The chemical composition of the young, Inter-Cloud population
W.R.J. Rolleston 1,
P.L. Dufton 1,
N.D. McErlean * 1 and
K.A. Venn 2
Received 9 February 1999 / Accepted 4 June 1999
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 1.1 dex lower than their Population I Galactic analogues, and 0.5 dex 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 10-40 Myr, 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 200 Myr 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 8.5 Gyr 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.
Key words: stars: abundances stars: atmospheres stars: early-type galaxies: Magellanic Clouds
Send offprint requests to: W.R.J. Rolleston
© European Southern Observatory (ESO) 1999
Online publication: August 13, 199