A Study of π Aquarii during a Quasi-normal Star Phase: Refined Fundamental Parameters and Evidence for Binarity

We present the results of recent multicolor photometric and high-resolution spectroscopic observations of the bright Be star π Aqr. Observational data collected from the literature were used to study the star's variations over the last four decades. The star is identified with the IR sources F22227+0107 in the IRAS Faint Point Source Catalog and MSX5 G066.0066-44.7392 in the MSX Catalog. The variations in near-IR brightness of π Aqr are found to be among the largest reported for Be stars. Since 1996, the star has shown only weak signs of circumstellar emission, which has allowed us to refine the fundamental stellar parameters: AV = 0.15 mag, Teff = 24,000 K, log g = 3.9, and MV = -2.95 mag. A weak emission component of the Hα line has been detected during the recent quasi-normal star phase. From analysis of the Hα line profiles, we find antiphased radial velocity variations of the emission component and the photospheric absorption, with a period of 84.1 days and semiamplitudes of 101.4 and 16.7 km s-1, respectively. This result suggests that π Aqr may be a binary system consisting of stars with masses of M1 i = 12.4 M☉, M2 i = 2.0 M☉. We also estimate the orbital inclination angle to be between 50° and 75°. We suggest that the photometric, spectroscopic, and polarimetric variations observed during the second half of the 20th century may be due to variable mass transfer between the binary components.