The nature of an extreme B supergiant in Cygnus

Image of the Cygnus-X region near the Cygnus OB2 association. 2MASS J20395358+4222505 is the star with the red border (revealing the high extinction) near the top left corner. (Courtesy of the GALANTE project, I.P. J. Maíz Apellániz).

Advertised on
Authors
References

2MASS J20395358+4222505 is an obscured early B supergiant near the massive OB star association Cyg OB2. Despite its bright infrared magnitude (Ks= 5.82) it has remained largely ignored because of its dim optical magnitude (B= 16.63, V= 13.68). In a previous work we classified it as a highly reddened, potentially extremely luminous, early B-type supergiant. We obtained its spectrum in the U, B and R spectral bands during commissioning observations with the instrument MEGARA@GTC. It displays a particularly strong Hα emission for its spectral type, B1 Ia. The star seems to be in an intermediate phase between super- and hypergiant, a group that it will
probably join in the near (astronomical) future. We observe a radial velocity difference between individual observations and determine the stellar parameters, obtaining Teff =24 000 K, log gc= 2.88±0.15. The rotational velocity found is large for a B-supergiant, v sin i= 110±25 km/s. The abundance pattern is consistent with solar, with a mild C underabundance (based on a single line). Assuming that J20395358+4222505 is at the distance of Cyg OB2 we derive the radius from infrared photometry, finding R= 41.2±4.0 R⊙, log(L/L⊙)= 5.71±0.04 and a spectroscopic mass of 46.5±15.0 M⊙. The clumped mass-loss rate (clumping factor 10) is very high for the spectral type, M= 2.4×10 M⊙/yr. The high rotational velocity and mass-loss rate place the star at the hot side of the bi-stability jump. Together with the nearly solar CNO abundance pattern, they may also point to evolution in a binary system, J20395358+4222505 being the initial secondary.