Workshop Gaia Benchmark stars

The Núcleo Milenio ERIS, together with the European Southern Observatory (ESO) hosted a workshop entitled “Extending the Gaia Benchmark stars” throughout November. From the 14th until the 18th of the last month and thanks to the collaboration between Ditte Slumstrup and Paula Jofré, it was possible to bring together a team of experts to work on this particular project. The team included several members of ERIS.

Ditte Slumstrup, is a young scientist from the ESO, who took the initiative to carry out  infrared spectroscopic observations of the Benchmarks stars, using the  instrument CRIRES+ at VLT.  In the last year and a half, it has been possible to obtain more than 30 new spectra, which are currently under analysis. Thanks to the workshop, the team that began the study of these stars in 2011 in Bordeaux, France, was able to collaborate with the ERIS members who study stellar spectra, such as Álvaro Rojas, Sara Vitali, Claudia Aguilera, Danielle de Brito and Scarlet Elgueta.

What are the Gaia Benchmark stars?

These are very bright and well known stars that have been studied since the first spectral catalogs at the beginning of the XX century, being analyzed by the famous group of women known as The Harvard Computers.

Along with the improvements in technology, our understanding of stellar astrophysics has also increased and a huge number of  spectral lines can now be resolved with great detail. Furthermore, thanks to modern telescopes, it is now possible to know the sizes of stars through measuring their angular diameters, to perform time-domain observations of their brightnesses, to study how they oscillate, etc. Nevertheless, this still can’t be done for all the stars of the Galaxy. In order to constrain the physical properties of the million of stars we can observe today thanks to Gaia and its complementary spectroscopic surveys, we need to know how well we understand stellar atmospheres, that’s how Gaia Benchmark stars can help us.

Paula Jofré is one of the leaders of the characterization of these stars. She began this project during her first postdoctoral position in Bordeaux, France, when she started collaborating with Caroline Soubiran y Ulrike Heiter. 

That project further motivated the development  of the computational package called iSpec by the then PhD  student Sergi Blanco Cuaresma from Bordeaux. With this program, the team is able to perform detailed and careful spectroscopic analysis using the high accuracy data of   the Gaia Benchmark stars.

Between the years 2014 and 2016, the team published their first series of articles, making these stars a reference for validating and calibrating the first generation of high resolution spectroscopic surveys, such as Gaia-ESO, GALAH, APOGEE and RAVE. These stars became fundamental material for the success of the aforementioned projects. 

The benefits of ERIS with the Gaia Benchmark stars

ERIS greatly benefits from this project since it relies on accurate chemical abundances which can be interpreted as the stellar DNA. ERIS aims to use thousands of stars of the Milky Way to build a phylogenetic tree and reconstruct the history of galaxies.

Over the years,  Gaia has published new parallaxes,  and  more stellar angular diameters have become available.  It is thus crucial  to keep improving  this very important stellar sample. This is particularly important given the new generation of spectroscopic surveys planned or on-going: SDSS, MOONS, WEAVE, 4MOST, among others, which require calibration and validation of their spectroscopic analyses.

Recent results  show that the chemical abundances obtained from spectral lines at  different wavelengths but from the same star do not agree. The workshop aimed at understanding if  this is because of a limitation of our understanding in stellar atmospheres, the atomic transitions or simply the methods to analyze spectra. Answering this question is  important to further improve the stellar spectroscopic analysis and thus have accurate stellar DNAs.