Clues from the Cosmic Past: Unraveling the Chemical History of NGC 2298
Globular clusters are dense collections of ancient stars, often seen as the Milky Way’s fossil record. Once believed to host stars with identical compositions, many globular clusters are now known to contain multiple generations of stars with distinct chemical fingerprints. These differences—especially in elements like carbon (C), nitrogen (N), sodium (Na), and aluminum (Al)—suggest that stars in these clusters formed in successive waves. In this study, Bandyopadhyay et al. explore the chemical landscape of NGC 2298, a particularly metal-poor and compact globular cluster, to better understand the processes that shaped its early formation and chemical enrichment.
Observations and Data Collection
The team selected 13 red giant branch (RGB) stars in NGC 2298 using precise motion and distance data from Gaia, filtering out non-members and poorly measured stars. Observations were carried out with the Gemini South telescope using its new GHOST spectrograph, which offers high spectral resolution across a wide range of wavelengths. Each star was observed long enough to gather spectra of sufficient quality to determine abundances for 36 species across 32 elements, including 16 neutron-capture elements. The resulting data were processed with DRAGONS, a data reduction software tailored to GHOST.
Classifying Stars by Generation
Using light element abundance patterns, particularly the Na-O and Mg-Al anticorrelations, the stars were divided into two groups: eight first-generation (1G) stars and five second-generation (2G) stars. The 2G stars were found to be enhanced in nitrogen, sodium, and aluminum while being depleted in carbon and magnesium, a pattern consistent with what’s seen in other globular clusters. These differences indicate that 2G stars likely formed from gas polluted by the nucleosynthetic products of 1G stars. Interestingly, while metallicity was similar across both groups, specific light element signatures clearly separated them.
Element Abundance Patterns: α and Fe-Peak Elements
When comparing alpha (α) elements like calcium (Ca) and iron-peak elements such as scandium (Sc), nickel (Ni), and zinc (Zn), the authors found that most elements showed little variation between stars. However, Sc, Ni, and Zn exhibited significantly different dispersions between the two populations. Sc showed a particularly pronounced difference, with 1G stars displaying much higher variability. These findings suggest that while most of the cluster formed from well-mixed material, certain elements may reflect early, localized enrichment from a narrow range of stellar sources, such as specific types of supernovae.
Heavy Element Clues: The r-Process Signature
The study also focused on neutron-capture elements—especially those formed through the r-process, a rapid neutron-capture process responsible for creating heavy elements like europium (Eu) and barium (Ba). The team found that both 1G and 2G stars display a general r-process abundance pattern, though 1G stars show much greater scatter in some elements like strontium (Sr) and Eu. This suggests that the earliest stars in the cluster formed from gas that had been unevenly enriched by rare r-process events, such as neutron star mergers or unusual supernovae, while later generations formed from more uniformly mixed material.
Intrinsic Scatter and Chemical Diversity
To confirm whether the observed variations were real and not just due to measurement uncertainty, the team applied a statistical method to estimate the intrinsic scatter for each element. They found meaningful intrinsic scatter in several elements, particularly among 1G stars. For example, Sc, Ba, and Eu had higher scatter in 1G stars, reinforcing the idea of inhomogeneous early enrichment. In contrast, 2G stars appeared more chemically uniform, except for cobalt (Co), which showed higher scatter in this group—possibly pointing to different nucleosynthetic sources influencing their formation.
Looking for Trends: Light vs. Heavy r-Process Elements
The researchers also investigated whether lighter r-process elements like Sr and Ba showed any patterns relative to magnesium, an α-element. Although the sample size limited the statistical strength, they noticed mild trends in how [Sr/Eu] and [Ba/Eu] ratios changed with [Mg/H], especially in 2G stars. These trends may hint at different nucleosynthetic processes at work across generations, but more data is needed to confirm these signals. Notably, they did not find evidence of an "actinide boost"—a strong overproduction of very heavy elements like thorium—indicating that extreme r-process events were not dominant in this cluster’s history.
Conclusions: A Chemically Complex Past
This study provides strong evidence that even a small and metal-poor globular cluster like NGC 2298 experienced complex chemical evolution shaped by multiple generations of stars. The presence of both well-mixed and inhomogeneously enriched elements points to a dynamic early environment influenced by a variety of nucleosynthetic sources, including core-collapse supernovae and rare r-process events. These results not only deepen our understanding of NGC 2298 but also contribute to the broader effort of piecing together the formation history of the Milky Way.
Source: Bandyopadhyay
