The Lithium problem. Primordial, cosmological or stellar?

Fabio Iocco is one of the Postdoc working within the OKC on Dark Matter. He is also interested in one of the puzzles keeping astronomers and cosmologists busy: the Lythium problem. Fabio has recently organized a conference dedicated to this mystery and he is getting ready to give the next OKC colloquium, this is why I asked him to tell us a bit more about this topic.

Credit: Belèn Roncoroni

Alright: the “Lithium problem”.
The “Cosmological” Lithium Problem.
The “Primordial” Lithium problem.
We all have heard about it since kindergarden, but would you bet 5000 SEK you know exactly what it is? I did not, so had to look it up. And here is what I have learned.
First of all, let’s play it fair: there’s two stable lithium isotopes, lithium-six and lithium-seven. In the last years it seemed both had problems, but we are talking about the bigger brother here, the one who has had problems for a longer time. Since 1982, 30 years ago -when my brother was born- there have been observations of lithium-seven in metal poor stars of the galactic halo. The most metal poor stars, the smallest mass, therefore the oldest stars to be around. Or at least a good approximation of a lot, a lot old. Ancient, pristine maybe. Ay, there’s the rub: “maybe”. Would you bet they were the first stars to be formed? I would not, but that’s another story. What matters here is that the stars were not the first generation, but the stuff in their atmosphere, what you observe when you take spectra of their surface may have been whatever had been produced “as far back in the past” as we could get with chemistry observations in our galaxy.
So they took a bunch of them, spanning two orders in magnitude in (low) metallicities (metallicity is a good proxy for the generation in which the star has been born) and surprise surprise: they found that the lithium-seven abundance in their atmosphere was constant: stars of different metallicities had the same surface abundance. Well, then that unique value of lithium found there must have had something very very special, perhaps it was there before the stars were born, because how could stars so different in behavior (they *do* behave differently at different metallicities) could have synthesized lithium at that very same abundance? And then, if it was there before the stars had formed at such low metalicities, it must have been there before anything else, therefore leftover there from the beginning of times, maybe from the Primordial Nucleosynthesis, the famed Big Bang Nucleosynthesis (BBN) itself? Maybe. And maybe not. But to spice things up, it turns out that Lithium-seven is a good thing in BBN, because it depends a lot from the baryon density, the very amount of normal matter (not Dark Matter, just the normal one), as much as Deuterium. And people were looking for that number, in 1982. Alas, it turns out that if you take the baryon number you need to to produce the right (observed) amount of Deuterium in BBN, you would expect twice as much lithium seven than the one you observe.
Hence, we have a problem. Or not? Well, if you believe that lithium you observe in stars is the primordial, we definitely have a big, cosmological, primordial problem. But if you don’t believe that is the primordial one, well… we have a problem as well. A big, fat, juicy astrophysical problem. Not so sexy to be cosmological, not so enthralling as primordial, but still a good old fashioned astrophysical problem. The Lithium Problem. Either that is primordial, cosmological, or stellar.
And the smaller brother, lithium-six? And all the stuff that goes on between BBN and the stars we observe? And stars in other systems? And thirty more years of observations? Goes without saying, there’s much more sauce to the pasta than what I am feeding you now, but for that you have to purchase your ticket for the next OKC colloquium.
See you on Tuesday.

Fabio Iocco

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