Multiple stellar populations (MPs) with different chemical compositions are not exclusive features of old GCs (older than 10 Gyr). Indeed, recent studies reveal that younger clusters ($\sim$2--6 Gyr-old) in the Magellanic Clouds also exhibit star-to-star chemical variations among evolved stars. However, whether MPs are present among less evolved dwarfs of these intermediate-age clusters is still unclear. In this work, we search for chemical variations among GK-type dwarfs in the $\sim$2 Gyr-old cluster NGC 1978, which is the youngest cluster with MPs. We exploit deep ultraviolet and visual observations from the Hubble Space Telescope to constrain the nitrogen (N) and oxygen (O) variations among MS stars. To do this, we compare appropriate photometric diagrams that are sensitive to N and O with synthetic diagrams of simple stellar populations and MPs. We conclude that the G- and K-type MS stars in NGC\,1978 host MPs. Our statistical analysis shows that the fraction of N-rich stars ranges from $\sim$40\% to $\sim$80\%, depending on the detailed distributions of nitrogen and oxygen.

中文翻译：

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处于低演化阶段的多个恒星群：NGC 1978 中主序矮星化学变化的检测

具有不同化学成分的多个恒星群 (MP) 并不是旧 GC（大于 10 Gyr）的独有特征。事实上，最近的研究表明，麦哲伦星云中较年轻的星团（$\sim$2--6 Gyr-old）在演化恒星之间也表现出星与星之间的化学变化。然而，这些中年星团的进化程度较低的矮人中是否存在 MP 尚不清楚。在这项工作中，我们在 $\sim$2 Gyr-old 星团 NGC 1978 中寻找 GK 型矮星之间的化学变化，这是最年轻的 MPs 星团。我们利用哈勃太空望远镜的深紫外和目视观测来限制 MS 恒星之间的氮 (N) 和氧 (O) 变化。去做这个，我们将对 N 和 O 敏感的适当光度图与简单恒星群和 MP 的合成图进行比较。我们得出结论，NGC\,1978 中的 G 型和 K 型 MS 恒星是宿主 MP。我们的统计分析表明，富含 N 的恒星的比例从 $\sim$40\% 到 $\sim$80\% 不等，具体取决于氮和氧的详细分布。