Multiple stellar populations at less evolved stages-II: no evidence of significant helium spread among NGC 1846 dwarfs

Abstract

The detection of star-to-star chemical variations in star clusters older than 2 Gyr has changed the traditional view of star clusters as canonical examples of "simple stellar populations" (SSPs) into the so-called "multiple stellar populations" (MPs). Although the significance of MPs seems to correlate with cluster total mass, it seems that the presence of MPs is determined by cluster age, however. In this article, we use deep photometry from the Hubble Space Telescope to investigate whether the FG-type dwarfs in the $\sim$1.7 Gyr-old cluster NGC 1846, have helium spread. By comparing the observation with the synthetic stellar populations, we estimate a helium spread of $\Delta{Y}\sim0.01\pm0.01$ among the main-sequence stars in NGC 1846. The maximum helium spread would not exceed $\Delta{Y}\sim0.02$, depending on the adopted fraction of helium-enriched stars. To mask the color variation caused by such a helium enrichment, a nitrogen enrichment of at least $\Delta{\rm [N/Fe]}$=0.8 dex is required, which is excluded by previous analyses of the red-giant branch in this cluster. We find that our result is consistent with the $\Delta{Y}$--mass relationship for Galactic globular clusters. To examine whether or not NGC 1846 harbors MPs, higher photometric accuracy is required. We conclude that under the adopted photometric quality, there is no extreme helium variation among NGC 1846 dwarfs.