Astronomers discover new insights into multiple stellar populations in Messier 92 using James Webb

The study of multiple stellar populations in globular clusters (GCs) has been a topic of interest for astronomers for several decades. Recently, a team of astronomers from the University of Padua, Italy, and elsewhere have used the James Webb Space Telescope (JWST) and the Hubble Space Telescope (HST) to explore the multiple stellar populations in Messier 92 (M92), a metal-poor GC located in the constellation of Hercules. The findings from this study have been published on the arXiv pre-print server.

Background

Globular clusters are among the oldest and most massive star clusters in the Milky Way galaxy. Almost all GCs exhibit star-to-star abundance variations of light elements such as helium, oxygen, nitrogen, carbon, and calcium, indicating self-enrichment in GCs. This suggests that star clusters are composed of at least two stellar populations.

Messier 92 is a GC with a metallicity of -2.31 and a mass of about 200,000 solar masses, located approximately 26,700 light years away from us. The cluster is estimated to be 14.2 billion years old and is known to host at least two stellar generations of stars, named 1G and 2G. Previous studies have found that Messier 92 has an extended 1G sequence, which hosts about 30.4% of cluster stars, and two distinct groups of 2G stars (2GA and 2GB).

Observations

The team of astronomers led by Tuila Ziliotto from the University of Padua decided to inspect these stellar populations of Messier 92, taking advantage of data collected by JWST and HST. The observations identified the three previously reported stellar groups, namely 1G, 2GA, and 2GB.

Results

The three stellar populations were found to share similar radial distributions of the proper motion dispersion, which range from about 0.2 mas/year near the cluster center to 0.15 mas/year around a distance of about 2.2 half-light radii. Moreover, the 1G, 2GA, and 2GB stars turned out to exhibit isotropic motions in the studied radial interval within about 1.5 half-light radii.

Based on the comparison between the observed colors of the cluster stars and the colors derived by synthetic spectra, the astronomers found that helium abundances of 2GA and 2GB stars have higher mass fractions than that of the 1G stars by 0.01 and 0.04, respectively. They noted that helium difference between 2GB and 1G MS stars is consistent with the maximum helium variation inferred for red giant branch (RGB) stars.

Furthermore, color-magnitude diagrams (CMDs) show that main sequence (MS) stars of Messier 92 fainter than the so-called MS knee exhibit an intrinsic color spread, which is present among stars with masses of about 0.1-0.4 solar masses. These low-mass stars appear to exhibit a continuous color distribution and do not showcase evidence for distinct groups of 1G and 2G stars.

Journal Information: Tuila Ziliotto et al, Multiple Stellar Populations in Metal-Poor Globular Clusters with JWST: a NIRCam view of M 92, arXiv (2023). DOI: 10.48550/arxiv.2304.06026