HAT-P-7B: an Extremely Hot Massive Planet Transiting a Bright Star in The

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University Keck by Technology. the granted of by Institute California operated the is which servatory, DC Washington, CA nvriy a rnic,CA Francisco, San University, rlbnso h et n hp fpiaytransits, primary of shape and depth ob- the Such spec- of different 2007). bands in al. observations tral et with Harrington together 2005; 2007; servations, al. al. et for Deming et and e.g. Knutson (see eclipse orbit secondary their emission of throughout times thermal some, parent at their their both that measurable, about sufficiently is orbits heated tight Tran- are very controlling stars atmospheres. in with their role planets of major siting dynamics a and play parent structure to the the expected from their radiation be of the may orbits Many case star close-in which quite objects. in in stars, these found parent are of planets mass these data) of because velocity (together and planets radial radius extrasolar the with of of measurement direct evolution yield they and nature the A [email protected] MA, aet Hungary. dapest, † 8 6 7 2 5 4 3 rf eso coe 8 08 ESO 21 L VERSION using 2018– typeset 28, Preprint October version Draft 1 about information unique provide planets Transiting .A Fischer A. D. A--b NETEEYHTMSIEPAE RNIIGABRIG A TRANSITING PLANET MASSIVE HOT EXTREMELY AN HAT-P-7b: ae npr nosrain banda h .M ekOb- Keck M. W. the at obtained observations on part in Based ugra srnmclAscain uaet Hungary Budapest, Association, Astronomical Hungarian eateto hsc n srnm,SnFacsoState Francisco San Astronomy, and Physics of Department eateto ersra ants,Crei nttt o Institute Carnegie Magnetism, Terrestrial of Department eateto srnm,Eovo oń nvriy Bu- Eötvös University, Loránd Astronomy, of Department eateto srnm,Uiest fClfri,Berkel California, of University Astronomy, of Department Hungary Budapest, Observatory, Konkoly Fellow NSF avr-mtsna etrfrAtohsc,Cambridge, Astrophysics, for Center Harvard-Smithsonian .P A. ( is nteaalbesetocpcdt ntehs tradphotomet and star host the on of data mass spectroscopic a available the on lnt il eyhg lntr raineo (4 of irradiance planetary high very a yield planet, vle 6sa with star F6 evolved [Fe lntna h o ftep ls firdae lnt sdfie b (2730 about defined be as abso should planets its temperature re-radiates irradiated day-side planet the of the side, class (2007) night pM al. the the et Fortney of by top predicted the near planet ujc headings: secon Subject and possible. upco transit Jup be the primary hot of will only field asteroseismology, current not the including of in follow-up, falls predictions star space-borne the host the compare Moreover, fo to feasible observations. opportunity be should good eclipse a secondary the of measurement bright, V erpr ntelts icvr fteHTe rjc;avr h very a project; HATNet the of discovery latest the on report We P / al ´ 10 = ]=+0 = H] 2 = 1,2 G. , 6 . 1. )sa ihasalsm-ao xsof axis semi-major small a with star 5) .P Butler P. R. , . 2047299 M INTRODUCTION .Bakos A. ´ A . p 26 T E 1 = tl mltajv 05/04/06 v. emulateapj style X ± 0 ± spectroscopic lntr ytm tr:idvda HTP7 S 03547-0140 GSC (HAT-P-7, individual stars: — systems planetary . . 78 8 h eaieyhtadlrehs tr obndwt h ls or close the with combined star, host large and hot relatively The 08. 0 1,3 . M − +0 004 as i-rni time mid-transit days, 0000040 0 .Torres G. , 7 ⋆ . . .D Sasselov D. D. , 08 05 1 = M Jup rf eso coe 8 08 ESO 21 VERSION 2018– 28, October version Draft . 47 − +0 n aisof radius and .L J. 0 . . 1 08 05 .W Noyes W. R. , az ´ M ar ´ ⊙ 1 , .Sip B. , part n and a 8 R ABSTRACT .Papp I. , ey, ⋆ FIELD f 1 = R a ocz p ˝ 0 = 1 . 84 1 = .W Latham W. D. , . 8 hs aaicuigdrvto fselradplanetary and stellar of derivation including data these eyhtJptrta rnisabright, a transits that Jupiter hot very a et (2006). Hubeny al. (2007), et al. Burrows Jupiter” et (2003), Fortney “hot al. e.g. of the (See of dynamics understanding atmospheres. and detailed structure, of possibility composition, the up open tr iha2 a with star, ) with 7), 0 cpcflo-pobservations; follow-up scopic ino h A--bpae yteHTe monitoring HATNet the by planet system; HAT-P-7b the of tion eunfo ae bevtoso h trb Kepler. by star scientific the the of increase observations later advance significantly from in could return undertaken mission system the HAT-P-7 secondary of the and in transits as of such eclipses rea- observations system, this For preparatory the in son, satellites. Trojan bodies or other indi- planets terrestrial-mass of could detected, presence 2009. if the in likely transits, cate starting the very Kepler in precision will of variations Timing photometric phase transits operational high mis- is many the Kepler extremely during star that forthcoming with host the recorded means of the be This view Notably, of field sion. ground. the in and stud- follow-on also space interesting of from number ies a of possibility the any of of highest temperature planet. the surface transiting among known the planet close-orbiting make very to this as luminosity high 71 2,1 . .S P. & − +0 3 U h aetsa hratrdntdHAT-P- denoted (hereafter star parent The AU. 038 . eew eoto h ANtpoetsdtcinof detection project’s HATNet the on report we Here h etscin( section next The up opens this bright, relatively is HAT-P-7 Because − +1 E 0377 .A Esquerdo A. G. , 0 . . 36 . 0 23 11 . . 44 05 2 = − +0 R ) 0 ± ari ´ § ⊙ . . × 20 09 , T − +150 , 0 ulnsbt h htmti n spectro- and photometric the both outlines 3 453 eff igKpe iso;hneextensive hence mission; Kepler ming 8 10 . 100 R bdeeg eoedsrbtn tto it distributing before energy rbed T rudbsdtlsoe,providing telescopes, ground-based r 05A.Ehmrsfrtesystem the for Ephemeris AU. 0005 eff Jup 9 . , .Bcuetehs tri quite is star host the Because K. ) tramshrcmdl ihthe with models atmospheric iter 30Kadrdu 1 radius and K 6350 = a eidadsm-ao xso only of axis semi-major and period day 2 ayelpeosrain u also but observations eclipse dary 790 yo h ytm h lnthas planet the system, the of ry 1 r cm erg tgatpae riigabright a orbiting planet giant ot 6350 = h aetsa saslightly a is star parent The . G , ote ta.(07.I as If (2007). al. et Fortney y . 2593 z Kov eza ´ § 1 G , )dsrbstepooercdetec- photometric the describes 2) − 2 ± ± TSA NTEKEPLER THE IN STAR HT brKov abor ´ s 0 0K n metallicity and K, 80 − . acs ´ 1 00(J) Based (BJD). 0010 hc lcsthe places which , § 4 xlisoraayi of analysis our explains 4 .W Marcy W. G. , )techniques: 2) acs ´ 1 i fthe of bit . .Stefanik R. , 8 R ⊙ V a uha such has 10 = 5 , . F6 5 1 , 2 Pál et al. parameters as well as verification that the signal derives 3.1. Low-resolution Spectroscopy from a transiting planet rather than a “false-positive”; Following the HATNet photometric detection, HAT-P- and finally, 5 discusses the results and implications for § 7 (then a transit candidate) was observed spectroscopi- further studies of this intriguing system. cally with the CfA Digital Speedometer (DS, see Latham 1992) at the FLWO 1.5 m Tillinghast reflector, in order 2. PHOTOMETRIC DETECTION to rule out a number of blend scenarios that mimic plan- The HATNet telescopes HAT-7 and HAT-8 (HATNet; etary transits (e.g. Brown 2003; O’Donovan et al. 2007), Bakos et al. 2002, 2004) observed HATNet field G154, as well as to characterize the stellar parameters, such as centered at α = 19h12m, δ = +45◦00′, on a near-nightly surface gravity, effective temperature, and rotation. Four basis from 2004 May 27 to 2004 August 6. Exposures spectra were obtained over an interval of 29 days. These of 5 minutes were obtained at a 5.5-minute cadence observations cover 45 A˚ in a single echelle order centered whenever conditions permitted; all in all 5140 expo- at 5187 A,˚ and have a resolving power of λ/∆λ 35,000. ≈ sures were secured, each yielding photometric measure- Radial velocities were derived by cross-correlation, and − ments for approximately 33, 000 stars in the field down have a typical precision of 1 km s 1. Using these mea- to I 13.0. The field was observed in network mode, surements, we have ruled out an unblended companion exploiting∼ the longitude separation between HAT-7, sta- of stellar mass (e.g. an M dwarf orbiting an F dwarf), tioned at the Smithsonian Astrophysical Observatory’s since the radial velocities did not show any variation (SAO) Fred Lawrence Whipple Observatory (FLWO) in within the uncertainties. The mean heliocentric radial − Arizona (λ = 111◦W), and HAT-8, installed on the velocity of HAT-P-7 was measured to be 11 kms 1. − rooftop of SAO’s Submillimeter Array (SMA) building Based on an analysis similar to that described in Tor- atop Mauna Kea, Hawaii (λ = 155◦W). We note that res et al. (2002), the DS spectra indicated that the host each light curve obtained by a given instrument was star is a slightly evolved dwarf with log g = 3.5 (cgs), −1 shifted to have a median value to be the same as cat- Teff = 6250K and v sin i 6kms . ≈ alogue magnitude of the appropriate star, allowing to merge light curves acquired by different stations and/or 3.2. High resolution spectroscopy detectors. For the characterization of the radial velocity varia- Following standard frame calibration procedures, as- tions and for the more precise determination of the stellar trometry was performed as described in Pál & Bakos parameters, we obtained 8 exposures with an iodine cell, (2006), and aperture photometry results were subjected plus one iodine-free template, using the HIRES instru- to External Parameter Decorrelation (EPD, described ment (Vogt et al. 1994) on the Keck I telescope, Hawaii, briefly in Bakos et al. 2007), and also to the Trend between 2007 August 24 and 2007 September 1. The Filtering Algorithm (TFA; Kovács et al. 2005). We width of the spectrometer slit was 0′′. 86 resulting a re- searched the light curves of field G154 for box-shaped solving power of λ/∆λ 55,000, while the wavelength ≈ transit signals using the BLS algorithm of Kovács et al. coverage was 3800 8000 A.˚ The iodine gas absorp- ∼ − (2002). A very significant periodic dip in brightness was tion cell was used to superimpose a dense forest of I2 lines detected in the I 9.85 magnitude star GSC 03547- ≈ on the stellar spectrum and establish an accurate wave- 01402 (also known as 2MASS 19285935+4758102; α = length fiducial (see Marcy & Butler 1992). Relative ra- ◦ ′ ′′ 19h28m59s.35, δ = +47 58 10 .2; J2000), with a depth dial velocities in the Solar System barycentric frame were of 7.0mmag, a period of P =2.2047 days and a relative derived as described by Butler et al. (1996), incorporat- ∼ duration (first to last contact) of q 0.078, equivalent ing full modeling of the spatial and temporal variations ≈ to a duration of P q 4.1 hours. of the instrumental profile. The final radial velocity data ≈ In addition, the star happened to fall in the overlap- and their errors are listed in Table 1. The folded data, ping area between fields G154 and G155. Field G155, with our best fit (see 4.2) superimposed, are plotted in ◦ ′ centered at α = 19h48m, δ = +45 00 , was also ob- Fig. 2a. § served over an extended time in between 2004 July 27 and 2005 September 20 by the HAT-6 (Arizona) and HAT-9 3.3. Photometric follow-up observations (Hawaii) telescopes. We gathered 1220 and 10260 data- Partial photometric coverage of a transit event of HAT- points, respectively (which independently confirmed the P-7 was carried out in the Sloan z-band with the Kepler- transit), yielding a total number of 16620 data-points. Cam CCD on the 1.2 m telescope at FLWO, on 2007 The combined HATNet light curve after the EPD and November 2. The total number of frames taken from TFA procedures is plotted on Figs. 1a,b, superimposed HAT-P-7 was 514 with cadence of 28 seconds. During the on these plots is our best fit model (see 4). We note that § reduction of the KeplerCam data, we used the following TFA was run in signal reconstruction mode, i.e. system- method. After bias and flat calibration of the images, atics were iteratively filtered out from the observed time an astrometric transformation (in the form of first or- series assuming that the underlying signal is a trapeze- der polynomials) between the 450 brightest stars and shaped transit (see Kovács et al. 2005, for additional de- the 2MASS catalog was derived,∼ as described in Pál & tails). Bakos (2006), yielding a residual of 1/4 pixel. Aper- In addition to having a significant overlap with each ture photometry was then performed∼ using a series of other, we note that fields G154 and G155 both intersect apertures of with the radius of 4, 6 and 8 pixels in fixed the field of view of the upcoming Kepler mission (Borucki positions calculated from this solution and the actual et al. 2007). 2MASS positions. The instrumental magnitude transformation was obtained using 350 stars on a frame 3. FOLLOW-UP OBSERVATIONS taken near culmination of the field.∼ The transformation HAT-P-7b 3

TABLE 1 9.82 a) Relative radial velocity measurements of HAT-P-7

9.84 BJD RV σRV −1 −1 I mag (2,454,000+) (m s ) (ms ) 9.86 336.73958 ...... +124.40 1.63 336.85366 ...... +73.33 1.48 9.88 337.76211 ...... −223.89 1.60 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 338.77439 ...... +166.71 1.39 9.82 b) 338.85455 ...... +144.67 1.42 339.89886 ...... −241.02 1.46 343.83180 ...... −145.42 1.66 9.84 344.98804 ...... +101.05 1.91 I mag 9.86 on the HATNet, the radial velocity (RV) and the high precision photometric follow-up (FU) data, respectively.

9.88 Analysis of the HATNet data yielded an initial value for -0.1 -0.05 0 0.05 0.1 the orbital period and transit epoch. The initial period c) 9.85 and epoch were used to fold the RV’s, and phase them with respect to the predicted transit time for a circular I mag 9.86 orbit. The HATNet and the RV epochs together yield a

-0.1 -0.05 0 0.05 0.1 more accurate period, since the time difference between Phase the discovery light curve and the RV follow-up is fairly 9.840 d) long; more than 3 years. Using this refined period, we 9.850 can extrapolate to the expected center of the KeplerCam 9.860 partial transit, and therefore obtain a fit for the two re- z mag (inst) maining key parameters describing the light curve: a/R⋆ 9.870 -0.005 e) where a is the semi-major axis for a circular orbit, and 0.000 the impact parameter b (a/R⋆)cos i, where i is the 0.005 ≡ z residual inclination of the orbit. -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 T - T (days) Second, using as starting points the initial values as de- C rived above, we performed a joint fit of the HATNet, RV Fig. 1.— (a) The complete light curve of HAT-P-7 with all of the and FU data, i.e. fitting all of the parameters simultane- 16620 points, unbinned instrumental I-band photometry obtained ously. The reason for such a joint fit is that the three sep- with four telescopes of HATNet (see text for details), and folded arate data-sets and the fitted parameters are intertwined. with the period of P = 2.2047299 days (the result of a joint fit to all available data, § 4.2). The superimposed curve shows the best For example, the epoch (depending partly on the RV fit) model fit using quadratic limb darkening. (b) Same as a), with has a relatively large error, affecting the extrapolation of the transit zoomed-in (3150 data points are shown). (c) Same as the transit center to the KeplerCam follow-up. b), with the points binned with a bin size of 0.004 in phase. (d) In all of the above procedures, we used the downhill Unbinned instrumental Sloan z-band partial transit photometry acquired by the KeplerCam at the FLWO 1.2 m telescope on 2007 simplex method to search for the best fit values and the November 2; superimposed is the best-fit transit model light curve. method of refitting to synthetic data sets to find out (e) The difference between the KeplerCam observation and model the error of the adjusted parameters. The latter method (same vertical scale as in panel d). yields a Monte-Carlo set of the a posteriori distribution of the fit parameters. fit was initially weighted by the estimated photon- and The third step of the analysis was the derivation of background-noise error of each star, then the procedure the stellar parameters, based on the spectroscopic anal- was repeated weighting by the inverse variance of the ysis of the host star (high resolution spectroscopy using light curves. From the set of apertures we have cho- Keck/HIRES), and the physical modeling of the stellar sen the aperture for which the out-of-transit (OOT) rms evolution, based on existing isochrone models. As the of HAT-P-7 was the smallest; the radius of this aper- fourth step, we then combined the results of the joint ture is 6 pixels. This light curve was then de-correlated fit and stellar parameter determination to determine the against trends using the OOT sections, which yielded a planetary and orbital parameters of the HAT-P-7b sys- light curve with an overall rms of 1.9mmag at a cadence tem. Finally, we have done a blend analysis to confirm of one frame per 28 seconds. This is a bit larger than the that the orbiting companion is a hot Jupiter. In the fol- expected rms of 1.5mmag, derived from the photon noise lowing subsections we give a detailed description of the (1.2mmag) and scintillation noise – which has an ex- above main steps of the analysis. pected amplitude of 0.8mmag, based on the observational conditions and the calculations of Young (1967) – pos- 4.1. Independent fits sibly due to unresolved trends and other noise sources. For the independent fit procedure, we first analyzed The resulting light curve is shown in Fig. 1d. the HATNet light curves, as observed by the HAT-6, HAT-7, HAT-8 and HAT-9 telescopes. Using the initial 4. ANALYSIS period and transit length from the BLS analysis, we fit- The analysis of the available data was done in five ted a model to the 214 cycles of observations spanned steps. First, an independent analysis was performed by all the HATNet data. Although at this stage we 4 Pál et al.

200 were interested only in the epoch and period, we have a) used the transit light curve model with the assump- 150 tion quadratic limb darkening, where the flux decrease 100 was calculated using the models provided by Mandel 50 & Agol (2002). In principle, fitting the epoch and pe- 0 riod as two independent variables is equivalent to fitting -50 -100 the time instant of the centers of the first and last ob- -150 Radial velocity (m/s) served individual transits, Tc,first and Tc,last, with a con- -200 straint that all intermediate transits are regularly spaced -250 with period P . Note that this fit takes into account -300 all transits that occurred during the HATNet observa- b) tions, even though it is described only by Tc,first and 20 Tc,last. The fit yielded Tc,first = 2453153.0924 0.0021 0 -20 (BJD) and T = 2453624.9044 0.0023 (BJD).± The O-C (m/s) c,last ± period derived from the Tc,first and Tc,last epochs was (1) c) P = 2.20480 0.00049 days. Using these values, we 20 ± found that there were 326 cycles between Tc,last and the 0 -20 end of the RV campaign. The epoch extrapolated to Span (m/s) 0 0.2 0.4 0.6 0.8 1 the approximate time of RV measurements was Tc,RV = Phase 2454343.646 0.008 (BJD). Note that the error in T ± c,RV Fig. 2.— (a) Radial-velocity measurements from Keck for HAT- is much smaller than the period itself ( 2.2 days), so P-7, along with an orbital fit, shown as a function of orbital phase, there is no ambiguity in the number of∼ elapsed cycles using our best fit as period (see § 4.2). The center-of-mass velocity when folding the periodic signal. has been subtracted. (b) Phased residuals after subtracting the orbital fit (also see § 4.2). The rms variation of the residuals is We then analyzed the radial velocity data in the follow- about 3.8ms−1. (c) Bisector spans (BS) for the 8 Keck spectra ing way. We defined the Ntr 0 transit as that being plus the single template spectrum, computed as described in the closest to the end of the radial≡ velocity measurements. text. The mean value has been subtracted. Due to the relatively This means that the first transit observed by HATNet small errors comparing to the RV amplitude, the vertical scale on the (b) and (c) panels differ from the scale used on the top panel. (at Tc,first) was the Ntr,first = 540 event. Given the short period, we assumed that the− orbit has been circu- larized (Hut 1981) (later verified; see below). The orbital and period was obtained through performing a joint fit, (as described later in 4.2). We note that in order to fit is linear if we choose the radial velocity zero-point γ § and the amplitudes A and B as adjusted values, namely: get a reduced chi-square value near unity for the radial velocity fit, it was necessary to quadratically increase the 2π 2π noise component with an amplitude of 3.8 ms−1, which v(t)= γ + A cos (t t ) + B sin (t t ) , P − 0 P − 0 is well within the range of stellar jitter observed for late (1) F stars; see Butler et al. (2006). (3) where t0 is an arbitrary time instant (chosen to be t0 = Using the improved period P and the epoch Tc,0, 2454342.6 BJD), K √A2 + B2 is the semi-amplitude we extrapolated to the center of KeplerCam follow-up ≡ of the RV variations, and P is the initial period P (1) transit (Ntr = 29). Since the follow-up observation only recorded a partial event (see Fig. 1d), this extrapola- taken from the previous independent HATNet fit. The tion was necessary to improve the light curve modeling. actual epoch can be derived from the above equation For this, we have used a quadratic limb-darkening ap- since for circular orbits transit center occurs when the RV proximation, based on the formalism of Mandel & Agol curve has the most negative slope. Using the equations (2002). The limb-darkening coefficients were based on above, we derived the initial epoch of the Ntr = 0 transit (1) the results of the SME analysis (notably, Teff ; see 4.3 center to be Tc = 2454343.6462 0.0042 Tc,0 (BJD). (z) § ± ≡ for further details), which yielded γ1 = 0.1329 and We also performed a more general (non-linear) fit to the (z) RV in which we let the eccentricity float. This fit yielded γ2 = 0.3738. Using these values and the extrapolated an eccentricity consistent with zero, namely e cos ω = time of the transit center, we adjusted the light curve 0.003 0.007 and e sin ω = 0.000 0.010. Therefore, parameters: the relative radius of the planet p = Rp/R⋆, −we adopt± a circular orbit in the further± analysis. the square of the impact parameter b2 and the quan- 2 −1/2 Combining the RV epoch T (1) with the first epoch ob- tity ζ/R⋆ = (a/R⋆)(2π/P )(1 b ) as independent c,0 parameters (see Bakos et al. 2007,− for the choice of pa- served by HATNet (Tc,first), we obtained a somewhat re- (2) rameters). The result of the fit was p =0.0762 0.0012, fined period, P =2.204732 0.000016 days. This was 2 −±1 ± b =0.205 0.144 and ζ/R⋆ = 13.60 0.83day , where fed back into phasing the RV data, and we performed the uncertainty± of the transit center time± due to the rela- the RV fit again to the parameters γ, A and B. The fit tively high error in the transit epoch Tc,0 was also taken yielded γ = 37.0 1.5ms−1, K √A2 + B2 = 213.4 − ± ≡ ± into account in the error estimates. 2.0ms−1and T (2) = 2454343.6470 0.0042 (BJD). This c,0 ± epoch was used to further refine the period to get P (3) = 4.2. Joint fit 2.204731 0.000016d, where the error calculation as- The results of the individual fits described above pro- ± sumes that Tc,0 and Tc,−540 are uncorrelated. At this vide the starting values for a joint fit, i.e. a simultaneous point we stopped the above iterative procedure of refining fit to all of the available HATNet, radial velocity and the epoch and period; instead a final refinement of epoch the partial follow-up light curve data. The adjusted pa- HAT-P-7b 5

uncertainties quoted here and in the remaining of this TABLE 2 discussion are twice the statistical uncertainties for the Stellar parameters for HAT-P-7 values given by the SME analysis. This reflects our at- Parameter Value Source tempt, based on prior experience, to incorporate system- atic errors (e.g. Noyes et al. (2008); see also Valenti & a Teff (K)...... 6350 ± 80 SME Fischer (2005)). Note that the previously discussed limb [Fe/H]...... +0.26 ± 0.08 SME (z) (z) (I) (I) −1 v sin i (km s )... 3.8 ± 0.5 SME darkening coefficients, γ1 , γ2 , γ1 and γ2 have been +0.08 2 b M⋆ (M⊙)...... 1.47−0.05 Y +LC+SME taken from the tables of Claret (2004) by interpolation +0.23 2 R⋆ (R⊙)...... 1.84−0.11 Y +LC+SME to the above-mentioned SME values for Teff , log g⋆, and +0.04 2 log g⋆ (cgs)...... 4.07−0.08 Y +LC+SME [Fe/H]. +1.5 2 L⋆ (L⊙)...... 4.9−0.6 Y +LC+SME As described by Sozzetti et al. (2007), a/R⋆ is a bet- 2 MV (mag)...... 3.00 ± 0.22 Y +LC+SME ter luminosity indicator than the spectroscopic value of 2 Age(Gyr)...... 2.2 ± 1.0 Y +LC+SME log g⋆ since the variation of stellar surface gravity has +50 2 Distance (pc) . . . . . 320−40 Y +LC+SME a subtle effect on the line profiles. Therefore, we used aSME = ‘Spectroscopy Made Easy’ package for analysis the values of Teff and [Fe/H] from the initial SME anal- of high-resolution spectra Valenti & Piskunov (1996). See ysis, together with the distribution of a/R⋆ to estimate text. b 2 the stellar properties from comparison with the Yonsei- Y +LC+SME = Yale-Yonsei isochrones (Yi et al. 2 2001), light curve parameters, and SME results. Yale (Y ) stellar evolution models by Yi et al. (2001). Since a Monte-Carlo set for a/R⋆ values has been derived during the joint fit, we performed the stellar parameters were T − , the time of first transit center in c, 540 rameter determination as follows. For a selected value the HATNet campaign, m, the out-of-transit magnitude of a/R , two Gaussian random values were drawn for of the HATNet light curve in I-band and the previously ⋆ T and [Fe/H] with the mean and standard deviation as defined parameters of γ, A, B, p, b2 and ζ/R . We note eff ⋆ given by SME (with formal SME uncertainties doubled that in this joint fit all of the transits in the HATNet as indicated above).Using these three values, we searched light curve have been adjusted simultaneously, tied to- the nearest isochrone and the corresponding mass by us- gether by the constraint of assuming a strictly periodic ing the interpolator provided by Demarque et al. (2004). signal; the shape of all these transits were characterized Repeating this procedure for values of a/R , T , [Fe/H], by p, b2 and ζ/R (and the limb-darkening coefficients) ⋆ eff ⋆ the set of the a posteriori distribution of the stellar pa- while the distinct transit center time instants were inter- rameters was obtained, including the mass, radius, age, polated using T − = T and A, B via the RV fit. c, 540 c,first luminosity and color (in multiple bands). The age deter- For initial values we used the results of the independent mined in this way is 2.2 Gy with a statistical uncertainty fits ( 4.1). The error estimation based on method refit- of 0.3 Gy; however, the uncertainty in the theoretical ting§ to synthetic data sets gives the distribution of the isochrone± ages is about 1.0 Gy. Since the corresponding adjusted values, and moreover, this distribution can be value for the surface gravity of the star, log g =4.07+0.04 used directly as an input for a Monte-Carlo parameter de- ⋆ −0.08 termination for stellar evolution modeling, as described (cgs),is well within 1-σ of the value determined by the later ( 4.3). SME analysis, we accept the values from the joint fit as § the final stellar parameters. These parameters are sum- Final results of the joint fit were: Tc,−540 = 2453153.0924 0.0015 (BJD), m = 9.85053 marized in Table 2. 0.00015 mag, γ±= 37.0 1.5ms−1, A = 33.8 0.9ms−±1, We note that the Yonsei-Yale isochrones contain the B = 210.7 1.9ms− −1±, p = 0.0763 0.0010,± b2 = absolute magnitudes and colors for different photometric +0.149 ± ± −1 bands from U up to M, providing an easy comparison 0.135−0.116 and ζ/R⋆ = 13.34 0.23 day . Using the distribution of these parameters,± it is straightforward to of the estimated and the observed colors. Using these data, we determined the V I and J K colors of the obtain the values and the errors of the additional pa- − − best fitted stellar model: (V I)YY = 0.54 0.02 and rameters derived from the joint derived fit, namely Tc,0, − ± (J K)YY = 0.27 0.02. Since the colors for the in- a/R⋆, K and P . All final fit parameters are listed in − ± Table 3. frared bands provided by Yi et al. (2001) and Demarque et al. (2004) are given in the ESO photometric standard 4.3. Stellar parameters system, for the comparison with catalog data, we con- verted the infrared color (J K) to the 2MASS sys- The results of the joint fit enable us to refine the pa- − YY tem (J KS) using the transformations given by Car- rameters of the star. First, the iodine-free template spec- penter (2001).− The color of the best fit stellar model trum from Keck was used for an initial determination of was (J KS)YY = 0.25 0.03, which is in fairly good the atmospheric parameters. Spectral synthesis model- agreement− with the actual± 2MASS color of HAT-P-7: ing was carried out using the SME software (Valenti & (J K )=0.22 0.04. We have also compared the Piskunov 1996), with wavelength ranges and atomic line − S ± (V I)YY color of the best fit model to the catalog data, data as described by Valenti & Fischer (2005). We ob- and− found that although HAT-P-7 has a low galactic lat- tained the following initial values: effective temperature ◦ itude, bII = 13.8, the model color agrees well with the 6350 80K, surface gravity log g⋆ = 4.06 0.10 (cgs), observed TASS color of (V I) = 0.60 0.07 (see iron abundance± [Fe/H] = +0.26 0.08, and± projected − TASS ± ± −1 Droege et al. 2006). Hence, the star is not affected by the rotational velocity v sin i = 3.8 0.5kms . The rota- interstellar reddening within the errors, since E(V I) tional velocity is slightly smaller± than the value given − ≡ (V I)TASS (V I)YY =0.06 0.07. For estimating by the DS measurements. The temperature and surface the− distance of− HAT-P-7,− we used the± absolute magnitude gravity correspond to a slightly evolved F6 star. The 6 Pál et al.

MV = 3.00 0.22 (resulting from the isochrone analy- ± TABLE 3 sis, see also Table 2) and the VTASS = 10.51 0.06 observed magnitude. These two yield a distance modulus± of Orbital and planetary parameters V M =7.51 0.28, i.e. distance of d = 320+50 pc. TASS − V ± −40 Parameter Value 4.4. Planetary and orbital parameters Light curve parameters The determination of the stellar properties was fol- P (days) ...... 2.2047299 ± 0.0000040 lowed by the characterization of the planet itself. Since E (BJD − 2,400,000) ...... 53, 790.2593 ± 0.0010 a T14 (days) ...... 0.1685 ± 0.0110 Monte-Carlo distributions were derived for both the light a T12 = T34 (days) ...... 0.0150 ± 0.0036 curve and the stellar parameters, the final planetary and +0.28 a/R⋆ ...... 4.35−0.38 orbital data were also obtained by the statistical analysis Rp/R⋆ ...... 0.0763 ± 0.0010 of the a posteriori distribution of the appropriate combi- +0.15 b ≡ a cos i/R⋆ ...... 0.37−0.29 nation of these two Monte-Carlo data sets. We found i (deg) ...... 85◦. 7+3.5 +0.077 −3.1 that the mass of the planet is Mp = 1.776− MJ, 0.049 Spectroscopic parameters the radius is R = 1.363+0.195 R and its density is p −0.087 J K (m s−1)...... 213.5 ± 1.9 +0.17 −3 −1 ρp = 0.876−0.24 g cm . We note that in the case of γ (km s )...... −37.0 ± 1.5 binary systems with large mass and radius ratios (such e ...... 0(adopted) as the one here) there is a strong correlation between Planetary parameters Mp and Rp (see e.g. Beatty et al. 2007). This correlation +0.077 Mp (MJ) ...... 1.776−0.049 is also exhibited here with C(Mp, Rp)=0.89. The final +0.195 Rp (RJ) ...... 1.363−0.087 planetary parameters are also summarized at the bottom C(Mp, Rp)...... 0.89 −3 +0.17 of Table 3. ρp (g cm ) ...... 0.876−0.24 Due to the way we derived the period, i.e. P = a (AU) ...... 0.0377 ± 0.0005 log g (cgs) ...... 3.31 ± 0.08 (T T − )/540, one can expect a large correlation p c,0 − c, 540 between the epochs Tc,0, Tc,−540 and the period itself. a T14: total transit duration, time between first to last Indeed, C(Tc,−540, P )= 0.783 and C(Tc,0, P )=0.704, − contact; T12 = T34: ingress/egress time, time between first while the correlation between the two epochs is negligi- and second, or third and fourth contact. ble; C(T − ,T ) = 0.111. It is easy to show that c, 540 c,0 − if the signs of the correlations between two epochs TA selected near the top and bottom of the mean bisectors and TB (in our case Tc,0 and Tc,−540) and the period (Torres et al. 2005). If the velocities were the result of are different, respectively, then there exists an optimal a blend with an eclipsing binary, we would expect the epoch E, which has the smallest error among all of the line bisectors to vary in phase with the photometric pe- interpolated epochs. We note that E will be such that it riod with an amplitude similar to that of the velocities. also exhibits the smallest correlation with the period. If Instead, we detect no variation in excess of the measure- σ(TA) and σ(TB) are the respective uncorrelated errors ment uncertainties (see Fig. 2c). We have also tested the of the two epochs, then significance of the correlation between the radial veloc- 2 2 ity and the bisector variations. Therefore, we conclude TAσ(TB) + TBσ(TA) E = 2 2 (2) that the velocity variations are real and that the star is σ(TB) + σ(TA) orbited by a Jovian planet. We note here that the mean where square brackets denote the time of the transit bisector span ratio relative to the radial velocity ampli- event nearest to the time instance t. In the case of tude is the smallest ( 0.026) among all the HATNet ∼ HAT-P-7b, T T − and T T , the correspond- planets, indicating an exceptionally high confidence that A ≡ c, 540 B ≡ c,0 ing epoch is the event Ntr = 251 at E Tc,−251 = the RV signal is not due to a blend with an eclipsing 2, 453, 790.2593 0.0010 (BJD).− The final ephemeris≡ and binary companion. planetary parameters± are summarized in Table 3. 5. DISCUSSION 4.5. Excluding blend scenarios With orbital period of only 2.2 days, implying a semi- Following Torres et al. (2007), we explored the possi- major axis of only 0.038AU, HAT-P-7b, is a “very hot bility that the measured radial velocities are not real, but Jupiter”, a name frequently given to giant planets with instead caused by distortions in the spectral line profiles orbital period less than 3 days (see Bouchy et al. 2004). due to contamination from a nearby unresolved eclipsing However, what really matters is the incident flux from binary. In that case the “bisector span” of the aver- the parent star, which for HAT-P-7b is exceptionally high age spectral line should vary periodically with amplitude because (a) the large radius of the parent star (1.84 R⊙) and phase similar to the measured velocities themselves combined with the small semi-major axis yields a very (Queloz et al. 2001; Mandushev et al. 2005). We cross- small geometrical factor a/R⋆ =4.35 (whose inverse de- correlated each Keck spectrum against a synthetic tem- scribes the stellar flux impinging on the planet); and plate matching the properties of the star (i.e. based on (b) its parent star has relatively high effective tempera- the SME results, see 4.3), and averaged the correlation ture: Teff = 6350 K. The flux impinging on the planet is functions over all orders§ blueward of the region affected 4.7 109 erg cm−2s−1. This places it among the most by the iodine lines. From this representation of the aver- highly× irradiated members of the “pM” class of planets, age spectral line profile we computed the mean bisectors, as discussed by Fortney et al. (2007). and as a measure of the line asymmetry we computed the For a pM class planet, Fortney et al. (2007) argue “bisector spans” as the velocity difference between points that the incident radiation is absorbed by TiO and VO HAT-P-7b 7 molecules in a hot stratosphere, and almost immediately of secondary eclipse as seen in z′-band should be similar re-radiated, rather than being partially advected to the to that for HAT-P-7b, namely 0.2 mmag: although night side of the planet, as would be the case if the radia- with its lower dayside temperature∼ the surface brightness tion were absorbed deeper down in the cooler atmosphere of WASP-3b should be smaller than for HAT-P-7b, the 2 of a “pL” planet, which would not have gaseous TiO and geometrical factor (Rp/R⋆) is larger, because WASP-3 VO in its atmosphere. is a less evolved star. For a pM planet, the dayside temperature would be HAT-P-7b has an additional important property in given by that it is in the field of view of one of the detectors on the forthcoming Kepler mission (Borucki et al. 2007), R 1/2 T = T ⋆ [f(1 A )]1/4. (3) currently scheduled for launch in early 2009 to moni- p eff a − B tor stars for photometric signals from transiting planets and other forms of stellar variability. Hence, HAT-P-7 where AB is the Bond albedo, which we assume to be should be subjected to intense and regular photometric essentially zero (see, e.g. Rowe et al. 2007), and the re- observations for several years starting in 2009. This will distribution factor f =2/3 (see Lopez-Morales and Sea- permit extremely high precision light curves of the pri- ger 2007). For HAT-P-7 this yields a dayside tempera- mary transit, the secondary transit, and perhaps also the +150 ture of (2730−100)K. If at the other extreme, contrary to variation of thermal and reflected light emission from the the theory of Fortney et al. (2007), the incident energy planet over the course of its orbit as described above. In absorbed by HAT-P-7b were to be distributed isotrop- addition, it will be possible to search intensively for tran- ically over the planet’s surface before being re-emitted, sit timing variations over the several year lifetime of the as is predicted for “pL” planets too cool to have signif- mission. Such observations could yield important infor- icant TiO and VO in their atmospheres, then f = 1/4 mation about orbital variations due to other companions in Equation 3, and the day-side temperature would be in the system including terrestrial-mass or smaller plan- +110 (2140−60 ) K. Thus, detection and measurement of the ets or Trojans, or to other effects such as precession of emission from the dayside of HAT-P-7b would yield an the orbital plane or the orbital line of nodes. To maxi- important test of the theory of Fortney et al. (2007). mize the scientific return from Kepler in this regard, it Lopez-Morales and Seager (2007) and Fortney et al. would be very useful to obtain high precision light curves (2007) noted that the thermal emission in the optical and both from the ground, and from space instruments such near infrared from very hot Jupiters should be detectable as Epoxi and MOST, in the very near term for later com- from the ground. Lopez-Morales and Seager (2007) iden- parison with Kepler observations. tified OGLE-TR-56b, the only other known very hot Asteroseismology of HAT-P-7 using Kepler could yield jupiter with predicted dayside temperature as high as an independent measurement of the star’s mean density that of HAT-P-7b, as a particularly promising candi- from the “large separation” ∆ν between p-modes of the dates. However, since HAT-P-7 is 6 magnitudes brighter same low angular degree ℓ (e.g. Stello et al. 2007). This than OGLE-TR-56, it is a far better candidate for sec- would provide a check on the mean density of the star ondary eclipse studies from the ground. For HAT-P-7, completely independent of that derived from the transit ′ the brightness decrease in z -band at secondary eclipse measurements of the a/R⋆ parameter. Equivalently, the should be 0.2 mmag or 0.04 mmag in the respective asteroseismology measurements would yield an indepen- cases of zero∼ (pM-type) or∼ complete (pL-type) planet- dent measurement of the stellar radius, and hence a more wide re-distribution of irradiated stellar flux. This im- precise value for the radius of the planet HAT-P-7b. Such plies that in the former case the eclipse could be detected a rich combined data set could have additional benefits at the 2-sigma level with a 1.2-m telescope with a z′-band for understanding stellar structure and evolution. integration of 4 hours during eclipse (that is, the total eclipse duration), plus 4 more hours of out-of-eclipse observation. We plan to carry out such observations in the Operation of the HATNet project is funded in part near future with the KeplerCam on FLWO 1.2 m and by NASA grant NNG04GN74G. Work by G.A.B.´ was with MMT 6.5 m (also at FLWO) telescopes. supported by NASA through Hubble Fellowship Grant Recently Pollacco et al. (2007) reported the discovery HST-HF-01170.01-A and by the Postdoctoral Fellow- of the transiting exoplanet WASP-3b, located 0.0317 AU ship of the NSF Astronomy and Astrophysics Program. from an F star with Teff = 6350K and radius R⋆ = We acknowledge partial support also from the Kepler 1.31R⊙; this indicates a very high radiative flux at the Mission under NASA Cooperative Agreement NCC2- planet of 2.5 109 erg cm−2s−1, comparable to but about 1390 (D.W.L., PI). G.T. acknowledges partial support 25% smaller× than the radiative flux at HAT-P-7b. For from NASA under grant NNG04LG89G, G.K. thanks the same reasons as for the case of HAT-P-7b discussed the Hungarian Scientific Research Foundation (OTKA) above, this almost certainly will be a pM-class planet, for support through grant K-60750. A.P. would like to so that its dayside temperature should be about 2530K, thank the hospitality of the Harvard-Smithsonian Center as indicated by Equation 3 with f = 2/3 and AB = 0. for Astrophysics, where most of this work has been car- This planet should also be detectable in visible or near ried out. This research has made use of Keck telescope infrared radiation. In fact, if it is a pM planet, the depth time granted through NOAO (program A285Hr).

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