In this project, Dr. McKee will undertake a theoretical study of magnetic fields and their role in star formation. Magnetic fields extract angular momentum from the gas that forms a star, thereby determining whether a gas cloud forms a single star or a binary. The radiation emitted by stars as they form can suppress fragmentation, thereby enabling just one or two stars to form in a region that would have formed many in the absence of thermal feedback. For the first time, frequency-dependent radiative transfer will be used to study the effects of thermal feedback in the formation of low-mass stars. Previous theoretical research has shown that ionized regions associated with the first stars are of particular importance since they determine the masses of these stars.
In addition, this project will explore the formation of stars in groups and clusters. Most stars form in clusters, and indeed it is likely that the Sun formed in a large cluster that contained high-mass stars as well as low-mass stars like the Sun. The formation of star clusters is affected by the powerful feedback mechanisms that accompany star formation, including protostellar outflows and, for massive stars, photoionization and radiation pressure. Both semi-analytic theory and simulation will be used to investigate the formation of clusters of high-mass stars. The results will be compared with observations of star clusters in the Galaxy and of super star clusters in other galaxies. Star formation on galactic scales is determined by the properties of the interstellar medium of the galaxy.
Star formation is central to contemporary astrophysics. As the process by which gas is transformed into stars, it determines the structure and evolution of galaxies. By tapping the nuclear energy in the gas left over from the Big Bang, it determines the luminosity of galaxies and, quite possibly, leads to the reionization of the universe. Most of the elements, including those that make up the world around us, are formed in stars. Finally, the process of star formation is inextricably tied up with the formation and early evolution of planetary systems. A theory of star formation must address both the formation of individual stars and the formation of systems of stars, ranging from clusters to galaxies.
This grant will fund the education of one or more graduate students who will contribute to education and add to the diversity of the scientific work force. Developments in computational methodology will be published for use by the community.
Intellectual Merit: Stars are the "atoms" of the universe: although they are parts of clusters and galaxies, they themselves are indivisible. The first stars formed shortly after the Big Bang, and are still forming today. The elements that we are made of were transmuted from hydrogen in the nuclear furnaces of stars. The Earth formed shortly after the Sun, just as the thousands of known exoplanets formed shortly after their host stars. The focus of the research supported by this grant was to address the question of how it is possible for stars to form out of the tenuous interstellar medium, which has a density so low that a cubic inch contains only about 20 atoms. A major approach used in this research was a computer code developed over the past two decades that can simulate the flow of interstellar gas under the influences of gravity, magnetic fields and radiation. With calculations that can run for several months on many hundreds of computer processors, it was possible to show that the combination of these effects prevented the fragmentation of gas clouds into very small pieces and resulted in a distribution of stellar masses similar to that observed. This result is relatively insensitive to the amount of heavy elements in the gas, which is consistent with observation. Several theoretical advances were also made: A theory was developed that explains both the rate of star formation and the amount of gravitationally bound gas in disk galaxies like the Milky Way. Analysis of observational data was used to show that about 1% of the mass of a collapsing gas cloud is converted into stars in the time it would take the gas to collapse; this inefficiency of star formation holds in our galaxy, in nearby galaxies, and in very distant galaxies. A striking feature of star formation is that stars are observed to form only in gas that is molecular, not atomic; this was shown to follow from the fact that the condition for the gas to be cold enough to form stars is essentially the same as that for it to be molecular. Most of the molecular gas is hydrogen, but that is very difficult to observe; instead, astronomers rely on observations of carbon monoxide. Since the molecular gas that does not contain carbon monoxide is so hard to observe, it is called the "dark molecular gas." The amount of this gas was calculated and shown to be consistent with observation. Broader Impacts: Research in astronomy and astrophysics is critical in developing the evolving view that our civilization has of its position in the universe. Historically, astronomy has had a revolutionary impact in this regard. The increased understanding of star formation resulting from the research supported by this grant is a contribution to the world-wide effort to understand humankind's origins. Star formation is particularly important in this effort since the formation of planets is inextricably bound up with the formation of stars. Several students were supported by this grant; one has gone to work for the computer industry, one is planning working on software development, and one is planning on teaching at the college level.
