Spectroscopic monitoring of bright, oscillating Algol-type stars

• Applicant: Dr. Holger Lehmann
• Subject Area: Astrophysics and Astronomy
• Term: from 2015 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 275029694

The class of oEA stars consists of eclipsing Algol-type systems with mass transfer where the mass-accreting primary shows delta Scuti-like oscillations. These systems are extraordinary objects for asteroseismic studies since they allow for an investigation of short-term dynamical stellar evolution during mass-transfer episodes, most probably caused by the magnetic activity cycle of the less massive secondary. Basic principles of the interaction between the magnetic cycle of the cool secondary, the occurrence of rapid mass-transfer episodes, the dynamical behavior of the system, and the excitation of different non-radial pulsation modes of the mass-gaining primary component can be studied in great detail. Advantages for the analysis of oEA stars follow from the fact that they are eclipsing binaries. This enables us to derive precise stellar and system parameters from the combined photometric-spectroscopic analysis. The identification of non-radial pulsation modes is simplified by the spatial filtration effect that occurs due to the scanning of the disk of the oscillating primary component by the secondary during the eclipses.The aim of this proposal is to derive the final results of 15 years of international spectroscopic and photometric monitoring of the key oEA star RZ Cas and of shorter time series of TW Dra and AS Eri, and to get new results from recent years of observations obtained by our international team. We want to use state-of-the-art methods for the investigation of a huge amount of collected data, in particular an improved method of least squares deconvolution that we want to develop in the framework of this project. It will enable us to decompose the time series of high-resolution composite spectra into time series of spectra of the individual components without losing information about the intrinsic oscillation signals. Our analysis will comprise the determination of precise stellar and system parameters, the search for episodes of rapid mass transfer including the calculation of mass-transfer rates during different epochs of observation, the identification of non-radial pulsation modes and, for the first time, an asteroseismic modeling of the oEA stars in active and inactive phases. The results from different methods, including 3D-hydrodynamic simulations, will be compared. Finally, we want to establish a consistent picture of the changes in all stellar and system parameters caused by the magnetic cycle of the cool secondary component, like orbital and rotation period, mass-transfer rate, and frequencies and amplitudes of the excited non-radial pulsation modes. The obtained results will help to clarify the questions about the role of magnetic activity in Algol-type stars and about the kind of mass and angular momentum transfer between their components.

• DFG Programme: Research Grants
• International Connection: Belgium, Russia, Thailand, Ukraine
• Partner Organisation: Russian Foundation for Basic Research
• Cooperation Partners: Dr. David Mkrtichian; Andrew Tkachenko, Ph.D.; Professor Vadim Tsymbal