2019ApJ...879...62M

Most aspects of stellar evolution proceed far too slowly to be directly observable in a single star on human timescales. The thermally pulsing asymptotic giant branch (AGB) is one exception. The combination of state-of-the-art modeling techniques with data assimilated from observations collected by amateur astronomers over many decades provide, for the first time, the opportunity to identify a star occupying this precise evolutionary stage. In this study, we show that the rapid pulsation period change and the associated reduction in radius in the bright, northern variable star T Ursae Minoris are caused by the recent onset of a thermal pulse (TP). We demonstrate that T UMi transitioned into a double-mode pulsation state, and we exploit its asteroseismic features to constrain its fundamental stellar parameters. We use evolutionary models from MESA and linear pulsation models from GYRE to track simultaneously the structural and oscillatory evolution of models with varying mass, and we apply a sophisticated iterative sampling scheme to achieve time resolution <=10 yr at the onset of the relevant TPs. We report an initial mass of 2.0 ± 0.15 M_☉ and an age of 1.17 ± 0.21 Gyr for T UMi. This is the most precise mass and age determination for a single AGB star ever obtained. The ultimate test of our models will be the continued observation of its evolution in real time: we predict that the pulsation periods in T UMi will continue shortening for a few decades before they rebound and begin to lengthen again, as the star expands in radius.