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

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 (P orbit = 78.3 years) and eccentric orbit (e = 0.63). We discuss the probability of a 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.

First results on RR Lyrae stars with the TESS space telescope: untangling the connections between mode content, colors and distances

“…Despite numerous hypotheses, the Blazhko effect has not been explained satisfactorily. Mass determinations remained controversial thanks to uncertainties in the pulsation models and the nearly complete lack of confirmed RR Lyrae binaries (Hajdu et al 2015;Liška et al 2016;Skarka et al 2018). Long-term photometric and spectroscopic monitoring and high-precision astrometry have recently begun to also reveal RR Lyrae stars with companions (Sódor et al 2017;Kervella et al 2019a;Barnes et al 2021;Hajdu et al 2021), but the first candidate RR Lyrae-type variable in an eclipsing binary system for which mass could be determined was ultimately identified as a low-mass binary pulsator rather than a bona fide RR Lyrae star (Pietrzyński et al 2012).…”

Synergy between professional and amateur astronomers

“…The Czech AMs participated in very nice projects and discovered interesting binaries. For example, Cagaš & Pejcha (2012) studied the double eclipsing binary with periods near 3:2 ratio, AMs contributed to the study of multiple stellar systems (Zasche et al, 2017) and eruptive stars ( Šmelcer et al, 2017), an amateur observer allowed for the precise description of modulation properties of Z CVn (Skarka et al, 2018).…”

On the interpretation of the long-term cyclic period variations in RR Lyrae stars

“…In Skarka et al (2018), we performed an extensive study of Z CVn employing various methods and approaches: from photometry (53 nights during four observing seasons) and spectroscopy (eleven nights during two seasons) to LTTE analysis (227 maximum times, 72 new). We also used archival data, when available.…”