Red Giant Brightness Variations Still Mysterious

Article written: 7 Dec , 2009
Updated: 24 Dec , 2015
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Like everything else in the Universe, stars get old. As they become older, stars like our own Sun “puff up”, becoming red giants for a period before finally settling down into white dwarfs. During this late period of their stellar lives, about 30% of low-mass red giants exhibit a curious variability in their brightness that remains unexplained to this day. A new survey of these types of red giants rules out most of the current explanations put forth, making it necessary to find a new theory for their behavior.

Red giants are a stage in the later part of a Sun-like star’s life when most of the fuel powering nuclear fusion in the core of the star is exhausted. The resulting lack of light pressure pushing out against the force of gravity causes the star to collapse in on itself. When this collapse occurs, though, it heats up a shell of hydrogen around the core enough to reignite fusion, resulting in an increase in nuclear fusion that causes the star to become bigger due to the increased light pressure. This can result in the star becoming 1,000 to 10,000 times more luminous.

Variability in the light output of red giants is natural -they swell up and shrink down in a consistent pattern, resulting in brighter and dimmer light outputs. There is, however, a difference in the brightness of roughly a third to one half of these stars that happens over longer time periods, to the tune of up to five years.

Called the Long Secondary Period (LSP), the changing brightness of the star happens over longer timescales than the shorter period pulsation. It is this long-term variation in brightness that remains unexplained.

A new detailed study of 58 variable red giants in the Large Magellanic cloud by Peter Wood and Christine Nicholls, both of the Research School of Astronomy and Astrophysics at the Australian National University, shows that the proposed explanations of this mysterious variability fall short of the measured properties of the stars. Nicholls and Wood used the FLAMES/GIRAFFE spectrograph on ESO’s Very Large Telescope, and combined the information with data from other telescopes like the Spitzer Space Telescope.

There are two leading explanations of the phenomenon: the presence of a companion object to the red giants that orbit in such a way to change their brightness, or the presence of a circumstellar dust cloud that somehow blocks the light coming from the star in our direction on a periodic scale.

A binary companion to the stars would change their orbit in such a way that they would approach and recede from the vantage point of the Earth, and if the companion passed in front of the star it would also dim the light streaming from the red giant. In the case of a binary companion, the spectra of the brightness change among all of these stars is relatively similar, meaning that for this explanation to work, all of the red giants exhibiting the LSP variation would have to have a companion of a similar size, approximately 0.09 times the mass of the Sun. This scenario would be extremely unlikely, given the large number of stars that show this brightness variation.

The effect of a circumstellar dust cloud could be a possible explanation. A cloud of circumstellar dust that obscures the light from the star once per orbit would dim its light enough to explain the phenomenon. The presence of such a dust cloud would be revealed by an excess of light coming from the star in the mid-infrared spectrum. The dust would absorb light from the star, and re-emit it in the form of light in the mid-infrared region of the spectrum.

Observations of LSP stars show the mid-infrared signature that’s a telltale sign of dust, but the correlation between the two doesn’t mean that the dust is causing the brightness variation. It could be that the dust is a byproduct of ejected mass from the star itself, the underlying cause of which could be associated with the change in brightness.

Whatever the cause of the oscillation of brightness in these red giants may be, it does make them eject mass in large clumps or in the form of an expanding disc. Obviously, further observations will be necessary to track down the reason for this phenomenon.

The results of the observations made by Nicholls and Wood have been published in The Astrophysical Journal. Two articles describing their findings are available on Arxiv, here and here.

Source: ESO, Arxiv papers

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20 Responses

  1. SuperKevin says

    “a third to one half of these stars”

    Wow, I can’t wait for an explantion of why this is happening.

  2. Member
    IVAN3MAN says

    No doubt that our resident “Electric Universe” proponent will explain everything! πŸ™„

  3. Trippy says

    Question regarding the floowing sentence:

    This can result in the star becoming 1,000 to 100,00 times more luminous.

    Is that a dropped zero, or a misplaced comma?

  4. Trippy says

    (and of course I transpose my L’s and O’s in my own post)

  5. Lawrence B. Crowell says

    Those planetary nebula, such as the highly symmetrical Cat’s Eye nebula appear to be due to waves of material blown off with each oscillation of the red giant. This seems to be a manifestation of the osicllating pressure model. As for this longer variation in brightness, we need more data on that.

    LC

  6. Hon. Salacious B. Crumb says

    Very confusing story here.

    What about the vast majority of red giants that show no variability at all?

    Theo other question is what is happening during the AGB (asymptotic giant branch) is highly complex. The outer expanded atmosphere of the star is fairly tenuous and general behaviour is subject to internal and external forces acting on it. No doubt the causes of the variations in brightness are a combination of multiple effects and not one tell all solution.

    The question here is; Are all red giant long-term secular variables?

  7. Hon. Salacious B. Crumb says

    Lawrence B. Crowell said;

    “Those planetary nebula, such as the highly symmetrical Cat’s Eye nebula appear to be due to waves of material blown off with each oscillation of the red giant. This seems to be a manifestation of the osicllating pressure model.”

    No quite. The loss of material during the AGB phase varies by the rate of the superwind emanating from the star. Mostly they form large halos, whose dissipation varies from the equatorial to poles. These is almost certainty influenced by the rotation of the star. The “puffing” from the AGB star converting into the PNN (Planetary Nebula Nucleus) is not like a switch that is instantly turned off, but is likely an intermittent affair. Its influence is to causes variations in the superwind strength.
    When the PNN finally switches on an the white dwarf inner atmosphere is exposed (the white dwarf surface, as often imagined, is never actually exposed but is a hot dense atmosphere of hydrogen, helium, or carbon or oxygen – depending on the mass) the UV illuminates the gas by fluorescence, exposing the evolutionary behaviour of the AGB star just before it died.
    I’d say the expulsions during the earlier red giant phase are long gone into interstellar space. The gas visible in planetary nebulae are 2.25 solar masses, it is about 1/1000 of a solar mass (10^-4). Star below <2.25 tend to loss mass more slowly.

  8. Lawrence B. Crowell says

    It has been a while since I thought about these matters. A standard middle of the main sequence star is a red giant for only a few million to maybe 100 million years. so maybe that is long enough for any material blown off in these oscillations to be disippated. So planetary nebula are the result of a last bursting out or so of the red giant. I had labored under the impression for some time that these types of red giants, even though called nova, tend to more puff up and fade out over time.

    Thx. LC

  9. Hon. Salacious B. Crumb says

    Lawrence B. Crowell said;

    So planetary nebula are the result of a last bursting out or so of the red giant. I had labored under the impression for some time that these types of red giants, even though called nova, tend to more puff up and fade out over time.

    No, theory is well past this notion. There is no final bursting out (like the nova) in planetary nebula. The nebula is passively ejected by the last superwind event, and is then excited by the appearance of the PNN.

    In the last stage of the stellar evolution process, the internal thermal pressure on the outer convection region becomes so much great, that significant mass loss occurs, producing powerful stellar winds – spewing much of the outer atmosphere out into nearby interstellar space. These strong winds persist for less than several thousand years. This very short period of time is called the Pre-Planetary Nebula (PPN) phase. One of the first example, CW Leonis ‘Frosty Leo’, which was discovered in 1971.

    Regardless, novae or caused by thermonuclear ignition of the dense atmosphere of white dwarfs – nearly all being in very close binary systems. There is no such corresponding ‘fiery’ ignition in planetary nebulae.

  10. Member
    Jon Voisey says

    Lawrence:

    If you’re interested in a more comprehensive review of planetary nebulae, I wrote one of my term papers on the subject a few years back. Given how broad of a topic it was, I couldn’t get into great depth on it so it should be pretty manageable, even without a deep understanding of the topic.

    Here’s a link to one of my blog posts that contains the material: http://angryastronomer.blogspot.com/2006/12/on-planetary-nebulae.html

  11. Hon. Salacious B. Crumb says

    Jon,
    This link of yours is not a bad approach to explaining PNe. As a comment it is interesting your discussion doesn’t attempt to join PNe theory and that of the bipolar PNe. More often than not the deeper explanation of the emission lines is mostly avoid because of the target audience. Also the proof of the Rossland Theorem is well into advanced technical studies – even by astrophysicists standards.

    In the end it is different than the more traditional “visual approach” I’ve seen on a number of occasions (and even have done myself)

    ** My recommendation for those interested in the technical side is “The Physics and Dynamics of Planetary Nebulae” by Grigor A. Gurzadyan (1997) published by Springer (Chapter 9 is especially useful) Like most text, the role of bipolar PNe is poorly covered, but this has been only because of advances in recent years.)

  12. Jon Hanford says

    @ HSB Crumb & Jon,

    A few days back, Sun Kwok published a paper on arXiv.org that specifically looks at planetary nebulae with bipolar and multipolar features ( “Morphological Structures of Planetary Nebulae” http://arxiv.org/PS_cache/arxiv/pdf/0911/0911.5571v1.pdf ). While this appears to be an overview paper (math-free), Kwok puts forward the hypothesis that the visible bright regions in the lobes of bi/multipolar PNe are both low density and illuminated by UV photons from the central star. Any thoughts on this mechanism?

    Btw, thanks, Jon, for the link to your paper and presentation.. Hopefully you were rewarded for your efforts at school. Nice job πŸ™‚

  13. vino says

    will someone recommend a book to study the stellar cycle in detail ??

  14. vino says

    Jon,
    Thanks for a wonderful link…

  15. Lawrence B. Crowell says

    So based on what is being said above I was originally not too far off in my estimate of how stars age and produce nebula. When it comes to stellar astrophysics I am not exactly much of an expert.

    LC

  16. Hon. Salacious B. Crumb says

    @ Lawrence

    Actually, you were not far of at all. Stellar astrophysics is a minefield, because thing regarding what is in vogue or not in vogue. Hell I just have trouble keeping up with the lingo and what all the actual abbreviations mean!
    There are a lot of misconceptions about planetary nebulae, and even general text on the subject is often in conflict.
    Suggest you do have a read of Sun Kwok’s article – he’s been selling the subject for years!

    Cheers

  17. Lawrence B. Crowell says

    I am looking at Kwok’s paper. It is not really easy reading for me. In fact I find reading a paper on Grothendieck’s category theory on algebraic varieties of sheaves easier reading. Yet I suppose I am somewhat on track.

    As I understand things the fusion process occurs on a shell that encloses the spent core, which collapses and provides more gravitational pressure on the fusion shell. Eventually the shell “migrates” outwards to the point such pressure can’t be sustained in a static or stationary way. So it is natural to suppose this instability will at first manifest itself in oscillations. The fusion expands the outer layers out, slowing down the fusion, then the outer layers fall back down to initiate fusion, where radiation then again expands the outer layers and so forth.

    LC

  18. Jon Hanford says

    @ vino,

    A good intro book on stellar evolution (a classic) is James Kaler’s “Stars and their Spectra: An Introduction to the Spectral Sequence”. Earlier editions served me well when I attended school.

    @ Lawrence Crowell,

    As pointed out in the Kwok paper (and by HSB Crumb), the jury is still out on the precise mechanism(s) responsible for planetary nebulae formation. Perhaps we will need to await precise kinematic mapping of planetary nebulae to tease out the predominant factors involved.

  19. Hon. Salacious B. Crumb says

    Lawrence said;

    “As I understand things the fusion process occurs on a shell that encloses the spent core, which collapses and provides more gravitational pressure on the fusion shell. Eventually the shell “migrates” outwards to the point such pressure can’t be sustained in a static or stationary way. So it is natural to suppose this instability will at first manifest itself in oscillations. The fusion expands the outer layers out, slowing down the fusion, then the outer layers fall back down to initiate fusion, where radiation then again expands the outer layers and so forth.”

    This is correct during the AGB phase, and are called thermal pulses.
    At the planetary nebulae stage, with the UV energy generated is mostly radiated is by the exposed WD core Fusion has then completely stopped and all you are left with is the hot burnt-out remnants. Fusion cannot start again unless there is a source to feed it.

  20. jan hoos says

    im curious. could stars coal core split in two components which collide and separate again in same period?

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