Stars and how we study them?

Stars are continuously burning gigantic balls of gas held together by their gravity. They are born by the collapse of large dust and gas clouds in space and are made mostly of hydrogen (H) and helium (He). Increasing pressure in their core due to gravitational contraction leads to a rise in temperature, and once the temperature is sufficiently high, the H atoms fuse to form He. The enormous amount of energy released by nuclear fusion counters the inward gravitational pressure and gives stars their stable state. Such stars powered only by the fusion of H in their core are known as main-sequence (MS) stars. Depending on initial mass, stars also run out of H fuel in their core on a time scale of millions to billions of years. When that happens, gravity starts dominating and makes the He core to contract, resulting in an increase in temperature, which reignites the H fusion in the shell around the core. At this point, the star leaves the main sequence phase and enters the red giant branch (RGB) phase. During the RGB and more evolved stages, stars also produce other heavier elements. The first 26 elements of the periodic table, i.e., up to iron, can be produced in the stars’ interior through nuclear fusion. When a star runs out of all possible options of fusing light elements to a stable heavy element, it starts collapsing very rapidly, which can result in an extremely energetic explosion known as a supernova. During such energetic explosions, elements heavier than iron are also formed and spilt into space. This thrown out material mixes with existing gas and dust in the space, and whenever there are favourable conditions, the cloud of this recycled material collapses again and gives rise to new stars. Well, that is how most of the stars are born and die! The newly born stars move in the Galaxy in the same sense as their natal gas cloud. Hence, by observing the current trajectory of a star, one can predict its origin site. Also, similar to any other object with a temperature, stars' also emit electromagnetic (EM) radiation throughout their lives. When this radiation from the inner regions of a star passes through its atmosphere, the signatures of chemical elements present in the atmosphere are printed on the spectrum (distribution of EM radiation as a function of wavelength or frequency) due to the absorption of some of the radiation by these chemical elements. Hence, by observing and analysing a star's spectrum, one can determine its chemical compositions.

Puzzling evolution of lithium in the low-mass giants and the Galaxy

The evolution of Li, however, is fascinating and a little different from other chemical elements. It is one of the three primordial elements (apart from H and He) produced during the Big Bang Nucleosynthesis (BBN). The standard BBN models, in combination with the input parameter value of baryon density measured from space missions such as the Wilkinson Microwave Anisotropy Probe, predict about 524 Li atoms per trillion H atoms. However, observations of the old, metal-poor MS stars suggest about 186 Li atoms per trillion H atoms. On the other hand, young metal-rich MS stars are found to have about 1584 Li atoms per trillion H atoms suggesting Li enrichment in the Galaxy. This increased Li in the Galaxy has been puzzling astrophysicists for decades.

The story of Li's evolution gets more complicated because of its fragile nature as it easily gets destroyed at temperatures of about 2.5 million kelvin, which is much lower than the core temperature of a star. In the atmosphere of Sun-like MS stars, it can survive. However, when a star evolves from MS to RGB phase, the outer layer material comes in contact with hotter material from the inner layers resulting in Lidestruction. Stellar evolution models have suggested a depletion of Li by a factor of about 30 to 60 in a solar metallicity star with masses ranging from 1 to 1.5 solar mass. Considering this, if a star exits the MS branch with the maximum possible amount of Li (i.e., about 1584 Li atoms per trillion H atoms), then it is expected to have a maximum of about 32 to 63 Li atoms per trillion H atoms in the RGB phase. However, in 1982, the discovery of a giant star with almost the same amount of Li as in the MS phase puzzled the researcher. Since then, nearly 300 giant stars with unexpectedly high Lihave been discovered and are known as Li-rich giant stars. These Li-rich giant stars are about one per cent of the total giant stars' population. For the last four decades, researchers worldwide have been trying to understand how a giant star can have such a high amount of Li. Also, as Li in stars gets depleted during their evolution, then how in the Galaxy it is increasing as none other process is known to contribute such an enormous amount of Li throughout the Galaxy.

What did we do to uncover the mystery of Li enrichment in the low-mass giants and the Galaxy?

To find answers to the mystery of Li enrichment in low-mass stars and the Galaxy, we started with a sample of about half a million stars collected from the Galactic Archaeology with HERMES (GALAH) survey. The GALAH survey is a large-scale observing program from the Anglo-Australian Telescope of the Australian Astronomical Observatory and provides stellar parameters along with quantitatively derived abundances of 23 chemical elements (including Li) for about half a million stars. We further added stars' positions, distance, motions (space velocities), and photometric information from the recently released Gaia survey from the Gaia space observatory of the European Space Agency. From this combined sample, we discovered 335 new Li-rich red giant stars, which is more than the total number of known Li-rich stars before the publication of our results in January 2019. Also, with the help of accurate distances from the Gaia survey, we discovered that most of these stars belong to the red-clump phase. In this phase, a star is primarily powered by the fusion of He in the core. Also, at the onset of this phase, stars go through a brief thermal runaway nuclear fusion of a large amount of He into carbon and release a huge amount of energy. This brief event is known as the He flash. During the He flash, many structural changes happen in a star, and some of the material from the inner region gets transported to the surface. In this process, beryllium from the core can also get transported to the surface. Through the well-known Cameron-Fowler mechanism, beryllium can capture an electron and get converted into Li and make a giant Li enriched. We also discovered that the rate of occurrence of Li-rich giants is not constant throughout our Galaxy, and they are relatively more prevalent among giants of the Galactic disc compared to the Galactic halo (the outer component of the Galaxy). We reported these results in the Monthly Notices of the Royal Astronomical Society in 2019. To further understand the evolution of Li-rich giant stars, we compared their various properties (like metallicity, chemical compositions, rotation about their axis, masses, etc.) with that of the Li normal giants. We found that even though about one per cent of giants are Li-rich, their frequency of occurrence is metallicity dependent and slightly increases with an increase in metallicity. Li-rich and normal giants are found to have similar chemical compositions suggesting Li addition to create a Li-rich giant may occur independently of the abundance changes wrought by the mixing processes. We also found that the probability of becoming a Li-rich giant is approximately independent of the star's mass, although the majority of the Li-rich giants are low-mass stars (with masses lower than two solar mass). We also found that the Li-rich and Li normal giants have similar rotational velocities about their axis, suggesting that Li enrichment is not linked to scenarios such as mergers and tidal interaction between binary stars. We published these results in the Monthly Notices of the Royal Astronomical Society in 2020. To understand Li's evolution in the Galaxy, we also studied a sample of about sixty thousand low-mass MS stars selected from our initial sample from the GALAH survey. We found that Li is continuously increasing with time in the Galaxy and this increase can be explained by Li's contribution from the evolved low-mass giant stars. About 70 per cent of stars in the Galaxy are of low mass. These stars go through the He-flash and Li enrichment phases as they evolve. This enriched Li can be added to gas and dust clouds in the Galaxy through mass loss during the He-core flash. The new stars forming from these Li enriched clouds will be Li enriched. This process keeps on repeating endlessly and can lead to the enrichment of Li in the Galaxy. We reported these results in the Proceedings of the International Astronomical Union and the Memorie della Societa Astronomica Italiana in 2020.

Note: After writing this small piece, we have published two more articles in MNRAS about 1) Lithium abundances and asteroseismology of red giants: understanding the evolution of lithium in giants based on asteroseismic parameters, and 2) Lithium in red giants: the roles of the He-core flash and the luminosity bump

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