Progenitors of GRBs Originated in the Dense Star Clusters

Abstract

The suggestion formulated in the title of this paper arised first to explain the origin of the giant (170–300 pc in radii) stellar arcs in the region of the supershell LMC4 in the LMC [4,5,7]. The formation of stellar arcs in the swept up gas shells formed by the central sources of pressure needs some 10 52 ergs, yet neither multiple SNe in suggested central clusters [6] nor cloud impacts were able to form these arcs. I believe the first suggestion failed to explain why are all the arcs so rare features, all the arcs in the LMC being close to each other, and the second one – why are the ages of arcs different. Yet another source of the energy imput to ISM to produce star-forming HI shells was suggested to be the GRB events [8,15] Along with the only known in the LMC Soft Gamma Repeater SGR 0526-66, within the same region of ∼ 1k pc in diameter there are HI supershell and three or four arc-shaped star complexes. There should be in this region the common source for the progenitors of all these objects! And there is indeed an unique star cluster in the same region: the NGC 1978 cluster, 2 Gyr old and 10 6 suns massive. There are no clusters of similar mass and age in the Milky Way galaxy, and only a handful of these is in the LMC, NGC 1978 being the most massive and the most elongated one. The binaries of compact objects, the progenitors of GRB, might not be results of two SN outbursts in the primeval binaries of massive stars. Instead of this, the single stellar remnants could form the hard binaries in result of dynamical interactions in dense cluster cores. Many of them might then have been dynamically ejected from the cluster, to merge after escaping in GRB events, and to form the giant arcs not centered on NGC 1978 itself [4,7]. This conclusion was supported by observation that X-ray binaries concentrate near NGC 1978. The recent data for the LMC X-ray sources [19] suggest that in 10 × 10 degrees square there are nine X-ray binaries, whereas within 0.6 × 0.6 degrees square including NGC 1978, there are four of these nine stars. It is tempting to explain this with the origin of these four X-ray binaries in NGC 1978, facing the high rate of occurence of X-ray binaries in globular clusters and the high chance for them to be dynamically ejected from the cluster (see refs. in [4,7]). The recent studies of dynamical evolution of star clusters suggested the high rate of the binary formation and star coalescences and ejections. Portegies Zwart and MacMillan [18] have argued that the BH/BH binaries are formed in (and many of these then ejected from) the massive clusters in a few Gyr after a number of close encounters. The ejected pairs are close enough to merge in a few Myr and the GRB events should be then rather close to the parent cluster. The