Special Session SS19
3 – 6 April 2018 (TBD)
Multiple populations in massive star clusters, a common thread through cosmological ages?
Aims and scope
Once thought to be the best example of a simple stellar population, it is now well-established that globular clusters (GCs)
are not consistent with this paradigm. This is due to the presence of star-to-star abundance variations of light elements
which reflect in a collection of photometric splits or spreads; observations being explained nowadays by models invoking
multiple star formations events. However, despite the ubiquitous nature of different stellar populations among these ancient
massive clusters, there is a striking heterogeneity of their spectroscopic and photometric properties between GCs. Without
mentioning the complex nature of some GCs displaying intrinsic spreads of heavy elements. In turn, the growing amount
of data accumulated from observations complicates the picture instead of simplifying it. As a result, to date, it has never
been more complicated to explain the origin of multiple populations. In addition, it is increasingly urgent to understand this
phenomenon since numerous recent observations highlight the fact that these multiple populations are not restricted to GCs.
Indeed there is evidence of their presence in young stellar clusters (down to ?2 Gyr), in the bulge and the halo of the Milky
Way, in massive early type galaxies and also in ultra-compact dwarf galaxies. On top of that, recent observations of younger
clusters have shown that they appear not to be simple stellar populations either, further complicating the picture.
While the multiple populations are now indissociable from the ancient massive globular clusters, young clusters also ques-
tion a simple formation and a quiescent evolution. Indeed several evidence such as split main sequences and extended main
sequence turn-off, are incompatible with the simple stellar population assumption. However although the exact origin of
these observations being still debated, it seems there is not a direct connection with the different stellar populations observed
in GCs as no abundance variations have been detected (the most favorite cause being currently a distribution of rotational
velocity). Finally, young massive clusters might be the youthful counterparts of GCs, thus they may be used to constrain
their early formation. However, they do not show evidence of the presence of multiple populations so far which might question
their role. Moreover, young massive clusters do not have any gas left, which also questions current theories. Therefore in
addition of the strong diversity of cluster systems, they are all imposing genuine challenges to our understanding of their
formation and evolution.
The goal of this special session is to generate brand-new perspectives by bringing together the large communities of observers and theorists working on several aspects of this phenomenon. The main purpose is to review and critically discuss
our current understanding of the complexity of stellar populations from both the observational and theoretical sides in the
different stellar systems and find a common thread between them (such as the connection between observed young clusters
and GCs at young ages). Finally, it will also be important to forecast what we might learn on multiple populations using
future facilities such as JWST.
- William Chantereau (LJMU)
- Corinne Charbonnel (UniGe/IRAP)
- Nate Bastian (LJMU)
- Eva K. Grebel (Heidelberg University)
- Carmela Lardo (EPFL)
- Joel Pfeffer (LJMU)
w.chantereau @ ljmu.ac.uk
Updated on Fri Sep 15 11:32:16 CEST 2017