Title: Pre-versus post-main sequence evolutionary phase of B[e] stars

Abstract:
Many galactic B[e] stars suffer from improper distance determinations, which make it difficult to 
distinguish between a pre- and post-main sequence evolutionary phase on the basis of luminosity 
arguments. In addition, these stars have opaque circumstellar material, obscuring the central star, 
so that no detailed surface abundance studies can be performed. We propose a different indicator 
for the supergiant status of a B[e] star, based on the enrichment of its circumstellar matter by 
13C, and detectable via its 13CO band emission in the K band spectra.  Based on stellar evolution 
models, we calculate the variation of the 12C/13C isotopic surface abundance ratio during the 
evolution of massive stars. This ratio is found to decrease strongly during the post-main sequence 
evolution, from its interstellar value of about 70 to a value of about 15-20 for stars with initial 
masses higher than 7 M_sun, and to a value of less than 5 for stars with initial masses higher than 
25 M_sun. We find that detectable 13CO band head emission is produced for isotope ratios 12C/13C < 
20, and can easiest be detected with a spectral resolution of R ~ 1500. Such a resolution can be 
achieved with VLTI/AMBER in the K band, and some first test observations of the unclassified 
galactic B[e] star GG Car revealed the presence of detectable 13CO band emission, classifying GG 
Car as an evolved (i.e., supergiant) star.