Mysteriously Hot "Blue Hook" Stars Explained

The giant globular cluster has the greatest known collection of “blue hook” stars. Hubble Space Telescope/NASA

Astronomers think that they have explained the puzzle of how certain stars, dubbed “blue hooks” can be exceptionally hot, while only having mass of around half that of the sun. The theory suggests that the strange combination is a result of events ten billion years earlier.

Most stars follow a pattern whereby the more mass they have, the hotter and brighter they will be, leading to larger stars burning through their gas much faster than smaller ones. Most anomalies in this pattern have been explained; stars are fainter earlier in their evolution, for example.

Blue hook stars, however, have been a puzzle up until now. They are very hot indeed but are only about half as massive as the sun and are not very bright, at least in the visible part of the spectrum. Moreover, most of the blue hook stars that we have found exist in globular clusters, particularly larger clusters such as Omega Centuari.

A paper in Nature presents the theory that disturbances when modestly-sized stars are forming create a legacy that only becomes visible billions of years later, as they approach the ends of their lives.

While most stars shine by turning hydrogen into helium, blue hook stars turn helium into carbon, which requires temperatures of over 100 million degrees Kelvin in the core. Fusing helium is common as stars age, but Dr Aaron Dotter, of the Australian National University, says that blue hook stars do it at temperatures double that of more massive stars. The question is: why?

Dotter and his coauthors propose that blue hook stars were disrupted during formation by the nearby passage of an existing star. Stars form from disks of ionized gas. A combination of rotational inertia and magnetic effects in the gas determine the spin of the star that forms. However, the authors' modeling suggests that if a passing star makes a mess of the disk, the result is a much faster spinning star. This extra rotation acts as a partial balance for gravity.

Artist's impression of a star disrupting the accretion disk around another, still forming, star. Marco Galliani, INAF

“It's surprising having the disk driven away leaves a fingerprint that survives throughout the lifetime of the star,” Dotter told IFLS. The team's modeling suggests that this does indeed occur, with a chain of processes producing a heavier core in disrupted stars. Once helium fusion begins, it operates at an even higher temperature than in other helium-burning stars.

The result is stars so hot, Dotter says, that they look faint in visible light because most of their energy is released in the ultraviolet range.

Blue hook stars are common in globular clusters because the tight packing of stars at the cluster's center gives plenty of opportunities for disruption. Until recently, such events would have been thought impossible because all the stars in globular clusters were believed to form at the same time. However, recent work has suggested that globular clusters contain a second, and possibly third, generation of stars that form a hundred million years or so after the first, making such disruptions very likely.

Dotter suggests that the few blue hook stars in the halo of the Milky Way may have formed in clusters that were subsequently swallowed by our galaxy.

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