A‐Type Supergiant Abundances in the Small Magellanic Cloud: Probes of Evolution

Abstract

New abundances of N, O, Na, Mg, Si, Ca, Sc, Ti, Cr, Fe, Sr, Zr, and Ba are presented for 10 A-type supergiants in the Small Magellanic Cloud (SMC), plus upper limits for C. In interpreting the CNO results for constraints on stellar evolution theories, careful attention has been paid to the comparison abundances, i.e., the present day abundances of SMC nebulae and B dwarf stars. These new results are also compared to published results from F-K supergiant analyses and found to be in good agreement when both sets of data are carefully examined as differential (SMC minus Galactic standard) abundances. With the exception of nitrogen, very small star-to-star abundance variations are found for all elements in this analysis. The N variations are not predicted by standard stellar evolution models. Instead, the results support the new predictions reported from rotating stellar models, where the range in nitrogen is the result of partial mixing of CN-cycled gas from the stellar interior due to main-sequence rotation at different rates. The overall overabundance of nitrogen in the sampled stars also implies that these stars have undergone the first dredge-up in addition to having been mixed while on the main sequence. The alpha elements (O, Mg, Si, Ca, Ti) have similar underabundances to Fe, which is not the same as seen in metal-poor stars in the solar neighborhood of the Galaxy. In addition, certain light s-process elements (Zr, Ba) are slightly more underabundant than Fe, which is predicted by the bursting chemical evolution model presented by Pagel & Tautvaisienė for the SMC.