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Project Progress

The composition of stars strongly influences their evolution and properties of their planets. Michael Pagano’s Ph.D. dissertation (ASU) is focused on measuring the composition of a large sample of potential planet host stars and statistically analyzing the variation in abundances and the distribution of elemental ratios that are important for stellar and planetary evolution. We have derived abundances of ~500 stars (from spectra obtained by Paul Butler, Carnegie Institute for Science). We have performed statistical analyses of abundances from this sample and five other groups. We have statistics for O/Fe (important to stellar evolution), Mg/Si, C/O, Ca/Fe, and Al/Fe (important to planetary mineralogy). In addition we have Eu/Fe, a proxy for the amount of long-lived radionuclides in the systems. A sample of >30 planets from the Carnegie data have one system with C/O > 0.8, which theoretically marks a transition from silicate to carbide mineralogy in planets. A significantly larger fraction of systems have Mg/Si ~2 or higher, marking a transition from an Earth-like lower mantle dominated by stishovite and spinel to perovskite and ferropericlase. This transition causes a large drop in the viscosity of the mantle. As an example of the utility of this process, we perform a composition-focused analysis of the Tau Ceti system that derived new abundances from high resolution spectra, created custom composition stellar evolution models for Tau Ceti and its habitable zone, and found that Mg/Si > 2 and Eu/Fe is substantially greater than solar. The last implies that the inventory of long-lived radionuclides important to planetary heating is potentially larger than Earth’s for planets in the system. We are in the process of characterizing what mineralogical properties may correspond with different abundance ratios in collaboration with Dan Shim, a mineral physicist at ASU.

As part of this task, our team organized an NAI-supported “Stellar Stoichiometry” Workshop Without Walls, hosted at ASU in April 2013. This workshop included ~ 20 in-person participants and ~ 50 virtual participants. As an outcome of the workshop, we began a collaboration to determine why measurements of abundances for a given star by different research groups often differ by more than the quoted observational errors. We now have six separate research groups systematically analyzing a set of data with their own techniques and various control situations such as fixed stellar parameters and line lists. The abundance determinations have been completed and are currently being analyzed. This work will result in a journal article and a set of recommended best practices for stellar abundance determinations.

The Hypatia abundance catalog previously developed as part of this task is being made available for use by Margaret Turnbull (Global Science Institute), who is supplying a catalog of target systems for the AFTA and NRO telescope probe teams (both starshade and coronagraph) developing mission concepts for direct exoearth detection and spectroscopy. Turnbull’s catalog is now being used for running DRMs to estimate mission yields in terms of habitable zones searched. Preliminary results indicate that it will be possible to detect earths and do rudimentary spectroscopy on major atmospheric features for the price of a probe ($1 billion), which was not initially considered possible. The next step is more in-depth characterization of each top target, which would make use of the abundance catalog.