Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000cem..conf..156o&link_type=abstract
Catastrophic Events and Mass Extinctions: Impacts and Beyond, p. 156
Astronomy and Astrophysics
Astronomy
Galactic Clusters, Solar Neighborhood, Deceleration, Local Group (Astronomy), Stellar Orbits, Galactic Rotation, Astronomical Models, Mathematical Models, Oort Cloud, Interstellar Matter
Scientific paper
The local galactic environment within 1 Kpc of the Sun comprises a massive, expanding ring of interstellar matter associated with star-forming molecular clouds and a group of relatively young OB stars known as Gould's Belt. This structure is likely the result of the disintegration of a formerly bound supercloud. Gould's belt is a discrete system that forms part of the so-called Orion or local arm, which is thought to be a material interarm. branch or spur. Another characteristic of the solar neighborhood is the existence of at least three superclusters near the Sun: (1) Sirius, (2) Pleiades, and (3) Hyades. The central thesis of this paper is that Gould's Belt, the local arm and the superclusters were formed in different epochs within a supercloud of about 2x107 Solar masses and about 500 pc of radius that has been being decelerated for a long period of time. The stars of the older generations, i.e. the Sirius supercluster whose age is around 500 Myr, tend to conserve the kinematics of the pre-braking phase of the supercloud, while the gas and early stars complexes reflect the recent kinematics, resulting from the braking process. We calculated back in time the epicyclic galactic orbits of the Sirius supercluster and the supercloud, starting from their current state as initial conditions and having into account the action of a friction force on the gas. From the condition that the Sirius supercluster and the supercloud shared the same orbits before the separation of gas and stars due to the braking of the gas, we determined the model's free parameters. 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 local interstellar matter, toward the galactic longitude of about 240 deg. An interesting prediction of the model is that the Sun has been gravitationally bound to the supercloud, rotating in sense contrary to the supercloud's rotation. With the values obtained for the model's parameters, we calculated the Sun's orbit with respect to the supercloud's gravitational center, for the last 100 Myr; time in which the braking force acted on the supercloud. A plausible explanation for the capture of the Sun by the supercloud is that while the Sun was passing through an extended concentration of gas, around 500 Myr ago, this gas was suddenly accelerated and organized into a supercloud, making the difference between the supercloud's barycenter velocity and the Sun velocity relative to Regional Standard of Rest lower than the escape velocity. This explains why the Sun current velocity of about 11 km/s with respect to LSR is significantly lower than the average of 60 km/s typical for similar G-type stars in the Galaxy. Additional information is contained in the original extended abstract.
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