[Paper Review] Mixing in magnetic OB stars

T. Morel|arXiv (Cornell University)|Oct 20, 2010

Stellar, planetary, and galactic studies1 references3 citations

TL;DR

This study investigates CNO surface abundances in magnetic OB stars using NLTE spectroscopic analysis to assess whether magnetic fields drive enhanced mixing. Despite strong magnetic fields in two main-sequence stars (NGC 2244 #201 and HD 57682), no nitrogen excess or core-processed material is detected, challenging the assumption that magnetic fields alone cause N-rich surface anomalies. The results suggest additional, still-unknown parameters control mixing efficiency in massive stars.

ABSTRACT

Recent observations have revealed the existence of a population of slowly-rotating, nitrogen-rich B dwarfs that are not predicted by evolutionary models including rotational mixing. However, as theoretical arguments suggest that magnetic processes may significantly increase the efficiency of the transport of the chemical elements, it is of importance to assess the extent of mixing in some known magnetic OB stars. We review our knowledge of the CNO abundance properties of these objects and present the first results of an NLTE abundance study of massive stars identified as being magnetic by the MiMeS collaboration. Although a nitrogen excess is often associated with the presence of a magnetic field, there is no evidence for a strict one-to-one correspondence between these two phenomena. This therefore suggests that other (still elusive) parameters may control the amount of mixing experienced by main-sequence OB stars.

Motivation & Objective

• To assess whether magnetic fields in OB stars enhance internal mixing, as predicted by theoretical models.
• To resolve the discrepancy between observed nitrogen-rich, slowly rotating B stars and current rotational mixing models.
• To investigate whether a direct link exists between magnetic field presence and surface nitrogen enrichment in main-sequence OB stars.
• To determine if magnetic fields alone are sufficient to explain the observed CNO abundance anomalies in massive stars.
• To evaluate the role of other physical parameters—beyond rotation and magnetic fields—in driving chemical mixing in massive stars.

Proposed method

• Acquired high-resolution (R ~ 46,000) FIES spectra of four O9–B2 IV–V stars using the Nordic Optical Telescope.
• Performed NLTE abundance analysis using Kurucz atmospheric models and the DETAIL/SURFACE line-formation codes.
• Determined atmospheric parameters (T_eff, log g, microturbulence) via ionisation balance of He, N, Ne, and Si species and Balmer line wings.
• Used classical curve-of-growth techniques to derive elemental abundances from observed spectral lines.
• Compared results with previous studies and with the magnetic, N-rich star τ Sco, analyzed under identical conditions.
• Evaluated the reliability of magnetic field detections using independent spectropolarimetric data from multiple collaborations.

Experimental results

Research questions

• RQ1 Is there a direct correlation between the presence of a magnetic field and surface nitrogen enrichment in main-sequence OB stars?
• RQ2 Do magnetic OB stars with strong fields exhibit measurable CNO abundance anomalies indicative of internal mixing?
• RQ3 How do the CNO surface abundances of magnetic stars compare to those of non-magnetic stars with similar evolutionary status and rotation?
• RQ4 What role do additional physical parameters—beyond magnetic fields and rotation—play in determining the extent of chemical mixing in massive stars?
• RQ5 To what extent do NLTE effects and model assumptions influence the interpretation of abundance anomalies in magnetic OB stars?

Key findings

• NGC 2244 #201, a slowly rotating magnetic star with a longitudinal field of ~500 G, shows no evidence of CNO-processed material, with [N/C] = –0.54 ± 0.14 and [N/O] = –0.95 ± 0.21, consistent with solar values.
• HD 57682, a magnetic O9 IV star with a polar field of ~1700 G, also shows no significant departure from solar CNO ratios, with [N/C] = –0.68 ± 0.30 and [N/O] = –0.79 ± 0.19.
• Despite similar masses and evolutionary stages, the magnetic star τ Sco exhibits a strong nitrogen excess ([N/C] ≈ –0.14), contrasting with the non-enriched NGC 2244 #201 and HD 57682.
• The absence of nitrogen excess in two magnetic stars with secure field detections challenges the hypothesis that magnetic fields alone drive significant mixing in massive stars.
• The results indicate that magnetic fields may not be the sole or dominant driver of surface abundance anomalies, suggesting the influence of other, still-unknown parameters.
• The study confirms that the relationship between magnetic fields and mixing is statistical rather than deterministic, as not all magnetic stars show N enrichment.