2025A&A...697A.209C

Context. The Be star π Aquarii shows a peculiar X-ray emission (γ Cas type) commonly attributed to its outer disk interacting with a low-mass companion star, probably a white dwarf.
Aims. In this work, we study the long-term variability of the optical spectra of π Aquarii in order to derive the stellar and disk parameters of specific epochs when the disk shows major changes.
Methods. We identified several spectral lines and studied specific observation dates for Balmer, helium, silicon, and iron emission lines in detail. We determined the stellar parameters using stellar atmosphere models, considering the oblate geometry due to the fast rotation, and we derived the disk parameters through Hα modeling and considering a viscous decretion disk parametric model.
Results. We found the following stellar parameters: M = (11.0 ± 1.9) M_⊙, < log g ≥ (4.03 ± 0.10) dex, Rp = (5 ± 0.8) R_⊙, R_eq = (1.15 ± 0.088) R_p, T_p = (25 000 ± 1174) K, v sin i = (271 ± 13) km/s, and a ω = 0.82 ± 0.05. Over five epochs, the Hα emission line evolved from shell profiles to double-peaked to triple-peaked to flat-topped structures. We found that the density in the disk decays faster (with n ∼ 3.9) on December 22, 2001. In this epoch, the Hα emission line presented a low-intensity shell profile modeled with a low base surface density (Σ₀ = 0.01g cm^–2), indicating a “small” disk. Then, from 2011 to (September) 2014, the disk decreased with a slowly decaying density distribution (n ∼ 3.2). The disk grew drastically from October 2014 until November 2022, increasing 18 times its initial Hα equivalent width value. At that time, we estimated an emitting region of ∼65 R_⊙. We also measured the inclination angle variation in ∼10° in 20 years, with the results indicating a likely precessing disk. We found that FeII 5018 Å covers an emission region larger than Hα and is the only FeII emission line showing a different shape profile than the rest of the lines.
Conclusions. We assert that the disk of π Aquarii is in a misaligned binary system going through shell profiles to double-peak separation to triple-peaked to flat-topped profiles, and it now shows an asymmetric double-peak separation profile again. The emission line FeII 5018 Å indicates changes in the outer disk probably related to the white dwarf. We propose that the white dwarf cross the Be disk at two points in its orbit, and it is at these moments that it captures material and temporarily increases its X-ray emission.