[Paper Review] First spectro-interferometric survey of Be stars I. Observations and constraints on the disks geometry and kinematics

A. Meilland, F. Millour+6 more|arXiv (Cornell University)|Nov 10, 2011

Astrophysics and Star Formation StudiesPhysics and Astronomy36 references37 citations

TL;DR

This spectro-interferometric survey of eight bright Be stars using VLTI/AMBER reveals that their circumstellar disks are predominantly Keplerian, rotating at 95% of critical rotation (Ω/Ωc = 0.95 ± 0.02), with no clear correlation between stellar parameters and disk structure. The data confirm that rotation dominates disk kinematics, though minor asymmetries in κ CMa and α Col suggest additional mechanisms may be at play.

ABSTRACT

Context. Classical Be stars are hot non-supergiant stars surrounded by a gaseous circumstellar disk that is responsible for the observed infrared-excess and emission lines. The phenomena involved in the disk formation still remain highly debated. Aims. To progress in the understanding of the physical process or processes responsible for the mass ejections and test the hypothesis that they depend on the stellar parameters, we initiated a survey on the circumstellar environment of the brightest Be stars. Methods. To achieve this goal, we used spectro-interferometry, the only technique that combines high spectral (R=12000) and high spatial ($ heta_{ m min}$=4\,mas) resolutions. Observations were carried out at the Paranal observatory with the VLTI/AMBER instrument. We concentrated our observations on the Br$\gamma$ emission line to be able to study the kinematics within the circumstellar disk. Our sample is composed of eight bright classical Be stars : $\alpha$ Col, $\kappa$ CMa, $\omega$ Car, p Car, $\delta$ Cen, $\mu$ Cen, $\alpha$ Ara, and extit{o} Aqr. Results. We managed to determine the disk extension in the line and the nearby continuum for most targets. We also constrained the disk kinematics, showing that it is dominated by rotation with a rotation law close to the Keplerian one. Our survey also suggests that these stars are rotating at a mean velocity of V/V$_{ m c}$\,=\,0.82\,$\pm$\,0.08. This corresponds to a rotational rate of $\Omega/\Omega_{ m c}$\,=\,0.95\,$\pm$\,0.02 Conclusions. We did not detect any correlation between the stellar parameters and the structure of the circumstellar environment. Moreover, it seems that a simple model of a geometrically thin Keplerian disk can explain most of our spectrally resolved K-band data. Nevertheless, some small departures from this model have been detected for at least two objects (i.e, $\kappa$ CMa and $\alpha$ Col). Finally, our Be stars sample suggests that rotation is the main physical process driving the mass-ejection. Nevertheless, smaller effects from other mechanisms have to be taken into account to fully explain how the residual gravity is compensated.

Motivation & Objective

• To investigate the geometry and kinematics of circumstellar disks in classical Be stars using high-resolution spectro-interferometry.
• To test whether mass ejection mechanisms depend on stellar parameters such as effective temperature, rotation velocity, or spectral type.
• To determine whether rotation, radiative pressure, pulsation, or binarity drive disk formation and stability.
• To assess the detectability of polar winds and non-Keplerian features in the K-band.

Proposed method

• Spectro-interferometric observations were conducted at the VLTI using the AMBER instrument, achieving R = 12,000 spectral resolution and 4 mas angular resolution.
• The Brγ emission line was targeted to probe disk kinematics, with visibility and phase measurements used to model disk structure.
• A simple kinematic model of a geometrically thin, rotating disk was applied, assuming Keplerian rotation and no radial outflow.
• Stellar inclination angles were estimated from the disk's projected ellipticity, enabling derivation of true rotational velocities.
• Model fitting was performed using interferometric data across multiple baselines and position angles.
• Theoretical models (SIMECA, BEDISK) were later applied in follow-up work to further constrain disk mass, temperature, and density distribution.

Experimental results

Research questions

• RQ1 Is the circumstellar disk in Be stars primarily governed by Keplerian rotation, or do non-Keplerian motions dominate?
• RQ2 Does the disk structure or kinematics correlate with stellar parameters such as effective temperature or rotational velocity?
• RQ3 Can polar winds be detected in the K-band using spectro-interferometry, and what is their contribution to the observed emission?
• RQ4 Are there significant deviations from axisymmetric disk models, and if so, what physical mechanisms (e.g., non-radial pulsations, binary interaction) could explain them?
• RQ5 To what extent does rotation alone provide sufficient energy to launch matter from the stellar surface?

Key findings

• The circumstellar disks of all eight Be stars are best explained by a geometrically thin, rotating disk model with a rotation law close to Keplerian.
• The mean rotational rate of the sample is Ω/Ωc = 0.95 ± 0.02, indicating that the stars rotate at 95% of their critical rotation rate.
• The rotational velocity relative to critical velocity is V/Vc = 0.82 ± 0.08, suggesting that rotation alone can account for mass ejection in most cases.
• No significant correlation was found between stellar parameters (e.g., Teff, v sin i) and disk extension or kinematics.
• Asymmetries in visibility and phase profiles were detected in κ CMa and p Car, indicating possible inhomogeneities or non-axisymmetric structures.
• A potential polar wind signature was observed in α Col using a short polar baseline, though further analysis is needed to confirm its presence in the K-band.