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Many galactic B[e] stars suffer from improper distance determinations, whichmake it difficult to distinguish between a pre- and post-main sequenceevolutionary phase on the basis of luminosity arguments. In addition, thesestars have opaque circumstellar material, obscuring the central star, so thatno detailed surface abundance studies can be performed. We propose a differentindicator for the supergiant status of a B[e] star, based on the enrichment ofits circumstellar matter by 13C, and detectable via its 13CO band emission inthe K band spectra. Based on stellar evolution models, we calculate thevariation of the 12C/13C isotopic surface abundance ratio during the evolutionof non-rotating stars with different initial masses. For different values ofthe 12C/13C ratio we then compute synthetic first-overtone vibration-rotationalband spectra from both the 12CO and 13CO molecule at different spectralresolutions. We further discuss the influence of stellar rotation on thevariation of the surface 12C/13C ratio. The surface 12C/13C isotope ratio isfound to decrease strongly during the post-main sequence evolution ofnon-rotating stars, from its interstellar value of about 70 to a value of about15-20 for stars with initial masses higher than 7 M_sun, and to a value of lessthan 5 for stars with initial masses higher than 25 M_sun. We find thatdetectable 13CO band head emission is produced for isotope ratios 12C/13C < 20,and can most easily be detected with a spectral resolution of R ~ 1500...3000.For the rotating stellar models, the drop in 12C/13C already occurs for allstars with M > 9 M_sun during the main-sequence evolution. The detection of13CO band head emission in such mid-resolution K band spectra of a B[e] starthus favours an evolved rather than a young nature of the object.

The pre- versus post-main sequence evolutionary phase of B[e] stars: Constraints from 13CO band emission