Multiple Populations in Globular Clusters: Where do we stand?

In the past two decades, our understanding of globular clusters (GCs) and their formation has undergone a radical change, revitalising cluster research with new discoveries and new questions. It is now clear that GCs host multiple stellar populations with variations in the abundances of He, light elements (C, N, O, F, K, Na, Mg, Al, Si), and even Fe or s-process elements in few cases. No such trends were found in open clusters or in the field population of the MW or external galaxies, implying that it is only within the environment of GCs that this peculiar chemical pattern can develop. Additionally, high precision colour-magnitude diagram of GCs have found that nearly all GCs display complex and unexpected features, with splits and/or spreads in the main sequences, red-giant branches, etc. A number of (qualitative) theories have been put forward to explain the observed anomalies, but none of them is able to entirely reproduce the observed properties of the GC system. Indeed, the origin of multiple population is still disputed and many question remain unanswered. Do we need a entirely new paradigm to explain these populations other than the stellar nucleosynthesis proposed so far? Or can uncertainties in the stellar models account for the discrepancies? How are the chemical anomalies connected with the complex colour-magnitude diagram features (i. e. extended HB, AGB)? Does a threshold in age or mass exist for GCs to develop chemical inhomogeneities? How do GCs relate to open clusters, ultra-compact dwarf galaxies and dwarf spheroidals? What fraction of the galactic field was formed in clusters? What was the rˆole of GCs in build up the Galaxy? How we can make the most effective use of the huge amount of data coming from from the ground-based large surveys (SEGUE, RAVE, LAMOST, Gaia ESO survey, APOGEE) and space missions (HST, GAIA)?

Recent high-resolution observations with the HST have revealed that GCs, once thought only be able to form in the special conditions present in the early Universe, are still forming today. Those young massive clusters, also known as super star clusters (SSCs), have been found in a variety of galactic environments characterised by elevated levels of star formation activity. They can be considered as young counterparts to the ancient GCs, as their sizes, masses, and luminosities are fully consistent with what is expected for young GCs in the Milky Way. What causes this extreme mode of star formation and what physical conditions are required for this SSCs to form? How the mode of star/cluster formation changed across the space and cosmic time? Which fraction of SSCs might eventually evolve in old GCs? Do SSCs show evidence for multiple generations of stars? Can they be used to constrain theories for the origin of the multiple populations observed in GCs. Do they show extended star formation histories or the cooling flows predicted in GC formation scenarios? Can SSCs retain the large gas/dust reservoirs required for theories to work? Finally, what observations are required to make definitive progress in our understanding of massive cluster formation and their role in galaxy evolution?

The origin of multiple populations within GCs is arguably the most active research field within the GC community at the present, with more than 350 published papers in the last 5 years. Despite the recent advances from the modelling side, theory is still far behind observations and is unlikely that most of the questions on GC formation and evolution will be answered without a new insight. This workshop is meant as a continuation of our previous workshops on stellar clusters in 2012 & 2014 and will compliment the approved summer 2016 workshop on “The Role of Feedback in the Formation and Evolution of Star Clusters” (led by Dr. James Dale). We expect that a number of participants will attend both meetings if held in back-to-back weeks. We will review the state-of-the-art data on multiple population formation from both the observational and theoretical side.

Our main goal is to generate fresh perspectives and unexplored avenues of study by bringing together theorists and observers that were focussed on a two related (SSCs and GCs) fields of study.

The general programme will consist of one or two keynote talks for a review of a particular subject as well as the open questions remaining, and what needs to be done to make progress.  After that, we will have contributed talks, along with posters on each subject.

The topics to be covered are: