Astronomy and Astrophysics, volume 348, 728-736 (1999/8-3)
The chemical composition of the young, inter-cloud population.
ROLLESTON W.R.J., DUFTON P.L., McERLEAN N.D. and VENN K.A.
Abstract (from CDS):
High-resolution AAT spectroscopy and lower resolution spectrophotometry are presented for three early B-type stars that are members of the young, Inter-Cloud population between the Magellanic Clouds. These spectra have been analyzed using LTE model-atmosphere techniques, to derive the stellar atmospheric parameters and photospheric chemical compositions. The latter should reflect that of the present-day interstellar medium (ISM) within the Inter-Cloud Region (ICR). From a differential analysis, the three ICR stars appear to have a mean metal abundance of ∼1.1dex lower than their Population i Galactic analogues, and 0.5dex lower than the SMC star (AV 304). Hence, the ICR gas does not reflect the present-day composition of either the SMC (or LMC) ISM. Age (and distance) estimates were obtained using the theoretical isochrones of Bertelli et al. (1994) these imply that the young, Inter-Cloud population has an age dispersion of at least 10-40Myr, and provide evidence for a distance gradient across the ICR. We discuss our results within the context of recent numerical simulations of the gravitational interactions between the Galaxy-LMC-SMC, that predict that the ICR was tidally disrupted from the SMC some 200Myr ago. If the SMC was chemically homogeneous, a comparison of the ICR abundance determinations with the SMC age-metallicity relationship would then imply that the formation of the ICR must have occurred ∼8.5Gyr ago. Alternatively and more plausible, we postulate that the ICR gas formed from a mixture of SMC gas and an unenriched component. This is consistent with model-predictions that both a halo and disc component should have contributed to the material within the ICR during the tidal disruption.