Recently, a team of researchers, led by Prof. Gang Zhao and Dr. Yerra Bharat Kumar from National Astronomical Observatories of Chinese Academy of Sciences (NAOC), studied Lithium (Li) con-tent in hundreds of thousands of Sun-like stars, using large spectroscopic surveys of GALAH and LAMOST. The result is published on Nature Astronomy on July 6, 2020. Then, how Lithium is made, and how it is destroyed. Let’s have a detailed explanation by the scientists themselves.
Lithium is becoming common in our everyday lives. It is the key ingredient in the batteries of our mobile phones and electric vehicles, but have you ever wondered where it comes from?
Lithium is a special element - it was the only metal produced in the Big Bang 13.7 billion years ago. While other elements were produced in copious amounts by stars over the eons (by factors of mil-lions in some cases), lithium has only increased by a relatively small amount. The source of even this small amount of lithium is still a matter of scientific debate. About half is thought to come from high-energy cosmic rays breaking up the heavier elements, like carbon and oxygen, in interstellar space.
Figure 1: Sketch of a lithium atom. Containing three protons in its nucleus, lithium is the third ele-ment in the periodic table. It was one of only three elements that formed in the Big Bang, along with hydrogen and helium. Wikipedia
Astronomers regard lithium as fragile, easily destroyed in the hot interiors of stars. This is confirmed by observations of lithium on the surface of stars, where we see that it becomes less abundant as they age, confirming that they gradually destroy it as they evolve.
However, there is one very notable exception to this rule of lithium destruction in stars - the 'lithium-rich giants'. These stars, first discovered about 40 years ago, have lithium contents up to 1000 times higher than other giant stars. Although they only comprise about 1% of giant stars, they have been a source of mystery and detailed astronomical study - eluding explanation for many dec-ades.
One of the main problems for identifying a way that these stars could produce so much lithium, was our lack of knowledge about exactly what type of giant stars they were. Since stars progress through three different giant phases (which all look quite similar in color and brightness), it is critical to understand just what phase lithium-rich stars are in when they produce lithium.
Of the many theories, one has now come to the fore. About 9 years ago, our group recognized that all the lithium-rich giants had particularly similar brightness's and colors, and were likely in the second giant phase. Also known as the red clump stage, these giants burn helium in their cores, for about 100 million years. Concrete confirmation of this theory hadn't come until a few years later, with the advent of precision asteroseismology, which looks at the oscillation signatures of stars to determine their precise evolutionary state. In the past year or two, with large surveys of stars com-binged with space-based asteroseismology, we now know for sure that the vast majority of very li-thium-rich giants are red clump stars.
Figure 2: This Hubble Space Telescope image shows stars at various stages of their lives, from young blue-hot stars to cooler red giants. Our study focused on the lithium content of red giant stars. Credit: NASA, ESA, and T. Brown (STScI)
For our current study, we also combined large stellar surveys to investigate the lithium-rich giants. In our case, we used a huge Chinese spectroscopic survey known as LAMOST, Australian stellar survey GALAH and another star survey from space mission known as GAIA. In our 200,000 star samples, we confirmed what others had recently found, that lithium-rich stars are in the red clump phase.
We also detected the normal strong destruction of lithium in the phase preceding this, the red giant branch phase. This lithium destruction phase is well known from studies of star clusters.
But then something strange stood out - the other red clump stars, although not extremely lithium-rich, contained much more lithium than the late-stage red giant branch stars. Since the red clump phase of evolution comes directly after the red giant branch phase, we concluded that the stars must be producing lithium when moving from one phase to the next. Furthermore, it appeared that all of these stars were higher in lithium than the preceding red giant branch phase - this was not just a 1% effect - up to 100% of the stars are experiencing lithium production!
In effect, by studying just the extremely lithium-rich stars, astronomers had been focusing on just the tip of the 'lithium iceberg' - it now appears that all red clump stars have been enriched with li-thium, and the extremely lithium-rich stars are only the tail end of the distribution. In our paper we show that, on average, the stars increase their lithium content by a factor of 40. The amount of li-thium produced would be enough to make electric car batteries for 20,000 trillion cars. How this lithium enrichment comes about is unknown, it is not predicted by our best models of stars. Clearly, there is some physical process missing in stellar theory.
What we can say with our data is when it occurs –some time between the end of the red giant branch phase and the beginning of the red clump phase.
For our next study, we will attempt to constrain the timing of the lithium-production phase more precisely. This information will help stellar theorists, including those in our group, to determine what physical process is leading to lithium production at this time in the lives of Sun-like stars.
Finally, since at least some of the newly created lithium will end up being blown off the star in stel-lar winds, it will also help us understand how much these stars enrich our Galaxy with lithium, and ultimately, planets like Earth.
The paper of this study can be accessed at https://www.nature.com/articles/s41550-020-1139-7
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