Spectroscopic eclipse mapping of mass-accreting Algol-type stars with pulsating components
Final Report Abstract
Asteroseismology is a modern tool for the investigation of stellar interiors. Preconditions for its application are the detection of non-radial pulsations, the measurement of their frequencies and the identification of the corresponding pulsation modes and a precise determination of the fundamental stellar parameters. Eclipsing binaries (EBs) are the most beneficial objects with respect to an asteroseismic modeling because their basic stellar properties can be derived from the light curve analysis. oEA stars are semi-detached, eclipsing Algol-type systems where the primary (gainer) shows δ Scuti-like oscillations. During episodes of rapid mass transfer from the cool secondary to the hot primary stellar evolution occurs on a short time scale. The investigation of the interaction between mass transfer and pulsations in oEA stars can lead to conclusions about the excitation mechanism of the observed pulsations, the internal structure of the pulsating, mass gaining star and about the change of the structure of its outer layers during rapid mass transfer episodes. The aim of this project was the investigation of oEA stars based on observed time series of highresolution spectra together with photometric data. For this purpose, we established a new program that computes synthetic, composite line profiles of EBs observed in disk-integrated light for all orbital phases. Algol-typical effects like gas stream or accretion disk can be included. The program is able to optimize the fundamental stellar and system parameters by a comparison of the synthetic with the observed spectra. We also implemented the calculation of pulsational perturbed line profiles (velocity field perturbations only). The program was applied to observed time series of spectra of the two oEA stars RZ Cas and TW Dra. From the light curve analysis in combination with the analysis of spectral line profile variations observed during out-of-eclipse and in-eclipse phases we extracted information on the fundamental stellar parameters of the binaries as well as on the pulsations of their oscillating components. We investigated the spatial filtration effect that occurs during the eclipses when the symmetry of the pulsation pattern on the stellar surface of the primary is lifted by the timely variable obscuration by the secondary and used it to obtain further constraints on the nature of the excited non-radial pulsation modes. In the case of RZ Cas, we could model the composite line profiles observed in 2006 in all orbital phases without taking Algol-typical effects like gas stream or accretion disk into account. The object was in a quiet state. To model the surface intensity distribution of the secondary, we had to include a large cool spot pointing towards the primary, presumably originating from a cooling mechanism by the enthalpy transport via the inner Lagrangian point. The time series observed in 2001, on the other hand, could not be fitted by such a simple model and required the inclusion of an accretion disk with a thickness comparable to the diameter of the primary. In the O-C residuals of the fit we clearly see the effects of dense circumbinary matter of complex structure. We conclude that in 2001 the star was undergoing a phase of rapid mass transfer. We could show that this episode changed the pulsation pattern observed in radial velocity (low-degree modes) and in the LSD profiles (high-degree modes). Moreover, we could measure a change of the orbital period by 2 seconds between 2001 and 2006. From this period change we estimate a mass transferred from the secondary to the outer layers of the primary of 6 × 10−5 M⊙ . Our investigation of TW Dra showed that this object was in a quiet state in both epochs of observations in 2007 and 2008. Our attempt to include the calculation of line profile variations due to non-radial pulsations gave encouraging results. We could derive common relationships between the inclination of the rotation axis of the oscillating primary and the amplitudes of the induced radial velocity variations in dependence on the pulsation characteristics and show that the spatial filtration effect amplifies these amplitudes during the eclipses remarkably. The derived relationship between the l, m wavenumbers of the pulsation modes and the amplification factors and the detection that the sectoral modes show a completely different behavior during the eclipses compared to all other modes can be used for a future mode identification based on radial velocity measurements or, in an improved version of our program, directly on the line or LSD profile variations.
Publications
- 2008, A&A, 480, 247: The eclipsing binary star RZ Cassiopeiae. II. Spectroscopic monitoring in 2006
Lehmann, H., & Mkrtichian, D.E.
- 2008, CoAst 157, 332: The oEA star TW Dra - a spectroscopic analysis
Lehmann, H., Tkachenko, A., Tsymbal, V., Mkrtichian, D.E.
- 2008, CoAst 157, 377: Spectroscopic solution for the oEA star RZ Cas using the SHELLSPEC code
Tkachenko, A., Lehmann, H., Tsymbal, V., Mkrtichian, D.E.
- 2009, A&A 504,991: Spectroscopic modeling of oscillating Algol-type stars. I. RZ Cassiopeia
Tkachenko, A., Lehmann, H., Mkrtichian, D.E.
- 2009, CoAst 159, 45: TW Dra: NRP mode identification with FAMIAS
Lehmann, H., Tkachenko, A., Mkrtichian, D.E.
- 2010, AJ 139, 1327: Spectroscopic modeling of the Algol-type star TW Draconis
Tkachenko, A., Lehmann, H., Mkrtichian, D.
