NATURE OF FS CMa STARS?

• Herbig Ae/Be stars

  • The spectra of FS CMa stars are almost identical with Herbig Ae/Be stars. However, FS CMa stars are too far from the star forming regions, therefore they can not be young.

• post-AGB stars

  • FS CMa stars have very similar spectral properties as post-AGB stars and some of them appear in the lists of post-AGB stars. However, they are differences (Miroshnichenko, 2018) rule out the post-ABG phase as an explanation for all FS CMa stars.

• extreme classical Be stars

  • classical Be stars do not have forbidden emission lines
  • classical Be stars do not have IR excess (If IR excess is present, it is weak and has different shape.)
  • classical Be stars are rapid rotators, while FS CMa stars are slow rotators

• binary?

  • The binarity offers a simple explanation of a large amount of the circumstellar gas and dust. Especially, the dust is very problematic to be explained. It needs very specific conditions for the creation, i.e., large density and low temperature.
  • There are some binaries among FS CMa stars.
  • The secondaries were found only in about 1/3 of FS CMa stars. In the rest of these stars, every sign of the binarity may be explained by another phenomenon or inaccurate observations. This persentage is in the agreement with the observed fraction of binaries in main sequence B-type stars. It may seem too low, because hot stars are mostly in binary or multiple systems. However, the binary radio rapidly decreases in late B-type stars (Vanbeveren & Mennekens) and most of FS CMa stars are of the late B-type.
  • The presence of the circumstellar matter may lead to the wrong interpretaion of the observations and results of the analysis (see here for more details).
  • warning: The literature is very biased in this field, especially notes on binary stars. It is necessary to follow the original papers with the observations.

• mergers

  • Mergers naturally explain all the observed phenomena.
  • The first indication that some FS CMa stars may be mergers was the discovery of two FS CMa stars in the central parts of two clusters (de la Fuente et al., 2015). (However, as showed (Dvořáková et al., 2024), it is not a necessary condition.)
  • Some observations cannot be explained by the secondary, on principle.
    • VLTI/PIONIER interferometric observations of Kluska et al. (2016)
    • Hα bisector variability of MWC 623 Polster et al. (2018)
    • polarimetric observations of MWC 623 Zickgraf & Schulte-Ladbeck (1989)
    • Moreover, signs of a secondary may be explained by another phenomenon in many cases (see here).
  • The strong magnetic field was discovered in IRAS 17449+2320 (Korčáková at al. 2022). The strong magnetic field may be generated during the merger (Schneider et al., 2019) .
    
  • FS CMa stars are slow rotators.
    As stronger the magnetic field is, as faster the star slows down (Schneider et al., 2019) .
  • About half of FS CMa stars have neutral lithium resonance lines in their spectra. For example, in MWC 623 Li I resonance doublet 6 708 Å is very strong. Lithium overabundance in half of red novae (nova-like events during the merger process) found Kamiński et al. (2023).
  • IRAS 17449+2320 is more likely a post-merger (Korčáková at al. 2022). The details can be found here.
  • N-body simulations of Dvořáková et al. (2024) show that 1.7% of stars is involved in mergers. 50% of mergers are B-type stars, that is 12.5% of B-type stars. The details are described here .
  • MHD simulations of a model of a FS CMa star Moranchel-Basurto et al. (2023) (2024) explained all the observed phenomena.
  • Based on the simulations of the merger evolution by Schneider et al. (2019), it was finally possible to explain the position of the FS CMa stars on the HRD .