A&A 480, 563–571 (2008) Astronomy DOI: 10.1051/0004-6361:20079000 & c ESO 2008 Astrophysics Extrasolar planet detection by binary stellar eclipse timing: evidence for a third body around CM Draconis H. J. Deeg1, B. Ocaña1,2, V. P. Kozhevnikov3, D. Charbonneau4,F.T.O’Donovan5, and L. R. Doyle6 1 Instituto de Astrofísica de Canarias, C. Via Lactea S/N, 38205 La Laguna, Tenerife, Spain e-mail: [email protected] 2 Instituto de Radio Astronomía Milimétrica (IRAM), Av. Divina Pastora 7, Núcleo Central, 18012 Granada, Spain 3 Astronomical Observatory, Ural State University, Lenin ave. 51, Ekaterinburg, 620083, Russia 4 Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA 5 California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA 6 SETI Institute, 515 N. Whisman Road, Mountain View, CA 94043, USA Received 5 November 2007 / Accepted 28 December 2007 ABSTRACT Aims. Our objective is to elucidate the physical process that causes the observed observed-minus-calculated (O–C) behavior in the M4.5/M4.5 binary CM Dra and to test for any evidence of a third body around the CM Dra system. Methods. New eclipse minimum timings of CM Dra were obtained between the years 2000 and 2007. The O–C times of the system are fitted against several functions, representing different physical origins of the timing variations. Results. Using our observational data in conjunction with published timings going back to 1977, a clear non-linearity in O–C times is apparent. An analysis using model-selection statistics gives about equal weight to a parabolic and to a sinusoidal fitting function. Attraction from a third body, either at large distance in a quasi-constant constellation across the years of observations or from a body on a shorter orbit generating periodicities in O−C times is the most likely source of the observed O−C times. The white dwarf GJ 630.1B, a proper motion companion of CM Dra, can however be rejected as the responsible third body. Also, no further evidence of the short- periodic planet candidate described by Deeg et al. (2000, A&A, 358, L5) is found, whereas other mechanisms, such as period changes from stellar winds or Applegate’s mechanism can be rejected. Conclusions. A third body, being either a few-Jupiter-mass object with a period of 18.5 ± 4.5 years or an object in the mass range of 1.5 Mjup to 0.1 M with periods of hundreds to thousands of years is the most likely origin of the observed minimum timing behavior. Key words. stars: individual: CM Dra – stars: binaries: eclipsing – eclipses – stars: planetary systems 1. Introduction successes of precise timing measurements to detect the pres- ence of planets. The first known extrasolar planets were CM Dra (LP 101.15, G225-067, GJ 630.1) is a detached spec- detected through light-time effects in the signals of the / troscopic eclipsing M4.5 M4.5 binary with one of the lowest Pulsar PSR 1257+12 (Wolszczan & Frail 1992) and recently, known total masses, of 0.44 M. With its nearly edge-on incli- ◦ sinusoidal residuals in the pulsation frequency of the sdB pulsat- nation of 89.59 (see Kozhevnikova et al. 2004 for the most re- ing star V391 Peg have been explained through the presence of cent orbital and physical elements) it was chosen as the target of a giant planet (Silvotti et al. 2007), leading to the first detection the first photometric search for planetary transits. Performed by of a planet orbiting a post-red-Giant star. In both cases, timing the “TEP” project, with an intense observing campaign during measurements have led to detections of planets that would have − the years 1994 1999 (Deeg et al. 1998; Doyle et al. 2000), over been difficult or impossible to find with other planet-detection 1000 h of coverage of that system were obtained with several methods, a situation that is similar to the detection of planets 1 m-class telescopes. This lightcurve was initially searched for around eclipsing binaries – unless they exhibit transits. the presence of transits from planets in circumbinary “P-type” orbits with 5−60 day periods, with a negative result (Doyle et al. A first analysis of 41 eclipse minima times for CM Dra, 2000). The same lightcurve provided, however, a further possi- presented in Deeg et al. (2000) (hereafter DDK00) gave a bility to detect the presence of third bodies, from their possible low-confidence indication for the presence of a planet of light-time effects on the binary’s eclipse minimum times. 1.5−3 Jupiter masses, with a period of 750−1050 days. This re- To date, no unambiguously circumbinary planets have been sult was based on a power-spectral analysis of the minimum tim- detected1, and their discovery would constitute a new class ings’ residuals against a linear ephemeris, indicating a periodic of planets. Motivation for this work also arises from previous signal with an amplitude of about 3 s. Motivated by the result of DDK00, we continued surveying the CM Dra system with oc- 1 A possible circumbinary planet is HD 202206c, orbiting around casional eclipse observations in the following years. During this HD 202206a and b. This depends however on the classification of the time, it became increasingly clear that a simple linear ephemeris ffi 17.4 Mjup object HD 202206b as being a planet or a brown dwarf would not provide a su cient description of the general trend (Correia et al. 2005). of the eclipse times any longer. This led to the objective of this Article published by EDP Sciences 564 H. J. Deeg et al.: Extrasolar planet detection by binary stellar eclipse timing paper – a thorough and systematic discussion of the possible pro- Table 1. CM Dra eclipse minimum times, given in International Atomic cesses acting on this interesting system, and an evaluation of the Time (TAI). presence of a third body of planetary mass or heavier. Tmin (TAI) σTmin I/II E O−CRef.Inst. In the following sections, we present the data for this anal- −6 ysis (Sect. 2), evaluate the effect of several physical processes BJD-2 400 000 10 ds 42 893.932818 51 I −5469 −31.8 1 − on the eclipse timings (Sect. 3), and compare the significance of 42 994.768729 36 II −5390 −24.7 1 − several numerical fits for the explanation of the observed trend 49 494.634037 32 I −265 1.2 2 − (Sect. 4). This is followed in Sect. 5 by a discussion of the phys- 49 497.803721 55 II −263 −9.6 2 − ical implications of these fits, with conclusions in Sect. 6. 49 499.707562 28 I −261 −1.7 2 − 49 500.975836 73 I −260 −11.7 2 − 49 501.609080 103 II −260 6.7 2 − 49 511.756133 58 II −252 1.1 2 − 2. The observational data 49 562.491845 27 II −212 11.3 2 − 49 815.536668 87 I −12 1.0 2 − The minimum times analyzed here include all minimum tim- 49 828.853584 24 II −2 −0.2 2 − − ings derived in DDK00. Since its publication, we performed 49 830.757404 14 I 0 5.9 2 49 833.927084 17 II 2 −5.3 2 − dedicated eclipse observations with the IAC80 (0.8 m) and the 49 840.904433 35 I 8 −1.9 2 − INT 2.5 m telescopes within the Canary Islands’ Observatories 49 853.588403 47 I 18 4.3 2 − and with the Kourovka 0.7 m telescope of the Ural State 49 855.489863 33 II 19 7.7 2 − University in Ekaterinburg, Russia. In summer 2004, one of the 49 858.661903 38 I 22 −0.9 2 − fields surveyed for planetary transits by the TrES collaboration 49 872.614147 44 I 33 −4.6 2 − (Alonso et al. 2007) contained CM Dra and time series spanning 49 881.492866 47 I 40 −5.5 2 − several weeks were obtained from the Sleuth 10 cm telescope 49 947.449171 18 I 92 −2.8 2 − (O’Donovan et al. 2004). In all cases, photometric time series 49 949.350561 15 II 93 −5.4 2 − − were obtained from which the eclipse times were measured us- 49 954.424251 33 II 97 5.9 2 50 221.421524 16 I 308 9.6 2 − ing the method of Kwee & van Woerden (1956). We accepted 50 222.689653 27 I 309 −12.9 2 − only results where the formal measurements error from that al- 50 243.617062 102 II 325 −0.7 2 − gorithm was less than 10 s, which required data with a largely 50 244.885447 53 II 326 −1.1 2 − complete and uninterrupted coverage of individual eclipses. The 50 252.495738 32 II 332 −5.3 2 − timings obtained were then corrected to solar-system barycentric 50 257.569367 36 II 336 0.7 2 − (BJD) times with the BARYCEN routine in the “aitlib” IDL li- 50 259.473057 51 I 338 −4.4 2 − brary2 of the University of Tübingen. 50 262.642977 33 II 340 5.1 2 − 50 272.790006 31 II 348 −2.7 2 − Only a few timings of CM Dra of comparable quality could 50 636.818094 41 II 635 14.4 2 − be found in the literature: Lacy (1977) gives a total of 9 min- 50 643.795304 35 I 641 5.8 2 − imum times from observations in 1976. These timings scat- 50 650.770213 55 II 646 −0.3 2 − ter in O−C by about 30 s, and were based on photoelectric 50 710.384658 40 II 693 10.2 2 − data with frequent gaps, including several incomplete eclipses.