We report here on two types of cyclic variations that can be observed in the periods of RR Lyrae stars, i.e., the Blazhko and the light-time effects. The former has been investigated by studying the amplitude variations recorded in RR Lyr itself, firstly by Kepler and then by the network of the ``Very Tiny Telescopes'' (VTTs), the latter on the basis of the new spectroscopic observations of the most promising candidate, KIC 2831097. The start of the search for binary candidates in the RR Lyrae stars observed with the TAROT telescopes is also announced

Correa, Mercedes

Klotz, Alain

Le Borgne, Jean-François

PORETTI, Ennio

RAINER, Monica

OA@INAF - Istituto Nazionale di Astrofisica

2018Publication Year2020-11-06T10:09:20ZAcceptance in OA@INAFCyclic Variations in the Periods of RR Lyrae StarsTitlePORETTI, Ennio; Le Borgne, Jean-François; Klotz, Alain; RAINER, Monica; Correa, MercedesAuthorshttp://hdl.handle.net/20.500.12386/28178HandlePROCEEDINGS OF THE POLISH ASTRONOMICAL SOCIETYSeries6NumberCyclic Variations in the Periods ofRR Lyrae StarsEnnio Poretti1·2·3, Jean-François Le Borgne4·5·2, Alain Klotz4·5·2,Monica Rainer1·6 and Mercedes Correa7·21. INAF-Osservatorio Astronomico di Brera,Via E. Bianchi 46, 23807 Merate (LC), Italy2. Groupe Européen d’Observations Stellaires (GEOS), Bailleau l’Evéque, France3. INAF – Fundación Galileo Galilei, Rambla José Ana Fernandez Pérez 7, 38712 –Breña Baja, Spain4. Université de Toulouse, UPS-OMP, IRAP, Toulouse, France5. CNRS, IRAP, Toulouse, France6. INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy7. Agrupació Astronòmica de Sabadell, Sabadell, SpainWe report here on two types of cyclic variations that can be observed in theperiods of RR Lyrae stars, i.e., the Blazhko and the light-time effects. The formerhas been investigated by studying the amplitude variations recorded in RR Lyritself, firstly by Kepler and then by the network of the “Very Tiny Telescopes”(VTTs), the latter on the basis of the new spectroscopic observations of the mostpromising candidate, KIC 2831097. The start of the search for binary candidatesin the RR Lyrae stars observed with the TAROT telescopes is also announced.1 IntroductionThe RR Lyrae variables are subjected to many other periodicities overlapped to themain pulsation cycle. The evolutionary changes, the excitation of other radial andnonradial modes, the modulation in amplitude and phase known as the Blazhko ef-fect, and period doubling are extensively reported in the literature. Unfortunately,many theoretical aspects are not well understood yet. Recently, many efforts havebeen made to detect the light-time effect, i.e., periodic variations of the O−C (Ob-served minus Calculated) values of the times of maximum brightness (Tmax) due tothe orbital motion of the RR Lyrae component in a binary system.In the present contribution, we report on the new observational efforts madejointly with the Brera team in Italy, the IRAP team in France, and by the amateurastronomers composing the Groupe Européen d’Observations Stellaires (GEOS).2 The Pulsation Period of RR LyrThe analysis of all the Tmax epochs listed in the GEOS database allowed the carefulreconstruction of the changes in the pulsation period of RR Lyr (Le Borgne et al.,2014), i.e., the alternation of two states characterized by the pulsation over a periodlonger than 0.56684 d and over another shorter than 0.56682 d. We emphasize theimportance of continuing to monitor RR Lyr: it is not only the eponym of the classpta.edu.pl/proc/v6p151 PTA Proceedings ? June, 2018 ? vol. 6 ? 151Ennio Poretti, et al.Fig. 1: The historical behaviour of the O−C values of RR Lyr. The new Tmax measuredby VTTs in 2015, 2016 and 2017 are on the extreme right side.of variable stars, but the ideal laboratory where all the tests of the modelling ofthe pulsation on horizontal branch stars can be performed. The persistence of thecurrent pulsation period in the low status was established by the new Tmax valuesobtained in 2015 (Poretti et al., 2016). The survey with the “Very Tiny Telescopes”(VTTs; Le Borgne et al., 2014) has continued in 2016 and 2017. Figure 1 shows thedevelopment of the pulsation period in the last three years: the new Tmax valuesindicate that the pulsation is still in the short-period status. The pulsation periodis actually the shortest ever experienced by the star.3 The Blazhko Period of RR LyrWe also determined the Blazhko period since 1910 to verify how it follows the changesin the pulsation period (Le Borgne et al., 2014). We could infer that the variationsof the pulsation and Blazhko periods are completely decoupled, since the Blazhkoperiod had just one sudden decrease from 40.8 d to 39.0 d in 1975.We remind the reader that the measurements obtained by the VTTs and byKepler have also been extensively used to record the monotonic long-term decreasein the amplitude of the Blazhko effect (Le Borgne et al., 2014). Such a decreaseresulted in an almost final damping in 2014. The subsequent Tmax values collectedwith the VTTs in 2015 showed a slight increase in the amplitude of the O−C values(Poretti et al., 2016). The new values collected in 2016-17 confirm that the Blazhkoeffect has resumed at a measurable amplitude (Fig. 2). However, the amplitudes in2015 and 2016-17 look very similar (Fig. 3) and hence the effect is still weak.152 ? PTA Proceedings ? June, 2018 ? vol. 6 pta.edu.pl/proc/v6p151Cyclic Variations in the Periods of RR Lyrae StarsFig. 2: The behaviour of the amplitude of the Blazhko effect of RR Lyr. The new deter-minations in 2016 and 2017 are shown in the bottom panels.pta.edu.pl/proc/v6p151 PTA Proceedings ? June, 2018 ? vol. 6 ? 153Ennio Poretti, et al.Fig. 3: The O−C values obtained in 2015 (filled black circles), 2016 (empty red circles) and2017 (blue crosses) folded with the Blazhko period PB=38.8 d.4 Hunting for the First RR Lyrae Star in a Binary SystemWe do not know of any binary system in which one of the components is an RR Lyraevariable. Searching for such a system is not only an observational exercise. Ourknowledge on the masses of RR Lyrae stars is based on evolutionary and pulsationmodels. A direct measure is still lacking. As in the case of many fields of stel-lar astronomy, binary stars can provide the laboratory where this measure can beextracted.The Kepler field includes many RR Lyrae variables, new and old (Benkö et al.,2010). The almost continuous light curves supply a testbed for the careful analysisof the O−C values in the search for cyclic patterns due to the light-time effect. Suchan analysis is particularly powerful in the case of Kepler stars since they have beenobserved for 4 years, investigating a wide range of orbital periods. A very promisingcandidate has been recently proposed: KIC 2831097 (Sódor et al., 2017). It is afirst-overtone RR Lyrae variable with a period P=0.337 d and a light amplitude of∆Kp=0.40 mag.The O−C curve shows two overlapping effects: a linear decrease of the pulsationperiod, as observed in many RR Lyrae variables (Le Borgne et al., 2007) and a well-defined periodicity of 753 d with a full-amplitude of 0.04 d. This periodicity seemsto be a well-established fact since it has been followed for almost two full cycles andrepeats itself in a very regular way (fig. 2, top panel, in Sódor et al., 2017). We canlook at KIC 2831097 as the current best target for hunting the first RR Lyrae starin a binary system.In our collaboration, we obtained a clear radial-velocity curve from the Hα lineby using the FIES instrument mounted at the Nordic Optical Telescope (Roque deLos Muchachos, La Palma, Canary Islands, Spain). Figure 4 (left panel) shows theradial velocity obtained from the Hα line, folded over the pulsation period; the meansystemic velocity has been subtracted. The spectra were taken in August 2017, nearone of the quadratures. A similar campaign is planned in May 2018 to obtain the154 ? PTA Proceedings ? June, 2018 ? vol. 6 pta.edu.pl/proc/v6p151Cyclic Variations in the Periods of RR Lyrae StarsFig. 4: Left panel: the radial velocity curve over the pulsation period obtained with FIESin August 2017. Different symbols for different nights. Right panel: the light curve fromsimultaneous CCD photometry.pulsation radial velocity at the other quadrature.The GEOS collaboration also ensured the simultaneous photometric coverage:Fig. 4 (right panel) shows the V light-curve obtained by the Agrupació Astronòmicade Sabadell (Spain) with a Meade 12" telescope equipped with a CCD Moravian G21600.5 Searching for Binary RR Lyrae Stars in the GEOS SurveySince 2004, the GEOS survey uses the TAROT telescopes to measure the O−Csof Tmax values of hundreds of Galactic RR Lyrae stars (Le Borgne et al., 2007).The O−C accuracy is about 3 minutes, and some stars are observed many timesper year. As a consequence, such a density of O−C values leads to identify newvariations with periods longer than ten years. They could be due to a companionof the RR Lyrae star, thus invoking a light-time effect. The baseline of only 13years of the GEOS survey introduces a bias leading to find suspect companionsnearer than 20 AU from their RR Lyrae star. It remains difficult to distinguish ifthe variations are due to the light-time effect or to a very long Blazhko period fromphotometric data only. Taking into account the large distance of the stars from theEarth, the apparent angular separation is typically lower than 10 mas, making it verychallenging for direct confirmation by using adaptive optics. We plan to continuethe TAROT observations to confirm the suspected periodicities and to find otherstars with longer periods.6 ConclusionsThe VTT monitoring showed that the pulsation of RR Lyr has continued in its short-period status in 2016 and 2017. Moreover, the VTT still measured a weak amplitudeof the Blazhko effect, but larger than in 2014. Both facts confirm the importancepta.edu.pl/proc/v6p151 PTA Proceedings ? June, 2018 ? vol. 6 ? 155Ennio Poretti, et al.of continuing the observation of this star, since its behaviour through the ages canbring important information on the evolution of horizontal-branch stars.As a new topic in the GEOS research, we investigated the TAROT database toidentify RR Lyrae stars in binary systems. At the moment, KIC 2831097 remains thebest candidate for such a system. We started an observational project to obtain theradial velocity curves of this star at the predicted quadratures. The first campaignwas successful, while the second is programmed in 2018.Acknowledgements. EP and MR acknowledge financial support from the PRIN-INAF 2014.ReferencesBenkö, J., Kolenberg, K., et al., S. R., MNRAS 409, 1585 (2010)Le Borgne, J. F., et al., A&A 476, 307 (2007)Le Borgne, J. F., et al., MNRAS 441, 1435 (2014)Poretti, E., et al., Communications from the Konkoly Observatory 105, 73 (2016)Sódor, A., Skarka, M., Liska, J., Bognár, Z., MNRAS 465, L1 (2017)Conference dinner in the Wieliczka Salt Mine, Jan Haluszka Chamber. From leftto right: Jean-François Le Borgne, Fouad Sefyani, Abdelmjid Benhida, Jan KareTrandem Qvam, Erika Pakstiene and Ennio Poretti.156 ? PTA Proceedings ? June, 2018 ? vol. 6 pta.edu.pl/proc/v6p151

Cyclic Variations in the Periods of RR Lyrae Stars

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Cyclic Variations in the Periods of RR Lyrae Stars

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