The Bulge Cluster Origin (BulCO) survey at the ESO-VLT: Probing the early history of the Milky Way assembly. Design and first results in Liller 1★

We present the scientific goals and the very first results of the Bulge Cluster Origin (BulCO) survey. This survey has been specifically designed to perform an unprecedented chemical screening of stellar systems orbiting the Milky Way bulge, with the aim to unveil their true origin. It takes advantage of the improved performances of the spectrograph CRIRES+ operating at the ESO Very Large telescope in the near-infrared domain. Due to the complex evolutionary history of the Milky Way, a variety of relics tracing different phenomena is expected to populate the bulge: Globular clusters formed in situ or accreted from outside the Galaxy, nuclear star clusters of cannibalized structures, and possibly a few remnants of the proto-bulge formation process (the so-called bulge fossil fragments). The signatures of the different origins are imprinted in the chemical properties of these stellar systems because specific abundance patterns provide authentic chemical DNA tests that unequivocally trace the enrichment process, and therefore, the environment in which the stellar population formed. Thus, each system can provide a new piece of information on the bulge formation and evolutionary history. As first results of the survey, we discuss the α−element and iron abundances of a sample of stars that was observed in the stellar system Liller 1, which is proposed to be a bulge fossil fragment. By combining this dataset with a recently published sample of high- to medium-resolution spectra, we show that the overall chemical properties of the stellar populations in Liller1 prove its tight link with the Galactic bulge and provide new constraints on its star formation history.