CHARA Array Measures Sub-Unity Roche-Lobe Filling in Symbiotic Giants

The study used H-band interferometry at the CHARA Array to resolve angular diameters of the cool giants in four symbiotic and related binaries. Model fits and image reconstructions were compared against circularly symmetric disks versus elongated Roche-lobe geometries, while near-simultaneous IRTF spectroscopy supplied effective temperatures and luminosities to derive physical radii. Parametric modeling consistently preferred symmetric disks; reconstructed images showed surface features on three stars.

Three systems with published orbital parameters yielded filling factors <1, directly constraining the relative roles of stellar winds and Roche-lobe overflow. This refines evolutionary pathways linking symbiotic stars to recurrent novae and Type Ia supernova progenitors by showing that mass transfer is predominantly wind-fed on average.

The work connects to earlier radial-velocity and photometric surveys that inferred high filling factors but lacked direct angular-size constraints. Future multi-epoch CHARA imaging at higher resolution could test whether episodic Roche-lobe contact occurs during pulsation maxima.

Main limitation is the small sample and single-epoch data; repeated observations across pulsation cycles would strengthen evidence for time-averaged versus instantaneous geometries.