Separating the causes of O–C variations in an RR Lyrae star with the Blazhko effect

The case of V1109 Cas

¹ Institute of Applied Nuclear Technology (ITNA), University of Cheikh Anta Diop, BP 5005 Dakar Fann, Senegal
² IMCCE, Observatoire de Paris, PSL Research University, CNRS UMR 8028, Sorbonne Université, Université de Lille, 77 av. Denfert-Rochereau, 75014 Paris, France
³ University of Antwerp, Physics Department, Groenenborgerlaan 171, Antwerpen B-2020, Belgium
⁴ KU Leuven, Institute of Astronomy, Celestijnenlaan 200D, Heverlee B-3001, Belgium
⁵ Vrije Universiteit Brussel – VUB, Department of Physics and Astronomy, Pleinlaan 2, Brussel B-1050, Belgium
⁶ Université de Toulouse, IRAP CNRS UPS, 14 Avenue Edouard Belin, Toulouse F-31400, France
⁷ Géosciences Environnement Toulouse UMR5563, CNRS, IRD & Université de Toulouse, 14 Avenue Edouard Belin, 31400 Toulouse, France
⁸ Université Félix Houphouët-Boigny, UFR Sciences de la Terre et des Ressources Minières Cocody, Abidjan, Côte d’Ivoire
⁹ LAM, Laboratoire d’Astrophysique de Marseille, 38 Rue Frédéric Joliot Curie, Marseille F-13013, France
¹⁰ GEOS (Groupe Européen d’Observations Stellaires), 23 Parc de Levesville, 28300 Bailleau l’Evêque, France
¹¹ Oukaimeden Observatory, High Energy Physics and Astrophysics Laboratory, Cadi Ayyad University, Marrakech, Morocco

^⋆ Corresponding author; salma.sylla@ucad.edu.sn

Received: 10 May 2024
Accepted: 24 August 2024

Abstract

Context. To date, puzzlingly few bona fide RR Lyrae stars have been identified in binaries. Binarity in pulsating stars can be revealed through well-timed photometric data over sufficiently long time bases because of the light travel time effect (LTTE) on the pulsations, which manifests as variation in the timings of maximum light in the O−C (observed minus calculated) diagram. However, O−C variations can also have other causes, such as the Blazhko effect or a sudden or gradual change in the main pulsation period.

Aims. We approach this challenge by disentangling the Blazhko effect and period changes from the potential LTTE on V1109 Cas, an RR Lyrae star suspected to be part of a binary system based on its O−C data in the GEOS database. In doing so, we aim to uncover the subtler signals indicative of a companion (LTTE).

Methods. We analysed nine years of ground-based photometric data, using Fourier analysis to model the pulsation modulated by the Blazhko effect. From the fit to the observed light curves, we constructed a refined O−C diagram without the scatter caused by Blazhko modulation. Subsequently, we considered different possible scenarios, because distinguishing intrinsic period changes or breaks from the LTTE is challenging.

Results. If the remaining O−C variation is due to a period break, refining the O−C diagram can almost entirely remove the trends. If we interpret the variation as an LTTE, we can consider possible configurations. While we currently favour a period break as the explanation for the residual O−C variations, more data on the star in the forthcoming years will help reveal whether the O−C variations are due to the LTTE or intrinsic period changes.

Conclusions. Our study, based on a detailed light-curve fitting of V1109 Cas, offers insights into the discernment between Blazhko modulation, intrinsic period breaks, and the LTTE in binary systems. Our results highlight the complexity of determining binarity from O−C data traditionally used for detecting binary motion, calling for caution in this process.