Multiple stellar populations at less evolved stages: detection of chemical variations among main-sequence dwarfs in NGC 1978

TL;DR

This study detects multiple stellar populations with chemical variations among less evolved main-sequence stars in the 2 Gyr-old cluster NGC 1978, using ultraviolet and visual Hubble observations, revealing MPs are present in younger clusters.

Contribution

It provides the first evidence of chemical variations among main-sequence dwarfs in an intermediate-age cluster, expanding understanding of MPs beyond old globular clusters.

Findings

G- and K-type main-sequence stars in NGC 1978 host MPs.

Fraction of N-rich stars varies from 40% to 80%.

Chemical variations are detectable among less evolved stars.

Abstract

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.