[Paper Review] The nature of B supergiants: clues from a steep drop in rotation rates at 22000 K. The possibility of Bi-stability braking

J. S. Vink, I. Brott+4 more|arXiv (Cornell University)|Mar 5, 2010

Stellar, planetary, and galactic studies19 references56 citations

TL;DR

The paper proposes that the steep drop in rotation rates of B supergiants at ~22,000 K is caused by bi-stability braking (BSB), a process where enhanced mass loss at the bi-stability jump increases angular momentum loss, slowing rotation. This mechanism, supported by stellar evolution models, offers a physical explanation for the slow rotation of cooler B supergiants and may resolve long-standing uncertainties in massive star evolution.

ABSTRACT

The location of B supergiants in the Hertzsprung-Russell diagram (HRD) represents a long-standing problem in massive star evolution. Here we propose their nature may be revealed utilising their rotational properties, and we highlight a steep drop in massive star rotation rates at an effective temperature of 22000 K. We discuss two potential explanations for it. On the one hand, the feature might be due to the end of the main sequence, which could potentially constrain the core overshooting parameter. On the other hand, the feature might be the result of enhanced mass loss at the predicted location of the bi-stability jump. We term this effect "bi-stability breaking" and discuss its potential consequences for the evolution of massive stars.

Motivation & Objective

• To resolve the long-standing problem of the evolutionary state of B supergiants, particularly why they rotate slowly despite being massive.
• To investigate whether the observed steep drop in rotational velocities at ~22,000 K is due to core hydrogen burning (main sequence) or post-main sequence evolution.
• To test the hypothesis that enhanced mass loss at the bi-stability jump causes significant angular momentum loss, termed 'bi-stability braking' (BSB).
• To determine whether BSB can explain the rotational properties of B supergiants across different masses and metallicities.
• To assess the implications of BSB for core overshooting parameters and the blue-to-red supergiant ratio in massive star evolution models.

Proposed method

• Stellar evolution models with rotation and mass loss were used to simulate the evolution of 40 M☉ stars, incorporating the bi-stability jump in mass loss at ~22,000 K.
• The models included wind-driven angular momentum transport, with mass loss rates increased by a factor of 5–7 at the bi-stability jump, as predicted by Vink et al. (1999).
• The terminal wind velocity was reduced by a factor of ~2 across the bi-stability jump, consistent with observational confirmation (Crowther et al. 2006).
• The models compared rotational velocity evolution with and without the bi-stability jump to isolate the effect of BSB on surface rotation.
• The critical mass threshold for BSB (≥30 M☉ in standard models) was determined by varying the core overshooting parameter α_ov.
• Sensitivity of BSB to α_ov was tested by increasing it to 0.5, showing that BSB can extend to lower masses (e.g., 20 M☉) under higher overshooting.

Experimental results

Research questions

• RQ1 Why do B supergiants exhibit a steep drop in rotational velocities at ~22,000 K, and what physical mechanism underlies this feature?
• RQ2 Can the bi-stability jump in mass loss explain the observed slow rotation of cooler B supergiants through enhanced angular momentum loss?
• RQ3 To what extent does bi-stability braking (BSB) depend on the core overshooting parameter α_ov, and how does this affect the predicted evolutionary tracks?
• RQ4 Is the observed rotational drop at 22,000 K better explained by a bimodal population of main-sequence and post-main-sequence stars, or by a continuous braking mechanism?
• RQ5 What are the implications of BSB for the evolutionary status of B supergiants, particularly regarding their core hydrogen-burning nature and the blue-to-red supergiant ratio?

Key findings

• The steep drop in rotational velocities at ~22,000 K is strongly correlated with the predicted location of the bi-stability jump, supporting the hypothesis of bi-stability braking (BSB).
• In models including the bi-stability jump, surface rotation rates drop drastically at ~22,000 K due to enhanced angular momentum loss, while models without the jump show no such drop.
• Bi-stability braking is effective only for stars with initial masses ≥30 M☉ in standard models with α_ov = 0.335, indicating a mass threshold for the effect.
• Increasing the core overshooting parameter α_ov to 0.5 extends the range of BSB to lower masses (e.g., 20 M☉), suggesting the effect is model-dependent.
• The BSB mechanism could explain why cooler B supergiants are slow rotators, potentially resolving the long-standing ambiguity about whether they are main-sequence or post-main-sequence objects.
• If BSB operates down to ~10 M☉, it would imply a significantly larger α_ov than currently constrained, with far-reaching consequences for massive star evolution models and the interpretation of the blue-to-red supergiant ratio.