Modeling Asymmetries in Be Star Disks Using Spectro-interferometric H-alpha Observations from the CHARA Array's VEGA Instrument

Be stars are rapidly rotating B-type stars with outflowing decretion disks. The disk gas emits light in the hydrogen H-alpha line, and the H-alpha profile often displays separate violet and red peaks caused by the Doppler shifts associated with those regions of gas approaching and moving away from the observer, respectively. The observational record shows that in many cases the relative flux in the two peaks varies on long timescales. These changes are probably caused by the development of disk flux asymmetries with azimuthal position that are related to global disk oscillation modes. Optical long baseline interferometry offers us the means to angularly resolve these disk asymmetries for disks surrounding nearby Be stars. Here I present spectro-interferometric observations made with the CHARA Array interferometer and the VEGA beam combiner that document the interferometric data across the H-alpha line for the Be stars gamma Cas, phi Per, zeta Tau, and beta CMi. The differential visibility amplitudes and phases across the H-alpha emission line reveal clues about the asymmetries present in their disks. I describe a model of the H-alpha formation that calculates the emission intensity at spatial positions around the star and disk. The models apply local variations in gas density associated with the global disk oscillation mode. The parameters describing the disk and the oscillation mode are optimized to match the interferometric data and contemporaneous H-alpha spectroscopy. The analysis confirms the presence of disk asymmetries at the time of the observations (2020) in the cases of gamma Cas and zeta Tau, and hints that oscillation modes were also present in the disks of phi Per and beta CMi. The success of this single epoch study means that future interferometric programs will be able to investigate how the modes rotate within the disk and grow and decline over time.