Astronomers have long been mystified by observations of a few hot, bright, apparently young stars residing in well-established neighborhoods where most of their neighbors are much older. It's like finding teen-agers hanging out in stellar retirement homes.
With the help of the Hubble Space Telescope, astronomers now have evidence that may eventually help solve the 45-year-old mystery of how these enigmatic stars, called blue stragglers, were formed. For the first time, astronomers have confirmed that a blue straggler in the core of a globular cluster (a very dense community of stars) is a massive, rapidly rotating star that is spinning 75 times faster than the Sun. This finding provides proof that blue stragglers are created by collisions or other intimate encounters in an overcrowded cluster core.
Astronomers studied a blue straggler in the tumultuous heart of the nearby globular cluster 47 Tucanae (47 Tuc), located 15,000 light-years away in the southern constellation Tucana. The observation was made October 27, 1995 by astronomers Michael M. Shara of the Space Telescope Science Institute in Baltimore, Md.; Rex A. Saffer of Villanova University in Villanova, Pa.; and Mario Livio, also of the Institute. Their analysis will appear in the Nov. 1 issue of the Astrophysical Journal Letters.
"This is an extremely exciting result," Saffer said, "because it may help distinguish between competing theories of blue straggler star formation and evolution.
"Allan Sandage (an astronomer with Carnegie Observatories in Pasadena, Calif.) discovered blue stragglers in the globular cluster M3 nearly 45 years ago. Since that time, they have been assumed to be stars much like the Sun, although their bluer color and larger brightness imply that they are more massive and much younger than normal globular cluster stars. Our analysis confirms that, but without having to make any assumptions about the state of blue straggler star evolution."
Using the Hubble telescope's Faint Object Spectrograph, which was removed during the Second Servicing Mission in February, the astronomers analyzed the spectrum of one blue straggler, measuring its temperature, radius, and rotation rate. The team then combined these measurements with the blue straggler's apparent brightness, taken from a Hubble telescope Wide Field and Planetary Camera archival image, to obtain the star's mass. The derived temperature and mass are consistent with the characteristics of a normal star with a mass about 1.7 times that of the Sun. However, the star is spinning at least two to three times faster than stars of its kind.
"Masses are among the most fundamental properties of stars," Saffer explained. "Yet, except for some stars in binary systems, they are difficult to measure directly. Measurements of the masses and rotation rates of blue stragglers in globular clusters are extremely important, since they allow us to trace the history of cluster formation and evolution. This is possible because different formation theories make different predictions about these characteristics."
"A direct mass measurement of a blue straggler is a kind of acid test of theory and observations," Shara added. "In this case, the mass theory has been tested and stands up well."
By analyzing a blue straggler's mass, temperature, and rotation velocity, astronomers can investigate how it formed. Astronomers now believe that blue stragglers are created by the merger of two low-mass stars. But they have two different views of how these stars interact to create blue stragglers. One merger theory proposes that a violent collision of two unrelated stars creates a blue straggler. Another hypothesizes that a slow coalescence of a gravitationally bound pair creates the straggler star.
Based on their analysis of the blue straggler in 47 Tuc, the team favors the slower, gentler merger scenario between binary stars. In double-star systems where the stars are close enough to touch each other, the more massive star can cannibalize its partner, producing a single, even more massive star. This process, astronomers believe, more likely results in a rapidly spinning merger product where the fast orbital motions of the binary star produces the rapid spin of the consolidated pair.
"Our measurement of a rapid rotation rate for the blue straggler star in 47 Tuc tends to strongly favor this second mechanism," Saffer said, "at least for this one star."
The second merger scenario involves a collision between two unrelated stars, which run into each other by chance in the dense star cluster core.
"It's a bit like a head-on wreck between two tractor trailers," Saffer explained, "where the enormous energy carried by the fast-moving stars is deposited in the debris from the collision."
The merged star is tremendously heated and swells into a red giant star, where it can easily lose its spin through magnetic activity. The swollen star's distended magnetic field throws off mass at high speed, like mud flung off the rim of a rapidly rotating wheel. Eventually, when the star shrinks to normal size after radiating the heat generated by the collision, it spins itself down and becomes a slow rotator, Shara said.
Saffer credits the Hubble telescope's superior spatial resolution with being able to peer into a swirling mix of stars to capture a blue straggler in the cluster core.
"While some blue stragglers are found in globular cluster outskirts, in 47 Tuc the blue stragglers are only found in the cluster core," Saffer said. "The crowding of the stars there is too severe for the current generation of ground- based telescopes to resolve them."
Globular clusters are massive systems of up to 1 million stars packed into a spheroid about 20 light-years in diameter. They also are among the oldest stellar systems in the Milky Way Galaxy. Stars speeding through the extremely crowded cluster core are far more likely to collide or experience other dynamic interactions with their neighbors than stars in the sparse neighborhood of the Sun. These processes can produce a zoo of stellar animals, such as X-ray binaries, millisecond pulsars, blue stragglers, and other exotic species, all of which have actually been observed in globular cluster cores.
47 Tuc (NGC 104) is one of the intrinsically largest and brightest globular clusters. A naked eye object, it is just west of the Small Magellanic Cloud. Blue stragglers were first discovered in its core by the Hubble telescope's Faint Object Camera in 1991.
