New insight into the variability of the Be star $π$ Aquarii: Determination of stellar and disk parameters

Abstract: The Be star $\pi$ Aquarii shows peculiar $\gamma$ Cas-type X-ray emission, likely caused by its outer disk interacting with a low-mass companion, probably a white dwarf. We study the long-term variability of its optical spectra to derive stellar and disk parameters during major changes. We identify and analyze Balmer, helium, silicon, and iron emission lines at selected epochs. Stellar parameters were derived using atmosphere models, considering oblate geometry due to fast rotation. Disk properties were constrained through H$\alpha$ line modeling using a viscous decretion disk model. The H$\alpha$ line evolved from shell to double-, triple-peaked, and flat-topped profiles. On Dec 22, 2001, the disk showed a low-intensity shell profile and fast density decay, indicating a small disk. From 2011 to 2014, the disk decayed slowly, then grew significantly until Nov 2022, increasing its H$\alpha$ equivalent width 18-fold and reaching an emitting region of ~65 solar radii. The inclination changed by ~10 degrees over 20 years, suggesting a precessing disk. The FeII 5018 A line traces a larger region than H$\alpha$ and is the only FeII line with a distinct profile. We conclude $\pi$ Aquarii is part of a misaligned binary system, with the white dwarf crossing the disk twice per orbit, capturing material, and enhancing X-ray emission.