ALMA REVEALS SPIRAL STRUCTURE AND AN OUTER SHELL AROUND DYING STAR.

ALMA REVEALS SPIRAL STRUCTURE AND AN OUTER SHELL AROUND DYING STAR.

ALMA REVEALS SPIRAL STRUCTURE AND AN OUTER SHELL AROUND DYING STAR.

Photo By: ESA/ESO Atacama Large Millimeter/Submillimeter Array (ALMA). CLICK for full size and all links below.

In 2012, astronomers at ALMA, the most powerful radio telescope ever constructed were viewing the star R-Sculptoris (AGB star at the end of its life) and soon noticed something odd about its basic features. Surrounding the near death red giant there is a shell of material which is not uncommon but within the shell lies a spiral structure beginning at the star itself and working its way outward.

This has led astronomers to conclude that the star must have a companion of some sort orbiting it. Simulations place the companion at about 60AU from R Sculptoris with an orbital period of about 350 years. At the time of this observation ALMA was already the most powerful radio telescope even though it only had about one half of its antennas constructed and in place.

Below is an excerpt from the ESO page in regards to this star and stars like it in general, enjoy:
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.
[1] R Sculptoris is an example of an asymptotic giant branch (AGB) star. These are stars with initial masses between 0.8 and 8 solar masses in the late stages of their lives. They are cool, red giants with large mass loss in the form of strong stellar winds, and are typically long-period variables. 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.
[2] The ejected shell formed around AGB stars is 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.
[3] A similar spiral, but without a surrounding shell, has been seen in NASA/ESA Hubble Space Telescope observations of the star LL Pegasi. But, unlike the new ALMA observations, these data did not allow the full three-dimensional structure to be studied. The Hubble observations detect the dust and ALMA the molecular emission.
[4] Unseen binaries have also been suggested as an explanation for the odd shapes seen in related objects, planetary nebulae. After the AGB phase, intermediate and low-mass stars (0.8–8 solar masses) will end their lives by forming a planetary nebula. These are the glowing remains of the stellar envelope of gas ejected during the AGB phase, which is ionised by ultraviolet radiation emitted by the central star. Many planetary nebulae have extremely complex and varied morphologies. Binary central stars, stellar discs and magnetic fields have been suggested as the mechanism that produces such a variety of shapes.
[5] The system modelled here consists of 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 astronomical units with a total mass of the system of two solar masses. The orbital period is 350 years.

NAME: R Sculptoris, SAO 193122, HD 8879.

WHAT IS IT?: AGB red giant star in the later stages of its life.

HOW FAR AWAY IS IT?: 1500 light years distant.

APPARENT MAGNITUDE?: About 5.7 so it’s just barely naked eye visible.

WHERE IS IT? (General): Constellation Sculptor.

WHERE IS IT? (Exact RA/DEC J2000): RA 01h 26m 58s / DEC −32° 32′ 35.43″.

European Southern Observatory (ESO) page for this object: http://www.eso.org/public/news/eso1239/

Phil Plait the Bad Astronomer article on this object: http://blogs.discovermagazine.com/badastronomy/tag/r-sculptoris/

SIMBAD data on R Sculptoris: http://simbad.u-strasbg.fr/simbad/sim-id?Ident=R+Sculptoris

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