Proxima Centauri: Alpha's sibling?

Generations of astronomy students have learned that the red dwarf star Proxima Centauri, the sun's closest stellar neighbor, tours the galaxy locked in a gravitational embrace with Alpha Centauri, a binary star system that shines in the southern hemisphere constellation Centaurus.

Now, two British astronomers are challenging this astronomical tenet. Their recalculation of Proxima's motion through space -- based on a widely quoted 1967 measurement--undermines the observational evidence that Proxima orbits Alpha Centauri, a pair of middle-aged stars similar in age and composition to the sun.

Robert Matthews, an amateur astronomer in Oxford, England, performed the labyrinthine calculations at the heart of the new study, described in the March 15 MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY. He and coauthor Gerard E Gilmore, an astrophysicist at the University of Cambridge, hope their challenge will stimulate wider interest in the Centauri system.

"Our understanding of the nearest star to the solar system is not in the sort of state [of certainty] that one might expect it to be," Matthews asserts.

Discovered in 1915, Proxima lies some 4.22 light-years from the sun --a quick hop in astronomical terms. Because Proxima and Alpha lie near each other and move through the galaxy at similar velocities, most astronomers believe they form a single system.

Initially, Matthews cast his mathematical eye on Proxima to figure out when the star would move far enough through the galaxy to lose its privileged status as our nearest stellar neighbor. But he also discovered a problem with an important and widely accepted measurement of Proxima's motion toward or away from Earth -- a quantity called radial velocity, Basing his calculations on the radial velocity cited in textbooks, Matthews found that Proxima may move too rapidly for the double suns of Alpha Centauri to hang on to it.

According to the standard description, Proxima's presumed orbit around Alpha is 330 times larger than Pluto's orbit around the sun. At that distance, Matthews calculates, the velocities of Proxima and Alpha must match within 1 percent for astronomers to conclude that they form a single system. Only a new and more precise measurement of Proxima's radial velocity-accurate to within a few hundred meters per second - can prove what astronomers have taken for granted, Matthews and Gilmore argue.

If Proxima indeed orbits Alpha - and therefore probably arose from the same stellar nursery at the same time -- then researchers will have to reconsider certain important theoretical issues, Matthews and Gilmore say, Proxima is a flare star, given to periodic and dramatic surface brightenings. Theory dictates that such stars should cease most flare activity before reaching Alphas estimated 5-billion-year age. Thus, proof of common lineage and similar age between Proxima and Alpha might encourage astrophysicists to rethink their ideas about flare stars, Matthews says.

"We need to sort out how old Proxima is so we can say something with confidence," he adds. "If a really hard, concerted effort were made [now] with the best technology, I think we could probably sort this out."

It won't be easy, warns Karl W. Kamper of the University of Toronto, who specializes in precision observations of the motions of stars and their distances from the Earth. Kamper says Proxima is so dim and cool that the details of its spectral fingerprint "blend and blur," making it difficult to obtain an unambiguous measurement of its motion. "The fact that Proxima is a very red star makes [the measurement of radial velocity] almost impossible to do," he says.