  The blue hypergiant Cyg OB2-12 (B3Ia+) is a representative member of the
class of very massive stars in a poorly understood evolutionary stage. We
obtained its high-resolution X-ray spectrum using Chandra observatory. PoWR
model atmospheres were calculated to provide realistic wind opacities and to
establish the wind density structure. We find that collisional de-excitation is
the dominant mechanism de-populating the metastable upper levels of the
forbidden lines of the He-like ions SiXIV and MgXII. Comparison between the
model and observations reveals that X-ray emission is produced in a dense
plasma, which could reside only at the photosphere or in a colliding wind zone
between binary components. The observed X-ray spectra are well fitted by
thermal plasma models, with average temperatures in excess of 10 MK. The wind
speed in Cyg OB2-12 is not high enough to power such high temperatures, but the
collision of two winds in a binary system can be sufficient. We used archival
data 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 new
Chandra observations of Cyg OB2-12 can be best explained if Cyg OB2-12 is a
colliding wind binary possessing a late O-type companion. This makes Cyg OB2-12
only the second binary system among the 16 known Galactic hypergiants. This low
binary fraction indicates that the blue hypergiants are likely products of
massive binary evolution during which they either accreted a significant amount
of mass or already merged with their companion.
