Science —

New telescope array reveals death spiral of old star

ALMA telescopes found unexpected structures in the gas shed by a dying star.

Gas shed by the dying star R Sculptoris forms a lumpy spherical shell with a strange spiral interior. Such a pattern indicates the presence of a hidden companion star.

As stars like our Sun die, they swell to a huge size and shed much of the gas in their outer layers. This process seeds the galaxy with raw material for the next generation of stars and planets. Studying the last stages of stellar evolution helps astronomers model the process by which heavier elements are distributed through interstellar space (and incidentally provides some of the most beautiful astronomical images).

New observations of one of these dying stars revealed both a fascinating spiral structure in the ejected gas and that about three times the expected mass has been jettisoned into space. Astronomers using the Atacama Large Millimeter Array (ALMA) in Chile constructed a three-dimensional view of the giant red star R Sculptoris. Based on the strange spiral pattern, they concluded the star has or had an unseen companion that shaped the gas it ejected. This observation was part of what's only a preliminary run for ALMA, which won't even be completed until 2013.

Millimeter and submillimeter astronomy straddles the (admittedly arbitrary) boundary between the infrared and microwave portions of the spectrum. Light at these wavelengths punches through obscuring gas and dust, allowing astronomers to look into optically-opaque regions, such as the nebulas surrounding dying stars like R Sculptoris. (The star's name comes from its location in the constellation Sculptor, visible in the Southern Hemisphere.)

While earlier observations showed a thin spherical shell of gas perfectly centered on R Sculptoris, the ALMA data revealed unexpected structure inside. The details included clumps in the gas shell and a winding spiral pattern, as seen in the image above. Additionally, the amount of mass contained in the surrounding matter is approximately three times what was estimated from earlier observations and models of similar stars.

R Sculptoris is in the asymptotic giant phase, the final stage in the evolution of stars similar to our Sun. Asymptotic giant stars use up most of their remaining nuclear fuel in a burst of fusion, which pushes their outer layers out to vast size. These objects are very bright and have relatively cool surface temperatures, so they appear red (in its prime the star may have appeared yellow or white). At least some asymptotic giant stars also undergo thermal pulses, where the nuclear fusion process restarts after a dormant period. The outer layers of these dying stars are only loosely bound, so thermal pulses can cause a lot of mass to be shed into interstellar space. This gas becomes available for forming new stars.

The ALMA researchers determined that R Sculptoris experienced a thermal pulse approximately 1,800 years ago, based on the observational data and computer simulations of the star's evolution. This pulse pushed out a lot more mass than previous models had suggested, based on the observed excess gas.

Spiral patterns like that exhibited by R Sculptoris are generally due to a companion star. The orbital motion of the asymptotic giant star around its companion twists the ejection of the gas, just as a spinning lawn sprinkler sends water out in a spiral pattern. Similarly, the perimeter of the sprinkler's reach is a circle centered on the sprinkler, and the outer shell of ejected gas from R Sculptoris maintains its spherical shape, even though the gas wasn't ejected as a smooth sphere. If R Sculptoris has a companion, it remains unobserved. That's less of a problem than it might seem, however, since the companion is likely much fainter than the main star (a white dwarf, for example).

The detailed interior structure revealed by the ALMA observations will help elucidate the late evolution of Sunlike stars, including how they distribute gas into interstellar space. The excess mass observed in the shell surrounding R Sculptoris will also assist in future models of thermal pulses in asymptotic giant stars. Since all of this was accomplished while ALMA was less than half complete, we can expect greater things still when the array is completed in 2013.

Nature, 2012. DOI: 10.1038/nature11511  (About DOIs).

You must to comment.

Channel Ars Technica