Gas distribution, kinematics, and excitation structure in the disks around the Classical Be Stars β Canis Minoris and ζ Tauri

Abstract

Using CHARA and VLTI near-infrared spectro-interferometry with hectometric baseline lengths (up to 330 m) and with high spectral resolution (up to λ/Δλ = 12, 000), we studied the gas distribution and kinematics around two classical Be stars. The combination of high spatial and spectral resolution achieved allows us to constrain the gas velocity field on scales of a few stellar radii and to obtain, for the first time in optical interferometry, a dynamical mass estimate using the position-velocity analysis technique known from radio astronomy. For our first target star, β Canis Minoris, we model the H+K-band continuum and Brγ-line geometry with a near-critical rotating stellar photosphere and a geometrically thin equatorial disk. Testing different disk rotation laws, we find that the disk is in Keplerian rotation (v(r)∝r –0.5 ± 0.1) and derive the disk position angle (140° ± 1fdg7), inclination (38fdg5 ± 1°), and the mass of the central star (3.5 ± 0.2 M ☉). As a second target star, we observed the prototypical Be star ζ Tauri and spatially resolved the Brγ emission as well as nine transitions from the hydrogen Pfund series (Pf 14-22). Comparing the spatial origin of the different line transitions, we find that the Brackett (Brγ), Pfund (Pf 14-17), and Balmer (Hα) lines originate from different stellocentric radii (R cont < R Pf < R Brγ ~ R Hα), which we can reproduce with an LTE line radiative transfer computation. Discussing different disk-formation scenarios, we conclude that our constraints are inconsistent with wind compression models predicting a strong outflowing velocity component, but support viscous decretion disk models, where the Keplerian-rotating disk is replenished with material from the near-critical rotating star.