A Lower Radius and Mass for the Transiting Extrasolar Planet HAT-P-8 B⋆

A&A 551, A11 (2013) Astronomy DOI: 10.1051/0004-6361/201220291 & c ESO 2013 Astrophysics A lower radius and mass for the transiting extrasolar planet HAT-P-8 b L. Mancini1,2, J. Southworth3,S.Ciceri1,J.J.Fortney4,C.V.Morley4, J. A. Dittmann5, J. Tregloan-Reed3, I. Bruni6, M. Barbieri7,D.F.Evans3,G.D’Ago2,N.Nikolov1, and Th. Henning1 1 Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany e-mail: [email protected] 2 Department of Physics, University of Salerno, Via Ponte Don Melillo, 84084 Fisciano (SA), Italy 3 Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK 4 Department of Astronomy & Astrophysics, University of California, Santa Cruz, CA 95064, USA 5 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 10 Cambridge, MA 02138, USA 6 INAF – Osservatorio Astronomico di Bologna, Via Ranzani 1, 40127 Bologna, Italy 7 INAF – Osservatorio Astronomico di Padova, Vicolo Osservatorio 5, 35122 Padova, Italy Received 27 August 2012 / Accepted 15 December 2012 ABSTRACT Context. The extrasolar planet HAT-P-8 b was thought to be one of the more inflated transiting hot Jupiters. Aims. By using new and existing photometric data, we computed precise estimates of the physical properties of the system. Methods. We present photometric observations comprising eleven light curves covering six transit events, obtained using five medium- class telescopes and telescope-defocussing technique. One transit was simultaneously obtained through four optical filters, and two transits were followed contemporaneously from two observatories. We modelled these and seven published datasets using the jktebop code. The physical parameters of the system were obtained from these results and from published spectroscopic measurements. In addition, we investigated the theoretically-predicted variation of the apparent planetary radius as a function of wavelength, covering the range 330–960 nm. Results. We find that HAT-P-8b has a significantly lower radius (1.321 ± 0.037 RJup)andmass(1.275 ± 0.053 MJup) compared to +0.08 +0.18 previous estimates (1.50−0.06 RJup and 1.52−0.16 MJup respectively). We also detect a radius variation in the optical bands that, when compared with synthetic spectra of the planet, may indicate the presence of a strong optical absorber, perhaps TiO and VO gases, near the terminator of HAT-P-8b. Conclusions. These new results imply that HAT-P-8b is not significantly inflated, and that its position in the planetary mass-radius diagram is congruent with those of many other transiting extrasolar planets. Key words. planets and satellites: fundamental parameters – stars: fundamental parameters – stars: individual: HAT-P-8 1. Introduction (Kipping et al. 2009; Tusnski & Valio 2011), gravity darkening (Barnes 2009; Szabó et al. 2011) and star spots (Pont et al. Transiting extrasolar planetary systems are of interest and 2007; Rabus et al. 2009; Désert 2011). Besides the Rossiter- importance, because precise measurements of their physical McLaughlin effect (Queloz et al. 2000; Gaudi et al. 2007), the properties can be achieved using spectroscopic and photomet- photometric follow-up on consecutive/close nights of transits ric observations. Atomic and molecular absorption within the of planets over parent-star starspots represents another fascinat- atmosphere of transiting extrasolar planets (TEPs) can also ing method (Sanchis-Ojeda & Winn 2011; Sanchis-Ojeda et al. be investigated through transmission spectroscopy (e.g. Swain 2011, 2012; Tregloan-Reed et al. 2012) to measure the sky- et al. 2008; Sing et al. 2009; Fossati et al. 2010)andsimul- projected spin-orbit alignment. taneous multi-colour photometry (e.g. Ballester et al. 2007; Southworth et al. 2012b; Mancini et al. 2013)ofthetran- The increasing number of TEPs discovered every year is pro- sits. High-quality photometric observations not only enable the gressively revealing a remarkable diversity. The improving sta- measurement of the masses and radii of TEPs to accuracies tistical weight of this sample is useful for establishing the cor- of a few percent (e.g. Torres et al. 2008; Southworth 2009), rect theoretical framework of planet formation and evolution. but also the detection of transit anomalies due to stellar pulsa- Accurate estimates of the planet properties (mass, radius, or- tions (Collier Cameron et al. 2010), tidal distortion (Li et al. bital semi-major axis, etc.) are vital for this purpose, and photo- 2010; Leconte et al. 2011), additional bodies (moons, planets) metric follow-up of known TEPs can dramatically improve our knowledge of the planet’s characteristics. Full Table 2 is only available in electronic form at the CDS via HAT-P-8 b is a transiting hot Jupiter found by the HATNet anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)orvia team (Latham et al. 2009), orbiting with a period of ∼3.07 days http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/551/A11 around a star of spectral type F8 (Jones & Sleep 2000)or Article published by EDP Sciences A11, page 1 of 11 A&A 551, A11 (2013) Table 1. Details of the observations presented in this work. Telescope Date Start/End times Nobs texp Filter Airmass Moon Aperture Scatter PSF β Kuiper 2009 10 22 04:03 → 09:10 1083 5 Cousins I 1.00 → 1.9 25% 10, 20, 25 1.54 Loiano 2009 11 12 16:52 → 22:09 103 118 Gunn r 1.02 → 1.01 → 1.55 23% 25, 45, 60 1.15 1963 1.4 INT 2010 08 28 20:44 → 04:02 131 140 Strömgren y 2.04 → 1.00 → 1.18 84% 45, 65, 90 0.47 3217 1.7 Loiano 2011 10 05 18:01 → 02:29 469 50 Gunn i 1.12 → 1.01 → 1.45 61% 19, 40, 60 1.15 908 1.4 CA 1.23 m 2011 10 05 19:27 → 03:07 166 70 Johnson R 1.08 → 1.05 → 1.54 61% 26, 38, 55 0.83 1662 1.0 CA 2.2 m 2012 08 26 20:31 → 04:48 160 80 sdss u 1.52 → 1.00 → 1.55 77% 25, 55, 75 2.37 2827 1.0 CA 2.2 m 2012 08 26 20:31 → 04:48 157 80 Gunn g 1.52 → 1.00 → 1.55 77% 30, 50, 70 0.97 2463 1.2 CA 2.2 m 2012 08 26 20:31 → 04:48 159 80 Gunn r 1.52 → 1.00 → 1.55 77% 30, 60, 90 0.71 2642 1.5 CA 2.2 m 2012 08 26 20:31 → 04:48 162 80 Gunn z 1.52 → 1.00 → 1.55 77% 28, 55, 80 0.92 1662 1.1 Loiano 2012 10 02 17:45 → 02:32 232 70 Johnson I 1.03 → 1.00 → 1.92 92% 23, 40, 65 0.79 2521 1.0 CA 1.23 m 2012 10 02 20:07 → 03:35 373 50 Johnson I 1.11 → 1.00 → 2.16 92% 35, 60, 90 1.69 2290 1.8 Notes. Nobs is the number of observations, Moon is the fractional illumination of the Moon at the midpoint of the transit, and texp is the exposure time in seconds. The aperture sizes are the radii in pixels of the software apertures for the star, inner sky and outer sky, respectively. Scatter is the rms scatter of the data versus a fitted model, in mmag. Target mean PSF area in px2 is also reported for each dataset. Times and dates are in UT. β is the ratio between the noise levels due to Poisson noise and to combined Poisson and red noise. F5 (Bergfors et al. 2012). At the time of its discovery it was Table 2. Excerpts of the light curves of HAT-P-8. labelled as one of the most inflated transiting giant planets, = +0.18 with a measured mass and radius of Mb 1.52−0.16 MJup and Telescope Filter BJD (TDB) Diff. mag. Uncertainty + = 0.08 ff Loiano r 2455148.210781 0.0009980 0.0018456 Rb 1.50−0.06 RJup, respectively. These values di er by 2–3σ from the theoretical predictions of Fortney et al. (2007). Loiano r 2455148.214867 0.0000050 0.0017916 The Rossiter-McLaughlin effect has been detected in the Loiano i 2455840.256418 0.0000785 0.0016988 HAT-P-8 system using radial velocity observations from the Loiano i 2455840.258316 0.0006256 0.0016925 SOPHIE and FIES spectrographs. Simpson et al. (2011) found CAHA R 2455840.319832 0.0007127 0.0013972 = − +9.0 CAHA R 2455840.329288 –0.0019752 0.0013798 a sky-projected orbital obliquity of λ 9.7−7.7 degrees and = − +9.2 INT y 2455437.412356 0.0052976 0.0009779 Moutou et al. (2011) found λ 17−11.5 degrees; both values are INT y 2455437.414575 0.0056092 0.0009998 consistent with alignment between the orbital axis of the planet Kuiper I 2455126.66906 –0.99507 − and the rotational axis of the star. The two studies between them Kuiper I 2455126.66930 –0.99959 − suggested lower values for Mb and Rb, but neither calculated the physical properties of the system. Notes. This table will be made available at the CDS. A portion is shown Bergfors et al. (2012) have found a faint companion to here for guidance regarding its form and content. the HAT-P-8 system using lucky imaging observations with the AstraLux Norte instrument at the Calar Alto 2.2 m telescope. all our transit observations were performed with the telescope The companion, a likely M2-4 dwarf, is at an angular distance defocussing method, in order to minimise the effect of Poisson, of 1.027 ± 0.011 arcsec and is fainter in the SDSS i and z pass- scintillation and flat-fielding noise (Southworth et al.

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