Year: 2025
Type: article
Abstract: Context. Supergiant B[e] (sgB[e]) stars are exceptionally rare objects, with only a select number of confirmed examples in the Milky Way. The evolutionary pathways leading to the sgB[e] phase remain largely debated, highlighting the need for additional observations. The sgB[e] star Wd1-9, located in the massive cluster Westerlund 1 (Wd1), is enshrouded in a dusty cocoon – likely the result of past eruptive activity – leaving its true nature enigmatic. Aims. We present the most detailed X-ray study of Wd1-9 to date, using X-rays that pierce through its cocoon with the aim of uncovering its nature and evolutionary state. Methods. We utilised 36 Chandra observations of Wd1 from the ‘Extended Westerlund 1 and 2 Open Clusters Survey’ (EWOCS), plus eight archival datasets, totalling 1.1 Ms. We used this dataset to investigate long-term variability and periodicity in Wd1-9, and analysed X-ray colours and spectra over time to uncover patterns that shed light on its nature. Results. Wd1-9 exhibits significant long-term X-ray variability, within which we identify a strong ∼14-day periodic signal. We interpret this as the orbital period, marking the first period determination for the system. The X-ray spectrum of Wd1-9 is thermal and hard (kT ∼ 3.0 keV), resembling the spectra of bright Wolf-Rayet (WR) binaries in Wd1, while a strong Fe emission line at 6.7 keV indicates hot plasma from a colliding-wind X-ray binary. Conclusions. Wd1-9, with evidence of past mass loss, circumbinary material, a hard X-ray spectrum, and a newly detected 14-day period, displays all the hallmarks of a binary – likely a WR+OB – that recently underwent early Case B mass transfer. Its sgB[e] classification is likely phenomenological, reflecting emission from the dense circumbinary material. This places Wd1-9 in a rarely observed phase, possibly revealing a newly formed WN star, bridging the gap between immediate precursors and later evolutionary stages in Wd1.
Authors: K. Anastasopoulou, M. G. Guarcello, J. J. Drake, B. Ritchie, M. De Becker, A. Bayo, F. Najarro, I. Negueruela, S. Sciortino, E. Flaccomio, R. Castellanos, J. F. Albacete-Colombo, M. Andersen, F. Damiani, F. Fraschetti, M. Gennaro, S. J. Gunderson, C. J. K. Larkin, J. Mackey, A. F. J. Moffat, P. Pradhan, S. Saracino, I. R. Stevens, G. Weigelt
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Funders: National Aeronautics and Space Administration, International Max Planck Research School for Environmental, Cellular and Molecular Microbiology, European Space Agency, European Commission, Deutsche Forschungsgemeinschaft, Universität Heidelberg, Consejo Nacional de Investigaciones Científicas y Técnicas, Generalitat Valenciana, Ministerio de Ciencia e Innovación, International Max Planck Research School for Advanced Methods in Process and Systems Engineering, Agencia Estatal de Investigación
Awards: G121809054, G2152447307, G2309174360, G2660406951, G3281880673, G34346518, G3480869486, G3518538256, G4010091782, G4066435496, G4140491619, EXC 2094: ORIGINS: Vom Ursprung des Universums bis zu den ersten Bausteinen des Lebens, G4442677895, G4526231891, G5080475149, G5368914293, G5414252573, G6285680282, G6286071373, G6848214322, G7084143925, G7434936867, G7925496166, G8746803448, G897484643
OpenAlex ID: W7082398050
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