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Journal Articles Astronomy and Astrophysics - A&A Year : 2022

The binary system of the spinning-top Be star Achernar

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Abstract

Context. Achernar, the closest and brightest classical Be star, presents rotational flattening, gravity darkening, occasional emission lines due to a gaseous disk, and an extended polar wind. It is also a member of a close binary system with an early A-type dwarf companion.
Aims: We aim to determine the orbital parameters of the Achernar system and to estimate the physical properties of the components.
Methods: We monitored the relative position of Achernar B using a broad range of high angular resolution instruments of the VLT/VLTI (VISIR, NACO, SPHERE, AMBER, PIONIER, GRAVITY, and MATISSE) over a period of 13 years (2006−2019). These astrometric observations are complemented with a series of ≈750 optical spectra for the period from 2003 to 2016.
Results: We determine that Achernar B orbits the primary Be star on a seven-year period, eccentric orbit (e = 0.7258 ± 0.0015) which brings the two stars within 2 au at periastron. The mass of the Be star is found to be mA = 6.0 ± 0.6 M for a secondary mass of mB = 2.0 ± 0.1 M (the latter was estimated from modeling). We find a good agreement of the parameters of Achernar A with the evolutionary model of a critically rotating star of 6.4 M at an age of 63 Ma. The equatorial plane of the Be star and the orbital plane of the companion exhibit a relative inclination of 30°. We also identify a resolved comoving low-mass star, which leads us to propose that Achernar is a member of the Tucana-Horologium moving group.
Conclusions: The proximity of Achernar makes this star a precious benchmark for stellar evolution models of fast rotators and intermediate mass binaries. Achernar A is presently in a short-lived phase of its evolution following the turn-off, during which its geometrical flattening ratio is the most extreme. Considering the orbital parameters, no significant interaction occurred between the two components, demonstrating that Be stars may form through a direct, single-star evolution path without mass transfer. Since component A will enter the instability strip in a few hundred thousand years, Achernar appears to be a promising progenitor of the Cepheid binary systems.

The series of high resolution spectra of Achernar (with the continuum normalized to unity) are available in FITS format at http://dx.doi.org/10.5281/zenodo.6977303 and also at the CDS via anonymous ftp to cdsarc.cds.unistra.fr (ftp://130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/667/A111

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Dates and versions

insu-03874873 , version 1 (28-11-2022)

Licence

Attribution - CC BY 4.0

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Pierre Kervella, Simon Borgniet, Armando Domiciano de Souza, Antoine Mérand, Alexandre Gallenne, et al.. The binary system of the spinning-top Be star Achernar. Astronomy and Astrophysics - A&A, 2022, 667, ⟨10.1051/0004-6361/202244009⟩. ⟨insu-03874873⟩
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