The evolutionary stage of Betelgeuse inferred from its pulsation periods

Abstract: Betelgeuse is a well known bright red supergiant that shows semi-regular variations with four approximate periods of 2200, 420, 230, and 185 days. While the longest period was customarily regarded as LSP (long secondary period) of unknown origin, we identify the ~2200-d period as the radial fundamental mode, and the three shorter periods as the radial first, second, and third overtones. From a linear nonadiabatic pulsation analysis including the pulsation/convection coupling, we have found that these radial pulsation modes are all excited in the envelope of a model in a late stage of the core-carbon burning. Models with similar pulsation property have masses of 11~12 M_\odot (19M_\odot at ZAMS) with luminosities (log L/L_\odot =5.27~5.28) and effective temperatures (log T_{eff}\approx 3.53) that are consistent with the range of the observational determinations. We also find that a synthetic light curve obtained by adding the fundamental and the first-overtone mode is comparable with the light curve of Betelgeuse up to the Great Dimming. We conclude that Betelgeuse is likely in the late stage of core carbon burning, and a good candidate for the next Galactic Type II supernova.

• The paper identifies the ~2200-day period as Betelgeuse’s radial fundamental mode, challenging previous mode interpretations.
• The paper employs linear nonadiabatic pulsation analysis with convection coupling to match observed luminosity, temperature, and pulsation modes.
• The paper concludes that Betelgeuse’s late core-carbon burning stage implies significant mass loss and a potential supernova in coming decades.

Analysis of Betelgeuse's Evolutionary Stage Through Pulsation Periods

The paper by Saio et al. offers a detailed investigation into the evolutionary stage of Betelgeuse, a prominent red supergiant star, by analyzing its pulsation periods. Betelgeuse exhibits semi-regular variability with periods approximating 2200, 420, 230, and 185 days. Traditionally, the longest of these, approximately 2200 days, has been considered a long secondary period (LSP) of unknown origin. However, the authors argue that this period actually corresponds to the radial fundamental mode of the star, while the three shorter periods align with the radial first, second, and third overtones.

The study employs a linear nonadiabatic pulsation analysis incorporating the coupling between pulsation and convection to establish that all these radial pulsation modes are excited in the envelope of a stellar model at a late stage of core-carbon burning. The models compatible with these pulsation properties have masses between 11 and 12 Mₒ (assuming an initial mass of 19 Mₒ at the zero-age main sequence) and exhibit luminosities and effective temperatures consistent with observed values.

Key Findings and Implications

1. Period Identification: The identification of the 2200-day period as the radial fundamental mode challenges previous interpretations that identified the 420-day period with the fundamental mode. This change in understanding significantly affects the inferred physical parameters of the star, such as its radius.
2. Theoretical Models: Models show that Betelgeuse is likely in a late stage of core carbon burning. The paper finds pulsation modes excited in models with physical characteristics matching observational data. These models predict significant stellar radius and mass loss, setting the groundwork for potential supernova conditions.
3. Synthetic Light Curve: By synthesizing a light curve using the fundamental and first overtone modes, the authors reproduce Betelgeuse's light variations up to the "Great Dimming" observed between 2019 and 2020. This supports the hypothesis that the dimming could have been partly caused by pulsation-related constructive interference.
4. Future Supernova Candidate: Given the late stage of core-carbon burning, Betelgeuse is a strong candidate for becoming the next Galactic Type II supernova. The timeline suggests core carbon exhaustion within approximately 300 years, followed by a possible core-collapse supernova in a few decades.
5. Discrepancies and Challenges: Despite robust modeling, certain discrepancies exist, such as the calculated versus observed surface CNO abundances and angular rotation rates. These require further investigation, possibly adjusting rotational mixing or considering past stellar mergers to reconcile differences.

Prospects for Future Research

The reinterpretation of Betelgeuse’s pulsation periods opens new avenues both observationally and theoretically. Continued monitoring of Betelgeuse’s variability will provide data to refine pulsation models. Additionally, further study into rotation and mixing processes can help address current discrepancies in properties like surface CNO ratios.

Such research improves our understanding not only of Betelgeuse but also other massive red supergiants, enhancing predictive models for stellar life cycles and supernova production. Continued advancement in these areas will deepen insights into massive star evolution and its broader cosmic implications.