First detection of a magnetic field in low-luminosity B[e] stars

New scenarios for the nature and evolutionary stages of FS CMa stars^★

¹ Charles University, Faculty of Mathematics and Physics, Astronomical Institute, V Holešovičkách 2, 180 00 Praha 8, Czech Republic
e-mail: kor@sirrah.troja.mff.cuni.cz
² Department of Physics and Astronomy, University of Victoria, Victoria, BC, V8W 3P2, Canada
³ Canada-France-Hawaii Telescope Corporation, 65-1238 Mamalahoa Hwy, Kamuela HI 96743, USA
⁴ Helmholtz-Institut für Strahlen- und Kernphysik, University of Bonn, Nussallee 14-16, 53115 Bonn, Germany
⁵ Institute of Theoretical Physics and Astrophysics, Masaryk University, 611 37 Brno, Kotlářská 2, Czech Republic
⁶ Astronomical Institute of the Academy of Science of the Czech Republic, Fričova 298, 251 65 Ondřejov, Czech Republic
⁷ Department of Physics and Astronomy, University of North Carolina at Greensboro, Greensboro, NC 27402, USA
⁸ Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India
⁹ Indian Institute of Astrophysics, Block II, Koramangala, Bangalore 560034, India
¹⁰ Department of Physics, Montana State University, PO Box 173840, Bozeman, MT 59717–3840, USA

Received: 7 April 2021
Accepted: 31 December 2021

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 Hα line shows quasiperiodic behavior on timescale of 800 days. We detected a strong red-shifted absorption in the wings of Balmer and O I 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 binary left its birth cluster. Another possible scenario is a magnetic Ap star around Terminal-Age Main Sequence. 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.