The mass distribution of stellar mergers

A new scenario for several FS CMa stars

¹ Charles University, Faculty of Mathematics and Physics, Astronomical Institute, V Holešovičkách 2, 180 00 Praha 8, Czech Republic
² Helmholtz-Institut für Strahlen- und Kernphysik, University of Bonn, Nussallee 14-16, 53115 Bonn, Germany
³ Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

Received: 13 February 2024
Accepted: 29 June 2024

Abstract

Context. FS CMa stars belong to a diverse group of stars exhibiting the B[e] phenomenon, which manifests itself mainly by the presence of forbidden emission lines and a strong infrared (IR) excess in their spectra. Only a few tens of FS CMa stars are known and their nature is still unclear. Recently, a strong magnetic field has been discovered in the FS CMa star IRAS 17449+2320. Its strength combined with an unusually high space velocity in the direction of the Galactic north pole point to the object having a post-merger nature. Such stellar mergers may provide an explanation for the complex and sometimes chaotic behaviour of some of the FS CMa stars.

Aims. In order to find out whether B-type stellar mergers are detectable, we did a statistical study of numerical simulations using Aarseth’s NBODY6 code. We show the importance of stellar mergers of low- to intermediate-mass stars (from ≈1.4 to ≈8 M_⊙) and for B-type stars in particular.

Methods. We analysed two sets of N-body simulations with different initial orbital period distributions. In the simulations, more massive binaries are treated differently than less massive binaries and the mass limit usually used is 5 M_⊙. In addition to this, we also used the value of 2 M_⊙ to test the influence of this ambiguous limit on the results. Looking at mass, distance from their birth cluster, and velocity distributions, we investigated the statistical significance of individual spectral types in terms of merger dynamics and how merger events affect the stellar evolution.

Results. We have found that around 50% of stars in the simulated open clusters involved in the formation of mergers are B-type stars. As a result, more than 50% of the merger products end up as a B-type star as well. Also, between 12.54% and 23.24% of all B-type stars are mergers. These results are a natural consequence of the initial mass function, initial distribution of the binary star parameters, and large range of masses for B-type stars. A non-negligible fraction of mergers occurred before entering the common envelope phase and we detected merger events at extragalactic distances. The total amount of detected mergers could have an impact on the chemical evolution of galaxies. The resulting mass distribution of merger products shows a peak for A-type stars, which is in agreement with observed massive Ap stars. Post-mergers among late B-type FS CMa stars could be the progenitors of very massive magnetic Ap stars. our results could also help to explain the nature of some magnetic white dwarfs. We present a comparison of the W component and the space velocity of the simulated mergers with a sample of observed FS CMa stars.