Newly found star cluster may have originated beyond the Milky Way

Spectroscopy enables scientists to determine cluster's chemical composition.
By Laurel Kornfeld | Sep 27, 2017
A massive star cluster discovered four months ago by the European Space Agency's (ESA) Gaia space observatory may have originated beyond the Milky Way.

Named Gaia 1 and located about 15,000 light years away, the cluster is puzzling scientists, who are attempting to determine its age and metallicity along with its origin.

Because its orbit extends 5,500 light years above the disk of the Milky Way, some astronomers believe its origin to be extra-galactic.

Initially, the 22,000-solar mass cluster was thought to be 6.3 billion years old and relatively metal-rich. The latter means it contains elements more complex than hydrogen and helium.

A subsequent study found the cluster to be much younger, about 3.3 billion years old and significantly more metal-rich than first suspected.

Now, a science team led by Andreas Koch of Lancaster University in the UK performed a more detailed chemical analysis of the star cluster and published their findings in the journal Astrophysics of Galaxies.

They specifically focused on four red giant stars in the cluster, for which they conducted detailed measurements of 14 chemical elements.

Using the Echelle spectrograph on the 2.5-meter du Pont telescope at the Las Campanas Observatory in Chile, the scientists observed these red giants and measured the amounts of various elements withing them, including magnesium, oxygen, carbon, lithium, silicon, aluminum, titanium, calcium, chromium, cobalt, nickel, zinc, scandium, and vanadium.

"This work focuses on a detailed chemical abundance analysis of four red giant members of Gaia 1, based on high-resolution spectroscopy, which we complement by an investigation of the orbital properties of this transition object," they noted in the journal paper.

While the process by which Gaia 1 formed remains unclear, this latest study indicates is now part of the Milky Way's thick disk and is a high-mass open star cluster rather than a low-mass globular cluster.

Open clusters are found in galactic disks and are younger than 10 billion years old while globular clusters are found in galactic halos and are at least 11 billion years old. The latter have tens to hundreds of thousands of stars as compared with hundreds to thousands for the former.

The study also found Gaia 1 to be more metal-poor than initially believed.

 

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