Stellar elemental abundances are important for understanding the fundamental properties of a star or stellar group, such as age and evolutionary history, as well as the composition of an orbiting planet. However, as abundance measurement techniques have progressed, there has been little standardization between individual methods and their comparisons. As a result, different stellar abundance procedures determine measurements that vary beyond the quoted error for the same elements within the same stars. The purpose of this paper is to better understand the systematic variations between methods and offer recommendations for producing more accurate results in the future. We invited a number of participants from around the world (Australia, Portugal, Sweden, Switzerland, and the United States) to calculate 10 element abundances (C, O, Na, Mg, Al, Si, Fe, Ni, Ba, and Eu) using the same stellar spectra for four stars (HD 361, HD 10700, HD 121504, and HD 202206). Each group produced measurements for each star using (1) their own autonomous techniques, (2) standardized stellar parameters, (3) a standardized line list, and (4) both standardized parameters and a line list. We present the resulting stellar parameters, absolute abundances, and a metric of data similarity that quantifies the homogeneity of the data. We conclude that standardization of some kind, particularly stellar parameters, improves the consistency between methods. However, because results did not converge as more free parameters were standardized, it is clear there are inherent issues within the techniques that need to be reconciled. Therefore, we encourage more conversation and transparency within the community such that stellar abundance determinations can be reproducible as well as accurate and precise.

tau Ceti (HD10700), a G8 dwarf with mass 0.78M(circle dot), is a close (3.65 pc) Sun-like star where five possibly terrestrial planet candidates (minimum masses of 2, 3.1, 3.5, 4.3, and 6.7 M-circle plus) have recently been discovered. We report abundances of 23 elements using spectra from the MIKE spectrograph on Magellan. We find [Fe/H] = -0.49 and T-eff = 5387 K. Using stellar models with the abundances determined here, we calculate the position of the classical habitable zone (HZ) with time. At the current best fit age, 7.63(-1.5)(+0.87) Gy, up to two planets (e and f) may be in the HZ, depending on atmospheric properties. The Mg/Si ratio of the star is found to be 1.78, which is much greater than for Earth (similar to 1.2). With a system that has such an excess of Mg/Si ratio it is possible that the mineralogical make-up of planets around tau Ceti could be significantly different from that of Earth, with possible oversaturation of MgO, resulting in an increase in the content of olivine and ferropericlase compared with Earth. The increase in MgO would have a drastic impact on the rheology of the mantles of the planets around tau Ceti.

We compile spectroscopic abundance data from 84 literature sources for 50 elements across 3058 stars in the solar neighborhood, within 150 pc of the Sun, to produce the Hypatia Catalog. We evaluate the variability of the spread in abundance measurements reported for the same star by different surveys. We also explore the likely association of the star within the Galactic disk, the corresponding observation and abundance determination methods for all catalogs in Hypatia, the influence of specific catalogs on the overall abundance trends, and the effect of normalizing all abundances to the same solar scale. The resulting large number of stellar abundance determinations in the Hypatia Catalog are analyzed only for thin-disk stars with observations that are consistent between literature sources. As a result of our large dataset, we find that the stars in the solar neighborhood may be reveal an asymmetric abundance distribution, such that a [Fe/H]-rich group near to the mid-plane is deficient in Mg, Si, S, Ca, Sc II, Cr II, and Ni as compared to stars further from the plane. The Hypatia Catalog has a wide number of applications, including exoplanet hosts, thick and thin disk stars, or stars with different kinematic properties.

Chemical composition affects virtually all aspects of astrobiology, from stellar astrophysics to molecular biology. We present a synopsis of the research results presented at the "Stellar Stoichiometry" Workshop Without Walls hosted at Arizona State University April 11-12, 2013, under the auspices of the NASA Astrobiology Institute. The results focus on the measurement of chemical abundances and the effects of composition on processes from stellar to planetary scales. Of particular interest were the scientific connections between processes in these normally disparate fields. Measuring the abundances of elements in stars and giant and terrestrial planets poses substantial difficulties in technique and interpretation. One of the motivations for this conference was the fact that determinations of the abundance of a given element in a single star by different groups can differ by more than their quoted errors. The problems affecting the reliability of abundance estimations and their inherent limitations are discussed. When these problems are taken into consideration, self-consistent surveys of stellar abundances show that there is still substantial variation (factors of ∼ 2) in the ratios of common elements (e.g., C, O, Na, Al, Mg, Si, Ca) important in rock-forming minerals, atmospheres, and biology. We consider how abundance variations arise through injection of supernova nucleosynthesis products into star-forming material and through photoevaporation of protoplanetary disks. The effects of composition on stellar evolution are substantial, and coupled with planetary atmosphere models can result in predicted habitable zone extents that vary by many tens of percent. Variations in the bulk composition of planets can affect rates of radiogenic heating and substantially change the mineralogy of planetary interiors, affecting properties such as convection and energy transport.

We report on the NASA Astrobiology Institute-funded Workshop Without Walls entitled "Stellar Stoichiometry," hosted by the "Follow the Elements" team at Arizona State University in April 2013. We describe several innovative practices we adopted that made effective use of the Workshop Without Walls videoconferencing format, including use of information technologies, assignment of scientific tasks before the workshop, and placement of graduate students in positions of authority. A companion article will describe the scientific results arising from the workshop. Our intention here is to suggest best practices for future Workshops Without Walls.

The high quality spectra required for radial velocity planet searches are well-suited to providing abundances for a wide array of elements in large samples of stars. Abundance ratios of the most common elements relative to Fe are observed to vary by more than a factor of two in planet host candidates. This level of variation has a substantial impact on the evolution of the host star and the extent of its habitable zone. We present stellar models of 1 solar mass stars with custom compositions representing the full range of these non-solar abundance ratios. We find that the effects derived from variation over the observed range of [O/Fe] are particularly dramatic. Habitability lifetimes for some classes of orbits can vary by gigayears for the observed range in [O/Fe].

We analyze published elemental abundances derived from high-resolution spectroscopy from radial velocity planet searches. We find that the sun has anomalous abundances compared to 130 nearby dwarfs. Since these are low mass main sequence stars this reflects differences in initial composition rather than self-enrichment. We propose individual [element/Fe] ratios as a function of [Fe/H] for 13 elements. Scenarios for enrichment of outliers are discussed.

During 2015, a unique launch opportunity exists that allows for a New-Frontiers-class mission to discover a compositional gradient of small bodies in our solar system. The proposed seven year mission includes a flyby of main belt asteroid (108144) 2001 HM1, a flyby and impactor release (a la Deep Impact) at the Trojan asteroid (624) Hektor (a suspected contact binary) with companion P/2006, and a flyby with impactor release at the Centaur asteroid 39P/Oterma. The variety of types and positions of these small bodies will help answer some of the fundamental questions we have for the evolution and composition of our solar system. The Jet Propulsion Laboratory (JPL) Planetary Science Summer School team has designed a mission designed to address all of these scientific questions and design objectives. We will describe instrument selection, launch dates and mission timetables, measurement and encounter strategies, impactor design and benefits, data acquisition and communication tradeoffs and provide background into the mission science goals. Furthermore, cost estimates and a work breakdown will be provided to prove the practicality 978-1-4244-2622-5/09/$25.00 copy2009 IEEE. IEEEAC paper #1634, Version 2, Updated Jan 7, 2009 of meeting all the science objectives within a short period of time. A strategy for the development of our system based upon previously used instruments and hardware will also be presented.