A Double Main Sequence in the Globular Cluster NGC 6397

TL;DR

High-precision HST photometry uncovered a double main sequence in NGC 6397, indicating two stellar populations with distinct chemical compositions and a slight helium enhancement, revealing complex cluster formation history.

Contribution

This study provides the first clear evidence of a double main sequence in NGC 6397, linking it to multiple stellar populations with different chemical signatures.

Findings

Two stellar populations with distinct chemical compositions identified.

The second sequence is anomalously faint in m_F336W, possibly due to NH depletion.

Approximately 30% of stars belong to the primordial population, 70% to the second generation.

Abstract

High-precision multi-band HST photometry reveals that the main sequence (MS) of the globular cluster NGC 6397 splits into two components, containing ~30% and ~70% of the stars. This double sequence is consistent with the idea that the cluster hosts two stellar populations: (i) a primordial population that has a composition similar to field stars, and containing ~30% of the stars, and (ii) a second generation with enhanced sodium and nitrogen, depleted carbon and oxygen, and a slightly enhanced helium abundance (Delta Y~0.01). We examine the color difference between the two sequences across a variety of color baselines and find that the second sequence is anomalously faint in m_F336W. Theoretical isochrones indicate that this could be due to NH depletion.