Daniela Korčáková

MERGERS

Mergers explain all the observed phenomena of FS CMa stars (see Chapter nature). However, it is not the explanation for all of them. The criteria that define the group Miroshnichenko (2007) are not sufficiently sharp and are similar to other spectral groups as young planetary nebulae, supergiants of lower luminosity, symbiotic stars or mass-transfer binaries.

Why are FS CMa mergers important?

The whole evolution of the mergers may be hidden in the FS CMa group.
- pre-merger phase
  - An important phenomena that may lead to a merger are dynamical processes in the birth cluster. Therefore the tertiary or multiple systems are not needed to create the merger.
  - Wind-wind or wind-atmosphere interaction that leads to the molecule and dust formation creates the similar situation as is in the common-envelope phase earlier than at the end of the lives on the main-sequence.
- merger phase
  - Some of the most covered stars may be in this phase.
- post-merger phase
  When the envelope starts to be transparent, the newly born star
  - is along the terminal main sequence,
  - may have strong magnetic field that causes the slow rotation
  - may have large space velocities,
  - has slightly peculiar spectra due to the fact that the star is still out of thermal equilibrium, the matter falls on the stellar surface heating the region, the strong magnetic field creates spots of the different composition.
  In this stage, it should be easiest to identify a merger.
The cooler FS CMa post-mergers may be the progenitor of magnetic Ap stars. Before the entering the main-sequence, the newly born star is overluminous, because the thermal equilibrium is not set there.

(Schneider et al., 2019) .
They also may offer the explanation of young strongly magnetic white dwarfs.
Mergers may contribute to the enrichment of the intestellar material.
Mergers may contribute to the enrichment of the intergalactic material. Some merger events occur out of the galaxies.
The wind-wind/wind-atmosphere interaction provides an excellent "laboratory" for the study of the dust creation.
As the newly born star is out of the thermal equilibrium, it is a perfect object to test the stellar structure and evolution models.

Once we found the first post-merger (IRAS 17449+2320) we started the work on their properties:

N-body simulations

MHD simulations

search for new post-merger candidates

spectroscopy

imaging