Astronomy & Astrophysics manuscript no. 19731 c ESO 2012 October 3, 2012 WASP-78b and WASP-79b: Two highly-bloated hot Jupiter-mass exoplanets orbiting F-type stars in Eridanus ⋆ B. Smalley1, D. R. Anderson1, A. Collier-Cameron2, A. P. Doyle1, A. Fumel3, M. Gillon3, C. Hellier,1, E. Jehin2, M. Lendl4, P. F. L. Maxted1, F. Pepe4, D. Pollacco5, D. Queloz4, D. S´egransan4, A. M. S. Smith1, J. Southworth1 A. H. M. J. Triaud4, S. Udry4, R. G. West6, 1 Astrophysics Group, Keele University, Staffordshire, ST5 5BG, United Kingdom e-mail: [email protected] 2 SUPA, School of Physics and Astronomy, University of St. Andrews, North Haugh, Fife, KY16 9SS, United Kingdom 3 Universit´ede Li`ege, All´ee du 6 aoˆut 17, Sart Tilman, Li`ege 1, Belgium 4 Observatoire de Gen`eve, Universit´ede Gen`eve, Chemin des maillettes 51, 1290 Sauverny, Switzerland 5 Astrophysics Research Centre, School of Mathematics & Physics, Queen’s University Belfast, University Road, Belfast BT7 1NN, United Kingdom 6 Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, United Kingdom Received ¡date¿ / accepted ¡date¿ ABSTRACT We report the discovery of WASP-78b and WASP-79b, two highly-bloated Jupiter-mass exoplanets orbiting F-type host stars. WASP- 78b orbits its V = 12.0 host star (TYC 5889-271-1) every 2.175 days and WASP-79b orbits its V = 10.1 host star (CD-30 1812) every 3.662 days. Planetary parameters have been determined using a simultaneous fit to WASP and TRAPPIST transit photometry and CORALIE radial-velocity measurements. For WASP-78b a planetary mass of 0.89 ± 0.08 MJup and a radius of 1.70 ± 0.11 RJup is found. The planetary equilibrium temperature of TP = 2350 ± 80 K for WASP-78b makes it one of the hottest of the currently known exoplanets. WASP-79b its found to have a planetary mass of 0.90 ± 0.08 MJup, but with a somewhat uncertain radius due to lack of sufficient TRAPPIST photometry. The planetary radius is at least 1.70 ± 0.11 RJup, but could be as large as 2.09 ± 0.14 RJup, which would make WASP-79b the largest known exoplanet. Key words. planets and satellites: general – stars: individual: WASP-78 – stars: individual: WASP-79 – techniques: photometry – techniques: spectroscopy – techniques: radial velocities 1. Introduction (Batygin & Stevenson 2010; Perna et al. 2010) and thermal tidal effects (Arras & Socrates 2010). The first exoplanets were discovered using the radial velocity In this paper we report the detection of WASP-78b and technique (Mayor & Queloz 1995). However, following the de- WASP-79b, two highly-bloated Jupiter-mass planets in orbit tection of a transiting exoplanet (Charbonneau et al. 2000), sev- around F-type stars. We present the WASP-South discovery pho- eral ground-based and space-based survey projects have dramat- tometry, together with follow-up optical photometry and radial ically increased the number of known systems. Transiting ex- velocity measurements. oplanets allow parameters such as the mass, radius, and den- sity to be precisely determined, as well as their atmospheric properties to be studied during their transits and occultations (Charbonneau et al. 2005; Southworth 2009; Winn 2009). 2. WASP-South photometry Most of the transiting exoplanets found by ground-based The WASP project has two robotic observatories; one on La arXiv:1206.1177v2 [astro-ph.EP] 2 Oct 2012 surveys are ’hot Jupiters’, with orbital periods of up to around Palma in the Canary Islands and another in Sutherland in South 5 days. Many of these have radii larger than predicted by irradi- Africa. The wide angle survey is designed to find planets around ated planet models (Fortney et al. 2007). Several have markedly relatively bright stars in the V-magnitude range 9 ∼ 13. A de- low densities, with WASP-17b (Anderson et al. 2010, 2011), tailed description is given in Pollacco et al. (2006). Kepler-12b (Fortney et al. 2011) and Kepler-7b (Latham et al. 2010) having a density less than 1/10 that of Jupiter. The mech- The pipeline-processed data were de-trended and anisms for producing such bloated planets are at present un- searched for transits using the methods described in clear (Fortney & Nettelmann 2010), but several have been pro- CollierCameronet al. (2006), yielding detections of pe- posed, including Ohmic heating in the planetary atmosphere riodic, transit-like signatures on two stars in the constel- lation Eridanus (Fig. 1). The V = 12.0 star WASP-78 (1SWASPJ041501.50-220659.0; TYC 5889-271-1) exhibited ⋆ Radial velocity and photometric data are only available in elec- ∼0.010 mag. transits every 2.175 days, while the V = 10.1 tronic form at the CDS via anonymous ftp to star WASP-79 (1SWASPJ042529.01-303601.5; CD-30 1812) cdsarc.u-strasbg.fr (130.79.128.5) or via showed ∼0.015 mag. transits every 3.66 days. A total of 16489 http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/???/A?? observations of WASP-78 were obtained between 2006 August 1 B. Smalley et al.: WASP-78b and WASP-79b and 2009 December and 15424 observations of WASP-79 were Table 1. Radial velocity (RV) and line bisector spans obtained between 2006 September and 2010 February. (Vspan) measurements for WASP-78 and WASP-79 obtained by CORALIE spectra. −1 −1 -0.02 BJD–2 400 000 (UTC) RV (km s ) Vspan (km s ) WASP-78 -0.01 55843.80914 0.3565 ± 0.0349 +0.0443 0 55844.85276 0.6078 ± 0.0404 +0.2036 0.01 55856.82996 0.3477 ± 0.0388 −0.0086 Relative mag WASP-78 ± + 0.02 55858.75524 0.3932 0.0409 0.0772 55859.84448 0.5282 ± 0.0339 +0.1423 0.6 0.8 1 1.2 1.4 55863.77620 0.4233 ± 0.0281 +0.1274 Phase 55864.77388 0.5390 ± 0.0284 +0.1252 -0.02 55868.78606 0.5463 ± 0.0320 +0.1018 55872.82630 0.5131 ± 0.0346 −0.1311 -0.01 55880.77994 0.3302 ± 0.0240 +0.1064 0 55881.82771 0.5734 ± 0.0272 +0.0760 0.01 55883.69133 0.5093 ± 0.0260 +0.1505 Relative mag WASP-79 0.02 55887.76423 0.4160 ± 0.0231 +0.0627 55888.64129 0.5431 ± 0.0281 +0.0722 0.6 0.8 1 1.2 1.4 55889.62002 0.3780 ± 0.0257 +0.0949 Phase 55894.74373 0.5875 ± 0.0238 +0.0426 Fig. 1. WASP photometry of WASP-78 and WASP-79 folded 55925.58825 0.5346 ± 0.0206 +0.0972 on the best-fitting orbital periods and binned into phase bins of WASP-79 0.001. The solid line is the model fit presented in Sect. 6. 55542.63132 4.8915 ± 0.0260 −0.2869 55573.65244 5.1141 ± 0.0225 −0.1877 55595.61945 5.0768 ± 0.0267 −0.2985 55832.81511 4.9912 ± 0.0280 −0.3792 There is a 15th mag. star, 2MASS04150416-2207189, lo- ′′ 55856.85501 4.9258 ± 0.0319 −0.2776 cated 42 away from WASP-78, which is just within the pho- 55858.83490 4.9823 ± 0.0315 −0.1275 tometric extraction aperture. However, the dilution is only 2% 55863.73949 4.9772 ± 0.0232 −0.2613 and does not significantly affect the depth of the WASP tran- 55864.82478 4.8688 ± 0.0236 −0.2901 sits. Around 24′′ away from WASP-79, is 6dFGSgJ042530.8- 55865.67114 4.9591 ± 0.0282 −0.3785 303554, a 16th mag. redshift z = 0.069 galaxy (Jones et al. 55867.86163 4.9566 ± 0.0241 −0.2958 2009). This is, however, too faint to significantly dilute the 55868.67269 4.9228 ± 0.0267 −0.2961 WASP-79 photometry. The spectral type of WASP-79 is listed 55869.70268 5.0298 ± 0.0266 −0.2612 as F2 in Jackson & Stoy (1954). 55870.84257 5.0119 ± 0.0249 −0.2504 55871.79821 4.9102 ± 0.0249 −0.3420 55872.78874 4.9724 ± 0.0251 −0.3896 55873.83187 5.0848 ± 0.0260 −0.3053 3. Spectroscopic observations with CORALIE 55874.83089 5.1331 ± 0.0486 −0.6088 55883.76896 4.9481 ± 0.0252 −0.2040 Spectroscopic observations were obtained with the CORALIE 55886.77560 4.9336 ± 0.0300 −0.3774 spectrograph on the Swiss 1.2m telescope. The data were 55888.67590 5.0633 ± 0.0258 −0.1902 processed using the standard pipeline (Baranne et al. 1996; 55925.65354 5.0572 ± 0.0200 −0.2109 Quelozet al. 2000; Pepe et al. 2002). A total of 17 and 21 ra- 55964.62745 5.0409 ± 0.0216 −0.3219 dial velocity (RV) and line bisector span (Vspan) measurements were made for WASP-78 and WASP-79, from 2011 October 09 to 2011 December 30, and 2010 December 12 to 2012 February mount, there are short gaps in the lightcurves at the times of cul- 07, respectively (Table 1). The bisector spans are a measure of mination due to meridian flips, when the telescope is reversed to the asymmetry of the cross-correlation function and, based on the other side the pier. our experience, have standard errors of ≈ 2σRV. The amplitude of the RV variations and the absence of any correlation with orbital phase of the line bisector spans (Vspan) 4.1. WASP-78b in Fig. 2 indicates that it is highly improbable that the RV varia- tions are due to an unresolvedeclipsing binary or chromospheric A total of three transits of WASP-78b were observed with activity (Queloz et al. 2001). TRAPPIST. The observations were made through a ‘blue- In the case of WASP-79 one RV point (HJD blocking’ filter with a transmittance >90% above 500nm.