The stars near the centre of supergiant shell LMC 4 : Further constraints on triggering scenarios ⋆

The huge supergiant shell (SGS) LMC4, next to the giant H II region 30Doradus the second most impressive feature of the Large Magellanic Cloud (LMC), has been the subject of many studies and was used to test various models of star forming mechanisms. In this paper we present a B,V photometry of the very centre of this SGS. The new data yield an age of 11(2) Myr and a B−V excess of 0.10(3) mag. Additionally, the two clusters near the geometric centre are too old to be related to the formation of LMC4: HS343 being ∼ 0.1 Gyr and KMHK1000 being ∼ 0.3 Gyr of age. This and existing photometries support that the young stellar population covering the entire inner region is nearly coeval. We compare these findings with predictions of models proposed for the creation mechanism of SGSs. We conclude that the large-scale trigger, necessary to explain the observations, comes from hydrodynamic interaction of the galactic halo with gas in the leading edge of the moving LMC.