[Paper Review] Weak magnetic fields in Ap/Bp stars: Evidence for a dipole field lower limit and a tentative interpretation of the magnetic dichotomy

M. Aurière, G. A. Wade+23 more|ArXiv.org|Oct 8, 2007

Stellar, planetary, and galactic studiesPhysics and Astronomy68 references155 citations

TL;DR

This study analyzes 28 Ap/Bp stars using high-precision spectropolarimetry to investigate weak magnetic fields, finding a consistent lower limit of ~300 G in dipole field strength across all detected stars. The authors propose that this threshold reflects a critical field strength required for large-scale magnetic field stability, explaining the rarity of magnetic fields in intermediate- and massive stars due to field decay below this threshold.

ABSTRACT

We have investigated a sample of 28 well-known spectroscopically-identified magnetic Ap/Bp stars, with weak, poorly-determined or previously undetected magnetic fields, with the aim of exploring the weak part of the magnetic field distribution of Ap/Bp stars. Using the MuSiCoS and NARVAL spectropolarimeters we have obtained 282 LSD Stokes V signatures of our 28 sample stars. All stars were detected, showing clearly that when observed with sufficient precision, all firmly classified Ap/Bp stars show detectable surface magnetic fields. To better characterise the surface magnetic field intensities and geometries of the sample, we have inferred the dipolar field intensity and the magnetic obliquity. The distribution of derived dipole strengths for these stars exhibits a plateau at about 1 kG, falling off to larger and smaller field strengths. Remarkably, in this sample of stars selected for their presumably weak magnetic fields, we find only 2 stars for which the derived dipole strength is weaker than 300 G. We interpret this "magnetic threshold" as a critical value necessary for the stability of large-scale magnetic fields, and develop a simple quantitative model that is able to approximately reproduce the observed threshold characteristics. This scenario leads to a natural explanation of the small fraction of intermediate-mass magnetic stars. It may also explain the near-absence of magnetic fields in more massive B and O-type stars.

Motivation & Objective

• To investigate the weak end of the magnetic field distribution in Ap/Bp stars, which remains poorly studied due to observational bias toward stronger fields.
• To determine whether the magnetic field distribution exhibits a lower cutoff or extends monotonically toward arbitrarily weak fields.
• To test the hypothesis that a critical magnetic field strength is required for the stability of large-scale magnetic configurations in these stars.
• To explore the implications of this threshold for the observed scarcity of magnetic fields in intermediate-mass and massive B/O-type stars.

Proposed method

• High-precision Stokes V Zeeman signatures were measured using the MuSiCoS and NARVAL spectropolarimeters at the Télescope Bernard Lyot.
• Least Squares Deconvolution (LSD) was applied to extract high-SNR longitudinal magnetic field measurements with a median uncertainty of 40 G.
• Rotational phase-folded longitudinal field data were modeled to infer the dipole field strength and magnetic obliquity for each star.
• A critical field strength threshold was derived based on the stability of large-scale magnetic fields against differential rotation-driven instabilities.
• Theoretical modeling compared the observed field threshold to known instability criteria, such as the Tayler instability and critical field scaling with rotation period.
• The model was extended to explain the low incidence of magnetic fields in massive B and O-type stars by comparing their required critical field strengths to observed values.

Experimental results

Research questions

• RQ1 Is there a lower bound in the dipole magnetic field strength distribution of Ap/Bp stars, or does it extend monotonically toward arbitrarily weak fields?
• RQ2 What physical mechanism could explain a lower limit in observed magnetic field strengths in Ap/Bp stars?
• RQ3 How does the critical field strength required for large-scale field stability scale with stellar rotation period and physical parameters?
• RQ4 Why are magnetic fields so rare in intermediate-mass and massive stars, despite theoretical expectations of fossil fields?
• RQ5 Can the observed magnetic threshold be quantitatively linked to known magnetic instability mechanisms such as the Tayler instability?

Key findings

• All 28 Ap/Bp stars in the sample showed detectable Stokes V Zeeman signatures, confirming that even weakly magnetic Ap/Bp stars possess measurable surface magnetic fields.
• Only two stars in the sample had derived dipole field strengths below 300 G, indicating a sharp lower limit in the observed magnetic field distribution.
• The distribution of dipole field strengths exhibits a plateau near 1 kG, with a decline toward both higher and lower values, suggesting a stable configuration at this field level.
• The observed 300 G threshold is consistent with a critical field strength required to stabilize large-scale magnetic configurations against differential rotation-driven instabilities.
• The model predicts that stars with initial fields below this threshold would be unstable and decay, explaining the apparent absence of magnetic fields in most intermediate-mass stars.
• The critical field strength increases with stellar mass and rotation rate, which naturally explains the near-absence of magnetic fields in massive B and O-type stars, as their required critical fields are substantially higher.