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Spiral Shell Sheds Light on Red Giant Star's Recent Convulsive History

October 11, 2012 | 3

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As stars age, they often shed their skins, so to speak, casting off expansive shells of dust and gas into interstellar space. A new look at the shell surrounding a type of aging star called a red giant shows that the star's ejected husk carries in its structure the imprint of its formation and subsequent evolution.

Matthias Maercker of the European Southern Observatory and the University of Bonn in Germany and his colleagues targeted R Sculptoris, a red giant star in the southern constellation of Sculptor, with the Atacama Large Millimeter/Submillimeter Array (ALMA). The radio telescope complex is already producing preliminary science, although it is still under construction in Chile's high-altitude Atacama Desert. ALMA will eventually consist of 50 widely spaced dish antennas, each of them 12 meters in diameter, plus a compact grouping of 16 additional dishes. As of last month 40 of the 66 planned antennas had been deployed. Data from the myriad dishes can be melded together by a computerized correlator to provide a single high-resolution radio image of an astronomical object.

The peek afforded by ALMA revealed that the shell around R Sculptoris is not spherical, as had been assumed, but has been twisted into a spiral, as depicted in the visualization of ALMA data above. Such spiral structure is thought to mark the presence of a stellar companion, so it now appears that R Sculptoris has an unseen binary partner. Maercker and his colleagues published the results of their observations in the October 11 issue of Nature. (Scientific American is part of Nature Publishing Group.)

The researchers could also infer from the shell's structure a timetable for its creation. R Sculptoris looks to have erupted the shell of material in a sort of stellar convulsion called a thermal pulse about 1,800 years ago. The thermal pulse lasted about 200 years, Maercker and his colleagues have concluded, during which time the star cast off about three times the mass of Jupiter in dust and gas.

—John Matson

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1. jtdwyer 08:11 PM 10/11/12

It seems that the otherwise undetected companion star must be accreting expelled matter, otherwise the expelled gas would likely form a typical spherical 'planetary nebula' rather than the unusual planar expulsion disk. Likewise, the expulsion flow velocity must exceed the ability of the companion star to accrete it, otherwise the outflow would not be 'channeled' around the companion.

Also see:
http://www.eso.org/public/news/eso1239/#4

“Unexpectedly large mass loss during the thermal pulse cycle of the red giant star R Sculptoris”, by Maercker et al. (2012).
http://www.eso.org/public/archives/releases/sciencepapers/eso1239/eso1239a.pdf

http://news.sciencemag.org/sciencenow/2012/10/scienceshot-star-sheds-a-thousan.html?ref=hp
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2. ec1966 09:01 PM 10/11/12

I wonder which would be the stronger effect on the companion - that is, accretion to it from the expelled gas, or if the expelled gas is moving out fast enough, having its own stellar atmosphere blown off, creating a "shadow" in the spiral (well, I guess that would happen either way, but with different composition).
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3. jtdwyer in reply to ec1966 07:52 AM 10/12/12

I think it is difficult to say, since we don't even know if the unidentified companion star if a small main sequence star or perhaps already a white dwarf.

The ESO link referenced previously states:
"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."

Since the mass of a typical white dwarf is around 0.6 Solar masses, a white dwarf companion would be consistent with the researcher's assumptions. A white dwarf would have very little atmosphere and its average density would be about 1M x the Sun's.

In that case, the dense white dwarf might gravitationally direct the AGB's outflow in it's direction but, if the outflow velocity is great enough much of its material might not be accreted.

This also raises the question of possible role reversal of binary stars. In this case, the the current companion star might have been the primary star when both star were at their greatest mass. The more massive primary would have entered its AGB phase sooner, producing a smaller white dwarf. At that point (the observed condition of R Sculptoris) the AGB star would be the more massive primary with a white dwarf companion. R Sculptoris, would likely then produce the small white dwarf: the current companion would once again become the more massive primary of the two white dwarf binaries. This process would make for some very interesting gravitational interactions, especially during periods of transition...
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