[Paper Review] The binary Be star $\delta$ Scorpii at high spectral and spatial resolution : II The circumstellar disk evolution after the periastron

A. Meilland, P. Stee+4 more|arXiv (Cornell University)|Jan 9, 2013

Astrophysics and Star Formation Studies22 references5 citations

TL;DR

This study uses high-spectral and spatial-resolution VLTI/AMBER interferometry to track circumstellar disk evolution in the binary Be star δ Scorpii one year before and after its 2011 periastron passage. It finds a 40% increase in Brγ line emission FWHM (from 5.5 to 7.8 R⊙) between 2010 and 2012, indicating a mean disk expansion velocity of 0.20 ± 0.11 km s⁻¹, with asymmetric disk morphology changes likely due to tidal interactions with the companion, though the asymmetry appears 'frozen' post-periastron.

ABSTRACT

Classical Be stars are hot non-supergiant stars surrounded by a gaseous circumstellar disk that is responsible for the observed infrared (IR) excess and emission lines. The influence of binarity on these phenomena remains controversial. We followed the evolution of the environment surrounding the binary Be star $\delta$ Scorpii one year before and one year after the 2011 periastron to check for any evidence of a strong interaction between its companion and the primary circumstellar disk. We used the VLTI/AMBER spectro-interferometric instrument operating in the K band in high (12000) spectral resolution to obtain information on both the disk geometry and kinematics. Observations were carried out in two emission lines: Br$\gamma$ (2.172\,$\mu$m) and $\ion{He}{i}$ (2.056\,$\mu$m). We detected some important changes in $\delta$ Scorpii's circumstellar disk geometry between the first observation made in April 2010 and the new observation made in June 2012. During the last two years the disk has grown at a mean velocity of 0.2\,km\,s$^{-1}$. This is compatible with the expansion velocity previously found during the 2001-2007 period. The disk was also found to be asymmetric at both epochs, but with a different morphology in 2010 and 2012. Considering the available spectroscopic data showing that the main changes in the emission-line profiles occurred quickly during the periastron, it is probable that the differences between the 2010 and 2012 disk geometry seen in our interferometric data stem from a disk perturbation caused by the companion tidal effects. However, taking into account that no significant changes have occurred in the disk since the end of the 2011 observing season, it is difficult to understand how this induced inhomogeneity has been "frozen" in the disk for such a long period.

Motivation & Objective

• To investigate the long-term evolution of the circumstellar disk in the binary Be star δ Scorpii after its 2011 periastron passage.
• To determine whether tidal interactions with the stellar companion induce measurable changes in disk geometry and kinematics.
• To assess the stability and growth rate of the disk over time using high-resolution interferometric data.
• To compare interferometric observations from 2010 (before periastron) and 2012 (after periastron) to detect post-periastron disk evolution.
• To evaluate the role of tidal forces in shaping disk asymmetries observed in emission-line profiles.

Proposed method

• Acquired high spectral resolution (R = 12,000) spectro-interferometric data using the VLTI/AMBER instrument in the K band.
• Observed two emission lines: Brγ (2.172 µm) and He i (2.056 µm), to probe disk structure and kinematics.
• Used baseline coverage and visibility/phase measurements across multiple baselines and position angles to reconstruct disk geometry.
• Fitted axisymmetric Keplerian disk models to the interferometric data, including parameters like disk FWHM, equivalent width, and position angle.
• Compared model fits to data from April 2010 (pre-periastron) and June 2012 (post-periastron) to detect temporal evolution.
• Accounted for instrumental and calibration effects using the calibrator star HD 139663 and fringe tracker FINITO.

Experimental results

Research questions

• RQ1 Has the circumstellar disk of δ Scorpii undergone measurable structural changes between 2010 and 2012, particularly after the 2011 periastron passage?
• RQ2 What is the rate of disk expansion, and is it consistent with viscous or ejection-driven disk models?
• RQ3 Are the observed disk asymmetries in 2010 and 2012 due to tidal perturbations from the binary companion?
• RQ4 Why does the disk asymmetry appear to remain unchanged in spectroscopic data despite the expected dynamical evolution of tidal inhomogeneities?
• RQ5 How do the kinematic and geometric properties of the Brγ and He i emission lines differ, and what do these differences imply about disk structure?

Key findings

• The circumstellar disk of δ Scorpii expanded by 40% between 2010 and 2012, with the Brγ line emission FWHM increasing from 5.5 ± 1.0 R⊙ to 7.8 ± 0.3 R⊙.
• The mean disk expansion velocity was measured at 0.20 ± 0.11 km s⁻¹, consistent with previous measurements from 2000–2007 and the viscous excretion disk model.
• The He i line showed only a minor 8% increase in FWHM (from 4.5 ± 0.5 R⊙ to 4.9 ± 0.2 R⊙), suggesting its emission region remains compact compared to Brγ.
• The disk was asymmetric at both epochs, but with distinct morphological differences between 2010 and 2012, indicating a structural change post-periastron.
• The spectroscopic V/R ratio change observed by Rivinius et al. (2012) during the periastron occurred over a few days, suggesting a rapid tidal perturbation that likely initiated the observed asymmetry.
• Despite the rapid change near periastron, no significant evolution in the disk asymmetry was observed in the 2012 data, suggesting the inhomogeneity may be 'frozen' in the disk for extended periods.