≡ Menu

Aftermath: Debris Disk around a Red Giant

Debris disks around young stars are keys to understanding how planets form. But what about debris around older stars? We now have the best view ever achieved of the dusty disk around an aging star — a red giant — and we’re forced to ask whether such a debris disk, so similar to what we see around young stars, could itself form a second generation of planets. The star in question is a binary designated IRAS 08544-4431, first detected by the Infrared Astronomical Satellite in the 1980s. Some 4000 light years from Earth in the direction of the southern constellation Vela (The Sails), the system contains a red giant (the source of the material in the surrounding disk) and a much smaller, less evolved companion star.

eso1608a

Image: The dusty ring around the aging double star IRAS 08544-4431. Credit: ESO.

Here we’re working with data from the Very Large Telescope Interferometer at the European Southern Observatory’s Paranal Observatory (Chile) using the PIONIER (Precision Integrated-Optics Near-infrared Imaging ExpeRiment) instrument that draws out the full power of the VLTI. Jacques Kluska (University of Exeter, UK) explains what we are looking at:

“By combining light from several telescopes of the Very Large Telescope Interferometer, we obtained an image of stunning sharpness — equivalent to what a telescope with a diameter of 150 metres would see. The resolution is so high that, for comparison, we could determine the size and shape of a one euro coin seen from a distance of two thousand kilometres.”

Have a closer look at the debris disk here. The inner dust rim of the circumbinary disk corresponds to where we would expect the dusty disk to begin, with any dust closer to the two stars evaporating in the flux. While there are a number of debris disks associated with nearby young stars, we have not until now had a detailed look at the disks around old stellar systems like this one. Our first comparison between the two types of disk shows that they look remarkably alike even though they form at opposite ends of the star’s life cycle.

eso1608b

The red giant in this system is an asymptotic giant branch (AGB) star (the ‘asymptotic giant branch’ is that part of the Hertzsprung-Russell diagram containing low- to medium-mass stars that are highly evolved; i.e., red giants that emerge late in the lifetime of the star). As for the binary companion, the researchers, led by Michel Hillen and Hans Van Winckel (Instituut voor Sterrenkunde in Leuven, Belgium) believe that it shows its own accretion disk. Hillen refers to “…a fainter glow that is probably coming from a small accretion disc around the companion star,” and adds, “We knew the star was double, but weren’t expecting to see the companion directly.”

The paper makes the case for the second accretion disk. In the passage below, Hα refers to the H-alpha spectral line for hydrogen. P Cygni is a hypergiant luminous blue variable (LBV) star in the constellation Cygnus to which this system is being compared:

IRAS 08544-4431 is the first post-AGB binary system in which direct emission from the secondary is detected, and even spatially separated from the primary. There are two hypotheses to explain the high companion flux at 1.65 µm: thermal emission from the surface of a 1.5−2.0 M red giant or emission from a compact accretion disk around a 1.5−2.0 M main-sequence star. We consider the second case more likely because observations of similar post-AGB systems indicate that circum-companion accretion disks may be common… The main evidence comes from the detection (in Hα and for more inclined systems) of fast outflows that originate from the companion. The Hα line of IRAS 08544-4431 has a P Cygni-like profile… which is consistent with this interpretation, given that we find the system to be viewed close to face-on.

Can a red giant form new planets from the debris disk that surrounds it? And what about surviving planets around an evolving red giant? These are interesting speculations, and they call to mind a 2005 paper from Bruno Lopez, Jean Schneider and William Danchi titled “Can Life develop in the expanded habitable zones around Red Giant Stars?” Here’s food for thought:

For a 1 M star at the first stages of its post main-sequence evolution, the temporal transit of the habitable zone is estimated to be of several 109 years at 2 AU and around 108 years at 9 AU. Under these circumstances life could develop at distances in the range 2-9 AU in the environment of sub-giant or giant stars and in the far distant future in the environment of our own Solar System. After a star completes its first ascent along the Red Giant Branch and the He flash takes place, there is an additional stable period of quiescent He core burning during which there is another opportunity for life to develop. For a 1 M star there is an additional 109 years with a stable habitable zone in the region from 7 to 22 AU.

We tend to rule out red giant stars when it comes to possible life, but perhaps we’re jumping to one conclusion too many. In any event, the new work on the IRAS 08544-4431 system reminds us we’re early in the game when it comes to understanding what goes on in this environment.

The paper on IRAS 08544-4431 is Hillen et al., “Imaging the dust sublimation front of a circumbinary disk,” Astronomy & Astrophysics 588, L1 (2016). Preprint available. The Lopez, Schneider and Danchi paper on red giants is “Can Life develop in the expanded habitable zones around Red Giant Stars?” Astrophysical Journal Vol. 627, No. 2 (2005). Abstract / preprint.

tzf_img_post

Comments on this entry are closed.

  • Joe March 10, 2016, 14:57

    No mention of pulsar planets? Last I heard on PSR B1257+12 (no I won’t use the ridiculous new IAU names), the thinking was they must have formed from a secondary accretion disk, perhaps what was left if a companion star had been stripped to spin up the pulsar (now a millisecond pulsar but too old to have started as one).

    • Paul Gilster March 10, 2016, 16:43

      As far as I know, pulsars aren’t associated with this class of red giant star. You need higher stellar masses to produce the needed supernova. Most AGB stars end up as white dwarfs.

      • Ashley Baldwin March 10, 2016, 17:41

        Yes. Generally above ten times the mass of the Sun. Asymptotic giant branch stars ,AGBs ,are crucibles for element formation though , by the “s” or slow process by which neutrons bombard a seed element , iron , in their fusion shell around an inert helium stellar core ,followed by beta decay to produce a proton and increase their atomic number up to the maximum size nucleus of bismuth. Over thousands of years, but critically spread by the subsequent stellar planetary nebula as the loosely held outer layer of the star dissipates over time.
        This as opposed to the “r” or rapid process whereby a vast amount of neutrons are released by the collapse of a greater than ten times mass of the sun star in a supernova and by which elements up to Uranium are created in the blink of an eye. AGBs are notoriously “dusty” because of the elements and minerals , particularly silicates , that they create as their loosely held outer layers disperse . All going to provide planet producing material both locally as shown here but also across these surrounding Galaxy in later generations of stars and protoplanetary disks .

    • andy March 19, 2016, 18:21

      I’ve always quite liked Gliese 86b as a candidate “second-generation planet”. It’s a giant planet in a 16-day orbit around a K-dwarf, with a companion white dwarf star at ~20 AU.

  • Andrew Palfreyman March 10, 2016, 16:39

    Saving our Earth from the forthcoming redgiantly barbeque means elevating its orbit slowly over time. Using rough numbers:
    Establishing a 2 AU orbit requires 10^33 Joules, which over 1 BY equates to an average power of 50 PW. Current insolation is 100 PW.

  • Ivan Vuletich March 10, 2016, 20:10

    Has a debris disk or planets been detected around Arcturus, according to Wikipaedia its the closest low mass giant star to Earth.

  • Stan Erickson March 11, 2016, 1:46

    You need angular momentum to form planets, and a star only retains a few percent of the original angmom in the originating cloud. When you have a binary, the other star can dump angmom into the disk, but for normal red giants, don’t waste your time looking for planets.
    http://stanericksonsblog.blogspot.com/2016/03/planetary-disks.html

    • Ashley Baldwin March 11, 2016, 12:02

      Fair point. How does it explain planets around pulsars though?

    • Michael March 11, 2016, 15:28

      Worlds in larger orbits can accreted material from the shroud of material the red giant gives off, planets with nicely orientated magnetic fields could collect a large amount of material perhaps fattening their moons up substantially as well.

      http://arxiv.org/abs/1505.02614

  • Ashley Baldwin March 11, 2016, 12:16

    Traditional planetary formation around a pre main sequence star occurs quickly in astronomical terms , a few million years or so max or less for rocky planets ,before ZAMs when high UV output and stellar wind from the nascent star drive off the remaining gas before it can be accreted . Driven by angular momentum and interaction but not for long. Other factors play a smaller role such as disk density and duration as well as temperature.

    In the case of any disk surrounding an AGB could these later factors play a far more significant role by being greater especially with the far lower UV output from the star ? The nature of the disk will be different too with high levels of silicate dust in particular rather than the gas and ices of a traditional protoplanetary disk.

    Im not sure that sort of disk will have ever been modelled in detail. All helping compensate for the lack of angular momentum.

  • Michael March 13, 2016, 12:31

    In the red giant stage the star can lose a lot of material, 10 to 50%, that’s is ~30 to ~150 thousand earth masses of material. If an alien race is smart enough it could capture some of this material to make habitats and return around the white dwarf later.

    http://www.ast.cam.ac.uk/~pettini/STARS/Lecture13.pdf

    I like this bit,

    ‘the energy generation rate of the triple alpha reaction has an extraordinarily steep dependence on T: E ~ Y^3 p^2 T^40. Thus, the rise in T leads to more efficient fusion, which in turn raises the T, and so on: a degenerate core that is ignited acts like a bomb! The thermonuclear runaway leads to an enormous overproduction of energy: at maximum, the local luminosity in the helium core is L 10^10L(sun)…!’

    The earth may fall outside of the red giant envelopment phase but it won’t survive the increase in luminosity.