Astronomers have discovered an unexpected spiral structure in the gas around the red giant star R Sculptoris.
“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,” said Dr Matthias Maercker of the University of Bonn’s Argelander Institute for Astronomy, lead author of a paper published in the journal Nature.
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.
Earlier observations had clearly shown a spherical shell around R Sculptoris, but neither the spiral structure nor a companion was found.
“When we observed the star with ALMA (Atacama Large Millimeter/Submillimeter Array), 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,” said study co-author Dr Wouter Vlemmings of the Chalmers University of Technology in Sweden.
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. During the red giant stage stars also periodically undergo thermal pulses. These are short-lived phases of explosive helium burning in a shell around the stellar core. A thermal pulse leads 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. After the pulse the rate at which the star loses mass falls again to its normal value.
Thermal pulses occur approximately every 10 000 to 50 000 years, and last only a few hundred years. The new observations of R Sculptoris show that it suffered a thermal pulse event about 1800 years ago that lasted for about 200 years. 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,” Dr Maercker said. “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.”
Bibliographic information: M. Maercker et al. 2012. Unexpectedly large mass loss during the thermal pulse cycle of the red giant star R Sculptoris. Nature 490, 232–234; doi: 10.1038/nature11511