Professor Stephen Smartt CBE FRS MRIA

Abstract:

In a previous paper we have published observational data for 6 early B-type stars having galactocentric distances of between 10 and 18 kpc. Using LTE line-blanketed model atmosphere techniques we derived their atmospheric parameters, finding that all our targets had similar effective temperatures and surface gravities. In the following study we additionally include two stars which have been presented previously (Rolleston et al. 1993) and found also to have compatible atmospheric parameters to the original programme stars. The homogeneity of this sample allows quantitative line-by-line differential abundance analyses to be carried out which should reliably detect variations in the chemical compositions of the stellar photospheres. We present differential abundances for eight stars, in either young open clusters or the field, with respect to an arbitrarily chosen standard which shows a normal abundance pattern. Our method of calculating distances from the derived atmospheric parameters means that the relative distance scale should be accurate. We find that three of these stars, probably all situated within the Perseus arm show relatively normal metal abundances and that one star which is metal deficient may have formed between the Local and Perseus spiral features. Beyond a galactic radius of 12 kpc, we detect depletions in most of the stars although a significant degree of scatter is also present. A negative linear abundance gradient appears too simple to account for the variations reported. A comparison between the nitrogen and oxygen differential abundances in each star shows a significant degree of scatter, whereas other reliably measured metal abundances (silicon and magnesium) appear well correlated. Hence we may be detecting core CNO cycled material which has been mixed to the surfaces of some of these objects.