Circumstellar Matter of B-type Stars

• Thesis title in Czech: Circumstellar Matter of B-type Stars
• Thesis title in English: Circumstellar Matter of B-type Stars
• Key words: spektrální diagnostika|látka v okolí hvězd|horké hvědy
• English key words: spectroscopic diagnosis|circumstellar matter|hot stars
• Academic year of topic announcement: 2022/2023
• Thesis type: dissertation
• Thesis language: angličtina
• Department: Astronomical Institute of Charles University (32-AUUK)
• Supervisor: doc. Mgr. Daniela Korčáková, Ph.D.
• Author: hidden - assigned and confirmed by the Study Dept.
• Date of registration: 02.08.2022
• Date of assignment: 02.08.2022
• Confirmed by Study dept. on: 20.09.2022

Guidelines

A small sample of B-type stars shows forbidden emission lines in their spectra and strong infrared excess due to the presence of circumstellar gas and dust. Such type of spectra has been found among supergiants, Herbig Ae/Be stars, compact planetary nebulae, and symbiotic stars (Lamers et al., 1998). However, there is a group called FS CMa stars, whose nature and evolutionary status has not been determined.

The focus of the thesis is the later one, FS CMa group. Currently, the most favourable scenario is the binarity. However, not sufficient binaries have been found among FS CMa stars. Moreover, almost every sigh of the binarity may be explained by another physical process or inappropriate observational technique. Recently, very strong magnetic field was discovered in IRAS 17449+2320 (Korčáková et al., 2022). This opened the possibility that some of the FS CMa stars are post-merger systems. This hypothesis is supported by large space velocity toward the galactic pole indicating that the star escaped from the birth cluster. The conservation of the momentum supports the companion merging.

One of the key tasks is to identify the candidates of post-mergers. Due to the huge amount of the circumstellar matter, the classification of the individual objects is very difficult. The list of the FS CMa stars is highly probably polluted by post-AGB objects, symbiotic stars, and Be/X-ray binaries. Currently, the amount of data is sufficient to study physical processes that take place in these objects, e.g. material ejecta or pulsations.

References

Dopita, M.A., Sutherland, R.S. Astrophysics of the Diffuse Universe (Springer), 3rd edition, 2003
Emerson, D., 1999, "Interpreting Astronomical Spectra", John Wiley & Sons
Gray, D.F., ``The Observation and Analysis of Stellar Photospheres'', Cambridge University Press, 2005
Jaschek, C., Andrillat, Y., Jaschek, M., 1996, A&AS 117, 281
Lamers, H.J.G.L.M., Zickgraf, F.J., de Winter, D., Houziaux, L., Zorec, J., 1998, A&A 340, 117
Mihalas, D. Stellar atmosheres (San Francisco: Freeman), 2nd edition, 1978
Miroshnichenko, A. S. 2007, ApJ, 667, 497
Miroshnichenko, A. S., Manset, N., Kusakin, A. V., et al. 2007, ApJ, 671, 828
Osterbrock, D. E. & Ferland, G. J. 2005, "Astrophysics Of Gaseous Nebulae And Active Galactic Nuclei" (University Science Books, 2nd edition)
Rivinius, T., Carciofi, A., & Martayan, C. 2013, A&ARv, 21, 69
Stars with the B[e] Phenomenon. ASP Conference Series, Vol. 355, M. Kraus & A. Miroshnichenko eds., San Francisco: Astronomical Society of the Pacific, 2006
The B[e] Phenomenon: Forty Years of Studies. ASP Conference Series, Vol. 508., A. Miroshnichenko, S. Zharikov, D. Korčáková & M. Wolf San Francisco: Astronomical Society of the Pacific, 2017
Zickgraf, F.J., Wolf, B., Stahl, O., Leitherer, C., & Klare, G. A&A, 143, 421

Preliminary scope of work in English

Among B-type stars are observed stars surrounded by the gas, eventually also by dust. The gas reveals 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 Rivinius, 2013). 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 (Lamers et al., 1998). 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 (Zickgraf et al., 1985). However, there are B[e] stars whose evolutionary status is not known, FS CMa stars (Miroshnichenko, 2007). 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. Recently, it has been shown that mergers played an important role in the formation of first stars and properties of young galaxies.