Since the Oort comet cloud extends to 1/3 the distance to the nearest stars, there is a strong connection between the galactic environment and the evolution of this cloud. Indeed the spike in semi-major axis distribution of observed new comets is due to the more significant torquing of more distant comets by passing stars and the Galactic tide as a whole. Given this connection, changes in the Galactic environment due to the orbit of the Sun in the Galaxy and drifting of the Sun from its formation environment will have significant effect on the distribution of comets in the Oort cloud and the subsequent delivery of those comets to the inner Solar System.
In this research program, Dr. Thomas Quinn and colleagues will first study the formation and evolution of the Oort Cloud over the 4.5 billion years of the Solar System's history assuming a number of different star forming environments. It is quite likely that the Sun formed in a dense stellar cluster, but the actual densities and timescale for leaving the cluster are uncertain. The work will consider a number of stellar densities and cluster dissipation timescales and compare the resulting Oort Cloud and subsequent comet shower strength with a control case with no cluster environment. The Solar System, however, does not occupy a static place within the Galaxy. The research will therefore go on to incorporate changes in Galactic perturbations as the orbit of the Sun in the Galaxy evolves. This will be accomplished in the context of a cosmologically motivated model of the Galaxy formed via hierarchical merging in a Cold Dark Matter (CDM) Universe. This model has been calculated using the research group's massively parallel cosmological code, GASOLINE, and the model has been shown to produce many observed characteristics of spiral galaxies. The evolution of the Oort Cloud will be calculated as it feels the perturbations of the gas, stars, and dark matter from the disk, spiral arms and other substructure that exists in the galaxy model. The study will pay particular attention to the subsequent changes to the comet flux into the inner Solar System and possible consequences for impact rates on the Earth.
Ever since the connection was made between the extinction event at the end of the Cretaceous and a comet impact, a strong connection has been made between comets and the disciplines of Paleontology and Geology. Both Dr. Quinn and the graduate student supported on this award are part of the University of Washington's Astrobiology program and therefore, will be connecting their research to work done by biologists and geologists in that program. The connection between orbital dynamics and comets also engages the public at large. The issues of the origin of our planetary system, how it fits into the galaxy as a whole and the implications for life are issues that draw the public into the results of dynamical research. Dr. Quinn and the graduate student will incorporate their results into their undergraduate classes and public lectures. ***
The Solar System is surrounded by a large number of comets, referred to as the Oort cloud, whose orbits extend to 1/3 the distance to the nearest stars. The existence of this cloud is inferred by the orbital distribution of ``new'' comets (like comet Ison), which are produced by the perturbation of this cloud by passing stars or the gravity of the Galaxy as a whole. Because of this connection between the orbits of Oort cloud comets and the Galactic environment, this project was able to search for evidence of a changing environment of the Sun during the 4.5 billion years of the Solar System's history. This work first studied the formation and evolution of the Oort Cloud over the 4.5 Gyrs of the Solar System's history assuming a number of different star-forming environments. It is quite likely that the Sun formed in a dense stellar cluster, but the actual densities and timescale for leaving the cluster are uncertain. The work considered a number of stellar densities and cluster dissipation timescales and compare the resulting Oort cloud and subsequent comet shower strength with a control case with no cluster environment. Furthermore, recent work on Galaxy formation and dynamics indicates that, contrary to a long established paradigm, stars like our Sun may have migrated large distances toward or away from the Galactic center. This large scale change in the Sun's orbit around the Galaxy would result in differences in the number of encounters with passing stars, and the overall gravitational force of the Galaxy. This study followed the effect of such changes on the orbits of Oort cloud comets by numerically simulating their gravitational effects. It was found that the inner and outer edges of the Oort Cloud can vary by an order of magnitude, and their radial extent from the Sun tends to be overestimated when it is assumed that the Oort cloud formed and evolved in a static Galactic context. A very intriguing result is that extreme cases of Solar migration in the Galaxy can produce comets in orbits similar to the object ``Sedna'', which has a closest approach to the Sun well outside the planetary region, but is too close to the Sun to be significantly perturbed by passing stars in the current Galactic environment. Hence Sedna may be evidence that our Sun did indeed undergo significant radial migration within the Galaxy. Further confirmation of this idea will require more observations. This project modeled the observed orbital distribution of outer Solar System comets as they are observed by recent surveys, such as the Sloan Digital Sky Survey (SDSS), and future surveys, such as the Large Scale Synoptic Telescope survey (LSST). This work suggests that an object discovered by the SDSS may be the most distant Oort Cloud object ever discovered. Deeper outer Solar System surveys like the LSST could discover a number of similar objects, the distribution of which will help constrain the most likely Solar System formation and migration scenarios. Most of the work performed here was by two graduate students. Both students were trained in techniques for performing large numerical simulations using high throughput computing systems, both on University of Washington computing resources, and on national facilities provided by the National Science Foundation. Both students were also mentored through the production of refereed papers and the preparation of conference presentations. A significant amount of work was also performed by an undergraduate student. The participation of the undergraduate was a direct result of the involvement of this project in the University of Washington Astronomy Pre-MAP (Pre-Majors in Astronomy Program). Pre-MAP is a program to increase diversity in undergraduate Astronomy majors by reaching out to entering UW students who are interested in math and science and who are traditionally underrepresented in astronomy (women, African Americans, Latinos, Native Americans, Asians/Pacific Islanders, low-income and first-generation college students).
