Stellar Evolution in Real Time. I. Models Consistent with the Direct Observation of a Thermal Pulse in T Ursae Minoris

Abstract

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 is one exception. The combination of state-of-the-art modelling 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 precisely this evolutionary stage. In this study, we show that the rapid pulsation period change and associated reduction in radius in the bright, northern variable star T Ursae Minoris are caused by the recent onset of a thermal pulse. 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. We apply a sophisticated iterative sampling scheme to achieve time resolution ≤ 10 years at the onset of the relevant thermal pulses.We report 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 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.

Standing on the Shoulders of Giants: New Mass and Distance Estimates for Betelgeuse through Combined Evolutionary, Asteroseismic, and Hydrodynamic Simulations with MESA

“…If we consider the progression of the models, we find that the period ratio drops in even younger, higher-mass models, with P 1 /P 0 as small as 0.30-0.36 at around 5 Myr. Older, lowermass models reach the 0.50-0.55 range, in agreement with period ratios observed in semiregular and Mira variables (Kiss et al 1999;Molnár et al 2019). We thus conclude that the signal detected in our photometric data is indeed consistent with the first overtone, making Betelgeuse a double-mode star.…”

“…Though we can explain and fully capture some pulsation physics in 1D stellar models (e.g., pressure and gravity modes; see the review by Aerts 2019), other mechanisms are not well understood (Wood et al 2004;Nicholls et al 2009). In this latter class fall many of the variations we observe on human timescales, as such behavior is, with rare exceptions, too rapid to be explained by classical stellar evolution (Molnár et al 2019). Modeling such processes may require 3D, time-dependent convection, or otherwise more sophisticated physical formalisms that are beyond the scope of typical 1D stellar evolution programs.…”

First results on RR Lyrae stars with the TESS space telescope: untangling the connections between mode content, colors and distances

“…Double-mode stars are valuable targets as they provide simultaneous period constraints for multiple modes that can help us determine their physical properties much more accurately than those of single-mode pulsating stars (see, e.g., Molnár et al 2015bMolnár et al , 2019Joyce et al 2020). If we place these five stars onto the Petersen diagram, four of them fall into the main locus of RRd stars where the other Galactic RRd stars reside (Fig.…”

The most metal-rich asymptotic giant branch stars

“…Thermally-pulsing AGB models are known to be greatly affected by modelling uncertainties, both in the numerical details and in the input physics used in calculations (e.g., Ventura & D'Antona 2005a,b;Stancliffe & Jeffery 2007;Karakas et al 2012;Rosenfield et al 2014;Pastorelli et al 2020;Marigo et al 2020;Molnár et al 2019;Gil-Pons et al 2021). The most significant for very metal-rich stars is the mass-loss rate, which is essentially unknown in evolved phases.…”