MNRAS 483, 824–839 (2019) doi:10.1093/mnras/sty3085 Advance Access publication 2018 November 15 Transit analysis of the CoRoT-5, CoRoT-8, CoRoT-12, CoRoT-18, CoRoT-20, and CoRoT-27 systems with combined ground- and space-based photometry Downloaded from https://academic.oup.com/mnras/article-abstract/483/1/824/5184487 by Inst. Astrofisica Andalucia CSIC user on 28 August 2019 St. Raetz,1,2,3‹ A. M. Heras,3 M. Fernandez,´ 4 V. Casanova,4 and C. Marka5 1Institute for Astronomy and Astrophysics Tubingen¨ (IAAT), University of Tubingen,¨ Sand 1, D-72076 Tubingen,¨ Germany 2Freiburg Institute of Advanced Studies (FRIAS), University of Freiburg, Albertstraße 19, D-79104 Freiburg, Germany 3Science Support Office, Directorate of Science, European Space Research and Technology Centre (ESA/ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands 4Instituto de Astrof´ısica de Andaluc´ıa, CSIC, Apdo. 3004, 18080 Granada, Spain 5Instituto Radioastronom´ıa Milimetrica´ (IRAM), Avenida Divina Pastora 7, E-18012 Granada, Spain Accepted 2018 November 8. Received 2018 November 7; in original form 2018 April 6 ABSTRACT We have initiated a dedicated project to follow-up with ground-based photometry the transiting planets discovered by CoRoT in order to refine the orbital elements, constrain their physical parameters, and search for additional bodies in the system. From 2012 September to 2016 December we carried out 16 transit observations of six CoRoT planets (CoRoT-5 b, CoRoT- 8 b, CoRoT-12 b, CoRoT-18 b, CoRoT-20 b, and CoRoT-27 b) at three observatories located in Germany and Spain. These observations took place between 5 and 9 yr after the planet’s discovery, which has allowed us to place stringent constraints on the planetary ephemeris. In five cases we obtained light curves with a deviation of the mid-transit time of up to ∼115 min from the predictions. We refined the ephemeris in all these cases and reduced the uncertainties of the orbital periods by factors between 1.2 and 33. In most cases our determined physical properties for individual systems are in agreement with values reported in previous studies. In one case, CoRoT-27 b, we could not detect any transit event in the predicted transit window. Key words: planets and satellites: individual: CoRoT-5 b, CoRoT-8 b, CoRoT-12 b, CoRoT- 18 b, CoRoT-20 b, and CoRoT-27 b – planetary systems – stars: individual: CoRoT-5, CoRoT- 8, CoRoT-12, CoRoT-18, CoRoT-20, and CoRoT-27. venture of the ultra high precision photometry from space’ (CoRot 1 INTRODUCTION Team 2016). Because of its low-earth orbit, CoRoT could point in The study of transiting extrasolar planets was revolutionized by one direction for not longer than 6 months per year to avoid the Sun the data obtained by space telescopes like CoRoT and Kepler,as entering in its field of view (FoV). The ∼6 month observing time they provide high-precision, high-cadence, continuous light curves in one direction was divided into two separate runs lasting ∼30 d (LCs) of a very high number of stars. Thanks to these extraordinary (short run, SR) and ∼150 d (long run, LR). CoRoT observed the capabilities, the first rocky super-Earths were detected (CoRoT-7b, fields with two cadence modes, a short cadence of 32 s exposure Kepler-10b, Queloz et al. 2009; Batalha et al. 2011), starting a new time and a long cadence with 16 exposures of 32 s stacked together era of exoplanet discoveries. resulting in 512 s cadence (Ollivier et al. 2016). While most stars CoRoT (convection, rotation, and planetary transits) was the first were observed in long cadence mode, the 32 s exposures were only space mission dedicated to the detection of transiting planets. The downloaded for selected targets i.e. after the detection of transit like mission was launched in 2006 December and started its first sci- events. ence observation in 2007 January. The spacecraft was equipped In 2009 March, the satellite suffered a loss of communication with a 27-cm telescope and a 4-CCD wide-field camera. Each pair with one of the data processing units (DPU), which reduced the of CCDs was designed for one of the two main goals of the mis- FoV by 50 per cent. In 2012 November, the second and last DPU sion, asteroseismology, or exoplanets. A complete overview on the failed resulting in the end of the mission in 2013 June. So far CoRoT mission can be found in ‘The CoRoT Legacy Book: The ad- 34 confirmed exoplanets have been published and ∼500 candidate exoplanets are awaiting evaluation (CoRot Team 2016). To truly benefit from CoRoT’s planet findings, the planet and E-mail: [email protected] orbit parameters need to be accurately determined. Since CoRoT C 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society Transit analysis of CoRoT-targets 825 could observe transiting planets only for a maximum duration of to the middle of the exposure. In step three, we normalized the LCs. 150 d, ground-based follow-up is mandatory to extend the observa- After the division by the average out-of-transit flux, additional light tional baseline. We have therefore initiated a dedicated project to (‘third’ light) L3 induced by contaminants in the aperture around combine the unprecedented precision of CoRoT LCs with ground- the target star was subtracted from the normalized flux before re- based follow-up photometry, in order to refine the planets orbital normalizing. In a last step we cleaned the LCs from outliers. By elements, constrain their physical parameters and search for addi- using a moving average of the time-series we created a smoothed tional bodies in the system. We selected 12 suitable targets that LC. Finally, we removed all data points that deviated more than 3σ Downloaded from https://academic.oup.com/mnras/article-abstract/483/1/824/5184487 by Inst. Astrofisica Andalucia CSIC user on 28 August 2019 fulfilled the following criteria: from this smoothed LC. (i) The brightness of the host star is V < 16 mag and the transit depth is at least 8 mmag, to ensure sufficient photometric and timing 2.2 Ground-based observations precision at 1–2 m class ground-based telescopes. (ii) The orbit of the known transiting planet is not well con- One observation in 2012 was carried out using the ‘Schmidt strained through radial velocity (RV) observations or shows non- Teleskop Kamera’ (STK, Mugrauer & Berthold 2010) mounted at zero eccentricity (though the circularization time-scale is much the 90 cm Schmidt telescope (60 cm in Schmidt mode) at the Univer- × shorter than the system age) and/or the data presents deviant RV sity Observatory Jena. With 2048 2048 pixels and a pixel scale of points, possibly indicating a perturber. 1.55 arcsec/pixel, we could observe a large FoV of 53 x 53 arcmin. (iii) Timing errors are critically large, which would impede the Most of the LCs (12 out of 16) were collected with the 1.5-m planetary transit observations within a few years. reflector at the Observatorio de Sierra Nevada (OSN), which is operated by the Instituto de Astrof´ısica de Andaluc´ıa, CSIC, Spain. A short description and first results of this study were published in Using a VersArray:2048B CCD camera (2048 × 2048 pixels, pixel Raetz et al. (2015). Here, we report on our observations of six of scale 0.23 arcsec/pixel) we covered a FoV of 7.85 × 7.85 arcmin. these targets, CoRoT-5, CoRoT-8, CoRoT-12, CoRoT-18, CoRoT- From 2015 November to 2016 June, we obtained three additional 20, and CoRoT-27. Table 1 summarizes the literature values of the LCs at ESA’s Optical Ground Station (OGS), a 1-m telescope lo- physical properties of these systems. Fig. 1 gives the propagation cated at the Observatorio del Teide on Tenerife. The mounted spec- of the original published ephemeris uncertainties for these targets trograph (Schulz et al. 2014) was used in imaging mode for the to the present. In particular, CoRoT-20 and CoRoT-27 are of spe- observations. The Roper Spec Camera provides 2048 × 2048 pix- cial interest as they have the largest uncertainties in our sample. els with a pixel scale of 0.403 arcsec/pixel. The initial FoV of Moreover, both targets are massive hot Jupiters and, hence, very 13.8 × 13.8 arcmin was windowed to shorten read-out time. interesting systems to study formation, migration, and evolution of Since the CoRoT targets are relatively faint (V ∼ 14 − 15.5mag, gas giant planets. see Table 1) all observations were carried out either in R-band or without any filter, with exposure times between 90 and 180 s. 2 OBSERVATION, DATA REDUCTION, AND Data reduction and photometry were performed following the PHOTOMETRY procedures described by us in e.g. Raetz et al. (2014, 2016). In short, we subtracted a bias (as overscan for the data of the STK) and We started our follow-up campaign in 2013 October after first test a dark frame (only for STK) and divided by a sky flat field using the observations that were carried out in 2012 September. In total, we IRAF1 routines zerocombine, darkcombine, flatcombine,andccd- collected 16 high-precision LCs of the six selected targets, CoRoT- proc. For the aperture photometry with 10 different aperture radii 5, CoRoT-8, CoRoT-12, CoRoT-18, CoRoT-20, and CoRoT-27, we used a script based on the standard IRAF routine phot. Finally, we from 2012 September to 2016 December. Our ground-based ob- derived differential magnitudes using an optimized artificial com- servations were performed with three 1-m class telescopes located parison star (Broeg, Fernandez´ & Neuhauser¨ 2005).