The production of chemical elements, luminous energy, and kinetic energy by stars is fundamental to the evolution of galaxies and the baryonic content of the cosmos. A recent National Academy report, "Connecting Quarks with the Cosmos: Eleven Science Questions for the New Century" identified the creation of the elements, particularly those beyond iron, as one of the most important problems facing modern science and concealing the answers to one of the most fundamental questions that humans strive to answer: "What is the Universe made of?". Massive stars hold a key to this mystery by serving as the main engines for creation of elements in the cosmos. However, there are known systematic effects and missing physics that inhibit creation of "standard abundance candles". Models of massive stars have long used spherical symmetry with mixing length treatments of convection and parameterized explosions. Recent breakthroughs in understanding 3D, compressible, multi-fluid, convection with nuclear burning have provided an improved treatment of convection and non-convective mixing for stellar evolution. In addition, observations of individual core-collapse supernovae suggest some fraction of them are strongly asymmetric.
Here, Dr. Young and collaborators will undertake a systematic survey of massive star evolution (1) that uses new results from 3D convection studies and (2) whose supernova abundance yields are calculated from 3D explosion models. These yields and evolutionary tracks will be validated with existing and future abundance determinations of stars and stellar populations. Methods and techniques of the analysis will be combined into an efficient pipeline comprised of analytical methods, stellar evolution models, multi-dimensional collapse and explosion simulations, and post-processing of the explosions with large nuclear reaction networks to establish detailed abundance levels. The results of this study are expected to significantly improve quantitative interpretation of abundance determinations being conducted by current and future instruments such as Spitzer's "Surveying the Agents of a Galaxy's Evolution" and NASA's "Space Interferometry Mission".
This research will have impacts on several areas of physics and astronomy as well as promoting national/international partnerships. Undergraduate and graduate students will be involved and exposed to a trans-disciplinary scientific process. Results from this work will be incorporated into Dr. Young's large classes for undergraduate non-science majors and disseminated to the public through talks and articles. Finally, the software developed during this project will be released as stand-alone software or as packages for use with extant software.
Stars provide much of the energy input that drives the evolution of normal matter in the universe and create all of the chemical elements heavier than boron. The work supported by this grant represents fundamental improvements in our understanding of how stars work, how they produce the chemical elements during their lives and violent deaths, and how those lements are distributed into the material from which new stars and planets form. Part of this work involved developing a new theory of how fluid motions inside a star transport energy and mix material, resulting in much more accurate and predictive models of the evolution of stars which we have made available to the astronomical community. This grant also supported three dimensional computer simulations of the deaths of massive stars in titanic supernova explosions. These are the first simulations to follow the explosion for decades as the material ejected by the supernova interacts with the surrounding gas and dust. This surrounding medium will eventually form new stars and planets. Often supernovae occur in areas where new stellar systems are actively forming. High resolution 3D simulations with physics of nuclear burning and cooling of gas included let is predict the size scales of clumpy material and other structures in a supernova and estimate their compositions. These simulations are being compared to observations of young supernova remnants by NASA space-borne observatories. Several lines of evidence, from isotopic analyses of meteorites to studies of the Sun's elemental and isotopic composition, indicate that the solar system was contaminated early in its evolution by ejecta from a nearby supernova. Part of this contamination was the short-lived radioactive isotope 26Al. This work showed how a clump of ejecta from a supernova can deliver enough of this isotope to the forming solar system while staying consistent with the abundance of other isotopes we find in meteorites, and showed which other elements can act as tracers of 26Al in observations of supernova remnants and perhaps even other stellar systems where 26Al itself is impossible to observe. Previous models have looked at supernova material being injected into an extant protoplanetary disk, or uniformly expanding ejecta sweeping over a distant cloud, simultaneously enriching it and triggering its collapse. This work introduced a new astrophysical setting: the injection of clumpy supernova ejecta, as observed in the Cassiopeia A supernova remnant, into the molecular gas at the periphery of a region created by the supernova's progenitor star. To track these interactions we conducted a suite of high-resolution 3D numerical hydrodynamic simulations that follow the evolution of individual clumps as they move into molecular gas. These simulations suggest isotropically exploding ejecta do not penetrate into the molecular cloud or mix with it, but, if cooling is properly accounted for, clumpy ejecta penetrate deeply and mix effectively with large regions of star-forming molecular gas. About 2 Msun of high-metallicity ejecta from a single core-collapse supernova is likely to mix with about 20,000 Msun of molecular gas material as it is collapsing. Thus, all stars forming late in the evolution of an H II region may be contaminated by supernova ejecta at the level 1 part per 10,000. This level of contamination is consistent with the abundances of short-lived radionuclides and possibly some stable isotopic shifts in the early solar system and is potentially consistent with the observed variability in stellar elemental abundances. Supernova contamination of forming planetary systems may be a common, universal process. This project also examined the abundances of elements measured in stars within a hundred parsecs of the sun. One research effort compiled abundance determinations for as many nearby stars as could be found in the literature and made the unsettling discovery that the abundances of elements measured in a single star by different research groups can differ by more than the observational errors calculated by each group. This has launched a multinational effort to find the causes of these discrepancies and ameliorate them. The other prong of this research examined the amount of variation in abundances in a group of stars surveyed by one group, on one intrument, with one method of analysis, so that the results are self-consistent. In this way the relative abundances can be compared. Often only the iron abundance of stars is measured, but it turns out that the ratios of other common elements to iron can vary by factors of more than two relative to the average even for a single iron abundance. Therefore the sun is not necessarily representative. Differences of this order can change the lifetime of a star by a third, among other effects. This also means that the reservoir of gas in the Galaxy from which new stars for is not well-mixed, and our models of stars and their companion planets must take this into account.
