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Spectroscopic study inspects blue straggler stars in NGC 3201

Using the Magellan Clay Telescope, astronomers have performed a spectroscopic study of blue straggler stars in the globular cluster NGC 3201. Results of the new study, May 21 on the arXiv preprint server, could help us better understand the properties and chemical composition of this cluster.

First identified in the 1950s, the blue straggler stars (BSSs) are unique main-sequence (MS) stars that are brighter, bluer, and appear younger than their coeval counterparts, hence more massive than MS stars. They are positioned to the left and above the main-sequence turnoff (MSTO) in the optical color-magnitude diagram (CMD).

One of the places to look for and investigate the BSS population are globular clusters (GCs)—gravitationally bound groups of stars. Due to their relatively high masses, the blue straggler stars can be used to probe the internal dynamics of GCs.

NGC 3201, also known as Caldwell 79, is a low galactic latitude GC in the constellation of Vela, at a distance of some 16,000 light years away. The cluster is estimated to be 12.2 billion years old, has a metallicity at a level of -1.59 dex, and its reddening is approximately 0.24 mag.

Previous observations have found that NGC 3201 hosts dozens of blue straggler stars, and now a group of astronomers led by Gourav Kumawat of the Indian Institute of Science Education and Research in Bhopal, India has employed the Magellan Clay Telescope at the Las Campanas Observatory in Chile to explore this population of stars.

As part of the new study, Kumawat's team analyzed the spectra of 39 BSSs in NGC 3201. The astronomers managed to derive radial velocities, effective temperatures, metallicities and chemical abundance ratio of magnesium to iron for this sample of stars.

The study found that the mean radial velocity of the investigated BSSs is 498 km/s, while their effective temperatures are within the range of 6,260–8,690 K. The metallicity values of the metallicity and magnesium to iron ratio were measured to be -1.42 and 0.36, respectively.

The researchers noted that their results still do not allow them to draw final conclusions regarding the formation of the blue straggler stars in NGC 3201.

"Chemical abundance is one such indicator that distinguishes the BSS formation scenario. (...) Our derived values for [Mg/Fe] have large uncertainty, which prevents us from making definitive statements about the formation mechanisms of these BSSs," the researchers wrote.

The authors of the paper added that further observations with broader spectral coverage in NGC 3201 should be conducted, which may provide important insights into potential correlations between the chemical abundances and atmospheric parameters of BSSs in this cluster.