Scientific motivation

The characterization of stellar properties is crucial to studies of Galactic archaeology, exoplanets, and stellar structure and evolution. To ensure the success of these science pillars, a well-established and well-understood set of reference stars is needed.

Current and future large surveys of the Milky Way are set to determine the evolutionary processes that took place in our galaxy, and others like it. This chemo-kinematical investigation of millions of stars relies on the accurate analysis of observed data, which has to be achieved by a myriad of automatic pipelines developed for each specific survey and instrument. If we are to trust the science extracted from thus derived astrophysical parameters and chemical abundances, it is of paramount importance that everyone agrees on the results obtained for reference stars and calibrates their analysis routines to them. Similarly, exoplanet searches are reaching levels of precision that demand exact characterization of the host star properties. A set of commonly observed reference stars thus serves as one of the most reliable inter-survey and interdisciplinary validators.

The astronomical community has already made great efforts towards establishing a reference set of stars, such as the atmospheric parameters and chemical abundances of the Gaia FGK benchmark stars, which are already being widely used for calibration by several pipelines. However, the HR diagram is diverse and many more reference stars are needed to fill the highly dimensional parameter space of stellar properties (e.g. age, multiplicity, temperature, surface gravity, rotation, abundances of many elements), and to calibrate the variance in stellar properties relative to the models or empirical relations. There are many questions that need to be answered in order to establish a widely useful reference set. For example:

• What are the magnitudes of stars suitable for ongoing and future surveys (can all surveys observe bright stars?)
• What spectral resolution is required for the characterisation of element abundances (e.g. by resolving lines of individual elements) and resolve binarity?
• Which stellar parameters exhibit the most complex behaviour and should thus be sampled more densely?
• What precision and accuracy is needed for such a reference sample?

We often forget that a significant number of FGK stars are multiple systems, thus their spectra carry information about several components, and these have to be disentangled or at least properly accounted for. Detection and analysis of single-epoch spectra of multiple stars can be easier in case of resolved multiple lines, or much more demanding in the unresolved case. Tools for automatic characterisation of these ''hidden gems'' need to be developed to the level of existing methodology for single stars, and reference sets of multiple stars have to be established. With the next generation of large surveys on the horizon, this is a crucial time for defining these reference stellar catalogues.

We believe the featured workshop is timely since we are concerned with the best possible scientific output of future large surveys, i.e. in a period where a number of such surveys (e.g. RAVE, Gaia-ESO, APOGEE, GALAH, LAMOST) have already been conducted and astronomers have significant experience with processing large datasets of observational data. This is a necessary requirement to make the best informed decisions about the calibration needs of surveys that are just starting to observe or are scheduled to begin in the following years (e.g. WEAVE, 4MOST, PLATO). Our goal is therefore to bring together people with relevant experiences to share their opinions on what would be the most suitable and widely applicable reference set of stars.