[Paper Review] Abundance analysis of prime B-type targets for asteroseismology I. Nitrogen excess in slowly-rotating beta Cephei stars

T. Morel, K. Butler+3 more|ArXiv.org|Jul 12, 2006

Stellar, planetary, and galactic studies79 references92 citations

TL;DR

This study presents a detailed non-LTE abundance analysis of nine prime B-type stars for asteroseismology, focusing on nitrogen, carbon, oxygen, and other elements. It finds a mild nitrogen excess (up to 0.6 dex) in four slowly-rotating β Cephei stars—including β Cep, V2052 Oph, δ Cet, and ξ¹ CMa—despite their low rotational velocities, challenging current evolutionary models and suggesting possible roles for radiative diffusion or magnetic fields in surface abundance anomalies.

ABSTRACT

We present the results of a detailed NLTE abundance study of nine beta Cephei stars, all of them being prime targets for theoretical modelling: gamma Peg, delta Cet, nu Eri, beta CMa, xi1 CMa, V836 Cen, V2052 Oph, beta Cep and DD (12) Lac. The following chemical elements are considered: He, C, N, O, Mg, Al, Si, S and Fe. Our abundance analysis is based on a large number of time-resolved, high-resolution optical spectra covering in most cases the entire oscillation cycle of the stars. Nitrogen is found to be enhanced by up to 0.6 dex in four stars, three of which have severe constraints on their equatorial rotational velocity, ΩR, from seismic or line-profile variation studies: beta Cep (ΩR~26 km/s), V2052 Oph (ΩR~56 km/s), delta Cet (ΩR < 28 km/s) and xi1 CMa (ΩR sin i < 10 km/s). The existence of core-processed material at the surface of such largely unevolved, slowly-rotating objects is not predicted by current evolutionary models including rotation. We draw attention to the fact that three stars in this subsample have a detected magnetic field and briefly discuss recent theoretical work pointing to the occurrence of diffusion effects in beta Cephei stars possibly capable of altering the nitrogen surface abundance. On the other hand, the abundances of all the other chemical elements considered are, within the errors, indistinguishable from the values found for OB dwarfs in the solar neighbourhood. Despite the mild nitrogen excess observed in some objects, we thus find no evidence for a significantly higher photospheric metal content in the studied beta Cephei stars compared to non-pulsating B-type stars of similar characteristics.

Motivation & Objective

• To determine precise stellar parameters and elemental abundances in key β Cephei stars for improved asteroseismic modeling.
• To investigate whether the observed nitrogen excess in slowly-rotating β Cephei stars can be explained by standard evolutionary models.
• To assess whether the metallicity of β Cephei stars differs significantly from that of non-pulsating B-type stars.
• To explore the potential role of radiative diffusion and magnetic fields in altering surface abundances in these stars.

Proposed method

• Conducted a curve-of-growth abundance analysis using high-resolution, time-resolved optical spectra covering the full pulsation cycle.
• Employed non-LTE (NLTE) modeling to account for departures from local thermodynamic equilibrium in the atmospheres of early B-type stars.
• Analyzed spectral lines of He, C, N, O, Mg, Al, Si, S, and Fe to derive surface abundances.
• Compared results with solar-scaled abundances and with those of nearby OB dwarfs to assess metallicity and chemical peculiarities.
• Used constraints on equatorial rotational velocity (ΩR) from seismic and line-profile variation studies to identify slowly-rotating targets.
• Evaluated the impact of magnetic fields and radiative forces on elemental separation, particularly for nitrogen.

Experimental results

Research questions

• RQ1 Do slowly-rotating β Cephei stars exhibit nitrogen excesses inconsistent with standard evolutionary models?
• RQ2 Is the observed nitrogen enrichment in these stars attributable to rotational mixing or alternative processes like radiative diffusion?
• RQ3 How do the surface abundances of β Cephei stars compare to those of non-pulsating B-type dwarfs in the solar neighborhood?
• RQ4 Can radiative forces explain the observed nitrogen excess, especially given the low rotational velocities?
• RQ5 What is the role of magnetic fields in enhancing or enabling surface abundance anomalies in these stars?

Key findings

• Nitrogen is enhanced by up to 0.6 dex in four β Cephei stars: β Cep, V2052 Oph, δ Cet, and ξ¹ CMa, despite their low rotational velocities.
• The nitrogen excess occurs in stars with severe constraints on equatorial rotational velocity (ΩR < 28 km s⁻¹ for δ Cet, ΩR ≈ 26 km s⁻¹ for β Cep, and ΩR sin i < 10 km s⁻¹ for ξ¹ CMa), contradicting predictions of standard models including rotation.
• The metallicity of the studied β Cephei stars is unremarkable and consistent with that of nearby early B-type dwarfs, with no significant excess in global metallicity (Z).
• The [N/C] and [N/O] ratios in ν Eri are approximately 0.25 dex above solar, indicating potential deep mixing or radiative diffusion effects.
• Three stars in the sample (β Cep, V2052 Oph, and ν Eri) have detected magnetic fields, suggesting a possible link between magnetism and abundance anomalies.
• Recent theoretical work indicates that radiative forces on nitrogen can exceed those on carbon and oxygen, potentially driving nitrogen accumulation at the surface under specific stellar conditions.