Star clusters: Anything but simple

Abstract

The heated debate on the importance of stellar rotation and age spreads in massive star clusters has just become hotter by throwing stellar variability into the mix.A quiet revolution has been sweeping the field of star-cluster astrophysics. A decade ago, we were reasonably convinced that we understood the formation and evolution of the massive, well-populated star clusters that can be used as a statistical tool for studies of stellar evolution. Groups of stars characterized by a common age and chemical composition were considered 'simple stellar populations', given that all of their stars had presumably formed from the same progenitor molecular gas cloud at approximately the same time. Admittedly, the oldest galactic building blocks, the globular clusters, were known to exhibit evidence of multiple stellar generations 1 , but clusters younger than a few billion years appeared to obey our simple models. Fast forward a decade, and we now know that the majority of 1-3 billion-year-old star clusters in the nearest galaxies, the Magellanic Clouds, are anything but simple. Indeed, writing in The Astrophysical Journal Letters, Ricardo Salinas and co-workers show that a significant population of pulsating stars can have a measurable effect on our interpretation of stellar evolution within such clusters 2 .Deviations from the simple stellar population model show up most readily in a cluster's colour-magnitude diagram. This type of plot is the observational counterpart to the theoretical Hertzsprung-Russell diagram, which relates the temperatures (or colours) of the cluster's stars to their luminosities. Instead of being randomly distributed, the stars tend to lie on bands. Most stars, including the Sun, belong to the 'main sequence', when they are fusing hydrogen into helium in their cores. By mapping a stellar population in this manner, it is possible to estimate the age of the stars in a given cluster.

Discovery of Extended Main-sequence Turnoffs in Four Young Massive Clusters in the Magellanic Clouds

“…Next came the rapid stellar rotation scenario (Bastian & de Mink 2009;Yang et al 2013;Li et al 2014b;Brandt & Huang 2015;Bastian & Niederhofer 2015;Li et al 2016a;Wu et al 2016). Recently, a new scenario has been suggested, i.e., that stellar variability may play a potential role in shaping the eMSTO regions as well (Salinas et al 2016;de Grijs 2017). The debate is indeed heating up.…”

Non-Gaussian error distributions of galactic rotation speed measurements

“…More details can be found in their paper [1]. The main goal of their paper (intended as a follow-up to a series of papers [15][16][17][18] looking for similar effects in other observables) was to undertake a meta-analysis of all the measurements of Θ 0 from published literature in order to look for intrinsic differences between the different categories of measurements of Θ 0 . They also wanted to see if there is evidence for publication bias or "bandwagon" effect.…”

The Effects of Binary Stars on the Color–Magnitude Diagrams of Young-age Massive Star Clusters

“…As an alternative interpretation, eMSTOs are thought to be due to coeval populations with different rotation rates (Bastian & de Mink 2009;Yang et al 2013;Li et al 2014;Brandt & Huang 2015;D'Antona et al 2015;Niederhofer et al 2015a) or due to interacting binaries (Yang et al 2011;Li et al 2012Li et al , 2015. Moreover, stellar variability (Salinas et al 2016;de Grijs 2017) and a variable overshoot of the convective core of stars (Yang & Tian 2017) may play a potential role in shaping the eMSTO regions as well. The nature of the eMSTO phenomenon is still highly debated.…”