Comment on the feasibility of carbon burning in Betelgeuse: a response to "The evolutionary stage of Betelgeuse inferred from its pulsation periods," arXiv:2306.00287

Abstract: The recent pre-print by Saio et al. 2023 argues that the supergiant Betelgeuse is already undergoing carbon burning, based on the assumption that all of its light variations are caused by radial pulsations. However, the angular diameter measurements of the star are in conflict with the stellar radius required by their models, as we show in this note. We discuss the feasibility that the Great Dimming was caused by constructive mode interference using long-term brightness measurements and comment on differences in modeling frameworks adopted in Saio et al. 2023 vs Joyce et al. 2020.

• The paper challenges Saio et al.'s assumption of a 2200-day fundamental mode, suggesting that Betelgeuse's radius measurements contradict this carbon-burning model.
• It probes the discrepancies between pulsation-derived radii (~1400 R⊙) and empirical measurements (600-1100 R⊙), undermining the proposed evolutionary stage.
• The study advocates refined models incorporating non-linear dynamics and multimodal observations to better represent complex stellar atmospheres.

Analysis of Carbon Burning Feasibility in Betelgeuse and its Evolutionary Implications

This paper provides a critical examination of the assertion that Betelgeuse, the well-known red supergiant, is in the carbon-burning phase of its stellar evolution, a topic recently put forward by Saio et al. The central critique focuses on discrepancies between the calculated stellar radius from pulsation periods and empirical angular diameter measurements, which inform the realism of the carbon-burning hypothesis.

Pulsation and Stellar Radius Discrepancies

The focal point of the discussion is Saio et al.'s model, which equates the star's Long Secondary Period (LSP) with the fundamental radial mode (FM) of pulsation. Evaluations of Betelgeuse's variability have traditionally attributed the longest 2200-day periodicity to external factors rather than intrinsic pulsation, with more immediate pulsations at around 380-430 days corresponding to the FM. Saio et al.'s proposal significantly upsizes the star's radius to accommodate a 2200-day FM, suggesting a radius of ~1400 $R_{\odot}$, contrasting with measurements calculating it between 600-1000 $R_\odot$ based on stellar brightness and angular diameter. Observational data, such as those derived by Haubois et al. and Dolan et al., indicate a maximum angular diameter translating to radii under 1100 $R_\odot$. This paper underscores these conflicting radius values as a challenge to classifying the 2200-day period as an FM pulsation.

Modeling and Great Dimming

Focusing on modeling approaches, the paper posits that the complexities of Betelgeuse's dimming phenomena weren't adequately addressed in Saio et al.'s framework. Specifically, the linear, adiabatic assumptions used don't fully encompass the non-linear dynamics and atmospherical extension, notably affecting opacities and the kappa mechanism responsible for driving pulsations. Notably, the Great Dimming event, characterized by significant temporary brightening, is better attributed to external dust clouds rather than intrinsic variability, as argued by the original observational framework and Joyce et al.

Theoretical and Practical Implications

The work implies a need for scrutiny of modeling parameters in stellar evolution, particularly when aligning pulsation periods with evolutionary phases. Constraining LSP phenomena to radial pulsations, as critiqued, could overlook crucial physical processes and interactions in extended stellar atmospheres. Additionally, understanding these mechanics is pivotal in modeling stellar lifecycles and predicting phenomena such as supernovae in supergiants. The insistence on detailed multimodal observations alongside advanced modeling helps ensure theoretical predictions match empirical evidence in luminous, complex systems like Betelgeuse.

In conclusion, this critique encourages refinement in the theoretical models describing evolutionary phases of massive stars based on pulsation analyses. Future advances could involve 3D hydrodynamic simulations to address non-linear atmospheric interactions more precisely. The research steers astrophysical discourse towards reconciling theoretical assertions with robust observational constraints, enabling accurate models of stellar evolution and lifecycle predictions.