Posted on by Mark Thompson

Stars Shrouded in Glittering Zirconium Light up the Sky

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Its been said that the Universe isn’t stranger than you can imagine, its stranger than you can’t imagine. Nowhere is this more true than the study of stars. Recently, a team of scientists from the Armagh Observatory in Northern Ireland have discovered a star that is enveloped by clouds of glittering zirconium! Its a metal you might be more familiar with in jewelry to make false diamonds but it now looks like stars are getting in on the act and becoming more sparkly than they are already.

The research team, led by graduate student N. Naslim and her supervisor Dr. Simon Jeffrey, were looking for clues to the lack of hydrogen on the surface of helium rich hot subdwarf stars, when compared to other similar stars. Using the 3.9m Anglo-Australian telescope at Siding Spring Observatory in New South Wales, the study focused on a star called LS IV-14 116 which lies at an incredible distance of 2000 light years.

By using a spectroscope attached to the telescope, the team was able to split the incoming starlight into its component parts (much like water droplets in the atmosphere do to sunlight to make a rainbow). Along with the expected patterns which showed the presence of certain elements, they were surprised to find lines in the spectrum which were not so easily identified. A careful study showed the lines were due to the presence of a form of zirconium that should only exist in temperatures in excess of 20,000 degrees. This was a first, no zirconium of this type had ever been found in a stellar spectrum before.

Team member Prof. Alan Hibbert built a computer model that enabled them to deduce that the zirconium existing on LS IV-14 116 was some ten thousand times more than the concentration found in the Sun. This highly unexpected result led the team to conclude that the abundance of zirconium is caused by the formation of cloud layers in the star’s atmosphere.

“The star doesn’t have a corona like the Sun. Our model shows the huge excess of zirconium that we discovered is on the photosphere (the visible ‘surface’ of the star), where it forms cloud layers much like stratus clouds on Earth.” Naslim told Universe Today. It seems that other elements, chiefly metals heavier than calcium, seem to form in high concentrations too but seem scarce in layers above and below. This could have a dramatic effect according to Dr. Natalie Behara from the Université Libre de Bruxelles appearing as many thin cloud layers in the atmosphere, each due to a different metal.

Further work from the team suggests that the star is shrinking from a bright cool giant to a faint hot subdwarf and as it does, different elements sink or float up in the atmosphere making the current composition very specific to the star’s recent history.

Naslim explains that “The huge excess of zirconium was a complete surprise. We had no reason to think this star was more peculiar than any other faint blue star discovered so far.” Its great to see that whilst we know so much about the Universe now, there are still discoveries that come along and surprise us. This latest discovery of zirconium rich stars has yet again shown us that we mustn’t become complacent and think we know everything, it keeps science interesting, it keeps it alive.

Source: from the Royal Astronomical Society.

Mark Thompson is a writer and the astronomy presenter on the BBC One Show. See his website, The People’s Astronomer, and you can follow him on Twitter, @PeoplesAstro

15 Replies to “Stars Shrouded in Glittering Zirconium Light up the Sky”

  2. This is a star also known as ALS 11334, which is a 12.6B, 13.03v magnitude star in the constellation Capricornus. It is a sdO type star (first found out in 2003). LS IV – 14 116 is Helium-rich, by B.C. Reed in “UBV beta database for Case-Hamburg Northern and Southern Luminous Stars.” (2005) and his subsequent catalogue which comes from “Photometry and Spectroscopy for Luminous Stars.“
    It was A. Ahmad & C.S. Jeffery, “Discovery of pulsation in a helium-rich subdwarf B star“; A&A., 437, L51 (2005), who found the star was variable, in which it came to prominence as unusual — being a so-called extreme horizontal branch star or EHB star. They are known to be hot compact fast pulsating stars whose main period is about 150 seconds or so. Due to these characteristics, they seem to be related to the V361 Hya stars discovered in or the PG1716 stars. (although no the same star, the paper by Schuh, S. “Discovery of a Long-Period Photometric Variation in the V361 Hya Star HS 0702+6043“, ASPC, 334,. 530 (2005) gives an excellent summary of the observational evidence of the fast oscillations expected in this new class of stars. (They also expectedly have high gravitational redshifts.) The originally nature of this particular EHB (LS IV – 14 116) was discovered as being significant by Viton et.al.”The Spacelab-1 very wide field survey of UV-excess objects. II. Analysis of 7 stars of various nature.” A&A., 242, 175 (1991)
    These are consider quite common stars, and were once featured in a Royal Astronomical Society Meeting ion their evolution in June 2003 held at Keele University in Staffordshire, England. One of the key points of the discussion is these stars display characteristics of asteroseimology, and have very turbulent atmospheres, whose velocities can exceed 200 metre per second. Some speculate they are formed by mergers of white dwarfs, though this is not clear. They seem similar to the Blue Horizontal Branch (BHB stars) which feature in globular clusters (BHB stars are also possibly formed by mergers.)
    The zirconium (Zr) here is visible because of the dredging from the core and then it is convected to the surface — probably some rare circumstances in the nucleosynthesis process.

    1. One small clarification. When I said; “These are consider quite common stars“, I was talking about sdO stars and not these rare-metal element stars.
      Also the star V361 HYa is of spectral type sdB (‘sd’ meaning subdwarf), and you can generally read about these kind of stars in the wiki article; Subdwarf B star“ (Look at “Pulsating white dwarf”
      Another interesting thing about these lightweight white dwarfs, is that they may not go through the planetary nebulae stage, as when they were single, they were >0.6⊙ Solar Masses. These are different than the ZZ Ceti type white dwarfs, which also pulsate but are instead main sequence versions. (These are of spectral class DAV and DBV, and have a significant range of varieties and subtypes. These too show some seismic activity, oscillating over minutes to hours, in numerous modes (overtones?)

      ★ I’ve once gave a lecture on white dwarfs a few years ago, whose interest in them comes mostly from interests in planetary nebulae

      1. Oops! “…they were >0.6? Solar Masses”, that is to say, theprogenitors were less than 0.6 Solar Masses. Energy wise, they did not emit enough UV energy to illuminate the often small amount of expelled nebulosity.

        Also the biggest question here; “Is where does the Zirconium comes from?” As single stars, at least in theory, should only be mostly Helium, as displayed in their atmospheres and most of their cores. Merging white dwarfs complicates things, as the original white dwarfs had to either expel a high percentage so they can stay so small; or instead, were even smaller stars in the first place I’d speculate, that these exotic metals were likely produced from the significant increased energy gained during this merger process.

        I’ll be interested in the future what the explanations the astrophysicists come up with!

    1. It is a hot subdwarf, sdB class. They are helium-core burning stars on the Extreme Horizontal Branch: they lost most of their H envelope, and are evolving bluewards from the HB to the white dwarfs, skipping the AGB phase.

      1. lacalaca
        I did said that a one and two day ago…
        It just proves proves that new UT system here is very confusing. You can write the last post and look like you have been preempted by just making a reply to an earlier remark!
        Comment therefore are not linear.

    1. Someone will have to break that legend… I remember actually emailing the BBC once about a similar story (to no avail) and a number of websites. Maybe even commenting on UT!
      It’s just that there is a huuuuge difference between crystallized carbon *atoms* which you might call diamond, and crystallized carbon *nuclei* which you sure can’t.
      Similarly neutron star crusts are not ‘iron’. And there is no ‘hydrogen’ nor ‘helium’ nor any other element in the core of stars, only nuclei of these elements.

  6. So this is a different sort of star. The zirconium emerges from the core merging of two white dwarves. This sounds like a pretty extreme sort of star. This is one of those things about stars, there always seem to be different types of stars that I have not heard about.

    LC

  8. Apart from zirconium, the elements yttrium, strontium and germanium were also detected in this star. From a quick scan of the paper, all these elements with the exception of germanium have been detected in other stars (pls correct me if I’m in error); the detection of this species of zirconium [Zr IV] in *optical* spectra of stars IS a first. The authors propose radiatively-driven diffusion combined (possibly) with strong magnetic fields might be responsible for the extreme overabundances seen in this star. They go on to discuss possible formation scenarios (WD mergers being one possibility) and the role variability may play in the unusual chemistry (no established links yet).

    “An extremely peculiar hot subdwarf with a ten-thousand-fold excess of zirconium, yttrium, and strontium” is available here: http://arxiv.org/PS_cache/arxiv/pdf/1010/1010.5146v1.pdf

    @Crumb,

    Thanks for the background history you posted on these stars and LS4 – 14 116 in particular. I haven’t really studied them in any detail.

  9. There are a couple of mysteries it seems here. These sdO stars are thought to have formed by the merger of .6M(sol) sub-dwarf stars. One question I have is how is it that such small stars reached that phase of their evolution? There were likely formed by red dwarf stars which have extraordinarily long life spans. There is also the question on the nucleosynthesis of these moderately heavy elements. The progenitor was likely mostly hydrogen and helium, so the hypothesis that these elements were formed in the merger might be reasonable. I would find it more likely the progenitor of these stars were heftier white dwarf stars that blew off lots of material in the merger.

    LC

    1. Some quite relevant comments here.

      In a nutshell, that is exactly the problem, and the only solution seems to be that these stars have had some kind of enhanced evolution. Against the cosmological limitations (the stars cannot be older than the universe c. 13.7 billion years) and the nucleosynthesis of metals (‘Z’).
      I agree with your view of the expected ‘purity’ of these types, but one thing you might consider is that these stars were once very close binaries, and during their evolution, the progenitors likely had significant mass exchange — effectively acting like a much bigger star. The mergers possibly happens when the stars have become white dwarfs, or late during the AGB phase (Asymptotic Giant Phase), so that interaction again enhances the ‘metal’ nucleosynthesis. (I’d think that the progenitors might have been bigger than ‘normal’, and instead had a greater mass loss during the AGB phase.) Such situations are far more unusual compared to run-of-the-mill single stars.

      For what I’ve read, you comment here surmises well the issues that have to be solved. Nonetheless, it is an interesting topic for future astronomers.

      (IMO, it is a pity stellar evolution is so mainstream. Common stories on the subject are often not really that exciting, but I think if others knew the basics, they might have a better understanding of these kinds of stories. I’ve considered perhaps contributing a few basic Universe Today stellar evolution stories (instead of bleating about the stories presented here.)

      Cheers

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