The Effects of Fractal Molecular Clouds on the Dynamical Evolution of Oort Cloud Comets

Thumbnail Image
Babcock, Carla
Oort Cloud , Molecular Clouds
The Oort Cloud (OC) is a roughly spherical cloud of comets surrounding the solar system, stretching from well beyond the orbit of Neptune, half way to the nearest star. This body of comets is interesting because it contains a record of the gravitational perturbations suffered by the solar system over its lifetime. Here, we investigate the effects of a particular class of perturbing objects - enormous complexes of molecular gas called giant molecular clouds (GMCs). Recent evidence has shown that the classical picture of Oort Cloud formation is inadequate to describe certain properties of the OC. To re-investigate the dynamical evolution of the Oort Cloud, we simulate the Sun's emergence from its natal molecular cloud, and its subsequent encounters with GMCs. While the role of giant molecular clouds in OC formation has been explored before, they have been implemented in a general way, not explicitly taking into account the 3D structure of the cloud. In this research, we draw on an extensive body of evidence which suggests that GMCs are not uniform, diffuse objects, but are instead organized into high density clumps, connected by a very diffuse inter-clump medium. Recent research has shown that GMCs are likely to be fractal in nature, and so we have modeled them as fractal distributions with dimension 1.6. We then perform N-body simulations of the passage of the Sun and its Oort Cloud through such a molecular cloud. We find that the fractal structure of the GMC is, in fact, an important parameter in the magnitude of the cometary energy change. The significant energy changes occur as a result of interactions with the GMC substructure, not simply as a result of its overall density distribution. We find that interactions with GMCs can be quite destructive to the OC, but can also serve to move comets from tightly bound orbits to less tightly bound orbits, thus partially replacing those lost to stripping. Simulations of the Sun's relatively slow exit from its birth GMC paint a picture of a potentially very destructive era, in which a large portion of the OC's evolution may have occured.
External DOI