[Paper Review] Evidence for rotation-induced mixing in evolved intermediate mass stars

R. Smiljanić, Beatriz Leonor Silveira Barbuy+2 more|arXiv (Cornell University)|Mar 6, 2006

Stellar, planetary, and galactic studies3 citations

TL;DR

This study presents observational evidence linking stellar rotation to enhanced internal mixing in evolved intermediate-mass stars (5–20 M⊙). Using high-resolution spectroscopy of 19 stars, the authors find a significant correlation between stellar mass and the [N/C] abundance ratio—specifically in red supergiants with vsini > 6 km s⁻¹—supporting rotation-induced mixing mechanisms such as meridional circulation and shear turbulence as key drivers of chemical mixing beyond standard non-rotating models.

ABSTRACT

Many observational results seem to indicate more efficient mixing processes in intermediate mass stars (5-20 M$_{\odot}$) than the expected by the standard models. These processes are usually thought to be caused by stellar rotation. Our recent analysis of 19 evolved intermediate mass stars has found them to display different efficiencies of internal mixing. The comparison of these results, and others from the literature, with rotating and non-rotating stellar evolutionary models led us to find, for the first time, an important correlation between stellar mass and the [N/C] ratio; the kind of correlation expected to be produced by a rotation-induced mixing.

Motivation & Objective

• To investigate the efficiency of internal mixing processes in evolved intermediate-mass stars (5–20 M⊙) beyond predictions of standard non-rotating stellar models.
• To determine whether stellar rotation can explain observed deviations in surface CNO abundances, particularly enhanced nitrogen and reduced carbon.
• To test the hypothesis that rotation-induced mixing, such as meridional circulation and shear turbulence, produces measurable correlations between mass and [N/C] ratios.
• To identify and quantify the role of initial rotation (via vsini) in shaping surface abundances during the main sequence and red giant phases.
• To provide observational constraints for rotating stellar evolution models by comparing observed [N/C] ratios with theoretical predictions across different masses and evolutionary stages.

Proposed method

• High-resolution FEROS spectroscopy (R = 48,000, S/N > 200) was obtained for 19 evolved intermediate-mass stars using the ESO 1.52 m telescope at La Silla, Chile.
• Atmospheric parameters (Teff, log g, [Fe/H]) and CNO abundances were derived from spectral analysis to determine surface composition.
• Stellar masses and evolutionary states were estimated using the Hertzsprung-Russell (HR) diagram, with evolutionary tracks from Schaller et al. (1992) and models from Meynet & Maeder (2000).
• The [N/C] ratio was calculated as a function of stellar mass and effective temperature, with a focus on the red supergiant phase (log Teff = 3.61–3.70).
• Stars were grouped by vsini, and the [N/C] vs. mass relation was analyzed separately for stars with vsini > 6 km s⁻¹ to isolate rotational effects.
• Comparative analysis was performed between observed [N/C] values and predictions from both non-rotating (Schaller et al. 1992) and rotating (Meynet & Maeder 2000) stellar evolution models.

Experimental results

Research questions

• RQ1 Is there a measurable correlation between stellar mass and the [N/C] abundance ratio in evolved intermediate-mass stars?
• RQ2 Do observed [N/C] ratios in red supergiants exceed predictions from non-rotating stellar models, indicating additional mixing processes?
• RQ3 Does the [N/C] ratio increase with mass in stars with high initial rotation (vsini > 6 km s⁻¹), as predicted by rotating models?
• RQ4 Can the observed abundance patterns be explained by rotation-induced mixing mechanisms such as meridional circulation and shear turbulence?
• RQ5 Are there discrepancies between observed mixing efficiencies and theoretical rotating models, particularly for stars more massive than 8 M⊙ or with anomalously high [N/C] for their mass?

Key findings

• Fifteen out of 19 evolved intermediate-mass stars show signs of enhanced internal mixing, with a mean [N/C] ratio of +0.95 dex, significantly higher than the non-rotating model prediction of +0.72 dex.
• A clear correlation between [N/C] and stellar mass is observed in red supergiants (log Teff = 3.61–3.70) with vsini > 6 km s⁻¹, indicating increasing nitrogen enrichment with mass.
• Some 5 M⊙ stars exhibit [N/C] ratios comparable to those expected for 12 M⊙ stars, suggesting more efficient mixing than predicted by standard models.
• The rotating models of Meynet & Maeder (2000) show better agreement with observations than non-rotating models, but still fail to fully reproduce the observed [N/C] values in the most massive stars.
• The observed scatter in [N/C] values is likely due to the high fraction of stars reaching the RGB with vsini > 6 km s⁻¹, indicating that rotation is a dominant factor in mixing efficiency.
• The presence of fully mixed 5 M⊙ stars in regions where blue loops are not expected suggests possible uncertainties in mass estimates, loop extent, or observational errors.