Professor Stephen Smartt CBE FRS MRIA

Abstract:

We have previously published intermediate to high resolution spectroscopic observations of approximately 80 early B-type main-sequence stars situated in 19 Galactic open clusters/associations with Galactocentric distances distributed over 6 ≤ Rg ≤ 18 kpc. This current study collates and re-analyses these equivalent-width datasets using LTE and non-LTE model atmosphere techniques, in order to determine the stellar atmospheric parameters and abundance estimates for C, N, O, Mg, Al and Si. The latter should be representative of the present-day Galactic interstellar medium. Our extensive observational dataset permits the identification of sub-samples of stars with similar atmospheric parameters and of homogeneous subsets of lines. As such, this investigation represents the most extensive and systematic study of its kind to date. We conclude that the distribution of light elements (C, O, Mg & Si) in the Galactic disk can be represented by a linear, radial gradient of -0.07 ± 0.01 dex kpc-1. Our results for nitrogen and oxygen viz. (-0.09 ± 0.01 dex kpc-1 and -0.067 ± 0.008 dex kpc-1) are in excellent agreement with that found from the study of H II regions. We have also examined our datasets for evidence of an abrupt discontinuity in the metallicity of the Galactic disk near a Galactocentric distance of 10 kpc (see Twarog et al. 1997). However, there is no evidence to suggest that our data would be better fitted with a two-zone model. Moreover, we observe a N/O gradient of -0.04 ± 0.02 dex kpc-1 which is consistent with that found for other spiral galaxies (Vila-Costas & Edmunds 1993).