Period change and stellar evolution ofβCephei stars

Abstract

The β Cephei stars represent an important class of massive star pulsators that probe the evolution of B-type stars and the transition from main sequence to hydrogen-shell burning evolution. By understanding β Cep stars, we gain insights into the detailed physics of massive star evolution, including rotational mixing, convective core overshooting, magnetic fields, and stellar winds, all of which play important roles. Similarly, modeling their pulsation provides additional information into their interior structures. Furthermore, measurements of the rate of change of pulsation period offer a direct measure of β Cephei stellar evolution. In this work, we compute state-of-the-art stellar evolution models assuming different amounts of initial rotation and convective core overshoot and measure the theoretical rates of period change, that we compare to rates previously measured for a sample of β Cephei stars. The results of this comparison are mixed. For three stars, the rates are too low to infer any information from stellar evolution models, whereas for three other stars the rates are too high. We infer stellar parameters, such as mass and age, for two β Cephei stars: ξ 1 CMa and δ Cet, which agree well with independent measurements. We explore ideas for why models may not predict the higher rates of period change. In particular, period drifts in β Cep stars can artificially lead to overestimated rates of secular period change.

Evolving pulsation of the slowly rotating magnetic β Cep star ξ1 CMa

“…Since the period is growing, this is qualitatively consistent with the increasing radius of the star as expected due to stellar evolution on the main sequence (e.g. Neilson & Ignace 2015). As reported by those authors, the fractional rate of change of the pulsation period of a radially pulsating star due to evolving mass M and radius R on evolutionary timescales can be computed according to:…”

“…Jerzykiewicz (1999), in his summary of period evolution of β Cep stars, cites a rate of period change of 0.37 ± 0.05 s/cen reported by Pigulski (1992a). Neilson & Ignace (2015) used those results to test the influence of rotation and convective core overshoot on models of massive star evolution, finding that the measured rate of period change of ξ 1 CMa was in good agreement with that predicted by models under the constraints applied by the physical parameters of Shultz (2016); Shultz et al (2017).…”

Asteroseismology of High-Mass Stars: New Insights of Stellar Interiors With Space Telescopes

“…This is because the southern CVZ of TESS includes the LMC galaxy, which allows pulsation excitation models to be tested for metal-rich and metal-poor stars (Bowman et al, 2019b). TESS also offers the opportunity to revisit "old friends" in terms of previously studied massive stars with high-precision photometry, which is particularly useful to probe the longterm stability in pulsation mode amplitudes and frequencies in relatively short-lived stars (e.g., Neilson and Ignace, 2015). The diverse variability of massive stars, which includes both coherent pulsation modes excited by the κ-mechanism and IGWs excited by core convection (Pedersen et al, 2019;Bowman et al, 2019b;Burssens et al, 2020), enables asteroseismology for a sample of massive stars larger by two orders of magnitude compared to any that came before.…”

HD 96446: a long-period binary with a strongly magnetic He-rich primary with β Cephei pulsations

“…Since the pulsational frequency is proportional to the square root of the mean stellar density, pulsation is a sensitive indicator of the stellar evolution (see e.g. Neilson & Ignace 2015). For a 9 M star, the expected period variation isṖ/P ∼ 7 × 10 −8 yr −1 .…”