Stellar Evolution in Real Time: Models Consistent with Direct Observation of Thermal Pulse in T Ursae Minoris
Molnár László
MTA CSFK CSI
Most aspects of stellar evolution proceed far too slowly to be directly observable in a single star on human timescales, but the thermally pulsing asymptotic giant branch is one exception. We show conclusively that the rapid pulsation period change and associated reduction in radius in the former Mira star T Ursae Minoris are caused by the recent onset of a thermal pulse. We discovered 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 and report initial mass of 2.0 +/- 0.15 M_Sun and an age of 1.17 +/- 0.21 Gyr for T UMi. This is the most precise mass and age determination for a single asymptotic giant branch 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.