Day 1: Variations in stellar composition

To accurately compare abundance data from multiple studies, several instrumental and methodological differences must be taken into account.  These differences include instrument zero points, resolution of the spectra, signal-to-noise ratios, oscillator strengths, line lists, equivalent widths, number of ionization stages used, LTE or non-LTE analysis, converged solar atmosphere models, curve-of-growth or spectral fitting, curve-of-growth program used, and adopted solar abundances. All of these factors may introduce systematic and stochastic differences between data sets. For example, the figure to the right shows the abundance measurements for six elements within five unique stars (Hinkel, N.R. 2012, PhD thesis, Arizona State University). The circles are as labeled with the element name while all triangles designate [Fe/H], each with respective errorbars from the catalog from which it was measured. The variation between catalogs per element, the largest of which we call the spread, is generally in the range of 0.1-0.15 dex.  Therefore, we ask the questions:

- How can the variations in observed stellar photospheres between data sets be explained?  Are the causes inherent to the telescope, the data reduction, or the stellar atmospheric models employed?

- How can we consistently measure the abundances of various elements in the atmospheres of stars?  

- How do abundance variations affect stellar properties?

Day 2: Effect on planetary systems

The detection of the first exoplanet orbiting a main-sequence star by Mayor & Queloz (1995) brought with it a number of questions regarding the conditions under which a star will harbor a planet, both of the star and the planet. Studies conducted by Bond et al. (2008, 2006); Fischer & Valenti (2005); G ́alvez-Ortiz et al. (2011); Gilli et al. (2006); Gonzalez & Laws (2000); Gonzalez (1997); Laws et al. (2003); Reid (2002); Santos et al. (2004, 2001); Sousa et al. (2011), and many others, examine the correlation between the metallicity of the host-star and the presence of an exoplanet, within both volume- and magnitude-limited samples. The independent conclusions of these analyses is that stars with orbiting giant exoplanets are generally more iron- rich than non-host stars.  However, elements such as H, C, N, O, Mg, Si, P, S, Mo, and Se are also paramount for the atmosphere, structure, and biogeochemistry found on Earth and are generally deemed “bio- essential.” Therefore, the chemical composition of an exoplanet in terms of the abundances within the host star is important when considering the habitability of the exoplanet.

- How does the initial composition of the host star and protostellar system affect the formation of both giant and terrestrial planets?

- What elemental variations most affect planet properties, and how?

- What can we say about planetary compositions in systems with non-solar compositions?

- How might a host star's composition affect planetary habitability?

Image courtesy Fahad Sulehria, www.novacelestia.com
Text from Hinkel, N.R. 2012, PhD thesis, Arizona State University.