Binary-object spectral-synthesis in 3D (BOSS-3D)

Modelling H_α emission in the enigmatic multiple system LB-1

¹ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
e-mail: levin.hennicker@kuleuven.be
² National Solar Observatory, 22 Ohi’a Ku Street, Makawao, HI 96768, USA
³ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
⁴ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁵ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Casilla, 19001 Santiago de Chile, Chile
⁶ Institut de Planétologie et d’Astrophysique de Grenoble, 414 Rue de la Piscine, 38400 Saint-Martin-d’Hères, France

Received: 20 July 2021
Accepted: 23 November 2021

Abstract

Context. To quantitatively decode the information stored within an observed spectrum, detailed modelling of the physical state and accurate radiative transfer solution schemes are required. The accuracy of the model is then typically evaluated by comparing the calculated and observed spectra. In the analysis of stellar spectra, the numerical model often needs to account for binary companions and 3D structures in the stellar envelopes. The enigmatic binary (or multiple) system LB-1 constitutes a perfect example of such a complex multi-D problem. Thus far, the LB-1 system has been indirectly investigated by 1D stellar-atmosphere codes and by spectral disentangling techniques, yielding differing conclusions about the nature of the system (e.g., a B-star and black-hole binary with an accretion disc around the black hole or a stripped-star and Be-star binary system have been proposed).

Aims. To improve our understanding of the LB-1 system, we directly modelled the phase-dependent H_α line profiles of this system. To this end, we developed and present a multi-purpose binary-object spectral-synthesis code in 3D (BOSS-3D).

Methods. BOSS-3D calculates synthetic line profiles for a given state of the circumstellar material. The standard pz-geometry commonly used for single stars is extended by defining individual coordinate systems for each involved object and by accounting for the appropriate coordinate transformations. The code is then applied to the LB-1 system, considering two main hypotheses, a binary containing a stripped star and Be star, or a B star and a black hole with a disc.

Results. Comparing these two scenarios, neither model can reproduce the detailed phase-dependent shape of the H_α line profiles. A satisfactory match with the observations, however, is obtained by invoking a disc around the primary object in addition to the Be-star disc or the black-hole accretion disc.

Conclusions. The developed code can be used to model synthetic line profiles for a wide variety of binary systems, ranging from transit spectra of planetary atmospheres, to post-asymptotic giant branch binaries including circumstellar and circumbinary discs and massive-star binaries with stellar winds and disc systems. For the LB-1 system, our modelling provides strong evidence that each object in the system contains a disc-like structure.