Over the course of the two-year MSCA funding period, the PI (Joyce) and host (Molnár) made significant advances in all three topics of the project: convection in 1D stellar models, globular cluster/isochrone analysis, and, especially, the modeling of high-mass variable stars. All of these three topics are areas of stellar physics with long-standing open questions, and the project was aimed at advancing the state of the art in each of them. And while these are all questions that concern fundamental science, they also help us create a broad understanding of how stars live and die, and how stars enrich their environments so that new stars and planets, similar to the Sun and Earth, may form from them.
This project has produced its most significant results through the advancement of our understanding of high-mass variable stars, especially two long-studied giants of different types: the asymptotic giant branch (AGB) star R Hydrae, and the nearby red supergiant and 10th brightest star in the sky, Alpha Orionis, popularly known as Betelgeuse.
As part of studying R Hya, this project produced the first ever, fully open-source grid of AGB evolutionary calculations including seismology. These models were computed with the stellar evolution code MESA and the accompanying pulsation code GYRE. Application to R Hya revealed that it is a few times heavier than the Sun, and it is likely actively enriching its environment.
Another significant result of the project was a collaborative work in which the PI participated, resulting in a Nature publication. The study found that one star out of a dozen has consumed one of its planets during their formation. The researcher contributed specialized MESA models to this end.
Important results were achieved on the ages of stars as well. The researcher found that the stars in the Galactic bulge, the center of the Milky Way, are predominantly old, exceeding 10 billion years. However, when she accounted not just for observational errors, but for modeling uncertainties properly, too, the uncertainties increased to 2-5 billion years.
In the final months of the project, the PI and host collaborated on a groundbreaking publication predicting the presence of a low-mass orbital companion to Betelgeuse: alpha Orionis B, or colloquially, “BetelBuddy.” This study has received considerable attention in popular science outlets and resulted in the PI and host winning time on both the Hubble Space Telescope and Chandra X-ray observatory to try to find the companion.
This action led to a better understanding of uncertainties in evolutionary stellar models, and especially their dependence on the mixing length parameter. The PI and the host will continue their collaboration to calibrate the parameter through the new observations they secured thanks to this action. The AGB model grid is available for modeling further AGB stars, either by the PI/Host or others. Their joint work in this action also led to successful observing proposals on the Hubble and Chandra space telescopes, which they will use to prove the existence of “Betelbuddy”.
