Thanks to the Gemini South telescope, a team of astronomers has explored the heart of Gran 5, a globular cluster near the galactic center. And what they discovered changes our understanding of these structures.
Gran 5 is not a cluster like the others. Located about 14,600 light-years away, it is composed of stars with distinct chemical compositions, which could hint at a complex evolutionary history. This recently discovered cluster has a mass of 22,900 solar masses and seems to be linked to the kinematic structures of the Milky Way.
Image of the Gran 5 field with the seven target stars marked in green. The yellow circle indicates the cluster's half-light radius. Credit: Lim et al., 2024.
Globular clusters like Gran 5 are natural laboratories for studying stellar evolution. The chemical composition of the stars in these clusters offers clues about the early phases of galactic formation, marked by explosions and star formation events. The study of stellar populations in these clusters thus helps researchers better understand the dynamics and history of the Milky Way.
To examine Gran 5 in detail, Dongwook Lim and his team from Yonsei University in Korea selected seven stars from the cluster and observed them with a high-resolution infrared spectrometer, IGRINS, installed on the Gemini South telescope. This technology enables the analysis of the chemical and kinematic characteristics of stars with great precision.
Among the seven stars observed, the researchers confirmed that six of them indeed belonged to Gran 5, while one was excluded due to a significantly different radial velocity. These six stars, with a radial velocity of -60 km/s (-37 mi/s), share a low metallicity of about -0.65 dex, confirming their membership in this globular cluster.
The chemical analysis of the six stars revealed two distinct populations, marked by different levels of metallicity, -0.76 and -0.55 dex, respectively. Since no other kinematic or atmospheric parameter explained this difference, the researchers concluded that two populations of stars with distinct compositions exist within the cluster.
This phenomenon is unprecedented in a globular cluster of such low mass. It challenges the idea that Gran 5 might be associated with the Gaia-Enceladus structure in the galactic halo. The observed characteristics suggest instead an origin in the galactic bulge or the disk of the Milky Way.
To explain this metallicity diversity, astronomers hypothesize that the cluster may have undergone complex evolutionary processes or even significant mass loss over time. This would help to understand why Gran 5 presents such unique characteristics compared to other low-mass globular clusters.
What is dex?
Dex is a logarithmic unit often used in astronomy to express metallicity or the difference in chemical abundance between celestial objects. It simplifies comparisons of element concentrations in stars or other objects.
The notation in dex is based on a base-10 logarithmic scale. For example, if a star has a metallicity of -0.5 dex compared to the Sun, this means that its concentration of elements heavier than hydrogen and helium is 0.5 powers of 10 lower than that of the Sun.
In practice, using dex allows astronomers to easily quantify chemical differences between various celestial objects. This unit is primarily used in metallicity studies to assess the chemical evolution of galaxies and star clusters.
Thus, dex is a valuable unit for precisely comparing chemical abundances while maintaining a compact and convenient notation for large variations.