The History of the Local ISM: The Last 50 Million Years

We present computational models where we reproduce the dominant largescale structures (radii =200-500 pc) in the solar neigborhood, namely the gaseous Lindblad's Ring and Gould's Belt of young stars. Specifically we use the energetics of the combined stellar winds of hot stars and supernovae to drive the expansion of a supershell. We further use star-formation criteria to show that one can reproduce the present day young (ages < 10 Myr) stellar associations, Ori OB2, Sco-Cen, and Cep OB2 of Gould's Belt. Realistic modeling implies that the age of the Lindblad's Ring is approximately 50 Myr. This age corresponds well with those of the Alpha Per and Cep OB6 clusters, which suggests that these older clusters may have been the seed associations producing the momentum that drove the expansion of the Lindblad Ring supershell. The younger stellar associations in Ori OB2, Sco-Cen and other Gould's Belt stars along the periphery of the Lindblad's Ring are driving the expansion of several well-known shells that are expanding into the pre-existing cavity formed by the older Lindblad's Ring. This suggests that the region identified as the ``Local Bubble'' in the LISM is not a bubble at all. In actuality, it appears to be delineated by the exteriors of these younger shell structures, where the low density region contained within the Local Bubble denotes part of the original pre-existing cavity of Lindblad's Ring. This model also explains qualitatively the peculiar motions, as deduced using Hipparcus data, for the OB associations of Gould's Belt.