On the Nature of the Herbig B[e] Star Binary System V921 Scorpii: Geometry and Kinematics of the Circumprimary Disk on Sub-AU Scales

Abstract

V921 Scorpii is a close binary system (separation 0farcs025) showing the B[e]-phenomenon. The system is surrounded by an enigmatic bipolar nebula, which might have been shaped by episodic mass-loss events, possibly triggered by dynamical interactions between the companion and the circumprimary disk. In this paper, we investigate the spatial structure and kinematics of the circumprimary disk, with the aim to obtain new insights into the still strongly debated evolutionary stage. For this purpose, we combine, for the first time, infrared spectro-interferometry (VLTI/AMBER, λ/Δλ = 12, 000) and spectro-astrometry (VLT/CRIRES, λ/Δλ = 100, 000), which allows us to study the AU-scale distribution of circumstellar gas and dust with an unprecedented velocity resolution of 3 km s–1. Using a model-independent photocenter analysis technique, we find that the Brγ-line-emitting gas rotates in the same plane as the dust disk. We can reproduce the wavelength-differential visibilities and phases and the double-peaked line profile using a Keplerian-rotating disk model. The derived mass of the central star is 5.4 ± 0.4 M ☉ ▪ (d/1150 pc), which is considerably lower than expected from the spectral classification, suggesting that V921 Sco might be more distant (d ~ 2 kpc) than commonly assumed. Using the geometric information provided by our Brγ spectro-interferometric data and Paschen, Brackett, and Pfund line decrement measurements in 61 hydrogen recombination line transitions, we derive the density of the line-emitting gas (Ne = (2-6) × 1019 m–3). Given that our measurements can be reproduced with a Keplerian velocity field without an outflowing velocity component and the non-detection of age-indicating spectroscopic diagnostics, our study provides new evidence for the pre-main-sequence nature of V921 Sco.