Libraries of stellar spectra (SSL) are at the crossroad of different fields of astrophysics. In particular, they serve as reference for the analysis of large spectroscopic surveys, and they are fundamental ingredients of the models of stellar populations used to study galaxies. These libraries may either consist of observed or theoretical spectra, and they vary by their spectral coverage/domain and resolution.

The goals of the WG are to identify the scientific and technical issues linked with SSL, in particular:

• The coverage in wavelength and parameter space of the current and scheduled libraries
• The dissemination of the libraries and their accurate description
• The characterization of the stars

A particular concern is that despite continuous progress on all aspects of SSL, considerable disagreements on the atmospheric parameters and chemical abundances of stars, and on the ages, metallicities of masses of galaxies, persist for decades. Whereas the internal precision these parameters is of the order of 0.02 or 0.03 dex, the actual accuracy is not better than 0.2 dex.

A number of projects faced these issues whose origins are complex, like for example the GAIA Benchmark Stars, a small library of primary calibrators, and the GAIA-ESO Survey. Even for the best studied stars, different approaches do not agree. These uncertainties on the stellar parameters further propagates to the stellar population models, adding up with our limited knowledge of the stellar evolution, contribution of binary stars, and other interpolation of tricky mathematical questions.

The WG will carry-on an inventory of the different attempts made to explain the discrepancies, and will summarize recommendations for actions that would improve the situation.

A report will be prepared before the next IAU GA in 2018.

The group takes actions to encourage:

• The open-source publication of the codes used to compute and analyse spectra of stars and stellar populations, and more generally actions helping to probe the reproducibility of the results.
• The dissemination of libraries together with accurate descriptions based on the best practices.
• Community collaborations aimed at comparing libraries or methods, or coordinating their development.