Dr. Constantine Deliyannis, at Indiana University, will study the abundance of the chemical Lithium (Li) in 7 primary and several secondary open star clusters, to address key issues in stellar structure and evolution that might ultimately have significant implications for light element chemical evolution, globular cluster ages, big bang nucleosynthesis, dark matter and cosmology. Specific areas of research address issues that follow from Dr. Deliyannis' work with the Wisconsin-Indiana-Yale-NOAO (WIYN) Open Cluster Study.
Two graduate students and at least six undergraduates will be trained and encouraged to pursue careers in science. Dr. Deliyannis has a record of recruiting women and minorities, and this is expected to continue. Dr. Deliyannis will also conduct exit and later interviews for assessment of student training, and he will incorporate his research results in classrooms, and give various public talks based, in part, on the research.
Modern "standard" models of the Sun are superb, as evidenced by helioseismology. However, the Solar Lithium Problem persists after over 50 years of study: inside stars, Li nuclei are destroyed by energetic protons, and mixing can then deplete the surface Li abundance. Standard theory predicts that the Sun's surface Li should have been depleted over the Sun's lifetime to 30% of its initial value -- yet, it has ACTUALLY been depleted to 0.7%. This huge discrepancy suggests the action of physical mechanisms not included in standard theory, such as rotationally-induced mixing, that must have been important in the past. Worse, nearly ALL prior stellar Li observations contradict standard theory, so the Li problem is quite broad. One Project goal was to help clarify empirically how some non-standard physical mechanisms might be acting inside stars. In the future, there may also be applications to other areas of astronomy. For example, confident determination of the Li depletion in the oldest (Galactic halo) dwarf stars can critically test Big Bang theory. Another goal was to search for evidence of the expected post-Big Bang Galactic production of Li. Tying both goals together, we carried out specific tests of standard theory, using sufficiently young stars. One key standard prediction, (A), is that Li depletion depends strongly on mass, so that, at a given age, lower mass dwarf stars have depleted more Li than higher mass stars, and (A2) above a certain mass (about 1.2 solar masses) the Li depletion is minimal, even zero. A second key prediction, (B), is that the depletion occurs during the early pre-main sequence, only, and not subsequently during the main sequence (MS is when stars use core fusion of hydrogen to helium as their main energy source). A third key prediction, (C), is that stars of higher metallicity deplete more Li. (Stars similar to the Sun are made up of about 98% by mass hydrogen and helium, and the sum total 2% of all the rest is their "metallicity".) Previous work using star clusters (where all stars have the same, determinable age) showed that: (A) is roughly correct, but stars have depleted more Li than predicted, and (A2, an undepleted Li-mass plateau) has never been observed; the Pleiades (age: 100Myr) mass-Li pattern suggests standard models DO deplete Li during the pre-MS, in the amount predicted, but older stars continue to deplete Li during the MS; and, five clusters at the Hyades age (of 700Myr) of different metallicities support (C) if the MS non-standard mechanism(s) have no metallicity dependence, but one cannot be sure that both of these conclusions apply simultaneously. The Hyades-aged clusters also provided indirect evidence for the Galactic production of Li. To eliminate complications due to MS Li depletion, our Project observed young clusters (<100Myr). We discovered the first, extended mass-Li plateau in M36, nearly certainly an undepleted plateau, and the lower-mass dwarfs were indeed depleted in Li by the predicted amount. Several other clusters of varying metallicity also exhibited Li plateaus and depletion for lower masses, that was consistent with the metallicity-dependence of standard theory. This also suggests that the MS-depleting mechanisms have little dependence on metallicity. Finally, the Li plateaus correlated tightly with metallicity, providing, for the first time, direct evidence for the Galactic Li production. Extrapolating backwards to zero metallicity implies a Li abundance consistent with the standard Big Bang model, but not precise enough to support it. (The standard Big Bang model produces just H and He, and no metals, other than a tiny amount of Li.) During the MS, stars near 1.2 solar masses deplete their Li drastically ("Li gap"), and non-standard mechanisms proposed to explain this include diffusion, mass loss, and rotation. Comparison of the Li-gap positions in several older clusters showed that (1) the gap's position as a function of stellar surface temperature is independent of metallicity, but (2) higher-metallicity clusters have their gaps in higher-mass stars. Also, the Li gap's position does not to evolve with age, which provides an important constraint for the Li-depletion mechanisms. Regarding broader impacts, the Project provided mentoring and professional development for a number of students: a) sixteen undergraduates (four at Indiana, four at the astronomy NSF-sponsored REU-site at Indiana, and eight at this Project's ROA-sponsored site SUNY-Geneseo; six of the sixteen are women, in a discipline where men outnumber the women) learned how to reduce, analyze and disseminate fundamental astronomical data (cluster photometry, and sometimes spectroscopy); two of the Indiana students were awarded Goldwater Scholarships, b) four Indiana graduate students further refined their knowledge of analyzing and interpreting spectroscopic data, c) one external assistant research professor further refined his knowledge of analyzing photometric and spectroscopic data, d) one junior faculty (now, tenured) at a primarily-undergraduate-institution (and former Indiana graduate student) developed skills at incorporating his own undergraduate students in research through the Project's ROAs.
