While there has been considerable progress toward the development of a standard model for the process of star formation in the case of low mass stars, we are much further from understanding how massive stars form. Massive stars produce most of the metals in the universe, dominate the energy balance of both the interstellar and intergalactic media via their ionizing radiation, winds, and supernovae, and shape the formation of planets through their interactions with nearby protoplanetary disks. No understanding of these processes is possible without a reliable model of massive star formation. In this project, Dr. Mark Krumholz (University of California - Santa Cruz) will develop such a model, using the complementary tools of analytic investigation and high resolution simulation including gravity, radiation, and magnetohydrodynamics.
As part of this research a number of questions and problems must be addressed. At the earliest stages of massive star formation, we must understand the fragmentation process. What physical mechanisms control whether gas fragments down to sizes less than one solar mass or collapses monolithically to a massive star? Once collapse begins, any model must describe how to overcome the two dominant barriers to accretion, angular momentum and radiation pressure. Disks are a likely solution, since they redistribute angular momentum while at the same time their high opacity shields gas from radiation. A second important part of this project is to develop an understanding of massive protostellar disks. At the end of the massive star formation process, what eventually halts accretion and clears away any remaining gas? In addition to working on these questions, and using material related to this research, Dr. Krumholtz will also offer math and science courses to prison inmates in prisons near Santa Cruz, California, building on Dr. Krumholz's decade of experience in prison education. Dr. Krumholtz will involve faculty, staff, and students at UC Santa Cruz in this activity, thereby facilitating interactions between the scientific community and a radically under-served, largely minority population.
We observe new stars forming out of interstellar gas throughout our Galaxy, and beyond, but the exact nature of the process is not entirely clear. Observations only take us so far, because the time scales involved are much longer than a human lifetime, or indeed than human civilization has existsed. Thus we only get to see a series of snapshots, and we must make models to figure out how they are connected to one another. One of the most significant mysteries is how the most massive stars form, those more than 10 times the size of the Sun. Although these stars are rare, due to their extreme luminosities, they outshine their far more numerous low mass cousins (like the Sun). This luminosity, however, also makes it hard to understand how they form in the first place: why doesn't the pressure exerted by their immense radiation repel interstellar gas, preventing them from growing? How does gas continue to flow onto these stars? Conversely, why doesn't the gas that is collapsing to make new stars fragment into many small stars like the Sun? Clearly it does most of the time, since Sun-like stars far outnumber massive ones . Under what circumstances can this fate be avoided, leading to the formation of a much larger single star rather than a group of smaller ones? To address these questions, I have used a mix of computer simulations, pencil-and-paper calculations, and astronomical observations. Together, these techniques have helped me construct a model for how massive stars form. Beyond gravity, which is obviously crucial to gathering diffuse interstellar material into stars, the next most important effect governing the entire process is the interaction of gas and radiation. Radiation from stars as they begin to form heats the gas around them, raising the gas pressure and making it harder for this gas to fragment into small stars. Under circumstances of extremely high density (by interstellar standards -- still vacuum by any terrestrial measure), this pressure increase is sufficient to choke of formation of small stars, allowing a few survivors to grab all the available gas in a region and grow to large masses. Accretion onto these stars is not halted by radiation pressure, however, because the interaction of radiation and matter leads to the radiation being beamed like a flaghslight. The beam of radiation repels matter it encounters, but other gas is able to slip around the beam and make its way to the star. This matter enters the star through a circumstellar disk, and the large flux of matter through the disk causes it to develop spiral arms, much like an entire galaxy. It should be possible to image these arms with the next generation of radio telescopes, particularly the Atacama Large Millimeter Array. The education component of this proposal involved the PI and a group of students at UC Santa Cruz establishing a program to teach mathematics in the Santa Cruz County Jail system. The PIE founded a group called the UCSC Project for Inmate Education, which now offers a free 14-week algebra class twice per year to inmates incarcerated in the Santa Cruz County Main Jail. The goal of this class is to help the inmates along the road to a community college degree upon release.
