From high resolution spectra of the C II lines at 6578 and 6582 recorded in a homogeneous sample of B-type stars, we have applied standard LTE model atmosphere techniques to determine accurate carbon abundances. Using differential methods we have been able to show that the abundances of carbon and nitrogen are closely correlated and unrelated to those of oxygen.
This result provides strong evidence for a common nucleosynthetic origin for carbon and nitrogen in the interstellar medium, distinct from that of oxygen and the other -processed elements, and offers little support to the theory that the enhanced nitrogen abundance measured in the stars S283-2, S289-4 and RLWT-41 is the result of CNO-cycled material mixing to the surface. This abundance pattern is consistent with carbon and nitrogen being mainly generated by nucleosynthesis in intermediate mass stars, while oxygen appears to have been created by an unrelated environment such as the interiors of massive stars.
On the basis that the CN abundances in this stellar sample appear not to have been contaminated by core CNO processed material, and that they hence reflect variations in the progenitor gas, we shall present a future consistent, systematic study of the large-scale Galactic variations of carbon, nitrogen, oxygen, silicon and magnesium (Hibbins et al, in preparation). These variations will play a crucial role in constraining future models of Galactic chemical evolution.
© European Southern Observatory (ESO) 1998
Online publication: March 23, 1998