The space photometry revolution and our understanding of RR Lyrae stars
R. Szabó, J. M. Benkő, M. Paparó, E. Chapellier, E. Poretti, A. Baglin, W. W. Weiss, K. Kolenberg, E. Guggenberger, J.-F. Le Borgne
aa r X i v : . [ a s t r o - ph . S R ] N ov The space photometry revolution and our understanding ofRR Lyrae stars
R. Szab´o , a , J. M. Benk˝o , M. Papar´o , E. Chapellier , E. Poretti , A. Baglin , W. W. Weiss , K.Kolenberg , , E. Guggenberger , , and J.-F. Le Borgne , Konkoly Observatory, MTA CSFK, Konkoly Thege Mikl´os ´ut 15-17. H-1121 Budapest, Hungary Laboratoire Lagrange, Universit´e Nice Sophia-Antipolis, UMR 7293, Observatoire de la Cˆoted’Azur 06300, Nice, France INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy LESIA, Universit´e Pierre et Marie Curie, Universit´e Denis Diderot, Observatoire de Paris, 92195Meudon Cedex, France Institute of Astronomy, University of Vienna, T¨urkenschanzstrasse 17, 1180 Vienna, Austria Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA 02138, USA Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200D, B-3001 Heverlee, Belgium Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 G¨ottingen, Ger-many Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade120, 8000 Aarhus C, Denmark Universit´e de Toulouse, UPS-OMP, IRAP, Toulouse, France CNRS, IRAP, 14, avenue Edouard Belin, F-31400 Toulouse, France
Abstract.
The study of RR Lyrae stars has recently been invigorated thanks to the long,uninterrupted, ultra-precise time series data provided by the
Kepler and CoRoT spacetelescopes. We give a brief overview of the new observational findings concentrating onthe connection between period doubling and the Blazhko modulation, and the omnipres-ence of additional periodicities in all RR Lyrae subtypes, except for non-modulated RRabstars. Recent theoretical results demonstrate that if more than two modes are present ina nonlinear dynamical system such as a high-amplitude RR Lyrae star, the outcome isoften an extremely intricate dynamical state. Thus, based on these discoveries, an under-lying picture of complex dynamical interactions between modes is emerging which shedsnew light on the century-old Blazhko-phenomenon, as well. New directions of theoreticale ff orts, like multidimensional hydrodynamical simulations, future space photometric mis-sions and detailed spectroscopic investigations will pave the way towards a more completeunderstanding of the atmospheric and pulsation dynamics of these enigmatic touchstoneobjects. High precision, uninterrupted, space-based photometric data sets obtained with MOST [27], CoRoT[1] and
Kepler [4] have transformed our view of exoplanetary systems and stellar variability, as well.RR Lyrae stars are no exception. New types of variations and dynamical phenomena have been discov-ered, like period doubling [10], [23], the presence of additional pulsational modes [14], [2], high-orderresonances [12], and maybe chaos [18]. In addition, the mysterious Blazhko e ff ect could be investi-gated in much greater detail than previously [7], [8], [13], [3]. While a great deal of advancement has a e-mail: [email protected] PJ Web of Conferencesbeen achieved on the Blazhko e ff ect itself, the recognition of multiple modulations based on ground-based [22], [21] and space data [3] being one particular example, we refer to a recent summary [24]regarding the Blazhko e ff ect. In this contribution we provide an update on the most recent results andhighlight some emerging trends focusing primarily on period doubling and the presence and dynamicsof additional modes in RR Lyrae stars. Period doubling (PD) is a well-known dynamical phenomenon. Its presence is betrayed by the al-ternating maxima and light curve shape in the light curve of a pulsating star, while in the frequencyspectrum half-integer multiples of the fundamental frequency appear. That means they are locatedhalfway between the dominant pulsation mode and its harmonics [23]. The importance of PD lies inthe fact that transition between regular to chaotic dynamics can occur through a series of PD bifur-cations. In RR Lyrae stars PD was first discovered in the
Kepler data [10], [23]. The origin of the PDcould be unambiguously traced back to a 9:2 resonance between the fundamental mode and the ninthradial overtone. In 2011 Buchler & Koll´ath [5] demonstrated that the same resonance may be able tocause light curve modulation, ie. the Blazhko e ff ect. By revisiting the CoRoT RR Lyrae light curveswe found signs of the PD in four Blazhko RRab stars [25] out of a sample of six. A recent compre-hensive work by Benk˝o et al. [3] on Kepler modulated RRab stars gave concordant results: six outof ten Blazhko-modulated RRab stars exhibited PD. Based on the latest space photometric results wecan conclude that altogether two-thirds of the Blazhko-modulated RRab stars exhibit this new type ofdynamical phenomenon [25]. It is important to emphasize that PD in connection with the dominantpulsation mode has been only seen in Blazhko-modulated RRab stars. No non-modulated RR Lyraeshow PD down to the outstanding precision delivered by CoRoT and
Kepler . RR Lyrae stars have been known for decades to pulsate exclusively in the fundamental (F) or firstovertone (O1) modes, or occasionally in both (RRd, double-mode stars). With the advent of high qual-ity space data it became obvious that most of the RR Lyrae stars show additional periodicities beyondthese dominant, low-order radial modes. In modulated RRab stars we find power in the frequency spec-trum around the theoretical location of the first and / or second radial overtone (see Fig 1 for the Kepler sample [2], [3] and the left panel of Fig 2 for the CoRoT Blazhko stars [25]). These can be eitherthe radial overtones themselves as was demonstrated by Moln´ar et al. [15], or nonradial modes closeto or exactly in 1:1 resonance with the radial modes predicted by earlier theoretical works [26] [6].Interestingly, none of the non-modulated RRab stars exhibit additional frequencies. This dichotomy isstrikingly demonstrated in the right panel of Fig 2. Overtone pulsators (RRc stars) also show additionalfrequencies, most prominent and most frequent of them is a probably nonradial pulsation mode witha period ratio of ∼ ff ect. In summary, Blazhko-modulation,period doubling and additional modes seem to be related phenomena, all being part of an intricate andcomplex underlying dynamics.We investigated the temporal variability of the additional modes in the CoRoT RR Lyrae sample[25]. It turned out that the amplitude or the shape of these frequencies vary in time in most cases wherewe could draw firm conclusions. This variability has been confirmed by Moskalik et al. [17] based onthe Kepler
RRc sample. The
Kepler data provide better frequency resolution compared to our CoRoTsample, hence the presence of potentially close-by, unresolved frequency components could be safelyexcluded as the main cause of the variations. Half-integer frequencies are nonstationary, because of theephemeral presence of PD itself. However, the variability of the other (presumably) nonradial modes ishe Space Photometry Revolution
V354 LyrV2178 CygV450 LyrV808 CygV353 LyrKIC 9973633V366 LyrV838 Cygf +0.2 1.5f −0.2 f f Frequency [d −1 ] A m p li t ude [ m ag ] −4 .0002.0003.00040.001.002.0030.000505 × −5 −4 V445 LyrV360 LyrV1104 CygKIC 11125706KIC 7257008V355 LyrV783 CygRR Lyrf +0.2 1.5f −0.2 f f Frequency [d −1 ] A m p li t ude [ m ag ] × −5 −4 .00015.000205 × −5 −4 .00015.00020.0005.001.0015.002.00250.0005.001.0015.00205 × −5 −4 .00015.00020.0025.005 Fig. 1.
Additional frequencies in
Kepler
Blazhko RRab stars [11], [3]. The frequency interval between the fun-damental mode and its first harmonic is plotted. From left to right the yellow stripes show the location of thefirst radial overtone, the half-integer frequencies caused by period doubling, and the second radial overtones. Thesignificant frequencies around the second overtone in V808 Cyg are barely visible because of the large amplitudeof the half-integer frequency, see Fig. 11 in [3] for a close-up view. more intriguing. While in Blazhko RRab stars the temporal changes may be related to the modulationmechanism (though more work should be done to corroborate this connection), in non-modulatedRR Lyrae, such as RRd and RRc stars, however, a di ff erent mechanism may be at work. The mostprobably culprit is an intricate dynamical interaction between the radial and nonradial modes. Themost important observational result based on the growing number of RR Lyrae stars observed fromspace is that temporal variability of the additional modes seems to be ubiquitous in these objects. Space photometry induced a veritable revolution in our understanding of RR Lyrae stars. New dynam-ical phenomena have been discovered, of which some are well-understood [12], while others are stillawaiting a theoretical explanation. If the ubiquity of nonradial modes is confirmed, the power of non-linear seismology using radial modes in RR Lyrae stars can be unleashed [15]. We hope to continuethis adventure using a larger sample of galactic and maybe extragalactic RR Lyrae stars by ongoingand future space photometric missions, like K2 [9], [16], TESS [20], and PLATO [19].
Acknowledgements.
This project has been supported by the Hungarian OTKA grant K83790 and the EuropeanCommunity’s Seventh Framework Program (FP7 / / ASK) and ERC grant agreement no. 338251 (StellarAges). RSz, MP, and JMB acknowledgethe support of the ESA PECS project No. 4000103541 / / NL / KML. WW was supported by the Austrian Science
PJ Web of Conferences
V1127 Aql112879339224345288363f +0.2 1.5f −0.2 f f Frequency [d −1 ] A m p li t ude [ m ag ] +0.2 1.5f −0.2 f f Frequency [d −1 ] A m p li t ude [ m ag ] Fig. 2.
Additional frequencies in CoRoT RRab stars. Left panel: Blazhko-modulated stars, right panel: non-Blazhko stars. The notation is the same as in Fig 1. The lack of any additional frequencies in non-modulated starsis remarkable.Fonds (FWF P22691-N16). KK acknowledges the support of FP7 Marie Curie Fellowship PIOF-255267 (SAS-RRL). This research made use of the ExoDat database, operated at LAM-OAMP, Marseille, France, on behalfof the CoRoT / Exoplanet program. Funding for the
Kepler mission is provided by the NASA Science Missiondirectorate. We are grateful to the Kepler Science Team for their extensive e ff orts in producing and makingpublicly available these high-quality and unique data sets. References
1. Baglin, A., Auvergne, M., Boisnard, L., et al. in , , 3749 (2006)2. Benk˝o, J. M., Kolenberg, K., Szab´o, R., et al., MNRAS , 1585 (2010)3. Benk˝o, J. M., Plachy, E., Szab´o, R., et al., ApJS , 31 (2014)4. Borucki, W. J., Koch, D., Basri, G., et al., Science , 977 (2010)5. Buchler, J. R., Koll´ath, Z. ApJ , 24 (2011)6. Dziembowski, W. A., Mizerski, T., AcA 54, (2004)7. Guggenberger, E., Kolenberg, K., Chapellier, E., et al. MNRAS , 1577 (2011)8. Guggenberger, E., Kolenberg, K., Nemec, J. M., et al. MNRAS , 649 (2012)9. Howell, S. B., Sobeck, C., Haas, M., et al., PASP, , 398 (2014)10. Kolenberg, K., Szab´o, R., Kurtz, D. W., et al., ApJ , L198 (2010)11. Kolenberg, K., Bryson, S. T., Szab´o, R., et al., MNRAS , 878 (2011)12. Koll´ath, Z., Moln´ar, L., Szab´o, R. MNRAS , 1111 (2011)13. Le Borgne, J. F., Poretti, E., Klotz, A., et al., MNRAS, , 1435 (2014)14. Moskalik, P., Proc. of IAU Symp. , 249 (2014a)15. Moln´ar, L., Koll´ath, Z., Szab´o, R., et al., ApJL , L13, (2012)16. Moln´ar, L., Plachy, E., Szab´o, R., IBVS (2014)17. Moskalik, P., Smolec, R., Kolenberg, K., et al., MNRAS submitted (2014b)18. Plachy, E., Koll´ath, Z., Moln´ar, L., MNRAS , 3590 (2013)19. Rauer, H., Catala, C., Aerts, C., et al. Exp. Astron. in press (2014)20. Ricker, G. R., Winn, J. N., Vanderspek, R., et al., Proc. of the SPIE, , 15 (2014)21. Skarka, M. A&A , A90 (2014)22. S´odor, ´A., Jurcsik, J., Szeidl, B. et al., MNRAS, , 1585 (2011)23. Szab´o, R., Koll´ath, Z., Moln´ar, L., et al., MNRAS , 1244 (2010)24. Szab´o, R., Proc. of IAU Symp. , 241 (2014a)25. Szab´o, R., Benk˝o, J. M., Papar´o, M., et al., A&A , A100, (2014b)26. Van Hoolst, T., Dziembowski, W. A., Kawaler, S. D., MNRAS, , 536 (1998)27. Walker, G., Matthews, J., Kuschnig, R., et al., PASP115