EWOCS-IV: 1Ms ACIS Chandra observation of the supergiant B[e] star Wd1-9

Abstract

Supergiant B[e] (sgB[e]) stars are exceptionally rare objects, with only a handful 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. We present the most detailed X-ray study of Wd1-9 to date, using X-rays that pierce through its cocoon with the aim to uncover its nature and evolutionary state. We utilize 36 Chandra observations of Wd1 from the 'Extended Westerlund 1 and 2 Open Clusters Survey' (EWOCS), plus eight archival datasets, totalling 1.1 Ms. This dataset allows investigation of long-term variability and periodicity in Wd1-9, while X-ray colours and spectra are analysed over time to uncover patterns that shed light on its nature. 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 approximately 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. 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.

accepted for publication in A&A, 17 pages, 14 figures, 5 tables

References in corpus (18)

• The Gaia mission
• Gaia Data Release 3: Summary of the content and survey properties
• The generalised Lomb-Scargle periodogram. A new formalism for the floating-mean and Keplerian periodograms
• HI4PI: A full-sky HI survey based on EBHIS and GASS
• Mass loss from hot massive stars
• The Galactic WN stars: Spectral analyses with line-blanketed model atmospheres versus stellar evolution models with and without rotation
• The empirical metallicity dependence of the mass-loss rate of O- and early B-type stars
• Introduction to astroML: Machine Learning for Astrophysics
• A census of the Wolf-Rayet content in Westerlund 1 from near-infrared imaging and spectroscopy
• The Galactic WN stars revisited. Impact of Gaia distances on fundamental stellar parameters
• A VLT/FLAMES survey for massive binaries in Westerlund 1. IV. Wd1-5 - binary product and a pre-supernova companion for the magnetar CXOU J1647-45?
• Variability of Young Massive Stars in the Galactic Super Star Cluster Westerlund 1
• A Rich Population of X-ray Emitting Wolf-Rayet Stars in the Galactic Starburst Cluster Westerlund 1
• CIR Modulation of the X-ray Flux from the O7.5 III(n)((f)) Star xi Persei?
• Westerlund 1 under the light of Gaia EDR3: Distance, isolation, extent, and a hidden population
• A VLT/FLAMES survey for massive binaries in Westerlund 1: VIII. Binary Systems and Orbital Parameters
• ALMA observations of the supergiant B[e] star Wd1-9
• Investigation of the nature of the wind interaction in HD93205 based on multi-epoch X-ray observations

Cited by in corpus (1)

• EWOCS-V: Is Wd1-72 a recent post-interaction WR+O binary?