely to disrupt habitable-zone planets.