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An international team of astronomers has employed NASA's Chandra X-ray observatory to perform the most detailed study of a supergiant star known as Wd1-9, which yielded important insights into the properties and nature of this star. The new findings are presented in a paper July 23 on the arXiv preprint server.

Supergiant B[e] (sgB[e]) stars are an enigmatic class of evolved massive stars. They are exceptionally rare, as only about a dozen such objects have been identified in the Milky Way. Observations show that these stars showcase strong Balmer emission lines, forbidden line emission from low-ionization metals, and significant infrared excess revealing dense, dusty circumstellar environments. However, the exact evolutionary state of sgB[e] stars still remains the subject of debate.

Wd1-9 is an sgB[e] star in Westerlund 1鈥攁 massive galactic open cluster located some 13,800 light years away. It is the brightest radio source in the cluster and optical observations have revealed that it displays a rich emission-line spectrum, with no detectable photospheric features and non-periodic photometric variability.

However, although Wd1-9 has been studied in various wavelengths, its true nature still remains unknown, given that the star is enshrouded in a dusty cocoon. In general, the previous observations suggest that it may be a cool hypergiant, a luminous blue variable (LBV), or an interacting binary system. That is why a group of astronomers led by Konstantina Anastasopoulou of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, decided to take a deeper look at Wd1-9 with Chandra.

"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," the researchers wrote in the paper.

Chandra observations found that Wd1-9 exhibits significant long-term X-ray variability. Within this variability, Anastasopoulou's team identified a strong 14-day periodic signal, which they interpreted as the orbital period. This is therefore the first period determination for Wd1-9.

The collected data show that Wd1-9 has a hard thermal X-ray spectrum, which is consistent with previous studies. However, this time, strong emission lines from various elements (like silicon, sulfur and argon) were detected and the iron emission line at 6.7 keV was identified for the first time. According to the astronomers, these findings are a clear signature of binarity.

The researchers noted that the spectrum of Wd1-9 closely resembles those of bright Wolf-Rayet (WR) binaries in Westerlund 1. Furthermore, by examining X-ray color-color diagrams, it was found that Wd1-9 showcases variations in thermal temperature across different observations.

Trying to explain the nature of Wd1-9, the authors of the paper concluded that the new findings, together with the evidence of mass loss found by previous studies, suggest that it is a binary system consisting of a WR donor star and an underluminous OB companion star.

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