In this project, previously obtained observational data are analyzed to determined the oxygen and iron abundances in solar-type dwarf stars of 15 open clusters of known age and different metallicity (i.e., the contents of the heavy chemical elements relative to hydrogen). The samples range by nearly a factor of ten in their iron-to-hydrogen ratios and cluster ages span from about 30 million to about 8 billion years. This study expands previous abundance data sets to cover about three times as many clusters, a higher sample of stars in each cluster, and a wider range in metallicity and age.
The abundances of oxygen and iron trace the galactic chemical evolution - the enrichment of the interstellar medium in chemical elements produced in stars. This study addresses whether the iron and oxygen abundances in clusters depend on cluster age and how iron and oxygen abundances relate to each-other. This is of interest because oxygen and iron productions vary among different types of massive stars that explode as supernovae. An additional study here checks on the relation of the alpha element (such as Mg and Si) production with age. Such relations may place constraints on the processes in stars and stellar nucleosynthetic yields of heavy elements that are released into the interstellar medium by massive stars.
This project provides a venue to attract many undergraduate and graduate students to scientific research as have previous projects by the principal investigator.
Oxygen is the third most abundant element in the Universe, after hydrogen and helium, and as such its origin is of intrinsic interest. Also, it has been long been suggested that oxygen may be the most useful and significant tool for understanding the production of elements in our Milky Way ("Galactic chemical evolution"), as opposed to the traditionally employed reference element, iron. While considerable effort has gone into studying the evolution of oxygen in field and metal-poor stars, studies of oxygen (and other elements) in (Milky Way Disk) "open" star clusters are few. (Stars similar to our Sun are made up of 98% by mass hydrogen and helium, and the remaining 2% is their "metallicity".) One shining exception (King PhD Dissertation 1993) took advantage of the age information contained nearly uniquely in star clusters to produce the surprising suggestions that, a) although there seems to be little or no age-Fe relation in the Disk, the Disk does produce more oxygen as it ages, even at constant Fe, and b) it is unclear whether the cluster [O/Fe] decreases with [Fe/H] as it does in field stars. ("[X/Y]" is the log of the quantity {the ratio of abundances, by number of atoms, of element X and Y in an object, divided by the Solar ratio of X and Y}.) We have re-examined these and related issues, and combining our results from seven open clusters with those of King and recent literature, we find: (1) Concerns existed that spectra from rapidly rotating and/or highly chromospherically active stars may fool us into deriving incorrect O abundances. Using such stars together with slow rotators and low-activity stars in the young (100Myr-old) Pleiades, we found, reassuringly, no correlation between oxygen abundances and any activity indicators, or rotation. (2) There is no robust [O/H]-versus-age correlation as implied by King. However, the young (<1Gyr-old) clusters are distinguished from the older clusters by their typically super-solar O abundances. These high O abundances contradict the [O/H]-versus-age relationship predicted by chemical evolution models which incorporate infall of metal-poor gas onto the Disk. We argue that such an increase in O would result if infall ceased about 1-2Gyr ago. (3) [O/H]-versus-age and [Fe/H]-versus-age both exhibit scatter in the 4-7Gyr age range, which may be evidence of an infall event. The reality of this scatter is supported by a constant, near-solar [O/Fe] for all ages greater than 1Gyr, implying a physical cause which preserves the abundance ratios. (4) The [O/Fe]-versus-[Fe/H] relationship for open clusters exhibits substantial scatter, and is in contradiction to the trend observed in field stars, suggesting that the chemical history of open clusters is, in fact, different than that of field stars. (5) The open clusters exhibit a clear, roughly linear trend in [Fe/O]-versus-[O/H], but shockingly, this trend contradicts the well-defined, roughly linear trend exhibit by field stars, which has opposite slope. This suggests that the chemical history of open clusters is different than that of field stars, and that self-enrichment may occur within cluster-forming regions. (6) Chemical evolution models can produce metallicities as high as a factor of 2-3 above solar ("super-metal-rich"), but only within the innermost 4-5 kiloparsecs of the Disk and the Bulge regions. The presence of three super-metal-rich clusters much closer to us is thus mysterious. (They are 6-8 kpc from the galactic center, and we are 8.5kpc from the GC.) The oldest cluster, NGC 6791 (8Gyr), may have been kicked out by the Galactic Bar, but no such explanation exists for the other two. We find scaled-solar abundances in NGC 6253 for O, Al, and Si, in agreement with abundances in the other two clusters and in nearby metal-rich field dwarf and giant stars. However, metal-rich Bulge stars seem to differ. These clusters' O abundances may be indicating a flattening of the field star [O/Fe]-versus-[Fe/H] relationship, and may suggest a decrease in supernova Fe yields at super-solar metallicities. Regarding broader impacts, the Project provided mentoring and professional development for a number of students: a) seven undergraduates (three at Indiana, one at the astronomy NSF-sponsored REU-site at Indiana, and three at this Project's ROA-sponsored site at SUNY-Geneseo) learned how to reduce, analyze and disseminate fundamental astronomical data (cluster photometry, and sometimes spectroscopy); one of the Indiana students (a woman) was awarded a Goldwater Scholarship; b) two Indiana graduate students refined their knowledge of analyzing and interpreting spectroscopic data; for one, much of this Project became his PhD Dissertation: besides reducing and analyzing data, he learned how to write successful proposals for extremely competitive telescope time (e.g. Multiple Mirror Telescope), and has developed an increasing maturity for writing papers and directing his own research; he is currently faculty at a primarily-undergraduate-institution; c) another faculty at a primarily-undergraduate-institution (and former Indiana graduate student) further developed skills at incorporating his own undergraduate students in research through the Project's ROAs.
