Using the Coolest Ae Stars to Constrain Circumstellar Disk Viscosity

Abstract Classical Ae (CAe) stars are main-sequence, A-type stars with H α emission but no signature of dust. They are thought to be the cool extension of the classical Be stars to lower masses. Recent surveys based on H α spectroscopy have significantly increased the number of known CAe stars, with the population extending to spectral types as cool as A4 ( T eff ≈ 8500 K). We compute the temperature structure of gaseous, circumstellar disks around A-type stars, including both radiative heating from the central star and viscous shear heating from the disk’s rotation. We find that shear heating can become important for spectral types A2 and later and can act to increase the low temperatures predicted by purely radiatively heated disks. Our modeling indicates that the presence and strength of H α emission for spectral types A2 and later significantly increases with the amount of shear heating included, and we propose that this dependence can be used to constrain the α viscosity parameter appropriate for CAe star disks.