Peanuts in Space: solving the thermodynamical conundrum in contact binary stars

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The two stars of a contact binary system orbit so close to each other that they share a common envelope, resulting in their distinctive "peanut" shape. Contact binaries are ubiquitous, easily observable thanks to their 0.2-1.2-day orbital periods and large brightness variations, and moreover appealing due to the physics governing their observed properties. Most perplexing is the fact that despite their generally different masses and radii, the two stars possess essentially identical temperatures, indicating that they are in thermal equilibrium and requiring a significant flow of mass and energy between the two stars' common envelope. However, current contact binary models do not account for mass/energy mixing. The principal investigator's (PI) team will correct this situation by upgrading their modeling software PHOEBE to account for mass/energy transfer by the introduction of a mixing parameter. They will analyze thousands of contact binary light curves and explore mixing parameter correlations to infer the physical processes driving this mass and energy transfer, leading to a better understanding of these systems. This award will support a postdoc and three undergraduate researchers each year, as well as supporting regular general audience level articles to be distributed online through the IAU Outreach office.The team will explore three simple 1-parameter model extensions to PHOEBE: radial mixing, lateral mixing, and magnetic activity. In each case mixing will be assumed to affect only the cooler star, and that the amount of mixing is approximated well enough by a power law. In each case the model will be parametrized by the single mixing parameter "p", serving as a proxy to the hydrodynamical underpinnings of mass and energy flows in the envelope. They will evaluate the effect of “p” on the light curve, and since “p” is at least somewhat degenerate with the remaining principal parameters, other parameters will be considered to generate a good fit to the data and account for mixing. Carrying out this same analysis on thousands of contact binary light curves available in public archives, they will be able to study the correlations between “p” and the degree of thermal and geometrical contact, and the distributions of parameter “p” for each of the single-parameter models. Those correlations are what will inform the team of the underlying physics, and “p” will provide them with an essential guidance on how to properly formulate a more advanced mixing model. This scheme is already implemented in PHOEBE for computing stellar fluxes, making contact binary atmospheres a natural extension to PHOEBE’s existing capabilities.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.