SAAO - South African Astronomical Observatory
11.10.2012 12:24 Age: 5 yrs
Category: SAAO Press Releases
By: Dr Nicola Loaring

Unwinding the secrets of a dying star


Image 1: Observations using the Atacama Large Millimeter/submillimeter Array (ALMA) have revealed an unexpected spiral structure attached to a bright shell in the material around the old star R Sculptoris. This feature has never been seen before and is probably caused by a combination of dramatic changes in the old star’s wind and a hidden companion star orbiting the star. On the left is a slice through the new ALMA data which reveals the shell around the star, the outer circular ring, as well as a very clear spiral structure in the inner material. The 3D computer model of the system is shown on the right. Credit: ALMA/ESO/NAOJ/NRAO and Dr. Shazrene Mohamed

Astronomers looking at the star R Sculptoris have recently discovered that the cloud of dust and gas immediately surrounding the star forms a spiral shape. These observations suggest that the star is not alone, but rather has an unseen orbiting companion star and thus forms a binary star system. Previous observations of R Sculptoris initially found a bright, thick cloud (shell) of gas and dust around the star consisting of material from the star’s outer atmosphere that was lost into space.  More detailed observations using the Atacama Large Millimeter/submillimeter Array (ALMA), the most powerful millimetre/submillimetre telescope in the world, have been able to look at the cloud in greater detail and have revealed a spiral shape inside the bright layer. The spiral is thought to have been created as the faint undetected companion star drags the gas along with it[1] as it orbits around R Sculptoris. Astronomer Dr. Shazrene Mohamed of the South African Astronomical Observatory (SAAO) has deduced the properties of the binary star system by conducting computer simulations which accurately reproduce the observations. This work is one of the first ALMA early science results to be published and it appears in the journal Nature.

A team of astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA), the most powerful millimetre/submillimetre telescope in the world, have discovered a spiral shape in the gas cloud (shell) around the old red giant star R Sculptoris. The spiral is thought to have been caused by an unseen companion star which has dragged the gas into a spiral as it travels around R Sculptoris.Stars with initial masses up to eight times that of our Sun will evolve to become a red giant star towards the end of their lives. During the red giant stage, the star swells up in size to tens or hundreds of times bigger than the Sun at the same time cooling and becoming redder in colour. Red giants are so big that they have trouble retaining their outer layers and a huge amount of their atmosphere is lost into space in an outward flowing wind. The material expelled from the star forms a cloud of gas and dust around the star. This gas and dust, lost into space by red giants like R Sculptoris, provide the raw materials for the formation of future generations of stars, planetary systems and subsequently for life. 

“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.”  says Maercker, lead author of the Nature paper. In order to explain the observations of R Sculptoris, Dr. Shazrene Mohamed from the SAAO has conducted computer simulations of different types of binary star systems for comparison with the ALMA observations. In her models she varied the mass of the unseen companion star, the separation between the companion star and the red giant, as well as how much red giant material is lost into space. A model in which the stars have a combined mass twice that of our Sun and complete one orbit around each other every 350 years fit the observed properties of R Sculptoris very well. "It’s a real challenge to describe theoretically all the observed details coming from ALMA, but our computer models show that we really are on the right track. ALMA is giving us new insight into what's happening in old stars and what might happen to the Sun in a few billion years from now," says Dr. Shazrene Mohamed (South African Astronomical Observatory), a co-author of the study.

Image 1: http://www.saao.ac.za/~shazrene/PressImage.jpg [Included in this press release above]

Observations using the Atacama Large Millimeter/submillimeter Array (ALMA) have revealed an unexpected spiral structure attached to a bright shell in the material around the old star R Sculptoris. This feature has never been seen before and is probably caused by a combination of dramatic changes in the old star’s wind and a hidden companion star orbiting the star. On the left is a slice through the new ALMA data which reveals the shell around the star, the outer circular ring, as well as a very clear spiral structure in the inner material. The 3D computer model of the system is shown on the right. Credit: ALMA/ESO/NAOJ/NRAO and Dr. Shazrene Mohamed

Video 1: http://www.saao.ac.za/~shazrene/RScl_slice.mov

This new video shows a computer simulation of how the material is distributed around the star. It presents a series of slices through a 3D model of the surroundings of R Sculptoris. The shell around the star shows up as a circular ring that appears to get bigger and then smaller in different slices. The newly discovered clear spiral structure in the inner material is best seen about half-way through the video sequence. Credit: Dr. Shazrene Mohamed, SAAO, L. Calçada, ESO.

Video 2: http://www.saao.ac.za/~shazrene/Rscl_evolve.mov

This video is a computer model of the evolution of the material around the old red giant star R Sculptoris over a period of 2000 years. This star experiences thermal pulses that lead to the ejection of material from its surface. Astronomers think that the strange spiral structure is the result of the presence of a companion star in orbit around the red giant. Credit: Dr. Shazrene Mohamed, SAAO.

Press release (pdf)

Contact:
Dr Shazrene Mohammed
shazrene(at)saao.ac.za / 021 447 0025

More information
This research was presented in a paper, “Unexpectedly large mass loss during the thermal pulse cycle of the red giant star R Sculptoris”, by Maercker et al. to appear in the journal Nature.

The research 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. A. Nyman (Joint ALMA Observatory).

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile.
ALMA is funded in Europe by the European Southern Observatory (ESO), in North America by the U.S. National Science Foundation (NSF) in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and in East Asia by the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Academia Sinica (AS) in Taiwan. ALMA construction and operations are led on behalf of Europe by ESO, on behalf of North America by the National Radio Astronomy Observatory (NRAO), which is managed by Associated Universities, Inc. (AUI) and on behalf of East Asia by the National Astronomical Observatory of Japan (NAOJ). The Joint ALMA Observatory (JAO) provides the unified leadership and
management of the construction, commissioning and operation of ALMA.

The ALMA Observatory will be inaugurated on 13 March 2013.