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

• Final Report Year: 2020

Final Report Abstract

In this joint DFG-RFBR project, we successfully developed and tested the new method and computer program LSDbinary and applied it to active Algol-type stars that show masstransfer episodes and δ Sct-like pulsations of the mass-gaining star (oEA stars). LSDbinary calculates separated least-squares decomposed (LSD) profiles and radial velocities (RVs) of the components of SB2 stars from observed time series of high-resolution spectra. R CMa is a short-period, Algol-type star of small mass ratio. We used it as the test case for LSDbinary. Results showed that very accurate RVs and LSD profiles can be obtained, even during the eclipses when the Rossiter-McLaughlin effect occurs, and also for binaries with very small light ratios. Our investigation confirmed the small mass ratio of 0.129, and also the existence of an oscillation frequency of 21.38 c d^−1 in the RVs, previously found from light curve analysis. A He I line in the spectra points to mass transfer and we found evidence from system geometry that the cool companion fills its Roche lobe. In the result we could show that R CMa is a semi-detached Algol belonging to the class of oEA stars. RZ Cas is one of the brightest and photometrically best investigated oEA stars. We monitored the star over a time span of more than 16 years. We analysed the obtained high-resolution spectra, aiming to find evidence for episodes of mass transfer and possible correlations with changes in system, stellar, and pulsation parameters. We find that the system can be best modelled when assuming two dark spots on the secondary star, S1 facing the primary component, and S2 on the opposite side. The variations observed for v sin i, amplitude of the Rossiter effect, and the spot strengths occur on a common time scale of 9 yr that can also be found in orbital period variations. We conclude that we see a 9 yr magnetic activity cycle of the cool companion, caused by an 18 yr dynamo cycle that includes a reversal of polarity. This is supported by the behaviour of the two dark spots which show the flip-flop effect (anti-correlation) in their strengths and by the close to 18 yr periodicity in longitudinal migration of S2. RZ Cas shows a damped activity cycle, starting with a high mass-transfer episode around 2001 with a calculated mass-transfer rate of 1.5 × 10^−6 Mꙩyr−1 , followed by a quiet period, slightly higher activity in 2013 and 2014, and again a quiet period until 2016. However, due to missing data in 2010 and 2011, we cannot exclude that a second high mass-transfer episode occurred in this time. The fact that we observed minimum strength of S1 in the high mass-transfer phase in 2001 let us assume that S1 represents a Wilson depression in the atmosphere close to Lagrangian point L1. In this case, mass transfer is controlled by the varying magnetic field strength of the cool companion via the variable depth of S1. From line profile variations, we find up to eight harmonics of the orbital (rotation) period, most likely caused by rotational modulation. In the 56 to 72 c d^−1 frequency range, three low-degree (l < 4) non-radial pulsation modes that were already known from previous studies and, for the first time, six high-degree (l up to 11) modes are found. Two of the low-degree modes are identified as zonal modes and at least one of them is a retrograde mode. Most of the high-degree modes are sectoral modes. All modes show variable frequencies and amplitudes, but we did not find correlations with the nine years magnetic activity cycle.

Publications

• 2018, PHOST conference in September 2018 at Banyuls-sur-mer, France ”Spectroscopic long-term monitoring of RZ Cas”
  Pertermann, F., Lehmann, H., Tsymbal, V. et al.
  
• 2016, ApJ 831, 17 ”Detection of Solar-like Oscillations, Observational Constraints, and Stellar Models for Theta Cyg, the Brightest Star Observed By the Kepler Mission”
  Guzik, J. A., Houdek, G., Chaplin, W. J.,...Lehmann, H., et al.
  
• 2018, A&A 615, 131 ”Spectroscopic time-series analysis of R Canis Majoris”
  Lehmann, H., Tsymbal, V., Pertermann, F., et al.
  
• 2018, MNRAS 475, 4745 ”The eclipsing binary star RZ Cas: accretion-driven variability of the multimode oscillation spectrum”
  Mkrtichian, D. E., Lehmann, H., Rodríguez, E., et al.
  
• 2018, MNRAS 481, 5660 ”Evidence for conservative mass transfer in the classical Algol system Delta Librae”
  Dervisoglu, A., Pavlovski, K., Lehmann, H., et al.
  
• 2019, Bulletin de la Société Royale des Sciences de Liège 88, 256 (Proceedings of the Second BINA workshop, October 2018, held in Brussels), ”A spectroscopic survey of oEA stars”
  Mkrtichian, D.; Engelbrecht, C.; Lampens, P., Lehmann, H., et al.
  
• 2020, Proceedings of the conference Stars and their Variability Observed from Space, held in Vienna on August 19-23, 2019, ”The non-radial pulsation pattern of the Algol star RZ Cas”
  Lehmann, H., Pertermann, F., Tsymbal, V., et al.