The Inner Structure Of A Stellar Cluster Revealed Thanks To 36 Pulsars

Location and direction of the millisecond pulsars inside Terzan 5 in an optical image taken by the Hubble space telescope. Pulsars in blue are accelerating toward Earth; those in red are accelerating away. B. Saxton (NRAO/AUI/NSF); GBO/AUI/NSF; NASA/ESA Hubble, F. Ferraro

Astronomers using the Green Bank Telescope (GBT) were able to study the internal structure of globular cluster Terzan 5 by precisely observing the trajectory of 36 millisecond pulsars, a rare type of degenerate star.

These special neutron stars pulsate hundreds of times every second (hence the name) and were used by the team to work out the dynamic of the cluster. As reported in the Astrophysical Journal, the cluster has a dense core that may or may not contain an intermediate mass black hole. If it does, it is not going to be bigger than 30,000 times the mass of the Sun.

“Pulsars are amazingly precise cosmic clocks," co-author Scott Ransom, from the National Radio Astronomy Observatory, said in a statement. "With the GBT, our team was able to essentially measure how each of these clocks is falling through space toward regions of higher mass. Once we have that information, we can translate it into a very precise map of the density of the cluster, showing us where the bulk of the ‘stuff’ in the cluster resides.”

The study was also able to clarify the nature of Terzan 5. Researchers have previously discovered multiple stellar populations, and it was suggested that Terzan 5 was the remnant of a dwarf galaxy absorbed by the Milky Way. The latest observations instead strongly suggest that Terzan 5 formed from an original fragment of our galaxy.

Terzan 5 is an impressive globular cluster, located 19,000 light-years away towards the center of the Milky Way. It is estimated to host millions of stars and astronomers believe that up to 200 millisecond pulsars might be found in it.

“That’s what makes Terzan 5 such an important target of study; it has an unprecedented abundance of pulsars – a total of 37 detected so far, though only 36 were used in our study,” lead author Brian Prager, from the University of Virginia in Charlottesville, added. “The more pulsars you can observe, the more complete your dataset and the more details you can discern about the interior of the cluster.”

Future observations and better models can be used to work out if a massive black hole is indeed hiding at its center and to confirm once and for all if it’s a globular cluster or a dwarf galaxy remnant.

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