Visible and near-infrared spectro-interferometric analysis of the edge-on Be star $o$ Aquarii

Abstract

We present a detailed visible and near-IR spectro-interferometric analysis of the Be-shell star $o$ Aquarii from quasi-contemporaneous CHARA/VEGA and VLTI/AMBER observations. We measured the stellar radius of $o$ Aquarii as 4.0 $\pm$ 0.3 ${\mathrm{R}}_{\odot }$. We constrained the disk geometry and kinematics using a kinematic model and a MCMC fitting procedure. The disk sizes in H$\alpha$ and Br$\gamma$ were found to be similar, at $\sim$10-12 ${\mathrm{D}}_{\star }$, which is uncommon since most results for Be stars show a larger extension in H$\alpha$ than in Br$\gamma$. We found that the inclination angle $i$ derived from H$\alpha$ is significantly lower ($\sim$15 deg) than the one derived from Br$\gamma$. The disk kinematics were found to be near to the Keplerian rotation in Br$\gamma$, but not in H$\alpha$. After analyzing all our data using a grid of HDUST models (BeAtlas), we found a common physical description for the disk in both lines: ${\Sigma }_0$ = 0.12 g cm\textsuperscript{-2} and $m$ = 3.0. The stellar rotational rate was found to be very close ($\sim$96\%) to the critical value. Our analysis of multi-epoch H$\alpha$ profiles and imaging polarimetry indicates that the disk has been stable for at least 20 years. Compared to Br$\gamma$, the data in H$\alpha$ shows a substantially different picture that cannot fully be understood using the current physical models of Be star disks. $o$ Aquarii presents a stable disk, but the measured $m$ is lower than the standard value in the VDD model for steady-state. Such long-term stability can be understood in terms of the high rotational rate for this star, the rate being a main source for the mass injection in the disk. Our results on the stellar rotation and disk stability are consistent with results in the literature showing that late-type Be stars are more likely to be fast rotators and have stable disks.

arXiv comment24 pages, 17 figures, accepted for publication in Astronomy & Astrophysics on 06/02/2020. Abstract shortened to fit within arXiv character limit

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