The Peculiar Chemistry of the Massive Globular Cluster NGC 6402 (M14)

Spectroscopic analyses indicated a possible gap in the light element composition pattern of NGC 6402 stars. The reason for such a composition gap is not clear, and may be indicative of an early termination of star formation in the cluster. HST follow-up observations showed a multi-modal, but otherwise continuous, “chromosome map” sequence. However, it is not yet clear if the composition discontinuity observed in heavier elements, such as O and Na, extends to the lighter elements, such as N, that heavily influence the chromosome map morphology. The HST observations also revealed the presence of a second main-sequence, which may support a discontinuity in He abundance. Additional HST data also indicate the possible presence of a more metal-rich population, which further complicates our understanding of the cluster’s formation and enrichment history.