First detection of a magnetic field in low-luminosity B[e] stars: New scenarios for the nature and evolutionary stages of FS CMa stars

TL;DR

This paper reports the first detection of a magnetic field in a low-luminosity B[e] star, IRAS 17449+2320, revealing new insights into the nature and evolutionary stages of FS CMa stars, and challenging previous classifications.

Contribution

It presents the first magnetic field measurement in an FS CMa star, suggesting new scenarios for their origin and evolution, including post-merger or magnetic Ap star hypotheses.

Findings

Magnetic field modulus of 6.2 kG detected in IRAS 17449+2320.

Spectral analysis indicates a hot source with temperature >50,000 K.

Quasiperiodic V/R variability of H alpha line on 800-day timescale.

Abstract

We report the first detection of the magnetic field in a star of FS CMa type, a subgroup of objects characterized by the B[e] phenomenon. The split of magnetically sensitive lines in IRAS 17449+2320 determines the magnetic field modulus of 6.2+/-0.2 kG. Spectral lines and their variability reveal the presence of a B-type spectrum and a hot continuum source in the visible. The hot source confirms GALEX UV photometry. Because there is a lack of spectral lines for the hot source in the visible, the spectral fitting gives only the lower temperature limit of the hot source, which is 50 000 K, and the upper limit for the B-type star of 11 100 K. The V/R ratio of the H alpha line shows quasiperiodic behavior on timescale of 800 days. We detected a strong red-shifted absorption in the wings of Balmer and OI lines in some of the spectra. The absorption lines of helium and other metals show no, or very small, variations, indicating unusually stable photospheric regions for FS CMa stars. We detected two events of material infall, which were revealed to be discrete absorption components of resonance lines. The discovery of the strong magnetic field together with the Gaia measurements of the proper motion show that the most probable nature of this star is that of a post-merger object created after the leaving the binary of the birth cluster. Another possible scenario is a magnetic Ap star around Terminal-Age Main Sequence (TAMS). On the other hand, the strong magnetic field defies the hypothesis that IRAS 17449+2320 is an extreme classical Be star. Thus, IRAS 17449+2320 provides a pretext for exploring a new explanation of the nature of FS CMa stars or, at least, a group of stars with very similar spectral properties.