Scientific Rationale



"Chemical Abundances in the Universe: Connecting First Stars to Planets"


This meeting aims to provide a unified picture of the production of chemical elements over cosmic time, and how this chemical evolution links together the early universe of metal-free and heavy-element poor first generations of stars (through the formation of galaxies and their diverse stellar populations), to a universe of heavy-element rich stars and planets.

A symposium on "Chemical Abundances in the Universe: Connecting First Star to Planets" is very timely. Due to the ever-expanding capabilities delivered by new arrays of instruments on large telescopes and a considerable number of upcoming large surveys, the quality and quantity of data available to probe the chemistry of the universe through time is increasing and will continue to do so. Coupling the data to analyses that are based on increasingly sophisticated modeling of stellar atmospheres, along with new, accurate nuclear, atomic, and molecular databases, will truly move the field of cosmic chemistry to a significantly higher level. As the observations continue to flow in and are scrutinized with ever-improving analysis tools used to extract accurate chemical abundances, new classes of extremely large telescopes and instruments are being planned. New large telescopes, when combined with future survey efforts, challenge the astronomical community to plan on how to optimize these new resources in order to be able to ask (and to answer) the big questions of the next decade. What is lacking in the astronomical future of cosmic chemistry? How best can we progress? What actions should be taken to prepare for the future? The IAU General Assembly in Rio de Janeiro in 2009 is a good time to survey the entire field of cosmic chemistry, with an emphasis on the connections between the various subfields within this broad topic.

In recent years, connections between seemingly different areas have become clearer. For example, the first generation of massive stars drives the era of reionization, as well as gives rise to the long duration Gamma-Ray Bursts (GRB). A confirmation of this link has been the detection of GRB 050904, which corresponds to the explosion of a massive star at a redshift, z=6.4. The oldest low-mass stars (still observable today) were chemically imprinted with the nucleosynthetic products from the energetic events driven by these first massive stars. Surprisingly, until recently it seemed that the predicted chemical compositions of such ejecta were not found in the abundances observed in the old, low-mass stars. However, it may be that several physical processes, e.g., neutrino interactions during the explosion, or the effects of rapid rotation of massive stars of close to zero metallicity, might reconcile the predicted yields with the observations. In order to find which proposed processes might be adequate in explaining the observations will require high-precision determinations of abundances in the oldest stars. On the other hand, the mass range of these first massive stars is poorly determined from theory and a precise determination of the abundances in the old stars is, at the moment, the best way to constrain this mass range. Moreover, why are the extremely iron-poor stars ([Fe/H] < -3.5) in the Milky Way typically very carbon- and nitrogen-rich? Are they really older than the other "normal" metal-poor stars? Do they provide the "rosetta stone" for examination of the IMF of the earliest stellar generations? Thus, comparisons between the yields from nucleosynthesis predictions, the few abundance measurements in GRBs, and abundances in old metal-poor halo stars is one of the aspects of this symposium, which will connect chemical abundances from different areas.

Another link is between the chemical abundances in QSO absorption lines and damped Lyman-alpha (DLA) systems as a function of redshift, which can be connected to detailed stellar abundance distributions as a function of metallicity. Such a comparison provides a view into chemical evolution in the universe during the first gigayear, or so, of its existence. In the external galaxies, can an increase of metallicity with time be detected? Are there measurable changes in some of the key abundance ratios? In this case, it is the moderately metal-poor stars ([Fe/H] > -2.0) that provide the connection.

Current abundances in the disk of the Milky Way can be connected to models of the formation and evolution of the Galaxy. The abundances now found across the disk of the Milky Way were shaped to a large degree by the star formation history of the disk and different physical processes occurring over the life of the disk will predict different spatial variations in the chemical abundances (such as abundance gradients). Observationally, the gradients can be traced by a variety of stellar populations, such as H II regions, Cepheids, OB stars, planetary nebulae, or open clusters. These different types and their time evolution are some of the strongest constraints on the types of physical processes (such as outflow or infall) that need to be combined with star formation to understand the evolution of the Milky Way as a whole. Observationally, there is a significant amount of controversy at present from recent abundance results in this general field. Are the gradients from the different populations steep or flat? How do the gradients evolve with time? Is there a steepening or flattening of the gradients with time? How do these gradients connect with the formation of the bar?

The bulge of the Milky Way, along with the bulges in other spirals and ellipticals in general, contain keys to the early formation and evolution of a galaxy. The Galactic bulge itself is currently under detailed inspection, with a wealth of brand new results from 8m telescopes. The formation of our own bulge is controversial. Is it a pseudo-bulge generated by the secular evolution of a bar, or does it host the oldest stars in the Galaxy?

Controversies also apply to the Galactic halo and potential connections to satellite galaxies. Beyond the inner halo, is there an outer halo formed by accretion of dwarf-like satellite galaxies, as recent evidence suggests? If so, these different populations and formation scenarios could be tested by the determination of precise, detailed chemical abundance distributions.

Moving closer to home, it has been found that there is a significant link between the presence of a large planet around a host star and its metallicity. This apparently links large-planet formation to a heavy-element rich environment, or is it possible that planetary leftovers pollute the parent star's atmosphere? What is the underlying link and how might it affect the probability of planet formation, or the number of rocky planets in the Galaxy?

The list of proposed sessions follows this evolution and will provide a setting into which a modern picture of the chemical structure of the universe, and how it came to be can be built. From this point, the closing session will frame the next sets of questions to be posed and the large programs needed to tackle these questions, using the present and next generation of very large telescopes and their instruments.

The symposium will be focused on chemical abundances found within a broad range of astrophysical environments. The goal being to explore the links between abundances in these differing environments and to understand how their diverse chemical signatures have evolved. By design, the proposed symposium is intended to be inclusive of a variety of topics with chemical abundances being the link. A General Assembly, which naturally attracts astronomers from all fields in the astronomical community is the perfect venue for the type of symposium.