Claudia Aguilera-Gómez - Explaining Lithium Enriched Red Giant Branch Stars

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The current theory of stellar structure and evolution was born and grew up along with the revolutions of modern and quantum physics of the early twentieth century. It is arguably one of the most successful physical frameworks of modern astrophysics, explaining the inner workings of stars of all sizes and chemical compositions to a remarkably high level of agreement between theory and observation. The model not only accounts for the detailed interior of stars at any given time (their structure, state of the matter, energy generation, energy transport, internal dynamics) but also successfully explains precisely why and how these properties change in time almost since stars are born until their ultimate fate in a universe of a finite age. Moreover, when the machinery of the model is applied to large collections of stars, we are able to understand the behavior of the stellar populations that make up and drive the evolution of the variety of galaxies whose studies, in turn, teach us about cosmology and even provide tests of some of the fundamental physics on which the theory itself rests. Today, our understanding of stellar interiors is being further boosted by the maturity of the tool of asteroseismology, which provides a new ability to peer into the structure and dynamics of stellar interiors. At the same time, the asteroseismic advances on stellar astrophysics, along with the present astrometric revolution being enabled by the Gaia mission and various large spectroscopic surveys, are leading us today into a new era in the understanding of the formation, structure, and evolution of the Milky Way galaxy as a whole. Modern stellar astrophysics, therefore, is an extremely robust construction that keeps improving and upon which new and larger astrophysical enterprises rest. In this context, long-standing unsolved problems in the area are few, and any new advances that offer truly novel insight on those problems are, not surprisingly, extremely rare. Claudia Aguilera-Gmezs PhD thesis work is one of these rarities.

The existence of a small but ubiquitous fraction of red giant stars with large amounts of lithium in their surfaces is among those few old problems in stellar astrophysics that have eluded an explanation for a few decades now. The problem is made the more difficult due to the fact that the physics of lithium is relatively simple (and therefore all available options to solve the problem have been explored already, or seemingly so), and most observational data involving it are satisfactorily explained today with standard astrophysics. Lithium is consumed (burned) via proton capture at relatively low temperatures for stellar interior standards, and this fragility is exploited in stellar astrophysics to obtain a probe, a thermometer, for the deep interiors of stars, i.e., locations that we are prevented from seeing by direct means because stars are such opaque objects to the passage of light. Since we know for a fact that lithium is burned at temperatures above 1.52 million degrees, all that we know after almost a hundred years of research in stellar astrophysics indicates almost without a doubt that when a low-mass star like the Sun gets old enough and grows in size to become a red giant, the lithium present in its surface should (1) first dissolve, because the outer convective envelope of the star is getting a lot larger, and (2) burn and disappear almost completely, because the deeper layers of this envelope get hotter than the burning temperature of lithium. However, while most red giants seem to follow this expectation, it happens that wherever we look in the Milky Way and around it, we find some red giants with large amounts of lithium. Early since their discovery, and this fact is not anecdotal in this story, lithium-rich giants were defined to be those objects with more than three times more lithium in their surfaces than in the Suns. Even with observational instrumentation and data improving with the years, and models becoming more and more sophisticated, the decades have passed and the problem remains there, with basically no new insights on where to go.