A Be star with a low nitrogen abundance in the SMC cluster NGC 330

¹ The Isaac Newton Group of Telescopes, Apartado de Correos 321, 38700 Santa Cruz de La Palma, Canary Islands, Spain e-mail: djl@ing.iac.es
² Department of Pure & Applied Physics, The Queen's University of Belfast, BT7 1NN, Northern Ireland, UK

Received: 13 July 2004
Accepted: 12 April 2005

Abstract

High-resolution UVES/VLT spectra of B 12, an extreme pole-on Be star in the SMC cluster NGC 330, have been analysed using non-LTE model atmospheres to obtain its chemical composition relative to the SMC standard star AV 304. We find a general underabundance of metals which can be understood in terms of an extra contribution to the stellar continuum due to emission from a disk which we estimate to be at the ~25% level. When this is corrected for, the nitrogen abundance for B 12 shows no evidence of enhancement by rotational mixing as has been found in other non-Be B-type stars in NGC 330, and is inconsistent with evolutionary models which include the effects of rotational mixing. A second Be star, NGC 330-B 17, is also shown to have no detectable nitrogen lines. Possible explanations for the lack of rotational mixing in these rapidly rotating stars are discussed, one promising solution being the possibility that magnetic fields might inhibit rotational mixing.