2009A&A...494..253K

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. Instead of studying the surface abundances as a tracer of the evolutionary phase, 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 non-rotating stars with different initial masses. For different values of the ¹²C/¹³C ratio we then compute synthetic first-overtone vibration-rotational band spectra from both the ¹²CO and ¹³CO molecule at different spectral resolutions. We further discuss the influence of stellar rotation on the variation of the surface ¹²C/¹³C ratio and on the possibility of ¹³CO band detection. The surface ¹²C/¹³C isotope ratio is found to decrease strongly during the post-main sequence evolution of non-rotating stars, from its interstellar value of about 70 to a value of about 15-20 for stars with initial masses higher than 7M_☉, and to a value of less than 5 for stars with initial masses higher than 25M_☉. We find that detectable 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 all stars with M_in>9M_☉ during the main-sequence evolution. The detection of ¹³CO band head emission in such mid-resolution K band spectra of a B[e] star thus favours an evolved rather than a young nature of the object.