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  A cautionary tale of interpreting O−C diagrams: period instability in a classical RR Lyr Star Z CVn mimicking as a distant companion

Skarka, M., Liška, J., Dřevěný, R., Guggenberger, E., Sódor, Á., Barnes, T. G., et al. (2018). A cautionary tale of interpreting O−C diagrams: period instability in a classical RR Lyr Star Z CVn mimicking as a distant companion. Monthly Notices of the Royal Astronomical Society, 474(1), 824-837. doi:10.1093/mnras/stx2737.

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 Creators:
Skarka, M., Author
Liška, J., Author
Dřevěný, R., Author
Guggenberger, Elisabeth1, Author           
Sódor, Á., Author
Barnes, T. G., Author
Kolenberg, K., Author
Affiliations:
1Max Planck Research Group in Stellar Ages and Galactic Evolution (SAGE), Max Planck Institute for Solar System Research, Max Planck Society, Justus-von-Liebig-Weg 3, 37077 Göttingen, DE, ou_2265636              

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Free keywords: methods: data analysis, techniques: photometric, techniques: spectroscopic, binaries: general, stars: individual: Z CVn, stars: variables: RR Lyrae
 Abstract: We present a comprehensive study of Z CVn, an RR Lyrae star that shows long-term cyclic variations of its pulsation period. A possible explanation suggested from the shape of the O−C diagram is the light travel-time effect, which we thoroughly examine. We used original photometric and spectroscopic measurements and investigated the period evolution using available maximum times spanning more than one century. If the binary hypothesis is valid, Z CVn orbits around a black hole with minimal mass of 56.5 
M⊙
on a very wide (Porbit = 78.3 yr) and eccentric orbit (e = 0.63). We discuss the probability of the formation of a black hole–RR Lyrae pair, and, although we found it possible, there is no observational evidence of the black hole in the direction to Z CVn. However, the main objection against the binary hypothesis is the comparison of the systemic radial velocity curve model and spectroscopic observations that clearly show that Z CVn cannot be bound in such a binary. Therefore, the variations of pulsation period are likely intrinsic to the star. This finding represents a discovery/confirmation of a new type of cyclic period changes in RR Lyrae stars. By the analysis of our photometric data, we found that the Blazhko  modulation with period of 22.931 d is strongly dominant in amplitude. The strength of the phase modulation varies and is currently almost undetectable. We also estimated photometric physical parameters of Z CVn and investigated their variations during the Blazhko cycle using the inverse Baade–Wesselink method.

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Language(s): eng - English
 Dates: 2019-03-192018
 Publication Status: Published in print
 Pages: -
 Publishing info: -
 Table of Contents: -
 Rev. Type: Peer
 Identifiers: DOI: 10.1093/mnras/stx2737
 Degree: -

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Title: Monthly Notices of the Royal Astronomical Society
  Other : Mon. Not. R. Astron. Soc.
Source Genre: Journal
 Creator(s):
Affiliations:
Publ. Info: Oxford : Oxford University Press
Pages: - Volume / Issue: 474 (1) Sequence Number: - Start / End Page: 824 - 837 Identifier: ISSN: 1365-8711
CoNE: https://pure.mpg.de/cone/journals/resource/1000000000024150