[Paper Review] New Nitrogen and Carbon in AF-supergiants

Kim A. Venn, N. Przybilla|arXiv (Cornell University)|Dec 11, 2002

Stellar, planetary, and galactic studies3 citations

TL;DR

This study reanalyzes nitrogen and carbon abundances in 22 Galactic and 9 SMC A- and F-supergiants using improved non-LTE models and updated solar abundances, finding significantly higher nitrogen enrichments than previously reported. The results indicate widespread rotational mixing and possible first dredge-up in both populations, with stronger enrichment in the metal-poor SMC stars, supporting the role of rotation and metallicity in shaping surface abundances in massive stars.

ABSTRACT

The AF-supergiants in the Galaxy and the SMC allow us to examine predictions from evolution models through their CNO abundances. In these proceedings, we recalculate the NLTE nitrogen abundances in 22 Galactic and 9 SMC A-supergiants using improved atomic data and model atmospheres to compare with new evolution models. The new abundances are higher than previously published values, and suggest that most of these stars have undergone substantial mixing with CN-cycled gas. While there is no clear relationship with mass, there is an apparent relation with metallicity since the SMC stars (including B-stars) have larger nitrogen enrichments. We suggest that rotational mixing is indicated from the main-sequence throughout the supergiant range, with more substantial rotational mixing in the SMC stars. In addition, the SMC AF-supergiants appear to have undergone the first dredge-up during a previous red giant phase, and possibly the Galactic AF-supergiants have as well. All abundances are compared to the new solar abundances from M. Asplund (this conference).

Motivation & Objective

• To re-evaluate nitrogen and carbon abundances in A- and F-supergiants using improved non-LTE atomic data and model atmospheres.
• To test predictions of rotating stellar evolution models by comparing observed CNO abundances with theoretical mixing scenarios.
• To assess the role of metallicity in driving nitrogen enrichment through rotational mixing and first dredge-up processes.
• To determine whether Galactic and SMC supergiants have undergone the first dredge-up and/or significant rotational mixing during their evolution.
• To reconcile observed abundance patterns with new solar abundances from Asplund et al. (2002) and updated evolutionary models.

Proposed method

• Employed non-LTE (NLTE) radiative transfer calculations to derive nitrogen and carbon abundances from high-resolution optical spectra of A- and F-supergiants.
• Used updated model atmospheres with improved atomic data for nitrogen lines, particularly focusing on the N ii 4634–4636 Å lines.
• Applied the new solar CNO abundances from Asplund et al. (2002), which reduced the solar nitrogen abundance by ~0.3 dex compared to older values.
• Compared observed N/C and [N/Fe] ratios with predictions from rotating stellar evolution models (e.g., Meynet & Maeder 2002) and first dredge-up theory.
• Analyzed the correlation between nitrogen abundance, metallicity, and stellar mass to infer evolutionary history.
• Used HST-STIS UV data for boron measurements in main-sequence B-stars to test the timing of LiBeB depletion relative to nitrogen enrichment.

Experimental results

Research questions

• RQ1 Do the new NLTE nitrogen abundances in A- and F-supergiants indicate stronger surface enrichment than previously reported?
• RQ2 Is there a correlation between nitrogen enrichment and metallicity, as suggested by the higher N abundances in SMC supergiants compared to Galactic ones?
• RQ3 Can the observed spread in nitrogen and N/C ratios be explained by rotational mixing during the main-sequence phase?
• RQ4 Do the abundance patterns in Galactic and SMC supergiants support the occurrence of the first dredge-up during a previous red giant phase?
• RQ5 To what extent do the observed abundance trends in supergiants require enhanced rotational mixing efficiency beyond current theoretical predictions?

Key findings

• The new NLTE nitrogen abundances in Galactic and SMC A- and F-supergiants are significantly higher than previous LTE-based estimates, with a mean enhancement of ~0.5 dex in nitrogen relative to solar.
• SMC supergiants exhibit the largest nitrogen enrichments, with [N/Fe] values up to ~+0.7 dex, suggesting stronger mixing efficiency in metal-poor environments.
• The observed spread in nitrogen abundances across both Galactic and SMC supergiants is best explained by rotational mixing during the main-sequence phase, rather than mass or luminosity differences.
• The data indicate that both Galactic and SMC supergiants likely underwent the first dredge-up during a prior red giant phase, with stronger evidence in the SMC due to higher N enrichment.
• The correlation between boron depletion and lack of nitrogen enrichment in main-sequence B-stars supports the role of rotational mixing in depleting fragile elements before significant nitrogen production.
• The combination of boron depletion and high nitrogen enrichment in SMC stars points to both rotational mixing and post-red-giant branch (RGB) convective mixing as key drivers of surface abundance evolution.