The Origin of the Local System of Gas and Stars

We present a model for the local system of gas and stars that we associate essentially with Gould's Belt, the Sirius supercluster, and the Local arm. We consider that these subsystems were formed in different epochs within a supercloud that was initially moving almost ballistically in the Galactic field until an encounter with a major spiral arm started a braking process. The stars of older generations, i.e., the Sirius supercluster, tended to conserve the kinematics of the prebraking phase of the supercloud, while the remaining gas and the early star complexes reflect the recent kinematics, resulting from the braking process. We have calculated back in time the epicyclic orbits of the Sirius supercluster, as well as of the postulated supercloud, starting from adequate initial conditions and taking into account the action of a friction force on the gas. From the condition that the Sirius supercluster and the postulated supercloud shared the same orbits before the separation of gas and stars because of the braking forces, we have determined the free parameters of the model. The main evidence supporting our hypothesis is that the supercloud's track derived from the model coincides with a large "tunnel" in the distribution of the local interstellar matter, toward l ≈ 240°.

We conclude that a supercloud of ≈2 × 10⁷ M_☉ and a radius of ≈400 pc was the common precursor of the Sirius supercluster, Gould's Belt, and the Local arm. The Sirius supercluster was born ≈500 Myr ago in the supercloud, rotating in the Galactic sense and coexisting with the supercloud ever since. We show that the theoretical velocity distribution of the Sirius supercluster derived from the model is consistent with the observed vertex deviation and velocity dispersions of the Sirius supercluster. The supercloud entered into a major spiral arm 100 Myr ago. The interaction with the Galactic shock and the subsequent gas streaming generated a process of braking and compression of the gas in the supercloud. This led to the drifting out of the stars of the Sirius supercluster from the gas and to the formation of Gould's Belt and the Local arm.

The results of the model suggest that the interaction with the Galactic shock compressed the gas of the central regions of the supercloud into a flattened disk, precursor of Gould's Belt, whereas the peripheral gas generated an expanding superring, precursor of the Local arm. The observed inclination of Gould's Belt and the Galactic longitude of its nodal line are consistent with the results of the model. To study the initial configuration and the evolution of the kinematics of the local superring in the Galactic force field with interstellar friction forces, we assumed that initially the superring (1) had a cylindrical form with its axis perpendicular to the Galactic plane, (2) was centered at the barycenter that the supercloud had 100 Myr ago, and (3) had a velocity field resulting from the velocity of its own barycenter and the velocity of rotation about it (both conserved from the supercloud), plus a velocity of radial expansion from the center of the ring. The model of the superring fits the configuration of the Local arm and the kinematics of its interstellar matter, i.e., Lindblad's feature C/H, reasonably well. Computing the Sun's orbit for the last 100 Myr, with the help of the supercloud model, we find that the Sun has been attached to the local system. We quote that this interesting fact could give a firm foundation for a Galactic theory of terrestrial catastrophism. Finally, we describe the probable origins of the main stellar streams, i.e., the superclusters in the solar neighborhood, from the point of view of the supercloud model.