The birth of Be star disks

I. From localized ejection to circularization

¹ LIRA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, 92190 Meudon, France
² Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão 1226, Cidade Universitária, B-05508-900 São Paulo, SP, Brazil
³ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching b. München, Germany
⁴ European Organisation for Astronomical Research in the Southern Hemisphere (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching b. München, Germany
⁵ Institute for Astronomy, University of Hawaii, 2680 Woodlawn, Honolulu, HI 96822, USA
⁶ Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Av. Gran Bretana 1111, Valparaíso, Chile
⁷ Groupe d’Astrophysique des Hautes Energies, STAR, Université de Liège, Quartier Agora (B5c, Institut d’Astrophysique et de Géophysique), Allée du 6 Août 19c, 4000 Sart Tilman, Liège, Belgium
⁸ European Organisation for Astronomical Research in the Southern Hemisphere (ESO), Casilla 19001, Santiago 19, Chile
⁹ Department of Physics and Astronomy, Embry-Riddle Aeronautical University, 3700 Willow Creek Rd, Prescott, AZ 86301, USA
¹⁰ CHRIST (Deemed to be University), Bangalore, India
¹¹ Three Hills Observatory, The Birches, Torpenhow CA7 1JF, UK
¹² The Be Star Spectra (BeSS) database, operated at LIRA, Observatoire de Paris, Meudon, France
¹³ Piera Remote Observatory, C. de Jaume Balmes 2, 08784 Piera (Barcelona), Catalonia, Spain
¹⁴ Selztal Observatory, Bechtolsheimer Weg 26, 55278 Friesenheim, Germany
¹⁵ BRIXIIS observatory (MPC:B96), Kruibeke, Belgium
¹⁶ Huggins Spectroscopic Observatory, Rayleigh, Essex SS6 8AW, UK
¹⁷ Glenpiper Observatory, 165 Sievers Lane, Glenhope, 3444 Victoria, Australia
¹⁸ DogsHeaven Observatory X87, Brasilia, Brazil
¹⁹ La Montagne Observatory, 1 B Rue Jacques Prevert, 44620 La Montagne, France
²⁰ Observatoire Belle Etoile, Revel 38420, France
²¹ 2SPOT, 45, Chemin du Lac, 38690 Châbons, France
²² SMM Remote Observatory, Av. de Catalunya 38, 25345 Santa Maria de Montmagastrell (Tárrega), Catalonia, Spain

^⋆ Corresponding author: jbartz@udel.edu

Received: 5 December 2024
Accepted: 5 March 2025

Abstract

Context. Classical Be stars are well known to eject mass to build up a disk, but the details governing the initial distribution and subsequent evolution of this matter into a disk are in general poorly constrained through observations.

Aims. By combining high-cadence time-series spectroscopy with contemporaneous space photometry from the Transiting Exoplanet Survey Satellite (TESS), we have sampled about 30 mass ejection events in 13 Be stars. Our goal is to constrain the geometrical and kinematic properties of the ejecta as early as possible, facilitating the investigation into the material's initial conditions and evolution, and understanding its interactions with preexisting material.

Methods. The photometric variability is analyzed together with measurements of the at-times rapidly changing emission features in order to identify the onset of outburst events and obtain information about the geometry of the ejecta and how it changes over time. Short-lived line asymmetries display oscillation cycles (Štefl frequencies), which are compared to photometric and stable spectroscopic frequencies.

Results. All Be stars observed with sufficiently high cadence during an outburst are found to exhibit rapid oscillations of line asymmetry with a single frequency in the days following the start of the event. For a given star this circumstellar frequency may differ only slightly from event to event even when the outbursts they are associated with have different properties. These circumstellar frequencies are typically between 0.5 to 2 d⁻¹, and are generally near photometric frequencies. They are slightly below prominent (generally stable) spectroscopic frequencies seen in photospheric absorption lines. The emission asymmetry cycles break down after roughly 5–10 cycles, with the emission line profile converging toward approximate symmetry shortly thereafter. In photometry, several frequencies typically emerge at relatively high amplitude at some point during the mass ejection process.

Conclusions. In all observed cases, freshly ejected material was initially constrained within a narrow azimuthal range, indicating it was launched from a localized region on the stellar surface. The material orbits the star with a frequency consistent with the near-surface Keplerian orbital frequency. This material circularizes into a disk configuration after several orbital timescales. This is true whether or not there was a preexisting disk at the time of the observed outburst. We find no evidence for precursor phases prior to the ejection of mass in our sample. The several photometric frequencies that emerge during outburst are at least partially stellar in origin.