Inferring hemispheric asymmetries of stellar active regions through the information content of astrometric signals

Abstract Photometric light curves suffer from fundamental degeneracies that limit surface information recovery. We demonstrate that astrometry enables access to complementary information through photocentre variations induced by rotating surface features. The forthcoming commissioning of microarcsecond-precision astrometric missions presents an opportunity to improve stellar surface mapping. This paper extends a previous theoretical framework for stellar surface mapping, along three primary directions: (1) we derive analytical selection rules showing that astrometry is sensitive to spherical harmonic modes not detectable via photometry, particularly odd-ℓ modes that encode north-south asymmetries; (2) we quantify the information content of combined photometric and astrometric observations, showing that the rank of observable modes grows faster for combined observations than for either technique alone, though the fraction of recoverable modes still decreases asymptotically with increasing spatial resolution; and (3) we reframe astrometric jitter—traditionally treated as noise in exoplanet studies—as a signal encoding stellar surface structure. Given the limited proposed target lists of high-precision astrometric missions, this capability is particularly valuable: understanding host star surfaces is crucial for both removing stellar signals from exoplanet detections and characterising star-planet interactions. We show that while Sun-like stars require sub-microarcsecond precision, evolved stars with angular diameter and larger spots present immediate opportunities with current technology, such as the Gaia mission.