The blue supergiant MN18 and its bipolar circumstellar nebula

We report the results of spectrophotometric observations of the massive star MN18 revealed via discovery of a bipolar nebula around it with the Spitzer Space Telescope. Using the optical spectrum obtained with the Southern African Large Telescope, we classify this star as B1 Ia. The evolved status of MN18 is supported by the detection of nitrogen overabundance in the nebula, which implies that it is composed of processed material ejected by the star. We analysed the spectrum of MN18 by using the code CMFGEN, obtaining a stellar effective temperature of approximate to 21 kK. The star is highly reddened, E(B - V) approximate to 2 mag. Adopting an absolute visual magnitude of M-V = -6.8 +/- 0.5 (typical of B1 supergiants), MN18 has a luminosity of log L/L-circle dot approximate to 5.42 +/- 0.30, a mass-loss rate of approximate to(2.8-4.5) x 10(-7) M-circle dot yr(-1), and resides at a distance of approximate to 5.6-(+1.5)(1.2) kpc. We discuss the origin of the nebula around MN18 and compare it with similar nebulae produced by other blue supergiants in the Galaxy (Sher 25, HD 168625, [SBW2007] 1) and the Large Magellanic Cloud (Sk-69 degrees 202). The nitrogen abundances in these nebulae imply that blue supergiants can produce them from the main-sequence stage up to the pre-supernova stage. We also present a K-band spectrum of the candidate luminous blue variable MN56 (encircled by a ring-like nebula) and report the discovery of an OB star at approximate to 17 arcsec from MN18. The possible membership of MN18 and the OB star of the star cluster Lynga 3 is discussed.