Circumstellar Matter of B-type Stars
Advisor: Daniela Korcakova (AI MFF CUNI)
Funding: additional funds on top of basic stipend might be available
Among B-type stars are observed stars surrounded by the gas, eventualy also by dust. The gas revereals itself in the spectra as emission lines, while the dust is responsible for the infrared excess. These stars are very variable and many unsoved questions are related with them. The best studied are classical Be stars (see, e.g. review ). The gas emission comes from the decretion disk. Material outflow is supported by the fast rotation of the star and pulsations. However, the exact principle is not yet known. More complex circumstellar matter is around B[e] stars. The forbidden emission lines, pointing to very low density material in outer parts, and the infrared excess are detected. Such spectral properties show stars of different types and at different evolutionary stages. The B[e] stars have been found among supergiants, young planetary nebulae, Herbig Ae/Be stars, and symbiotic stars . The B[e] phenomenon is natural for young planetary nebulae and symbiotic stars. There has to be a phase during the expansion of the envelope when such spectral properties are observed. In Herbig Ae/Be stars, the accretion of the material is responsible for the B[e] phenomenon. The B[e] supergiants have the geometrically thick slowly expanding disk, surrounded by the dusty ring in the equatorial region, while the fast radiatively driven wind is around poles . However, there are B[e] stars whose evolutionary status is not known, FS CMa stars . These stars are the main topic of the proposed project.
Recently, it has been shown that one of the FS CMa stars is a post-merger system. The crucial question is whether this star was misclassified or whether all FS CMa stars are all post-merger systems. In the latter case, FS CMa stars provide an evolutionary sequence of the merging process. Understanding the merger process helps, among others, to understand the formation, evolution, and chemical composition of stars and interstellar matter. Resently, it has been shown that mergers played an important role in the formation of first stars and properties of young galaxies.
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