Sun Loses Lithium with Age

Observations of two solar twins — one old and one young — confirm that the Sun has probably destroyed its lithium over time.

Two exposures combine to create this image of HIP 102152, located 250 light-years from Earth in the constellation of Capricornus. HIP 102152 is a close solar twin — except it's nearly four billion years older.
ESO / Digitized Sky Survey 2; Acknowledgement: Davide De Martin
I’ve blogged repeatedly here about the universe’s missing lithium. But lithium is also a troublemaker in the solar system. Based on primitive meteorites that record the makeup of the nebula from which the solar system formed, the Sun seems to have destroyed more than 99% of its initial lithium.

This missing lithium is a different problem than that of cosmic lithium. The old stars that are used to estimate the universe’s primordial lithium levels still have something like 150 times more of the isotope lithium-7 than the Sun does.

Astronomers have mused over how the Sun lost so much lithium for a while. Studies of other stars have suggested that the Sun might have destroyed its lithium as it grew older. A new study in the September 10th issue of Astrophysical Journal Letters supports that idea.

TalaWanda Monroe (University of São Paolo, Brazil) and colleagues used the UVES spectrograph on the Very Large Telescope to study the chemical makeup of two Sun-like stars, HIP 102152 (in Capricornus) and 18 Sco. They found that HIP 102152 is perhaps the closest solar twin yet studied — except for two things: its age and its lithium level.

HIP 102152 is 8.2 billion years old, 3.6 billion years older than the Sun and about to transition to its golden years, when it stops fusing hydrogen in its core. Its lithium level is really low, about one-fourth the Sun’s.

Conversely, 18 Sco is only 2.9 billion years old and has nearly four times more lithium than the Sun does.

Given the close agreement of the level of other elements in the three stars, age seems to be a key factor in this development. “We’re pretty sure that lithium is somehow destroyed as a star ages,” Monroe says. The team is currently looking at more than a dozen other solar twins of different ages to see whether the correlation holds up.

Lithium is a fragile element, destroyed at temperatures above about 2.5 million Kelvin. This temperature is much lower than that required to destroy other elements found in the Sun, such as carbon and oxygen. But it’s also more than 300 times hotter than the Sun’s surface. The lithium would need to sink deep into the Sun to reach such temperatures.

The problem is, the Ferris-wheel-like motion that carries material from the interior to the surface and back again shouldn’t drag lithium deep enough into the star to hit these temperatures, at least according to standard solar models. But this so-called convective zone doesn’t exist in isolation: it’s above the hot radiative core, which has different properties than the overlying convective layer. Astronomers have suggested various mechanisms to encourage mixing between these two regions and heat up the bottom of the convective zone, but the right explanation is hard to pin down. Given the new observations, the heating definitely looks like it’s happening.

This destruction — however it happens — probably has no bearing on the cosmic lithium problem, says Christopher Howk (University of Notre Dame): the stars used to measure the universe’s lithium levels shouldn’t have outer convective zones.

And while the authors raise the question of whether the low, solar-like levels of lithium and rocky-body elements in HIP 102152 suggest it might have terrestrial planets, there’s debate about whether that connection exists.

Watch the video the ESO put together to show how a Sun-like star will develop with age (from presolar nebula on left to red giant on right), with markers showing where the Sun and the other two stars lie:

Credit: ESO / M. Kornmesser

Reference: T. R. Monroe et al. "High Precision Abundances of the Old Solar Twin HIP 102152: Insights on Li Depletion from the Oldest Sun." Astrophysical Journal Letters, September 10, 2013.

9 thoughts on “Sun Loses Lithium with Age

  1. Henrik

    In 1954, the US detonated it’s second thermonuclear device codenamed Castle Bravo. The yield was 2½ times that expected and subsequent investigations found this was due to the 60% Litium 7 used in the Lithium Deuteride mixture, where the Lithium 6 was expected to contribute to the reaction, was not inert as expected. As you specifically mention Litium 7, would this in anyway be connected and how is Lithium generated in stellar nucleosynthesis in main sequence G stars?

  2. Bruce

    Henrik, both Li6 and Li7 aren’t “generated in stellar nucleosynthesis,” they are distroyed in these reactions. The first paragraph of the wikipedia article “Lithium burning” says: “Lithium is generally present in brown dwarfs and not in low-mass stars. Stars, which by definition must achieve the high temperature (2.5 × 10^6 K) necessary for fusing hydrogen, rapidly deplete their lithium. This occurs by a collision of lithium-7 and a proton producing two helium-4 nuclei. The temperature necessary for this reaction is just below the temperature necessary for hydrogen fusion. Convection in low-mass stars ensures that lithium in the whole volume of the star is depleted. Therefore, the presence of the lithium line in a candidate brown dwarf’s spectrum is a strong indicator that it is indeed substellar.”
    This wiki article also shows that Li6 in stars is converted Li7, which is then distroyed. The main source of Lithuim of both isotopes is believed to have been the Big Bang.

  3. Rod

    My observation – the model is dependent upon a number of assumptions about the initial chemical composition of the early Sun (which no one observed), including any initial lithium present. It is an assumption that lithium in meteorites was similar to the Sun. The two stars in this report have different [Fe/H] ratios compared to the Sun also. HIP 102152 [Fe/H] is about 97% solar value and 18 Sco [Fe/H} is about 1.135 solar value so differences in lithium abundance or other metals could be expected without age being the primary factor.

  4. Anthony Barreiro

    Rod, medical doctors have a diagnostic aphorism: "when you hear hoofbeats, think first of horses, not zebras." In other words, presenting symptoms are usually caused by boring common illnesses, not rare exotic diseases. The same heuristic seems helpful here. The inverse correlation between these stars’ ages and their lithium concentrations presents a plausible hypothesis deserving further exploration.

  5. Henrik

    Anthony, while I like your aphorism/heuristic, lithium cannot be a "Big Bang"-only generated element as our sun and suns similarly middle-aged and especially younger ones are formed from regurgitated stellar, not primordial, matter. Therefore, the younger the star, the less lithium it should have to begin life with. Furthermore, as lithium is easily burnt at H-fusion temperatures and not only is able to escape witgh the solar wind but theoretically be "burnt" during the passage through the corona. Thus we should end up with far less lithium than actually observed in ancient G-dwarfs just before they move off the main sequence.

    Ergo, stars must be able to at least partially replenish their Li during their lifetimes, i.e. it is part of stellar nucleosynthesis. Bruce, you are correct, the Wiki-articles do not mention this, but I am rather skeptical to Wiki-content as all that is required is the abilities to register and do a cut-and-paste job, peer-reviewed by people with the same skill-set but not leading astrophysicists who have better things to do with their times.

  6. Bruce

    Henrik, you made some interesting points, I hope you and others find these friendly counterpoints to be interesting as well. In general, your statement, “the younger the star, the less lithium it should have” is true. But you might be assuming that the ratio of star processed to primordial gas is higher than it actually may be. And even gas that has passed through one or more generations of stars can still posses primordial Li due to the fact that the top layers of massive stars do not mix with the deeper levels toward the core where the fusion occurs. I also doubt that much Li is burnt in stellar corona. Yes, the temps are high enough, but coronal gas is so diffuse and the reaction cross-sections are so small that Li destruction there is unlikely. But my main question is how could Li be produced in stars when it is destroyed so easily?

  7. Henrik

    Bruce I quite agree with your last statement. We, or rather I as a layman, feel that there is a conundrum here and to me, the article raises more questions than it explains – which is A GOOD THING! But still have an itch to know the answers, now!

  8. Bruce

    Henrik, I completely agree, which is why we follow these stories isn’t it. The puzzling conundrums with Lithium are caused by the fact that it takes a hotter temperature to produce it than it takes to destroy it. Lithium is in fact produced all the time in stars via (He3 + He4 => Li7), but in the conditions inside stars it is then promptly destroyed via Li7 + H => Be8, and then unstable Be8 decays into 2 He4s. Li6 can also form via He4 + D, but it is also destroyed. What’s needed to both form and to preserve Lithium is an environment where the temperatures and pressures start very high (like the BB) but also rapidly decline (also like the BB). Are there sites like this today? Yes, in the accretion disks and jets of black holes. See
    http://www.skyandtelescope.com/community/skyblog/newsblog/Universes-Lost-Lithium-159192855.html and the conversation after the article, which relates very much to this discussion. And Peter, elderly black holes exibit histories of once being quite bi-polar. :)

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