The Circumstellar Disk of Omicron Aquarii

The Circumstellar Disk of the Be Star o~Aquarii

Authors:

Sigut et al

Abstract:

Omicron Aquarii is late-type, Be shell star with a stable and nearly symmetric Hα emission line. We combine Hα interferometric observations obtained with the Navy Precision Optical Interferometer (NPOI) covering 2007 through 2014 with Hα spectroscopic observations over the same period and a 2008 observation of the system's near-infrared spectral energy distribution to constrain the properties of o~Aqr's circumstellar disk. All observations are consistent with a circumstellar disk seen at an inclination of 75±3∘ with a position angle on the sky of 110±8∘ measured E from N. From the best-fit disk density model, we find that 90\% of the Hα emission arises from within 9.5 stellar radii, and the mass associated with this Hα disk is ∼1.8×10−10 of the stellar mass and the associated angular momentum, assuming Keplerian rotation for the disk, is ∼1.6×10−8 of the total stellar angular momentum. The occurrence of a central quasi-emission (CQE) feature in Mg\,{\sc ii} λ4481 is also predicted by this best-fit disk model and the computed profile compares successfully with observations from 1999. To obtain consistency between the Hα line profile modelling and the other constraints, it was necessary in the profile fitting to weight the line core (emission peaks and central depression) more heavily than the line wings, which were not well reproduced by our models. This may reflect the limitation of assuming a single power-law for the disk's equatorial density variation. The best-fit disk density model for o~Aqr predicts that Hα is near its maximum strength as a function of disk density, and hence the Hα equivalent width and line profile change only weakly in response to large (factor of ∼5) changes in the disk density.