Lithium reveals new pre-main sequence physics
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Photospheric lithium depletion is a sensitive test of stellar structure on the pre-main sequence, as the rate of early lithium burning depends strongly on the physical conditions at the center of the star. Mismatches between standard stellar model predictions and the empirical Li patterns of young clusters, notably the Pleiades, have long demonstrated that physical effects not treated in standard theory must be at work in young stars.
In a series of two papers in 2014 and 2015, Marc Pinsonneault and I argued that if the most rapidly rotating young stars were inflated by ~10%, their interiors would cool, thus decreasing the rate of lithium depletion. Several features of the Pleiades Li pattern can be explained by this phenomenon; namely, the Li-rotation correlation among the K-dwarfs, the Li dispersion at fixed Teff which increases towards lower temperatures, and the generic shape of the lithium depletion pattern. Shown to the right in black is the Li pattern of the Pleiades (larger points are faster rotating), and in red is a synthetic pattern tying radius inflation to rotation rate. |
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The influence of starspots on stellar structure and evolution
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I next attempted to identify the physical effect driving the radius inflation that I proposed to explain the Pleiades lithium pattern. Rotation, activity, and starspots have been posited as agents of radius inflation through studies of eclipsing binaries, so in a paper in 2015, I devised a method for calculating the impact of starspots on stellar structure and evolution with a modern evolution code.
Among my findings, a heavy starspot coverage (~50%) leads to radius inflation of about 5-10% in stars during the pre-main sequence and main sequence. Corresponding decreases in the Teff and luminosity shift the locations of stars on the color-magnitude and HR diagrams. This issue leads to systematically under-estimated masses and radii of young stars if not taken into account. Finally, this increased radius during the main sequence suppressed the rate of lithium destruction, so if the most rapidly rotating young stars are also the most heavily spotted, the Pleiades lithium pattern is naturally predicted by this methodology. |
Empirical measurements of the radii of Pleiades members
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I have also searched for direct empirical evidence of radius inflation among the most active, rapidly rotating stars. In a 2017 paper with Keivan Stassun, I measured the radii of several members of the Pleiades by measuring their Teff and Luminosity, and solving the Stefan-Boltzmann equation. I then compared them to stellar evolution models to determine which stars had radii that disagreed with predictions based on their Teff.
I found that for stars rotating at slower than a 2 day period, models and measurements agreed well. However, for faster rotating stars, there is a clear tendency to be larger by ~10-20% compared to standard expectations. This provides confirmation of radius dispersions among young, active stars, and the development of this method opens the door for applications in other age and mass regimes. Stay tuned! |
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