OBA Stars: Variability and Magnetic Fields
Published June 20, 2021 | Version v1
Presentation Open

The First Detection of a Strong Magnetic Field in a FS CMa Star

Authors/Creators

  • 1. Astronomical Institute, Charles University, Praha, Czech Republic
  • 2. Department of Physics and Astronomy, University of North Carolina at Greensboro, Greensboro, USA, Main (Pulkovo) Astronomical Observatory of the Russian Academy of Sciences, Saint-Petersburg, Russia, Fesenkov Astrophysical Institute, Almaty, Kazakhstan
  • 3. 5 Department of Physics and Astronomy, University of Victoria, Victoria, Canada
  • 4. Instituto de Astronomia, Universidad Nacional Autonoma de Mexico, Mexico
  • 5. Canada-France-Hawaii Telescope Corporation, USA
  • 6. Institute of Theoretical Physics and Astrophysics, Masaryk University, Brno, Czech Republic
  • 7. Astronomical Institute of the Academy of Science of the Czech Republic, Ondrejov, Czech Republic
  • 8. Department of Physics and Astronomy, University of North Carolina at Greensboro, Greensboro, USA
  • 9. Department of Physics and Astrophysics, University of Delhi, Indian Institute of Astrophysics, Bangalore, India
  • 10. Al-Farabi Kazakh National University, Almaty, Kazakhstan
  • 11. Fesenkov Astrophysical Institute, Almaty, Kazakhstan
  • 12. Department of Physics, Montana State University, USA

Description

We report the first detection of a magnetic field in a star of the FS CMa type, a subgroup of objects showing the B[e] phenomenon. These stars are surrounded by an extended circumstellar medium, which is responsible for emission lines from both permitted and forbidden transitions, a large infrared excess due to dust, and a very complicated variability. The split of magnetically sensitive lines in the spectra of IRAS 17449+2320 unexpectedly revealed a strong magnetic field with strength of 6.2+/-0.2 kG. Our discovery indicates the importance of considering magnetic fields in studies of FS CMa stars, whose evolutionary status is still unknown.

We found that IRAS 17449+2320 has an effective temperature of ~10,000 K. Several spectral lines affected by the circumstellar medium (e.g., O I 7772–7775 AA, Balmer lines) regularly exhibit redshifted absorption components with a period of 36.13 days. The same periodicity shows up in the equivalent width of the Halpha line.

The analysis of our data suggests three possible scenarios for the nature and evolutionary status of the object: i) a magnetic A-type star primary with a hot dwarf secondary, ii) a binary merger, or iii) a magnetic Ap star near the terminal age main sequence.  The latter one may represent an object which has been sought for decades and fills the gap in the observations and theory of magnetic Ap stars. 

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