[Paper Review] Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1

S. Bagnulo, J. D. Landstreet+4 more|ArXiv.org|Jan 23, 2006

Stellar, planetary, and galactic studiesPhysics and Astronomy42 references96 citations

TL;DR

This study presents a large-scale spectropolarimetric survey of A- and B-type stars in open clusters using FORS1 at the VLT, measuring longitudinal magnetic fields with ~100 G precision. It detects magnetic fields in 41 Ap/Bp stars—37 previously unknown—while finding no field in 138 normal A/B-type stars, confirming the method's reliability and providing critical age-tagged data to probe magnetic field evolution during main-sequence evolution.

ABSTRACT

About 5% of upper main sequence stars are permeated by a strong magnetic field, the origin of which is still matter of debate. With this work we provide observational material to study how magnetic fields change with the evolution of stars on the main sequence, and to constrain theory explaining the presence of magnetic fields in A and B-type stars. Using FORS1 in spectropolarimetric mode at the ESO VLT, we have carried out a survey of magnetic fields in early-type stars belonging to open clusters and associations of various ages. We have measured the magnetic field of 235 early-type stars with a typical uncertainty of about 100 G. In our sample, 97 stars are Ap or Bp stars. For these targets, the median error bar of our field measurements was about 80 G. A field has been detected in about 41 of these stars, 37 of which were not previously known as magnetic stars. For the 138 normal A and B-type stars, the median error bar was 136 G, and no field was detected in any of them.

Motivation & Objective

• To investigate how magnetic fields in A- and B-type stars evolve during the main sequence by studying stars in open clusters with known ages.
• To overcome the lack of reliable age estimates for field magnetic Ap stars by targeting cluster members with precise membership and age constraints.
• To test the sensitivity and reliability of FORS1 spectropolarimetry for detecting weak magnetic fields in faint, early-type stars.
• To identify new magnetic Ap/Bp stars in clusters and assess the prevalence of magnetic fields across different evolutionary stages.
• To provide a robust observational foundation for linking magnetic field strength and evolution in massive stars, enabling future theoretical modeling.

Proposed method

• Utilized the FORS1 spectropolarimeter on the ESO VLT to obtain circularly polarized spectra of 257 early-type stars in open clusters and associations.
• Measured the longitudinal magnetic field ⟨Bz⟩ using a combined analysis of H Balmer lines and metallic lines to enhance signal-to-noise for weak fields.
• Applied a linear least-squares fitting technique to the normalized Stokes V/I profiles, modeling the correlation between V/I and dI/dλ to detect magnetic signals.
• Employed a null-horizon technique to assess false-positive detection rates, ensuring that no field was detected in non-magnetic stars.
• Used high-precision astrometry from Hipparcos and Tycho-2 to confirm cluster membership and reduce contamination from field stars.
• Performed data reduction with careful sky subtraction, flat-fielding, and wavelength calibration to minimize systematics in polarized flux measurements.

Experimental results

Research questions

• RQ1 How does the magnetic field strength of Ap/Bp stars change during the main sequence evolution, as constrained by cluster age estimates?
• RQ2 What is the detection efficiency of FORS1 for weak magnetic fields in faint, early-type cluster stars?
• RQ3 Are magnetic fields in Ap/Bp stars correlated with rotation, chemical peculiarity, or evolutionary stage in cluster environments?
• RQ4 Is the observed magnetic field distribution in cluster A/B-type stars consistent with fossil field models?
• RQ5 Can the spectropolarimetric technique used here reliably distinguish true magnetic signals from spurious detections in non-magnetic stars?

Key findings

• The survey measured the longitudinal magnetic field ⟨Bz⟩ in 257 A- and B-type stars with a typical uncertainty of ~100 G, and detected a field in 41 Ap/Bp stars.
• Of the 41 detected magnetic stars, 37 were previously unknown as magnetic stars, significantly expanding the sample of age-tagged Ap stars.
• No magnetic field was detected in any of the 138 normal A- and B-type stars, with a median measurement uncertainty of 136 G, indicating high detection reliability.
• The null detection in non-peculiar stars strongly supports that the method is not prone to spurious detections, with only one potential outlier (CD-60 1996) requiring further scrutiny.
• The survey identified 90 new candidate Ap cluster members and added approximately 30 new clusters to the list of those searched for magnetic stars.
• The detection of a field in HD 298045 (M3 giant) is highly questionable due to spectral contamination from the LADC, highlighting the need for caution in late-type star applications.