By Wei Shen Oh
[email protected]
It has been known for several decades that globular clusters in the Milky Way display multiple stellar populations. This refers to a cluster having two main stellar groups: a first generation (1G), which are stars that are chemically similar to Milky Way halo stars; and a second generation (2G), consisting of stars rich in certain elements (e.g. He, N, Na and Al), and poor in other elements (e.g. C, O and Mg) with respect to 1G. In recent years however, we have also detected the presence of multiple populations in nearby dwarf galaxies such as the Magellanic Clouds, and photometric data seem to suggest that this phenomenon only occurs for massive clusters that are 2 Gyr and older. Currently, no stellar evolution theory has been able to predict this unusual behaviour.
In our paper recently published in MNRAS, we studied a 1.95 Gyr old massive cluster (NGC 1846) in the Large Magellanic Cloud using high-resolution spectroscopy to verify if it contains multiple populations. The data was obtained with the FLAMES instrument, which is a fibre-fed multi-object spectrograph mounted on the 8m ESO/VLT. Abundance measurements for 12 elements were analysed for 20 red giant branch (RGB) stars. Corrections for non-local thermodynamic equilibrium effects were also included for some elements.
Our results confirm that there is no evidence for multiple populations in the NGC 1846 cluster based on the lack of any intrinsic star-to-star spread in the abundances of Na and O. However, we do detect a significant spread in the carbon abundances, indicating varying evolutionary mixing occurring on the RGB that increases with luminosity. Overall, the general abundance patterns for NGC 1846 are similar to those seen in previous studies of intermediate-age LMC star clusters and field stars.
The figure above shows the comparison of the Na and O abundances in the NGC 1846 stars with literature globular cluster values (Carretta et al. 2009). Red triangles indicate the outlier points for O, while the red arrow indicates the outlier point for Na. Error bars denote the combined random and systematic uncertainties associated with each measurement. The orange shaded box covers the central 95 % of the likelihood distribution for the intrinsic spread in Na and O. The box plot below shows a zoomed-in version of the above plot.
