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Metals in ancient stars

  1. Nov 16, 2011 #1
    I was reading news article: http://www.sciencedaily.com/releases/2011/11/111115095911.htm
    Article explains in beginning that when star explodes as supernova, heavier elements are formed which later become part of new generations of stars. And in concluding paragraph, article says "reason why some of the old stars became abnormally rich in heavy elements must therefore be that exploding supernovae sent jets out into space".
    In the news, I was expecting to find how oldest stars got some metal contents (Since there were no stars present to go supernova). But if they say that supernova is the reason then I wonder what is the news.
    Am I missing something?
     
  2. jcsd
  3. Nov 16, 2011 #2

    D H

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    The Niels Bohr Institute press release that is the source of the ScienceDaily article:
    Some of the oldest stars in the Milky Way – a kind of stellar fossils in the outer reaches of our galaxy, contain abnormally large amounts of heavy elements like gold, platinum and uranium. Where these large amounts came from has been a mystery for researchers, since they are usually seen in much later generations of stars. ...​
    For more, see http://www.nbi.ku.dk/english/news/n...ed_light_on_the_prehistory_of_the_milky_way_/

    The underlying paper is Terese Hansen et al. 2011 ApJ 743, "The Binary Frequency of r-Process-Element Enhanced Metal-Poor Stars and its Implications: Chemical Tagging in the Primitive Halo of the Milky Way," published in the November 2011 issue of The Astrophysical Journal.
    Article link (subscription): http://iopscience.iop.org/2041-8205/743/1/L1/
    arXiv preprint: http://arxiv.org/abs/1110.4536

    __________________________________________________

    First off, those halo stars are not the oldest stars. The very first stars lived very briefly due to their (hypothetical) very large size and zero metallicity. None of those very first stars are around anymore. We will have to look very far away with telescopes much more powerful than those of today to see those very first stars.

    Those halo stars presumably formed from a mix of the primordial interstellar medium polluted by remnants of that first generation of stars. Because of the utter lack of metallicity in those very first stars, the supernovae deaths of those first stars would not have created elements beyond iron. Seeing elements such as uranium in those halo stars is a bit problematic. Scientists do see small amounts of these very massive elements in a small percentage of these halo stars. So, how to explain it?

    One explanation is that these halo stars with abnorminally-high metallicity were painted by the supernova remnants when some more massive second generation star died. The Type IA supernova is one obvious candidate. Another candidate is a core collapse supernova that ejected some of its material in the form of a jets. The article ruled out the first as a possibility, leaving being painted by a collimated jet as the most likely alternative.

    That still doesn't answer your question. Those halo stars are old because they formed a long time ago (duh) and because they are small (this is key). The star production regions that produced those halo stars almost certainly would have produced more massive as well. So why can't we see those larger, older stars? The answer is simple: The bigger a star is, the shorter its life span. We only see small stars in the galactic halo because the larger stars that formed along with those smaller ones we see now would have gone supernova long ago.
     
  4. Nov 16, 2011 #3

    Drakkith

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    Why is it that no elements heavier than Iron are capable of being made in the very first stars? (Or rather their supernova)
     
  5. Nov 16, 2011 #4
    It is my understanding that once the core of a star starts to create iron it quickly collapses in on itself and goes supernova because of the mass of the iron and no longer has the energy to convert the iron to anything else.

    I am a novice and may be wrong but this is how I understand it.
     
  6. Nov 16, 2011 #5
    Thank you DH.


    The Type IA supernova is one obvious candidate. Another candidate is a core collapse supernova that ejected some of its material in the form of a jets


    That means, smaller halo starts were born without heavy metal content, but they lived longer than large halo stars and small stars gathered some metal content after formation from jets ejected from large halo stars that went supernova.​

    Why is it that no elements heavier than Iron are capable of being made in the very first stars? (Or rather their supernova)
    Actually even Nickel-56 is also created in stars but it has very short half-life i.e. 6.077 days. Even heavier elements (Zinc) are not formed because fusion of Ni-56 require more energy than released in fusion because of higher binding energy of Ni-56. ​
     
  7. Nov 16, 2011 #6

    Drakkith

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    A supernova releases a tremendous amount of energy. Elements heavier than Iron are all made in the supernova process when the outgoing shockwave forces nuclei to fuse together into heavy elements.

    Of course, but I am referring to why elements cannot be created in the supernova of early stars like they are today.
     
  8. Nov 17, 2011 #7

    D H

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    Some caveats:
    1. This is not my area of expertise. I'm being an armchair scientist here.
    2. The consensus that has built up over the last couple of decades regarding population III stars appears to be unraveling a bit.

    This page, http://astronomy.swin.edu.au/cms/astro/cosmos/p/Population+III, summarizes the consensus view:
    The currently favoured explanation for the lack of observed Pop III stars, is that the Pop III generation of stars were all high mass stars, with masses ranging from 60 to 300 times that of the Sun. In other words, no low mass Pop III stars were ever formed. This is supported by recent theoretical models which show that primordial stars possessed much higher masses than the stars we see in the Universe today. If this bias in the mass distribution of primordial stars is the case, then all Pop III stars would have exhausted their fuel supplies long ago and would now be present only as remnants.​

    That consensus is falling apart a bit. The article cited in the original post looked at halo stars with abnormally high metallicity. Per the consensus view, there shouldn't be any halo stars with extremely low metallicity. The very recent article Caffau, E. et al. "An extremely primitive star in the Galactic halo." Nature. 477 (7362): 67-69 http://www.nature.com/nature/journal/v477/n7362/abs/nature10377.html says otherwise.


    One thing that is certain, assuming the big bang theory is correct, is that the CNO cycles that dominate in massive population I stars would not have been present in population III stars until they started burning helium. Up to the point of helium burning, the proton-proton chain would have been essentially the only fusion process in the most massive of those population III stars. This alone makes those population III stars behave quite different than population I and II stars. The temperatures and densities in large population III stars (per the consensus view, all population III stars), could have resulting in helium burning starting before hydrogen depletion. Now the CNO cycle is available, and the resulting nova would have been quite intense. Some of those stars may not have survived past this event.

    If they did survive and progress toward an even more calamitous death, that death may have been by means other than core collapse. The largest of those population III stars, 250 to 300 solar masses or more, would have formed a hypernova before they started producing iron in their core. Slightly less massive stars would have formed a pair-instability supernova, once again before they started producing iron in their core. Hypernova and pair instability supernova could have formed up to iron (and nickel and geranium), but because their wasn't an iron core already present, that is mostly where things would stop.


    This consensus view does explain the metallicity in most of the population II halo stars. The ratio of iron to carbon/oxygen to iron is much lower than in population I stars. The nucleosynthesis processes that formed the metals (lithium and up) that constitute those population II stars was somehow different than the nucleosynthesis that occurs in the core collapse supernova of population I stars.
     
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