Observations using the Alma telescope have revealed an unexpected spiral structure in the material around the old star R Sculptoris. This feature has never been seen before, and is probably caused by a hidden companion star orbiting the star. This slice through the new Alma data reveals the shell around the star, which shows up as the outer circular ring, as well as a very clear spiral structure in the inner material. Credit: ALMA (ESO/NAOJ/NRAO)
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.
Alma observes the Universe in radio waves: light which is invisible to our eyes. Extremely weak signals from space are collected by the Alma antennas. Image: ESO/B. Tafreshi
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.
More about AGB stars, red giants and Alma
R Sculptoris is an example of an asymptotic giant branch (AGB) star. These are old stars which began their lives with masses between 0.8 and 8 solar masses. They are cool, red giants which lose mass in the form of strong stellar winds, and vary slowly but regularly in brightness. Their structure consists of a tiny central core of carbon and oxygen surrounded by a helium and hydrogen burning shell, and then an enormous convective envelope. The Sun will eventually evolve into an AGB star.
The shells ejected by AGB stars are composed of gas and dust grains. The dust grains can be spotted by looking for thermal emission extending from the far infrared through millimetre wavelengths. At millimetre wavelengths, emission from the CO molecule allows astronomers to obtain high-resolution maps of the gas emission from the strong stellar wind generated by the AGB stars. These observations are also excellent tracers of the gas distribution around these objects. The high sensitivity of Alma makes it possible to directly image the dust condensation zone and the structure of the material around AGB stars, showing details smaller than 0.1 arcsecond.
Even in the Early Science phase, when the new observations were made, Alma greatly outperformed other submillimetre observatories. Earlier observations had clearly shown a spherical shell around R Sculptoris, but neither the spiral structure nor a companion was found.
In order to describe the observed structure around R Sculptoris, the team of astronomers has also performed computer simulations to follow the evolution of a binary system. These models fit the new Alma observations very well. The scientists modelled the system as a primary AGB star going through a thermal pulse and a small companion star. The separation between the stars used in the simulation is 60 au (1 au is the distance of the Earth from the Sun) with a total mass of the system of two solar masses. The orbital period is 350 years.
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More about the research and the team
This research was presented in the paper “Unexpectedly large mass loss during the thermal pulse cycle of the red giant star R Sculptoris”, by Maercker et al. in the journal Nature.
The team is composed of M. Maercker (ESO; Argelander Institute for Astronomy, University of Bonn, Germany), S. Mohamed (Argelander Institute for Astronomy; South African Astronomical Observatory, South Africa), W. H. T. Vlemmings (Onsala Space Observatory, Chalmers University of Technology, Onsala, Sweden), S. Ramstedt (Argelander Institute for Astronomy, University of Bonn; Uppsala University), M. A. T. Groenewegen (Royal Observatory of Belgium, Brussels, Belgium), E. Humphreys (ESO), F. Kerschbaum (Department of Astronomy, University of Vienna, Austria), M. Lindqvist (Onsala Space Observatory), H. Olofsson (Onsala Space Observatory), C. Paladini (Department of Astronomy, University of Vienna, Austria), M. Wittkowski (ESO), I. de Gregorio-Monsalvo (Joint Alma Observatory, Chile) and L.-Å. Nyman (Joint Alma Observatory).
More About Onsala Space Observatory
Onsala Space Observatory is Sweden's national facility for radio astronomy. The observatory provides researchers with equipment for the study of the earth and the rest of the universe. In Onsala, 45 km south of Gothenburg, it operates two radio telescopes and a station in the international telescope Lofar. It also participates in several international projects. The observatory is hosted by Department of Earth and Space Sciences at Chalmers University of Technology, and is operated on behalf of the Swedish Research Council.
Robert Cumming, astronomer and communications officer, Onsala Space Observatory, +46-31-772 55 00 or +46-70-493 31 14, email@example.com
Wouter Vlemmings, astronomer, Onsala Space Observatory, Chalmers University of Technology, +46-31-772 55 09 or +46- 733 544 667, firstname.lastname@example.org