Seven transiting hot-Jupiters from WASP-South, Euler and TRAPPIST: WASP-47b, WASP-55b, WASP-61b, WASP-62b, WASP-63b, WASP-66b&WASP-67b

We present seven new transiting hot Jupiters from the WASP-South survey. The planets are all typical hot Jupiters orbiting stars from F4 to K0 with magnitudes of V = 10.3 to 12.5. The orbital periods are all in the range 3.9--4.6 d, the planetary masses range from 0.4--2.3 Mjup and the radii from 1.1--1.4 Mjup. In line with known hot Jupiters, the planetary densities range from Jupiter-like to inflated (rho = 0.13--1.07 rho_jup). We use the increasing numbers of known hot Jupiters to investigate the distribution of their orbital periods and the 3--4-d"pile-up".


INTRODUCTION
Transiting exoplanets found by the ground-based transit searches are mostly "hot Jupiters", Jupiter-sized planets in ≈ 1-6-d orbits, since these are the easiest planets for such surveys to find. However, planet candidate lists from Kepler show that hot Jupiters are much less common than smaller planets (Batalha et al. 2012). This means that the much larger sky coverage of the ground-based surveys (e.g. WASP, Pollacco et al. 2006, andHATnet, Bakos et al. 2004) is needed to produce large samples of hot Jupiters that will enable us to understand the properties of this class. In addition, hot Jupiters from these surveys orbit stars of V ≈ 9-13, which are bright enough for radial-velocity measurements of the planetary masses and for many other types of study.
Here we present seven new transiting planets discovered by the WASP-South survey (Hellier et al. 2011a) in conjunction with the Euler/CORALIE spectrograph and the TRAPPIST robotic photometer (Jehin et al. 2011). These are all hot Jupiters with ∼ 4-d orbits that are compatible with being circular, and with masses and radii that are typical of the class. They all orbit relatively isolated stars of V = 10.3-12.5 which have metallicities and space velocities compatible with local thin-disc stars. WASP-South planets that are less typical of the class will be reported in other papers.

OBSERVATIONS
WASP-South uses an 8-camera array that covers 450 square degrees of sky observing with a typical cadence of 8 mins. The WASP surveys are described in Pollacco et al. (2006) while a discussion of our planet-hunting methods can be found in Collier-Cameron et al. (2007a) and Pollacco et al. (2007).
WASP-South planet candidates are followed up using the TRAPPIST robotic photometer and the CORALIE spectrograph on the Euler 1.2-m telescope at La Silla. About 1 in 12 candidates turns out to be a planet, the remainder being blends that are unresolved in the WASP data (which uses 14" pixels) or astrophysical transit mimics, usually eclipsing binary stars. A list of observations reported in this paper is given in Table 1 while the CORALIE radial velocities are listed in Table A1.

THE HOST STARS
The CORALIE spectra of the host stars were co-added to produce spectra for analysis using the methods described in Gillon et al. (2009). We used the Hα line to determine the effective temperature (T eff ), and the Na i D and Mg i b lines as diagnostics of the surface gravity (log g * ). The resulting parameters are listed in Tables 2 to 8. The elemental abundances were determined from equivalent-width measurements of several clean and unblended lines. A value for microturbulence (ξt) was determined from Fe i lines using the criteria of a null-dependence of line abundances with equivalent width (see Magain 1984). The quoted error estimates include that given by the uncertainties in T eff , log g * and ξt, as well as the scatter due to measurement and atomic data uncertainties. The projected stellar rotation velocities (v sin I) were determined by fitting the profiles of several unblended Fe i lines. We used values for macroturbulence (vmac) from the tabulation by Bruntt et al. (2010). A CORALIE instrumental FWHM of 0.11 ± 0.01Å was determined from the telluric lines around 6300Å.

Rotational modulation
We searched the WASP photometry of each star for rotational modulations by using a sine-wave fitting algorithm as described by Maxted et al. (2011). We estimated the significance of periodicities by subtracting the fitted transit lightcurve and then repeatedly and randomly permuting the nights of observation. For none of our stars was a significant periodicity obtained, with 95%-confidence upper limits being typically 1 mmag (as listed in Tables 2 to 8).

Proper motions
For each of our stars we list (Tables 2 to 8) the proper motions from the UCAC3 catalogue (Zacharias et al. 2010).
Combining these with the spectroscopic distances and the radial velocities in the same Tables gives space velocities in the range 19-55 km s −1 . Our stars are all compatible with the local thin-disc population, which typically has −0.6 < [Fe/H] < 0.3 and σv ≈ 50 km s −1 (Navarro et al. 2011) .

SYSTEM PARAMETERS
The CORALIE radial-velocity measurements were combined with the WASP, EulerCAM and TRAPPIST photometry in a simultaneous Markov-chain Monte-Carlo (MCMC) analysis to find the system parameters. For details of our methods see Collier Cameron et al. (2007b). The limbdarkening parameters are noted in each Table, and are taken from the 4-parameter non-linear law of Claret (2000). For all of our planets the data are compatible with zero eccentricity and hence we imposed a circular orbit (see Anderson et al. 2012 for a discussion of the rationale for this). The fitted parameters were thus Tc, P , ∆F , T14, b, K1, where Tc is the epoch of mid-transit, P is the orbital period, ∆F is the fractional flux-deficit that would be observed during transit in the absence of limb-darkening, T14 is the total transit duration (from first to fourth contact), b is the impact parameter of the planet's path across the stellar disc, and K1 is the stellar reflex velocity semi-amplitude.
The transit lightcurves lead directly to stellar density but one additional constraint is required to obtain stellar masses and radii, and hence full parametrisation of the system. We adopt the approach of Enoch et al. (2010), based on empirical calibrations of stellar properties from well-studied detached eclipsing binary systems, but we use the calibration coefficients calculated by Southworth (2011). These are improvements on the coefficients from Enoch et al. as they include far more stars (180 versus 38) and also restrict the calibration sample to objects with masses relevant to the study of transiting planetary systems (mass < 3 M⊙). The rms scatter of the calibrating sample around the best fit is 0.027 dex for log(mass) and 0.009 dex for log(radius).
Errors are 1σ; Limb-darkening coefficients were: (Euler r) a1 = 0.727, a2 = -0.653, a3 = 1.314, a4 = -0.594 For each system we list the resulting parameters in Tables 2 to 8, and plot the resulting data and models in Figures 1 to 7. We also refer the reader to Smith et al. (2012) who present an extensive analysis of the effect of red noise in the transit lightcurves on the resulting system parameters.
As in past WASP papers we plot the spectroscopic T eff , and the stellar density from fitting the transit, against the evolutionary tracks from Girardi et al. (2000), as shown in Fig. 8.   V mag = 11.8 Rotational modulation < 1 mmag (95%) pm (RA) 11.2 ± 1.0 (Dec) -8.2 ± 1.0 mas/yr Stellar parameters from spectroscopic analysis.
Parameters from MCMC analysis.
Parameters from MCMC analysis.

WASP-47
WASP-47 is a G9 star (V = 11.9) with a possibly elevated metallicity of [Fe/H] = 0.18 ± 0.07. There is no significant detection of lithium in the spectra, with an equivalent width upper limit of 3mÅ, corresponding to an abundance upper limit of log A(Li) < 0.81 ± 0.10. The temperature of 5400K along with the lithium abundance imply a lower age limit of around 0.6 Gyr when compared with the Hyades cluster (Sestito & Randlich 2005). The rotation rate (P = 15 ± 3 d) implied by the v sin I (assuming that the planet's orbit is aligned, and thus that the star's spin axis is perpendicular to us) gives a gyrochronological age of ∼ 1.0 +0.7 −0.4 Gyr using the Barnes (2007) relation.
With an orbital period of 4.16 d, a mass of 1.14 MJup and a radius of 1.15 RJup WASP-47b is an entirely typical hot Jupiter.
2MASS images of WASP-55 show a star approximately 2 ′′ away and about 5 magnitudes fainter. This is sufficiently faint that it is unlikely to be affecting our results significantly.
WASP-55b is moderately inflated, with a mass of 0.57 MJup and a radius of 1.30 RJup, though this is in line with many known hot Jupiters.

WASP-61
WASP-61 is an F7 star (V = 12.5) with metallicity near solar (the poor quality of our spectrum prevents more detailed analysis than the [Fe/H] = -0.10 ± 0.12 reported in Table 4). There is no significant detection of lithium in the spectra, corresponding to an abundance upper limit of log A(Li) < 1.1 ± 0.1, which implies an age of several Gyr (Sestito & Randlich 2005). The rotation rate (P = 6.3 ± 0.9 d) implied by the v sin I gives a gyrochronological age of ∼ 0.7 +1.2 −0.4 Gyr using the Barnes (2007) relation.
WASP-61b has a high mass of M = 2.1 MJup and the highest density of the planets reported here, at ρ = 1.1 ρJup.

WASP-62
WASP-62 is an F7 star (V = 10.3) with a solar metallicity. For a star of this temperature (6230 ± 80 K) the presence of relatively strong lithium absorption in the spectrum does not provide a strong age constraint; this level of depletion is found in clusters as young as ∼0.5 Gyr (Sestito & Randlich 2005). The rotation rate (P = 6.3 ± 0.8 d) implied by the v sin I gives a gyrochronological age of ∼ 0.7 +0.4 −0.3 Gy using the Barnes (2007) relation. There are no emission peaks evident in the Ca ii H+K lines.
The EulerCAM transit lightcurve is badly affected by weather. Our MCMC analysis balances χ 2 across the different datasets, so inflates the error bars of this lightcurve. We also ran the analysis omitting this curve, which led to results that were the same within the errors.

WASP-63
WASP-63 is a G8 star (V = 11.2) with solar metallicity. There is no significant detection of lithium in the spectra, with an equivalent width upper limit of 11mÅ, corresponding to an abundance upper limit of log A(Li) < 0.96 ± 0.10. This implies an age of at least several Gyr (Sestito & Randlich 2005). The rotation rate (P = 37 ± 9 d) implied by the v sin I gives a gyrochronological age of ∼ 6 +5 −3 Gyr using the Barnes (2007) relation. There are no emission peaks evident in the Ca ii H+K lines.
The stellar radius is inflated for a G8 star and indicates that WASP-63 has evolved off the main sequence (see Fig. 8), with an age of ∼ 8 Gyr.
The planet WASP-63b is the least massive of those reported here, at 0.38 MJup, and also has the lowest density (ρ = 0.13 ρJup). This indicates that mechanisms causing inflated planet radii need to be able to operate late on in the evolution of a planetary system.
With a mass of 2.3 MJup WASP-66b is the most massive of the planets reported here.

WASP-67
WASP-67 is an K0V star (V = 12.5) with a solar metallicity. There is no significant detection of lithium in the spectra, with an equivalent width upper limit of 5mÅ, corresponding to an abundance upper limit of log A(Li) < 0.23 ± 0.11. This implies an age of at least ∼0.5 Gyr (Sestito & Randlich 2005). The rotation rate (P = 25±7 d) implied by the v sin I gives a gyrochronological age of 2.0 +1.6 −1.0 Gyr using the Barnes (2007) relation. There are no emission peaks evident in the Ca ii H+K lines.
WASP-67b has a mass of 0.42 MJup and a radius of 1.4 MJup, making it inflated (ρ = 0.16 ρJup). It also has a high impact factor of b = 0.94, making the transit curve Vshaped. If the criterion X = b + RP/R * > 1 is satisfied then the transit is grazing, with part of the planet not transiting the stellar face (see, e.g., Smalley et al. 2011). For WASP-67b this value is 1.07 +0.05 −0.03 , and, further, out of 375 000 MCMC steps only 17 had X < 1. This implies a >3σ probability that the transit is grazing, making WASP-67b the first hot Jupiter known to have a grazing transit, following WASP-34  Girardi et al. (2000). and HAT-P-27/WASP-40 that are possibly grazing (Smalley et al. 2011;Anderson et al. 2011).

DISCUSSION
It has often been noted that the hot-Jupiter population shows an apparent "pile up" at orbital periods of P = 3-4 d. We can use the increasing numbers of hot Jupiters, primarily from the ground-based transit surveys, to investigate this.
As a first step we take the sample of confirmed planets compiled by Schneider et al. (2011), as of March 2012, limiting this to periods less than 8 days and planetary masses of 0.1-12 MJup (the super-Earths may well be a different population dynamically). We show (Fig. 9) the cumulative distributions against orbital period, semi-major axis and Rochelimit separation. These confirm that we see more planets at periods P of ≈ 3-5 days, with fewer at shorter and longer periods. However, this compilation comes from many different surveys, each of which will have different selection effects, and so needs to be interpreted with caution.
We thus create a second sample of planets discovered by the transit surveys (P Roche-limit radii Figure 9. The cumulative distributions of (top) orbital period, (middle) semi-major axis, and (bottom) separations in units of Roche-limit separation for a sample of hot Jupiters (see text). The definition of Roche-limit separation is taken from Ford & Rasio (2006).
The inclination range that produces a transit scales with semi-major axis as cos −1 (R * /a). To compare this with the distribution of orbital periods, which are securely known, we can translate this to P by assuming a star of solar mass and radius. Further, the biggest factor affecting discovery probability in a WASP-like survey is the number of transits recorded, which will scale as P −1 . In Fig. 10  distribution of rejected WASP-South candidates, which indicates that a P −1 function, though imperfect, is a rough approximation.
We caution that this is only a very preliminary account of relevant selection effects, which will be different for each of the above surveys. For example, the number of transits required is likely to saturate above some number (this number depending on the survey and the amount of data), and WASP-like surveys at only one longitude will also suffer from sampling effects at integer-day periods.
Nevertheless, we can multiply together the transit probability and the P −1 function to produce the detectionprobability curve shown in Fig. 11, and we can use this to produce a "corrected" planet distribution curve.
One can interpret this curve as showing four regions with different slopes, the slopes having relative ratios (for corrected number of planets versus period interval) of 1 : 10 : 40 : 12. The need for different slopes in the different regions is significant on a K-S test at >95% probabilities. We caution, though, that we regard this as an indicative description of the distribution, rather than a unique one, and the relative slopes are of course dependent on the uncertain selection effects.
The four regions of the hot-Jupiter period distribution are: (1) P = 0.8-1.2 d, containing only 4 planets (WASP-19b, WASP-43b, WASP-18b & WASP-12b;Hebb et al. 2010;Hellier et al. 2011b;Hellier et al. 2009;Hebb et al. 2009), despite the probability of detection being greatest. These planets are thought to be tidally decaying on relatively short timescales, and so are rare, found only by the surveys sampling the most stars (see the discussion in Hellier et al. 2011b; note that WASP observes from one longitude with greater sky coverage than HATnet, whereas HATnet covers less sky but from several longitudes).
There are no known hot Jupiters with a period below Cumulative Number Period (days) Figure 11. The solid-black line is the cumulative distribution of orbital periods for transiting hot Jupiters. The solid-blue line is the same but corrected for the probability of detection (which is the dotted curve). The dotted straight lines are a suggested parametrisation of the distribution (see text).
the P = 0.79-d of . Despite the fact that the probability of detection of such planets in WASP data is at its highest (see Fig. 11 curve), the number of good candidates declines (Fig. 10), and, further, we have followed up over 40 such candidates without success (compared to an overall success rate of 1-in-12). Thus hot-Jupiter planets below P = 0.79-d must be very rare (there are several super-Earths with such periods, though their tidal-decay rate will of course be much lower).
(2) P = 1.2-2.7 d. The abundance is an order of magnitude greater than in the 0.8-1.2 d range, but still a factor ∼ 4 lower than in the range 2.7-5-d.
(3) P = 2.7-5-d. Our analysis confirms the existence of a pile-up of hot Jupiters, and suggests that it has a relatively well defined lower edge at P = 2.7-d. Ford & Rasio (2006) argued that a hot-Jupiter population resulting from circularisation of highly eccentric orbits will have an inner edge at 2 Roche-limit radii (2aR where aR = R P 0.462 For a planet of Jupiter mass and radius around a star of solar mass and radius, 2 aR corresponds to P = 1.2-d, and so would explain our finding of a break at that period. The few systems inside that limit are presumably spiralling inward relatively rapidly under tidal decay (e.g. Matsumura, Peale & Rasio 2010).
Further, for an inflated planet with a radius of R ≈ 1.8 RJup (as seen in the highly inflated planets WASP-12b, WASP-17b, HAT-P-32b & HAT-P-33b; Hebb et al. 2009;Anderson et al. 2010;Hartman et al. 2011) 2 aR equates to P = 2.7 d (again assuming a 1-MJup planet orbiting a sunlike star). Hence, if hot Jupiters arrive in the pile-up with a range of radius inflations, the Ford & Rasio argument would produce a cut-off ranging from ∼ 1.2 to 2.7 d, which might explain our finding of breaks at both those values.
(4) P = ∼ > 5-d. The upper edge of the pile-up appears to be near 5-d, although we caution that in WASP-like surveys the detection probability (and hence number of planets) decreases, and the selection effects get worse, as the period increases beyond P ∼ 5 d. For this reason we don't further interpret the longer-period range.
The period distribution of hot Jupiters is likely to result from several physical mechanisms. These include disk migration and possible 'stopping mechanisms' (e.g. Matsumura, Pudritz & Thommes 2007), third-body interactions, such as the Kozai mechanism, that can move planets onto highly eccentric orbits that are then tidally captured and circularise at short periods (e.g. Guillochon et al. 2011;Noaz et al. 2011), and orbital decay and in-spiral caused by tidal interactions with the host star (e.g. Matsumura et al. 2010).
Study of the angle between the planetary orbit and the stellar rotation axis indicates that many current orbits are likely to result from the Kozai mechanism (e.g. Triaud et al. 2010), but it is probable that the hot Jupiters are a composite population with differing past histories. Thus, to further investigate the pile-up, we need to accumulate statistics to look for differences in, for example, the orbital eccentricities and the spin-orbit angle between the different period ranges that we have outlined.