Benko, J. M.

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COMMISSIONS G1 AND G4 OF THE IAUINFORMATION BULLETIN ON VARIABLE STARSVolume 62 Number 6189 DOI: 10.22444/IBVS.6189Konkoly ObservatoryBudapest25 November 2016HU ISSN 0374 – 0676V620 Oph = CoRoT 104190253 – A MISCLASSIFIED RR LYRAE STARBENKŐ, J. M.Konkoly Observatory, MTA CSFK, 1121 Budapest, Konkoly Thege u. 15-17. Hungarye-mail: benko@konkoly.huWhile I was reading the recently appeared CoRoT Legacy Book (CoRoT Team, 2016) Inoticed the figures A.1 and A.2 in the paper of Ollivier et al. (2016). The authors call theattention to a rare but possible failure of the jump detection algorithm of the CoRoT datapipeline. If a large amplitude variable star has similar period than the CoRoT satelliteSouth Atlantic Anomaly passing through rate (0.499 d), this could lead to a false jumpdetection and the algorithm heavily distorts the light curve. The effect is illustrated withthe case of the RR Lyrae star CoRoT 104190253. Since the CoRoT Variable star Classifier(CVC, Debosscher et al. 2009) uses the processed light curves, this star was not classifiedas an RR Lyrae star. So it has not been reported by any of the specific CoRoT RR Lyraestudies nor by the two review papers (Szabó et al. 2014, Benkő et al. 2016).1 History and classificationThe SIMBAD database contains a Mira variable star V620 Oph at the position of CoRoT 104190253(α2000=18h33m47.s35, δ2000=+9◦06′58.′′608). The literature of V620 Oph is rather poor. Itsvariable nature was discovered by Hoffmeister (1936) who later gave the finding chart aswell (Hoffmeister, 1957). The only additional paper which mentioned the star is the workof Kinnunen & Skiff (2000), which provides proper positions for many variable stars. Nospecific study of V620 Oph has been found.The star was observed by the CoRoT satellite (Baglin 2006, 2016) in its fourth long runtowards the Galactic centre direction (LRc04) between 07 July and 29 September 2009(84 days). After the first two days the observation was taken in oversampled mode whichmeans 32 sec sampling rate. This resulted in more than 19 000 individual data points1.Considering the data it is immediately visible that the Mira classification is wrong. Theperiod, the amplitude, and the light curve shape are all typical of a fundamentally pul-sating RR Lyrae (RRab) star (Fig. 1).1The CoRoT N2 reduced data used this work can be downloaded from the IAS CoRoT Public Archive site at http://idoc-corot.ias.u-psud.fr/sitools/client-user/COROT N2 PUBLIC DATA/project-index.html2 IBVS 618955040.2 55040.4 55040.6 55040.8 55041 55041.2 55041.4.4.3.2.10−.1−.2−.3−.4−.5−.6Time [BJD − 2 451 545]Brightness [mag]Figure 1. A light curve part of the CoRoT observation of V620 Oph.2 Data analysis and resultsThe data was selected, processed, and analyzed in the same way as it was described inBenkő et al. (2016). The main pulsation period P0 is 0.501743±0.000003 d and the Fourieramplitude of the main frequency in CoRoT instrumental magnitude scale is A(f0) =0.3284 ± 0.0005 mag. All the errors given in this paper are estimated with the MonteCarlo Simulation tool of Period04 program package (Lenz & Breger 2005).The light curve does not show any serious modulation effects but the Fourier analysisshows some. If I pre-whiten the data with the main pulsation frequency and its harmonicsthe residual spectrum is dominated by side peaks around the harmonics and a highlysignificant (S/N=12.2) peak in the low frequency regime at f ′ = 0.016 ± 0.013 d−1.The side peak distances from the harmonics and f ′ frequency itself determine a possible∼60 d-long modulation.The reality of such a long period modulation needs a careful check which can distinguishit from the possible similar time-scale instrumental trends. For this purpose I chosethe Period04 amplitude/phase variation calculation tool which is sensitive for findingsmall amplitude or phase modulations (Nemec et al. 2011, 2013). To test the methoddependence on the bin size I run the program twice using either 3 days-long or 5 days-long bin sizes. The results concerning the amplitude and phase of the main pulsationfrequency are shown in Fig. 2.The variation is clearly seen in both panels of the figure. The different symbols (whichdenote the two bin sizes) show the same curve shape demonstrating that the variation isindependent of the used bin sizes. The amplitude and phase variation curves are highly(anti)correlated which strengthen the reality of the effect. Fourier spectra of these curvesIBVS 6189 3yield a period of P = 41.2± 11 d which is in agreement with the result of the above lightcurve Fourier analysis. The huge period error of both methods is on the one hand due tothe small number of covered cycles (∼ 2), on the other hand because of the irregularity ofthe cycles. The observed two Blazhko cycles have different amplitude and phase variationcurves suggesting multiperiodic and/or irregular nature of the effect. It is not a surprisebecause it was recently demonstrated that the multiperiodic Blazhko effects are verycommon (Benkő et al. 2014).The amplitude of this amplitude variation is ∆A1 = 0.042 ± 0.001 mag while theamplitude of the phase variation is ∆φ1 = 0.0018 ± 0.0008. Here I used the definition ofNemec et al. (2013) for ∆A1 and ∆φ1, the phases defined as in Lenz & Breger (2005) thatis they should be multiplied by 2π for obtaining the phases in radian. The amplitude of thedetected variations is low but not extremely: V620 Oph values are between the parametersof the two Kepler Blazhko stars KIC 11125706 and V838 Cyg (Nemec et al. 2013). (HereI assumed that the CoRoT white light amplitudes are similar to those that the Keplerunfiltered ones.) Otherwise, this star shows the smallest amplitude Blazhko effect amongthe non-blended CoRoT RRab stars.55020 55030 55040 55050 55060 55070 55080 55090 55100.325.33A(f0) [mag]Time [BJD − 2 451 545].019.02.021.022.023.024φ(f 0)Figure 2. The amplitude (bottom) and phase variation (top) of the main pulsation frequency f0.Phase units are rad/2π. The open circles denote the result obtained using 3 days-long bin size while xsymbols show the 5 days-long bin size result. The error of the individual data points is smaller than thesymbol sizes.4 IBVS 61893 ConclusionIn this short paper I demonstrated that the formerly misclassified V620 Oph is a fundamental-mode pulsating RR Lyrae star showing a small amplitude but real Blazhko effect. Theeffect has possible multiperiodic or irregular character.The lesson from the accidental finding of the star is that some RR Lyrae stars withsimilar period might be hidden in the CoRoT archive. This is the first and the onlyidentified RR Lyrae star within the period range of 0.4853-0.5385 d. In turn the maximumof the period distribution function of Galactic RR Lyrae stars (0.556 d, see e.g. Soszyńskiet al. 2011) is near of this interval.Acknowledgements: The author thanks K. Nagy for polishing the English style of thepaper. This research has been supported partially by the NKFIH Grant K-115709.References:Baglin, A., Fridlund, M., 2006, in The CoRoT Mission Pre-Launch Status – Stellar Seis-mology and Planet Finding, ESA SP-1306, p. 11Baglin, A., Lam-Trong, T., Vandermarcq, O., Donny, C., & Burgaud, S., 2016, in TheCoRoT Legacy Book, (EDP Sciences), p. 11 DOIBenkő, J. M., Plachy, E., Szabó, R. et al., 2014, ApJS, 213, 31 DOIBenkő, J. M., Szabó, R., Derekas, A., & Sódor, Á., 2016, MNRAS, in press, arXiv:1608.06418DOIDebosscher, J., Sarro, L. M., López, M. et al., 2009, A&A, 506, 519 DOICoRoT Team, 2016, The CoRoT Legacy Book, EDP Sciences DOIHoffmeister, C., 1936, AN, 259, 37 DOIHoffmeister, C., 1957, Mitteil. Veränd. Sterne, No. 245Kinnunen, T. & Skiff, B. A., 2000, IBVS, No. 4864Lenz, P. & Breger, M., 2005, CoAst, 146, 53 DOINemec, J. M., Smolec, R., Benkő, J. M. et al., 2011, MNRAS, 417, 1022 DOINemec, J. M., Cohen, J. G., Ripepi, V. et al., 2013, ApJ, 773, 181 DOIOllivier, M., Deru, A., Chaintreuil, S. et al., 2016, in The CoRoT Legacy Book, EDPSciences, p. 41 DOISoszyński, I., Dziembowski, W. A., Udalski, A. et al., 2011, Acta Astron., 61, 1Szabó, R., Benkő, J. M., Paparó, M. et al., 2014, A&A, 570, A100 DOI

V620 Oph = CoRoT 104190253 - a misclassified RR Lyrae star

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