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The blue hypergiant Cyg OB2-12 (B3Ia+) is a representative member of theclass of very massive stars in a poorly understood evolutionary stage. Weobtained its high-resolution X-ray spectrum using Chandra observatory. PoWRmodel atmospheres were calculated to provide realistic wind opacities and toestablish the wind density structure. We find that collisional de-excitation isthe dominant mechanism de-populating the metastable upper levels of theforbidden lines of the He-like ions SiXIV and MgXII. Comparison between themodel and observations reveals that X-ray emission is produced in a denseplasma, which could reside only at the photosphere or in a colliding wind zonebetween binary components. The observed X-ray spectra are well fitted bythermal plasma models, with average temperatures in excess of 10 MK. The windspeed in Cyg OB2-12 is not high enough to power such high temperatures, but thecollision of two winds in a binary system can be sufficient. We used archivaldata to investigate the X-ray properties of other blue hypergiants. In general,stars of this class are not detected as X-rays sources. We suggest that our newChandra observations of Cyg OB2-12 can be best explained if Cyg OB2-12 is acolliding wind binary possessing a late O-type companion. This makes Cyg OB2-12only the second binary system among the 16 known Galactic hypergiants. This lowbinary fraction indicates that the blue hypergiants are likely products ofmassive binary evolution during which they either accreted a significant amountof mass or already merged with their companion.

On the binary nature of massive blue hypergiants: high-resolution X-ray spectroscopy suggests that Cyg OB2 12 is a colliding wind binary