ar X iv : a st ro - p h / 98 11 21 6 v 1 1 3 N ov 1 99 8 Spontaneous and Induced Star Formation in the LMC

Spontaneous and triggered star formation in the LMC is discussed with data on star clusters ages and positions. The supershell LMC4 and the stellar arcs in the same region are suggested to be triggered by GRBs, the progenitors of which might have escaped the old elliptical cluster NGC1978, close to which are a number of X-ray binaries and the SGR/SNR N49. 1. Stochastic star formation The LMC is the best site to study large-scale features of star formation as displayed by mutual positions and ages of star clusters. The ages are now available for about 600 clusters. Efremov and Elmegreen (1998a) demonstrated recently that the larger the mutual distance between these clusters, the larger their age difference. This relation may be considered part of a general correlation between the size of a star formation region and the duration of star formation there. For example, OB subgroups each form in ∼ 3 My, the whole OB associations that surround them form in ∼ 10 My, and the star complexes, such as Gould's Belt, that include the OB associations, form in sim30 My; each scale is larger than the previous by a factor of 10 (e.g., 3 pc, 30 pc, and 300 pc). Thus the duration of star formation scales approximately with the square root of the region size in this example (see review in Elmegreen & Efremov 1999). Such scaling is expected for star formation in turbulent gas, because star formation usually operates on a time scale proportional to the cloud or clump crossing time, and this crossing time scales with the square root of region size. Thus the formation of hierarchical young stellar groups results from hierarchical cloud structure. There is no preferred scale for cluster formation.