On the Nitrogen variation in ∼2 Gyr old massive star clusters in the large Magellanic Cloud

Abstract

We present ESO/VLT FORS2 low resolution spectroscopy of red giant branch stars in three massive, intermediate age (∼1.7 − 2.3 Gyr) star clusters in the Large Magellanic Cloud. We measure CH and CN index bands at 4300Å and 3883Å, as well as [C/Fe] and [N/Fe] abundance ratios for 24, 21 and 12 member stars of NGC 1978, NGC 1651, NGC 1783, respectively. We find a significant intrinsic spread in CN in NGC 1978 and NGC 1651, a signal of multiple stellar populations (MPs) within the clusters. On the contrary, we report a null CN spread in NGC 1783 within our measurement precision. For NGC 1978, we separated the two populations in the CN distribution and we translated the CN spread into an internal N variation Δ[N/Fe]=0.63 ± 0.49 dex. For NGC 1651 and NGC 1783, we put upper limits on the N abundance variations of Δ[N/Fe]≤0.2, 0.4 dex, respectively. The spectroscopic analysis confirms previous results from HST photometry, where NGC 1978 was found to host MPs in the form of N spreads, while slightly younger clusters (e.g. NGC 1783, < 2 Gyr old) were not, within the limits of the uncertainties. It also confirms that intermediate age massive clusters show lower N abundance variations with respect to the ancient globular clusters, although this is in part due to the effect of the first dredge up at these stellar masses, as recently reported in the literature. We stress the importance of future studies to estimate the initial N abundance variations, free of stellar evolutionary mixing processes, by observing unevolved stars in young clusters.

Evidence of globular cluster abundance anomalies in the SMC intermediate-age cluster Kron 3

“…22, and we draw attention to a few particular clusters, including Kron 3 8 . As has been noted before (e.g., , Martocchia et al (2021)), Fig. 22 reveals that every cluster studied older than ∼ 2 Gyr and with a (present day) mass exceeding ∼ 10 5 M shows evidence for multiple populations, while the lowest mass clusters with MPs (Lindsay 113, Martocchia et al (2019); NGC 6535, Bragaglia et al (2017)) have masses of ∼ 2×10 4 M .…”

“…Further, the apparent cutoff of the presence of multiple populations at an age of ∼ 2 Gyr (e.g., see Martocchia et al (2021) for the presence of N-variations in NGC 1978 and NGC 1651, both at age ∼ 2 Gyr but absent in the ∼ 1.7 Gyr old cluster NGC 1783) remains an intriguing and unsolved puzzle, particularly as the masses of at least some of these 1-2 Gyr old LMC clusters that lack multiple populations will remain above 10 5 M when they are 2-4 Gyr older. Milone et al (2020) also find that the clusters NGC 419, 1783, 1806 and 1846, all of which have ages of ∼1.6 Gyr and present-day masses above 10 5 M , also lack any evidence for MPs, while for the slightly older cluster NGC 1978, evidence for the presence of multiple pops is confirmed, although the 1P fraction, at 0.85 ± 0.04 9 , is substantially larger than those of GCs of similar present-day mass, which further increases the questions about these young clusters.…”

“…The photometric analysis confirms the presence of MPs in all clusters studied older than 6 Gyr. An interesting finding is that the photometric evidence of MPs in the red giant branch of NGC 1978, a cluster of age ∼ 2.3 Gyr in LMC (Martocchia et al 2018b) has been confirmed by a spectroscopic analysis of CN and CH bands of 24 member stars (Martocchia et al 2021); see also the recent results of Li et al (2021). Also, Martocchia et al (2021) has found significant intrinsic spread in CN in 21 members of NGC 1651 (∼ 2 Gyr), which is the signal of the presence of MPs.…”

“…Several studies (e.g., Carretta et al 2010;Milone 2015;Salinas & Strader 2015;Lagioia et al 2019a;Milone et al 2020) have suggested that cluster mass plays an important role in generating the MPs. However, studies such as Mucciarelli et al (2008), Milone et al (2020) and Martocchia et al (2021) have shown that the abundance anomalies are not obviously present in LMC and SMC star clusters with ages less than ∼2 Gyr. These clusters have present-day masses comparable to the present-day masses of lower luminosity (ancient) GGCs that possess abundance anomalies.…”

The chemical composition of globular clusters in the Local Group

“…Nevertheless, these updates are of relatively minor consequence for this work, since the CN and CH lines mostly affect the spectra at wavelengths < 4400 Å. In principle, the CN and CH features in the wavelength range 4100 Å-4400 Å can be used to constrain the abundances of C and N, although the N abundances are better constrained by the stronger CN band near 3800 Å-3900 Å, espe-cially at lower metallicities (Cohen et al 2002;Graves & Schiavon 2008;Lardo et al 2012;Schiavon et al 2013;Martocchia et al 2021). However, the interpretation of C and N abundances in integrated-light spectra is complicated by stellar evolutionary effects (mixing) along the red giant branch (RGB) (Gratton et al 2000;Martell et al 2008), and we do not here quote abundances of these elements.…”