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WASP-78B and WASP-79B: Two Highly-Bloated Hot Jupiter-Mass Exoplanets Orbiting F-Type Stars in Eridanus

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WASP-78B and WASP-79B: Two Highly-Bloated Hot Jupiter-Mass Exoplanets Orbiting F-Type Stars in Eridanus Article WASP-78b and WASP-79b: two highly-bloated hot Jupiter-mass exoplanets orbiting F-type stars in Eridanus SMALLEY, B., et al. Reference SMALLEY, B., et al. WASP-78b and WASP-79b: two highly-bloated hot Jupiter-mass exoplanets orbiting F-type stars in Eridanus. Astronomy and Astrophysics, 2012, vol. 547, p. A61 DOI : 10.1051/0004-6361/201219731 Available at: http://archive-ouverte.unige.ch/unige:33858 Disclaimer: layout of this document may differ from the published version. 1 / 1 A&A 547, A61 (2012) Astronomy DOI: 10.1051/0004-6361/201219731 & c ESO 2012 Astrophysics 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. Ségransan4,A.M.S.Smith1, J. Southworth1 A. H. M. J. Triaud4,S.Udry4, and R. G. West6 1 Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK e-mail: [email protected] 2 SUPA, School of Physics and Astronomy, University of St. Andrews, North Haugh, Fife, KY16 9SS, UK 3 Université de Liège, Allée du 6 août 17, Sart Tilman, Liège 1, Belgium 4 Observatoire de Genève, Université de Genève, Chemin des maillettes 51, 1290 Sauverny, Switzerland 5 Astrophysics Research Centre, School of Mathematics & Physics, Queen’s University Belfast, University Road, Belfast BT7 1NN, UK 6 Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK Received 1 June 2012 / Accepted 28 September 2012 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: spectroscopic – techniques: radial velocities – techniques: photometric 1. Introduction In this paper we report the detection of WASP-78b and WASP-79b, two highly-bloated Jupiter-mass planets in orbit The first exoplanets were discovered using the radial veloc- around F-type stars. We present the WASP-South discovery pho- ity technique (Mayor & Queloz 1995). However, following the tometry, together with follow-up optical photometry and radial detection of a transiting exoplanet (Charbonneau et al. 2000), velocity measurements. several ground-based and space-based survey projects have dra- matically increased the number of known systems. Transiting exoplanets allow parameters such as the mass, radius, and den- 2. WASP-south photometry sity to be precisely determined, as well as their atmospheric The WASP project has two robotic observatories; one on properties to be studied during their transits and occultations La Palma in the Canary Islands and another in Sutherland in (Charbonneau et al. 2005; Southworth 2009; Winn 2009). South Africa. The wide angle survey is designed to find planets Most of the transiting exoplanets found by ground-based around relatively bright stars in the V-magnitude range 9 ∼ 13. surveys are “hot Jupiters”, with orbital periods of up to A detailed description is given in Pollacco et al. (2006). around 5 days. Many of these have radii larger than predicted The pipeline-processed data were de-trended and searched by irradiated planet models (Fortney et al. 2007). Several have for transits using the methods described in Collier Cameron et al. markedly low densities, with WASP-17b (Anderson et al. 2010, (2006), yielding detections of periodic, transit-like signatures on 2011), Kepler-12b (Fortney et al. 2011) and Kepler-7b (Latham = / two stars in the constellation Eridanus (Fig. 1). The V 12.0star et al. 2010) having a density less than 1 10 that of Jupiter. The WASP-78 (1SWASPJ041501.50-220659.0; TYC 5889-271-1) mechanisms for producing such bloated planets are at present exhibited ∼0.010 mag transits every 2.175 days, while unclear (Fortney & Nettelmann 2010), but several have been the V = 10.1 star WASP-79 (1SWASPJ042529.01-303601.5; proposed, including Ohmic heating in the planetary atmosphere CD-30 1812) showed ∼0.015 mag transits every 3.66 days. (Batygin & Stevenson 2010; Perna et al. 2010) and thermal tidal ff A total of 16489 observations of WASP-78 were obtained be- e ects (Arras & Socrates 2010). tween 2006 August and 2009 December and 15424 observations Photometric data is only available at the CDS via anonymous ftp to of WASP-79 were obtained between 2006 September and 2010 cdsarc.u-strasbg.fr (130.79.128.5) or via February. http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/547/A61 There is a 15th mag star, 2MASS 04150416-2207189, lo- Table 1 is available in electronic form at cated 42 away from WASP-78, which is just within the pho- http://www.aanda.org tometric extraction aperture. However, the dilution is only 2% Article published by EDP Sciences A61, page 1 of 7 A&A 547, A61 (2012) -0.02 0.65 -0.01 0.6 WASP-78 0 0.55 ) 0.01 -1 Relative mag 0.5 WASP-78 0.02 0.45 0.6 0.8 1 1.2 1.4 RV (km s Phase 0.4 0.35 -0.02 0.3 -0.01 ) -1 0.2 0 0.1 (km s 0 0.01 -0.1 Relative mag WASP-79 span V -0.2 0.02 0.6 0.8 1 1.2 1.4 0.6 0.8 1 1.2 1.4 Phase Phase Fig. 1. WASP photometry of WASP-78 and WASP-79 folded on the 5.15 best-fitting orbital periods and binned into phase bins of 0.001. The solid line is the model fit presented in Sect. 6. 5.1 WASP-79 ) -1 5.05 and does not significantly affect the depth of the WASP tran- 5 RV (km s sits. Around 24 away from WASP-79, is 6dFGS gJ042530.8- 4.95 303554, a 16th mag redshift z = 0.069 galaxy (Jones et al. 2009). This is, however, too faint to significantly dilute the WASP-79 4.9 photometry. The spectral type of WASP-79 is listed as F2 in 4.85 ) Jackson & Stoy (1954). -1 -0.1 -0.2 (km s -0.3 span -0.4 3. Spectroscopic observations with CORALIE V 0.6 0.8 1 1.2 1.4 Spectroscopic observations were obtained with the CORALIE Phase spectrograph on the Swiss 1.2 m telescope. The data were pro- Fig. 2. Radial velocity variations and line bisectors (Vspan)ofWASP- cessed using the standard pipeline (Baranne et al. 1996; Queloz 78 and WASP-79 folded on the best-fitting circular orbit periods (solid et al. 2000; Pepe et al. 2002). A total of 17 and 21 radial veloc- lines). The eccentric orbit solutions are shown as dashed-lines (see ity (RV) and line bisector span (Vspan) measurements were made Sect. 6 for discussion). The bisector uncertainties of twice the RV un- for WASP-78 and WASP-79, from 2011 October 09 to 2011 certainties have been adopted. The mean values of the bisectors are in- December 30, and 2010 December 12 to 2012 February 07, re- dicated by the dotted lines. There is negligible correlation between Vspan spectively (Table 1). The bisector spans are a measure of the and orbital phase. asymmetry of the cross-correlation function and, based on our experience, have standard errors of ≈2σRV . exposure times were 15 s and the telescope was defocused, the The amplitude of the RV variations and the absence of any mean full width at half maximum (FWHM) being 5 pixels (pixel correlation with orbital phase of the line bisector spans (Vspan) scale = 0.65 ). in Fig. 2 indicates that it is highly improbable that the RV varia- The first transit was observed on 2011 November 8 from tions are due to an unresolved eclipsing binary or chromospheric 01h23 to 08h00 UTC. The first ≈75% of the run was slightly activity (Queloz et al. 2001). cloudy. There was a meridian flip at HJD 2 455 873.748. The re- In the case of WASP-79 one RV point (HJD 2 455 874. sulting lightcurve has 949 measurements. 830894) was taken during transit. This is affected by the A second transit was observed on 2011 December 15 from Rossiter-McLaughlin effect and has been excluded from the fit- 00h23 to 06h44 UTC. Transparency was good this time. A ting process. meridian flip occurred at HJD 2 455 910.646. The resulting lightcurve has 918 measurements. 4. TRAPPIST photometry The third transit was observed on 2012 January 8 from 00h45 to 05h30 UTC. However, only part of the transit Photometric observations of transits were obtained using the was observed.
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