Solar and Stellar Photospheric Abundances

The determination of photospheric abundances in late-type stars from spectroscopic observations is a well-established field, built on solid theoretical foundations. Improving those foundations to refine the accuracy of the inferred abundances has proven challenging, but progress has been made. In parallel, developments on instrumentation, chiefly regarding multi-object spectroscopy, have been spectacular, and a number of projects are collecting large numbers of observations for stars across the Milky Way and nearby galaxies, promising important advances in our understanding of galaxy formation and evolution. After providing a brief description of the basic physics and input data involved in the analysis of stellar spectra, a review is made of the analysis steps, and the available tools to cope with large observational efforts. The paper closes with a quick overview of relevant ongoing and planned spectroscopic surveys, and highlights of recent research on photospheric abundances.Comment: Invited review to appear in Living Reviews in Solar Physics. 39 pages, 7 figure

Chemical Abundances from the Continuum

The calculation of solar absolute fluxes in the near-UV is revisited, discussing in some detail recent updates in theoretical calculations of bound-free opacity from metals. Modest changes in the abundances of elements such as Mg and the iron-peak elements have a significant impact on the atmospheric structure, and therefore self-consistent calculations are necessary. With small adjustments to the solar photospheric composition, we are able to reproduce fairly well the observed solar fluxes between 200 and 270 nm, and between 300 and 420 nm, but find too much absorption in the 270-290 nm window. A comparison between our reference 1D model and a 3D time-dependent hydrodynamical simulation indicates that the continuum flux is only weakly sensitive to 3D effects, with corrections reaching <10% in the near-UV, and <2% in the optical.Comment: 10 pages, 5 figures, to appear in the proceedings of the conference A Stellar Journey, a symposium in celebration of Bengt Gustafsson's 65th birthday, June 23-27, 2008, Uppsal

An Automated System to Classify Stellar Spectra I

Analyses of stellar spectra often begin with the determination of a number of parameters that define a model atmosphere. This work presents a prototype for an automated spectral classification system that uses a 15 nm-wide region around Hbeta, and applies to stars of spectral types A to K with normal (scaled solar) chemical composition. The new tool exploits synthetic spectra based on plane-parallel flux-constant model atmospheres. The input data are high signal-to-noise spectra with a resolution greater than about 0.1 nm. The output parameters are forced to agree with an external scale of effective temperatures based on the Infrared Flux Method. The system is fast -- a spectrum is classified in a few seconds-- and well-suited for implementation on a web server. We estimate upper limits to the 1-sigma random error in the retrieved effective temperatures, surface gravities, and metallicities as 100 K, 0.3 dex, and 0.1 dex, respectively.Comment: 8 pages, 5 figures; to appear in MNRA

Bridging Model and Observed Stellar Spectra

Accurate model stellar fluxes are key for the analysis of observations of individual stars or stellar populations. Model spectra differ from real stellar spectra due to limitations of the input physical data and adopted simplifications, but can be empirically calibrated to maximise their resemblance to actual stellar spectra. I describe a least-squares procedure of general use and test it on the MILES library.Comment: 7 pages, 6 figures, accepted for publication in MNRA