Multiple stellar populations (MPs) with different chemical compositions are not exclusive features of old globular clusters (older than 10Gyr). Indeed, recent studies reveal that younger clusters (~2-6Gyr-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 ~2Gyr old cluster NGC1978, 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 main-sequence (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 ~40% to ~80%, depending on the detailed distributions of N and O.

Cone search capability for table J/ApJ/906/133/fig2 (C_F275W,F343N,F438W_ vs. F814W diagram of NGC 1978)