A detailed study of the blue supergiant UIT 005 ( B2-2.5Ia ^ { + } ) in M 33 is presented .
The results of our quantitative spectral analysis indicate that the star is a very luminous , \log \mathrm { L } / \mathrm { L } _ { \odot } \sim 5.9 \mathrm { dex } , and massive , M \sim 50 M _ { \odot } , object , showing a very high nitrogen-to-oxygen ratio in its surface ( N/O \sim 8 , by mass ) .
Based on the derived Mg and Si abundances , we argue that this high N/O ratio can not be the result of an initial low O content due to its location on the disk of M 33 , a galaxy known to present a steep metallicity gradient .
In combination with the He abundance , the most plausible interpretation is that UIT 005 is in an advanced stage of evolution , showing in its surface N enrichment and O depletion resulting from mixing with CNO processed material from the stellar interior .
A comparison with the predictions of current stellar evolutionary models indicates that there are significant discrepancies , in particular with regard to the degree of chemical processing , with the models predicting a much lower degree of O depletion than observed .
At the same time , the mass-loss rate derived in our analysis is an order of magnitude lower than the values considered in the evolutionary calculations .
Based on a study of the surrounding stellar population and the nearby cluster , NGC588 , using HST/WFPC2 photometry , we suggest that UIT 005 could be in fact a runaway star from this cluster .
Regardless of its origin , the derived parameters place the star in a region of the Hertzsprung–Russell diagram where Luminous Blue Variables are usually found , but we find no evidence supporting photometric or spectroscopic variability , except for small H \alpha changes , otherwise observed in Galactic B-type supergiants .
Whether UIT 005 is an LBV in a dormant state or a regular blue supergiant could not be discerned in this study .
Subsequent monitoring would help us to improve our knowledge of the more massive stars , bridging the gap between regular and more exotic blue supergiants .