New Variable Stars Discovered in NGC 6558 Globular Cluster

In a recent study, an international team of astronomers focused their attention on the globular cluster NGC 6558 and successfully detected multiple variable stars within its vicinity. This research has unveiled two previously unidentified members of the cluster, marking an exciting discovery for the scientific community. Findings from this investigation have been documented in a research paper available on the pre-print server arXiv, published on June 29.

Variable stars play a crucial role in unraveling the mysteries of stellar structure and evolution. They offer valuable insights into the distance scale of the universe, helping scientists gain a deeper understanding of these astrophysical phenomena. Scientists are particularly interested in studying variable stars in star clusters as they contribute to identifying potential errors that impact measurements of stellar distances.

Situated approximately 24,100 light-years away in the constellation Sagittarius, NGC 6558, also recognized as GCL 89 or ESO 456-SC62, represents a globular cluster located within the Galactic bulge. Through previous research, it has been determined that NGC 6558 possesses a metallicity level of -1.32 dex and is speculated to be among the Milky Way's most ancient celestial objects, based on earlier observations.

Leading a team of astronomers, Armando Arellano Ferro from the National Autonomous University of Mexico spearheaded an investigation into NGC 6558, aiming to identify potential new members within the cluster. Through thorough observations performed at the National Observatory of Cordoba in Argentina and the Las Campanas Observatory in Chile, the researchers conducted an extensive survey of variable stars covering a 13.2 x 13.2 arcmin2 area centered on the Galactic bulge cluster NGC 6558.

The study unveiled a total of 78 variable stars in the field of NGC 6558. Subsequent proper motion analysis and an in-depth examination of these variables on the cluster's color-magnitude diagram (CMD) led to the discovery that nine of the candidates were indeed authentic NGC 6558 members.

Notably, two of the identified cluster members, named V18 and V19, represent newly detected variable stars. V18 is classified as an eclipsing variable, exhibiting a period of approximately 0.4 days, while V19 appears to be a semi-regular variable, displaying a period of roughly 346 days.

Intriguingly, by studying the properties of these member variables, astronomers have been able to compute the distance to NGC 6558 as well as its metallicity. According to their calculations, NGC 6558 is positioned approximately 27,500 light-years away, with a metallicity level estimated at approximately -1.33 dex.

The research team also analyzed a sample of RR Lyrae variables of RRab type within NGC 6558, leading to the suggestion that the cluster potentially belongs to the intermediate Oosterhoff type (Oo-Int) category. However, further observations are necessary to validate this classification fully.

The recent discovery of the two newfound variable stars and the comprehensive assessment of NGC 6558 represents a significant step towards expanding our knowledge of these fascinating celestial objects. As astronomers continue to conduct research, further insights into the behavior and characteristics of variable stars, as well as the properties of NGC 6558, will undoubtedly arise, shedding light on the broader understanding of the universe.