In the past decade there has been a revolution in our ability to carry out detailed abundance analyses of stars in the Galactic halo, distant Milky Way globular clusters, and the satellite galaxies of the Milky Way, because, prior to the current generation of large telescopes and efficient spectrographs, these stars were too faint for high dispersion spectroscopy. Under this award, Dr. Judith Cohen (California Institute of Technology) will carry out a study of the chemical evolution of the halo of the Milky Way galaxy in detail, taking advantage of this tremendous advance in technology as well as leveraging off her previous, NSF-supported efforts, in which large samples of extremely metal-poor stars were identified. The elemental abundance patterns in these halo stars, both in the field and in the globular clusters, have direct implications for the early epochs of our Galaxy, the onset of chemical evolution of the Galaxy, the possible stellar sources which produced many elements at very early epochs (very massive stars and Type II supernovae), the age of the Galaxy, the relationship between the halo field stars and the galactic globular clusters, and the composition of the interstellar medium and the intergalactic medium. Such stars provide a chance to study the general chemical abundances in local material representing the high redshift Universe. Through this research, new insight will be gained into the relationship between the Galactic stellar halo and the satellite galaxies around the Milky Way, both as seen now and in the distant past.
Under this award Dr. Cohen will also continue to be dedicated to the development of young scientists, particularly women and minorities, by providing valuable research experiences for undergraduates. In addition, lectures, both for the general public and at primarily undergraduate institutions, will continue. Dr. Cohen also will produce an exhibit intended for the general public in the lobby of the Cahill Center for Astronomy and Astrophysics, located on the campus of Caltech.
NSF grant AST-0908139 to Prof. Judith Cohen at the California Institute of Technology funded her study of the formation and the chemical inventory of dwarf galaxies and field stars in the outer part of the halo of the Milky Way. She and her group tried to establish the extent of contributions of various types of supernovae within our young, turbulent forming galaxy. In the scenario we have developed the most metal-poor stars, which presumably are the oldest stars, in these low mass Galactic satellites show abundance ratios that agree well with those of the Galactic halo globular cluster and field star population. We thus support the notion that in the outer halo we still see intact low mass Galactic satellites, most of which presumably are infalling, while in the inner halo, such objects were largely dispersed and disrupted long ago. We have also been studying Galactic globular clusters in the outer halo. Globular clusters are densely packed roughly spherical clusters of about a million stars and are mostly found in the halo of our galaxy. One of them, NGC 2419, in the outer halo with distance of about 80 kpc fromthe Earth, has turned out to be extremely peculiar. It has a unique, bizarre chemical inventory which we gradually revealed as our sample of starswith accurate abundances increased with time. J. Cohen, in more than 30 years of carrying out detailed stellar abundance analyses, has never seen anything resembling the stars in NGC 2419. Figuring out the cause of such peculiar behavior will be challenging, but several theoretically oriented groups working on the production of the elements in stars have begun relevant calculations and modeling. Twenty six scientific papers were prepared and published in refereed journals during the course of this grant. Three undergraduate students, two of whom are women, and one postdoctoral fellow participated in this study.
