Optically thin circumstellar medium in the β Lyr A system^⋆

¹ Astronomical Institute of the Charles University, Faculty of Mathematics and Physics, V Holešovičkách 2, 180 00 Praha 8, Czech Republic
e-mail: mira@sirrah.troja.mff.cuni.cz
² Université Côte d’Azur, OCA, CNRS, Lagrange, Parc Valrose, Bât. Fizeau, 06108 Nice, France
³ Astronomical Institute, Slovak Academy of Sciences, 059 60 Tatranská Lomnica, Slovak Republic
⁴ Naval Research Laboratory, Remote Sensing Division, Code 7215, 4555 Overlook Ave. SW, Washington, DC, 20375 USA
⁵ Univ Lyon, Univ Lyon1, Ens de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, 69230 Saint-Genis-Laval, France
⁶ Hvar Observatory, Faculty of Geodesy, University of Zagreb, Kačićeva 26, 10000 Zagreb, Croatia
⁷ The CHARA Array of Georgia State University, Mount Wilson Observatory, Mount Wilson, California, 91023 USA
⁸ Astronomical Institute, Czech Academy of Sciences, 251 65 Ondřejov, Czech Republic

Received: 26 July 2020
Accepted: 11 October 2020

Abstract

The complex binary system β Lyr A has an extensive observational dataset: light curves (from far UV to far IR), interferometric squared visibility, closure phase, triple product measurements, spectral-energy distribution, high-resolution spectroscopy, differential visibility amplitude, and also a differential phase. In particular, we used spectra from the Ondřejov 2m telescope from 2013 to 2015 to measure the emission in Hα, He I, Si II, Ne I, or C II lines, and differential interferometry by CHARA/VEGA from the 2013 campaign to measure wavelength-dependent sizes across Hα and He I 6678. This allowed us to constrain not only optically thick objects (primary, secondary, accretion disc), but also optically thin objects (disc atmosphere, jets, shell). We extended our modelling tool, Pyshellspec (based on Shellspec; a 1D local thermodynamical equilibrium radiative transfer code), to include all new observables, to compute differential visibilities/phases, to perform a Doppler tomography, and to determine a joint χ² metric. After an optimisation of 38 free parameters, we derived a robust model of the β Lyr A system. According to the model, the emission is formed in an extended atmosphere of the disc, two perpendicular jets expanding at ∼700 km s⁻¹, and a symmetric shell with the radius ∼70 R_⊙. The spectroscopy indicates a low abundance of carbon, 10⁻² of the solar value. We also quantified systematic differences between datasets, and we discuss here alternative models with higher resolutions, additional asymmetries, or He-rich abundances.