R Sculptoris is a dying red giant star located ~1,000 light-years from Earth. The star is surrounded by a detached shell of dust and gas that was created during a thermal pulse event where the star underwent a brief period of increased mass loss. A team of astronomers using the Atacama Large Millimeter/Submillimeter Array (ALMA) in the Atacama Desert of northern Chile discovered the presence of a spiral structure within the shell of material around R Sculptoris. The spiral structure extends from the central star outwards to the shell. “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.
Figure 1: A visualization of the spiral structure around R Sculptoris. Credit: ALMA (ESO/NAOJ/NRAO)
Towards the end of their lives, low-mass and intermediate-mass stars, such as the Sun, become red giant stars and start to lose a large amount of their mass. During the red giant phase of a star, it also periodically experiences thermal pulses. A thermal pulse occurs when a thin shell of helium surrounding the star’s core fuses the helium into carbon in an explosive fashion. The thermonuclear convulsion dumps a huge amount of energy into the star’s interior which causes the star to blast off a significant amount of material. For this reason, red giant stars like R Sculptoris are major contributors to the bulk of raw materials such as carbon and oxygen that are incorporated into the formation of future generations of stars and planets.
Figure 2: The detached shell and spiral structure observed in different velocity channels. Numbers in the top right corners indicate the velocity in km/s with respect to R Sculptoris. The detached shell is most pronounced at lower velocities while the spiral structure can be traced through all velocity channels. (M. Maercker et al., 2012)
Figure 3: The green curve outlines the spiral structure around R Sculptoris. (M. Maercker et al., 2012)
Observational data combined with hydrodynamic simulations suggests that the shell of dust and gas around R Sculptoris was created when the star underwent a thermal pulse ~1,800 years ago, lasting for ~200 years. The spiral structure observed around R Sculptoris is most likely caused by the presence of a companion star that is shaping the stellar wind into a spiral pattern, like a rotating garden sprinkler. Extending from the central star out to the shell, the spiral structure can be followed over about 5 windings. Based on the spacing of the windings and the present-day expansion velocity of the stellar wind streaming from R Sculptoris, the companion star around R Sculptoris is estimated to have an orbital period of 350 years.
The amount of material present in the shell of dust and gas around R Sculptoris is estimated to have a mass of ~0.003 times the Sun’s mass (~1000 times the Earth’s mass). All these material was blasted outward at 50,000 kilometres per hour. Since the thermal pulse lasted for ~200 years, the mass-loss rate of R Sculptoris during the thermal pulse is ~5 Earth masses per year. This is about 30 times lower than the present-day mass-loss rate of ~0.15 Earth masses per year. “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 these stars and what might happen to the Sun in a few billion years from now,” says Shazrene Mohamed, a co-author of the study.
M. Maercker et al., “Unexpectedly large mass loss during the thermal pulse cycle of the red giant star R Sculptoris”, Nature 490, 232-234 (11 October 2012)