[Paper Review] The binary Be star {\delta} Sco at high spectral and spatial resolution: Disk geometry and kinematics before the 2011 periastron

A. Meilland, O. Delaa+19 more|arXiv (Cornell University)|Jun 9, 2011

Stellar, planetary, and galactic studiesPhysics and Astronomy34 references29 citations

TL;DR

This study uses high-spectral and spatial-resolution interferometry with VLTI/AMBER and CHARA/VEGA to resolve the circumstellar disk of the binary Be star δ Sco before its 2011 periastron passage. It finds the disk is rotationally dominated with Keplerian kinematics, a disk expansion velocity of ~0.2 km s⁻¹, and geometry consistent with other quasi-critically rotating Be stars, despite the star rotating at only ~70% of its critical velocity, suggesting rotation alone may not drive mass ejection.

ABSTRACT

Classical Be stars are hot non-supergiant stars surrounded by a gaseous circumstellar disk that is responsible for the observed IR-excess and emission lines. The influence of binarity on these phenomena remains controversial. delta Sco is a binary system whose primary suddently began to exhibit the Be phenomenon at the last periastron in 2000. We want to constrain the geometry and kinematics of its circumstellar environment. We observed the star between 2007 and 2010 using spectrally-resolved interferometry with the VLTI/AMBER and CHARA/VEGA instruments. We found orbital elements that are compatible with previous estimates. The next periastron should take place around July 5, 2011 (+- 4,days). We resolved the circumstellar disk in the HAlpha (FWHM = 4.8+-1.5mas), BrGamma (FWHM = 2.9 0.,mas), and the 2.06$ \mu$m HeI (FWHM = 2.4+-0.3mas) lines as well as in the K band continuum (FWHM ~2.4mas). The disk kinematics are dominated by the rotation, with a disk expansion velocity on the order of 0.2km/s. The rotation law within the disk is compatible with Keplerian rotation. As the star probably rotates at about 70% of its critical velocity the ejection of matter doesn't seems to be dominated by rotation. However, the disk geometry and kinematics are similar to that of the previously studied quasi-critically rotating Be stars, namely Alpha Ara, Psi Per and 48 Per.

Motivation & Objective

• To constrain the geometry and kinematics of the circumstellar disk in the binary Be star δ Sco prior to its 2011 periastron passage.
• To investigate whether binarity influences the formation and dynamics of the circumstellar disk in Be stars.
• To determine whether the observed disk properties are consistent with Keplerian rotation and rotational ejection mechanisms.
• To assess the role of stellar rotation versus other physical processes in driving mass ejection in Be stars.
• To compare δ Sco’s disk properties with those of other well-studied Be stars such as α Ara, ψ Per, and 48 Per.

Proposed method

• Spectrally resolved interferometry using VLTI/AMBER and CHARA/VEGA instruments to obtain high angular resolution data in the Hα, Brγ, and He i (2.06 µm) emission lines and K-band continuum.
• Modeling of visibilities and differential phases across the spectral lines to infer disk geometry and kinematics.
• Orbital element determination using multi-epoch interferometric measurements and comparison with prior estimates.
• Application of a simple kinematical model assuming a rotating, Keplerian disk to fit observed differential phases and visibilities.
• Use of calibrated interferometric data with known baseline geometries and atmospheric conditions to minimize systematic errors.
• Comparison of line profile asymmetries and visibility asymmetries to assess disk inhomogeneities and potential non-axisymmetric structures.

Experimental results

Research questions

• RQ1 What is the kinematic structure of the circumstellar disk around δ Sco, and is it consistent with Keplerian rotation?
• RQ2 How does the disk's spatial extent and velocity structure compare to other Be stars with similar rotational properties?
• RQ3 What is the role of binarity in shaping the disk geometry and driving mass ejection in δ Sco?
• RQ4 Is the observed disk expansion velocity consistent with a rotational or outburst-driven ejection mechanism?
• RQ5 To what extent does the star’s rotation rate (vsini = 175 km s⁻¹) explain the formation of the circumstellar disk?

Key findings

• The circumstellar disk of δ Sco was spatially resolved in the Hα line with a full width at half maximum (FWHM) of 4.8 ± 1.5 mas.
• The Brγ line was resolved with a FWHM of 2.9 ± 0.5 mas, and the He i (2.06 µm) line with a FWHM of 2.4 ± 0.3 mas.
• The K-band continuum was resolved with a FWHM of approximately 2.4 mas.
• The disk kinematics are dominated by rotation, with a disk expansion velocity of 0.2 km s⁻¹, indicating negligible radial outflow.
• The rotation law within the disk is consistent with Keplerian rotation, with the inner boundary rotating at critical velocity.
• The star rotates at approximately 70% of its critical velocity (Vcrit ≈ 500 km s⁻¹), suggesting rotation alone may not be the primary driver of mass ejection.