Astronomers have discovered a totally unexpected spiral structure in space. The strange shape, discovered in gas and dust surrounding the red giant star R Sculptoris, was probably created by a hidden companion star orbiting the star. The research, which appears in the journal Nature this week, gives new insight on how stars like the Sun return material to space when they become old.
The discovery was made by an international team of astronomers, among them several scientists from Chalmers University of Technology and Onsala Space Observatory. The team used the Atacama Large Millimeter/submillimeter Array (Alma), the most powerful millimetre/submillimetre telescope in the world.
The surprising spiral structure was discovered in the gas around the red giant star R Sculptoris. This is the first time that such a structure, along with an outer spherical shell, has been found around a red giant star. It is also the first time that astronomers could get full three-dimensional information about such a spiral.
The presence of the spiral means that there is probably a previously unseen companion star orbiting the star.
"We've seen shells around this kind of star before, but this is the first time we've ever seen a spiral of material coming out from a star, together with a surrounding shell," says Matthias Maercker, the lead author on the paper presenting the results, and astronomer at ESO and the Argelander Institute for Astronomy, University of Bonn, Germany.
Because they blow out large amounts of material, red giants like R Sculptoris are major contributors to the dust and gas that provide the bulk of the raw materials for the formation of future generations of stars, planetary systems and subsequently for life. The astronomers were surprised to find that far more material than expected had been ejected by the red giant.
"When we observed the star with Alma, not even half its antennas were in place. It's really exciting to imagine what the full Alma array will be able to do once it's completed in 2013," says Wouter Vlemmings, a co-author of the study and astronomer at Chalmers and Onsala Space Observatory.
Late in their lives, stars with masses up to eight times that of the Sun become red giants and lose a large amount of their mass in a dense stellar wind.
The new observations of R Sculptoris show that it suffered a thermal pulse event about 1800 years ago that lasted for about 200 years. Thermal pulses are short-lived phases of explosive helium burning in a shell around the stellar core. The pulses lead to material being blown off the surface of the star at a much higher rate, resulting in the formation of a large shell of dust and gas around the star.
Thermal pulses occur approximately every 10,000 to 50,000 years, and last only a few hundred years. It was during the thermal pulse that the companion star shaped the wind from R Sculptoris into a spiral structure.
"By taking advantage of the power of Alma to see fine details, we can understand much better what happens to the star before, during and after the thermal pulse, by studying how the shell and the spiral structure are shaped," says Matthias Maercker.
"We always expected Alma to provide us with a new view of the Universe, but to be discovering unexpected new things already, with one of the first sets of observations, is truly exciting," he adds.
- M. Maercker, S. Mohamed, W. H. T. Vlemmings, S. Ramstedt, M. A. T. Groenewegen, E. Humphreys, F. Kerschbaum, M. Lindqvist, H. Olofsson, C. Paladini, M. Wittkowski, I. de Gregorio-Monsalvo, L.-A. Nyman. Unexpectedly large mass loss during the thermal pulse cycle of the red giant star R Sculptoris. Nature, 2012; 490 (7419): 232 DOI: 10.1038/nature11511
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