Abstract

We report the discovery of WASP-10b, a new transiting extrasolar planet (ESP) discovered by the Wide Angle Search for Planets (WASP) Consortium and confirmed using Nordic Optical Telescope FIbre-fed Echelle Spectrograph and SOPHIE radial velocity data. A 3.09-d period, 29 mmag transit depth and 2.36 h duration are derived for WASP-10b using WASP and high-precision photometric observations. Simultaneous fitting to the photometric and radial velocity data using a Markov Chain Monte Carlo procedure leads to a planet radius of 1.28RJ, a mass of 2.96MJ and eccentricity of ≈0.06. WASP-10b is one of the more massive transiting ESPs, and we compare its characteristics to the current sample of transiting ESP, where there is currently little information for masses greater than ≈2MJ and non-zero eccentricities. WASP-10's host star, GSC 2752−00114 (USNO-B1.0 1214−0586164) is among the fainter stars in the WASP sample, with V= 12.7 and a spectral type of K5. This result shows promise for future late-type dwarf star surveys.

1 INTRODUCTION

Photometric transit observations of extrasolar planets (ESP) are important because the transit strongly constrains their orbital inclination and allows accurate physical parameters for the planet to be derived. Their mass–radius relation allows us to probe their internal structure and is vital to our understanding of orbital migration and planetary formation. The radial velocity (RV) measurements which are used to confirm a candidate transiting ESP also provide more complete information on the orbital eccentricity.

As wide-field photometric transit surveys have collected additional sky and temporal coverage, and understood their noise components (Collier Cameron et al. 2007a; Smith et al. 2007), the number of transiting ESP has grown to over 50 in line with earlier predictions (Horne 2003). Recently, one such survey SuperWASP (Pollacco et al. 2006) published its first five confirmed ESP, all of which have periods of less than 3 d (Anderson et al. 2008; Collier Cameron et al. 2007b; Pollacco et al. 2008; Wilson et al. 2008), and reported an additional 101 (Hellier et al. 2008; Hebb et al. 2008; Joshi et al. 2008; West et al. 2008). SuperWASP is performing a ‘shallow-but-wide’ transit search, designed to find planets that are not only sufficiently bright (9 < V < 13) for high-precision RV follow-up to be feasible on telescopes of moderate aperture, but also for detailed studies such as transmission spectroscopy during transits. Details of the WASP project and observatory infrastructure are described in Pollacco et al. (2006).

In this paper, we present the WASP photometry of 1SWASP J231558.30+312746.4 (GSC 2752−00114), higher precision photometric follow-up observations with the MERCATOR and Tenagra telescopes and high-precision RV observations with the Nordic Optical Telescope new FIbre-fed Echelle Spectrograph (FIES) and the Observatoire de Haute-Provence (OHP) SOPHIE collaboration. These observations lead to the discovery and confirmation of a new, relatively high-mass, gas-giant exoplanet, WASP-10b.

2 OBSERVATIONS

1SWASP J231558.30+312746.4 (GSC 2752−00114) was monitored by SuperWASP-N starting in 2004. SuperWASP is a multi-camera telescope system with SuperWASP-North located in La Palma and consisting of 8 Canon 200-mm f/1.8 lenses each coupled to e2v 2048 × 2048 pixel back illuminated CCDs. This combination of lens and camera yields a field of view of forumla with an angular size of 14.2 arcsec per pixel. During 2004, SuperWASP was run with four or five cameras as the operations moved from commissioning to routine automated observing. We show the WASP-10b SuperWASP light curve in Fig. 1.

Figure 1

Shown is the un-binned SuperWASP folded light curve for WASP-100b (1SWASP J231558.30+312746.4). The data were phased using the ephemeris, TO= 2454357.85808+0.00041−0.00036P= 3.09276 d.

Figure 1

Shown is the un-binned SuperWASP folded light curve for WASP-100b (1SWASP J231558.30+312746.4). The data were phased using the ephemeris, TO= 2454357.85808+0.00041−0.00036P= 3.09276 d.

2.1 Higher precision photometry

We obtained photometry of WASP-10 with the MEROPE instrument on the 1.2 m MERCATOR Telescope in V band on 2007 September 1. Only a partial transit was observed due to uncertainties in the period and epoch from the SuperWASP data. Observations were in the V band with 2 × 2 binning over ∼2.9 h. Despite clear conditions, exposure times were varied from 25 to 30 s to account for changes in seeing and to keep below the saturation limits of the chip. This allowed 170 images to be taken. There was a drift of only 1 binned pixel in x and y on the chip during the run. The MEROPE images were first de-biased and flat-fielded with combined twilight flats using iraf and the apphot package to obtain aperture photometry of the target and five nearby companion stars using a 10 pixel radius. Finally, the light curve was extracted and normalized to reveal a depth of ∼33 mmag.

Further observations of WASP-10 were taken as part of an observing program sponsored by the Las Cumbres Observatory Global Telescope Network2 on the Tenagra II, 0.81 m F7 Ritchey–Chretien telescope sited in the Sonora desert in S. Arizona, USA. The science camera contains a 1 × 1 k SITe CCD with a pixel scale of 0.87 arcsec pixel−1 and a field of view of 14.8 × 14.8 arcmin2. The filter set is the standard Johnson/Cousins/Bessel UBVRI set and the data presented here have been taken in I band.

Calibration frames were obtained automatically every twilight, and the data were de-biased and flat-fielded using the calibration section of the SuperWASP pipeline. Object detection and aperture photometry were then performed using daophot (Stetson 1987) within iraf. Differential photometry was derived from a selection of typically 5 to 10 comparison stars within the frame.

These confirmed that the object had a sharp egress with an amplitude of 0.033 ± 0.001 mag. The MERCATOR V and Tenagra I light curves show consistent transit depths, confirming that the companion is a transiting ESP.

2.2 Spectroscopic follow-up

We obtained high-precision RV follow-up observations of WASP-10 with the 2.5 m Nordic Optical Telescope (NOT) new FIES, supplemented with observations from the Observatoire de Haute–Provence's 1.93 m telescope and the SOPHIE spectrograph (Bouchy et al. 2006). We present a summary of the FIES and SOPHIE RV data in Table 1.

Table 1

Journal of RV measurements for WASP-10 (1SWASP J231558.30+312746.4, USNO-B1.0 1214−0586164). Stellar coordinates are for the photometric apertures; the USNO-B1.0 number denotes the star for which the RV measurements were secured. The quoted uncertainties in the RV errors include components due to photon noise (Section 2.2) and 10 m s−1 of jitter (Section 3.2) added in quadrature.

BJD texp (s) Vr km s−1 
FIES 
2454437.540 2400 −11.028 ± 0.026 
2454463.377 2400 −11.941 ± 0.030 
2454465.342 2400 −11.003 ± 0.018 
2454466.335 2400 −11.804 ± 0.021 
2454490.329 2400 −11.013 ± 0.143 
2454490.358 2400 −10.990 ± 0.120 
2454491.340 2400 −11.955 ± 0.024 
SOPHIE 
2454340.569 3300 −11.657 ± 0.008 
2454342.505 3600 −11.575 ± 0.011 
2454508.262 1680 −11.027 ± 0.014 
2454509.268 1680 −11.336 ± 0.017 
2454510.276 1680 −11.990 ± 0.020 
2454511.262 1680 −11.135 ± 0.016 
2454512.262 1680 −11.244 ± 0.016 
BJD texp (s) Vr km s−1 
FIES 
2454437.540 2400 −11.028 ± 0.026 
2454463.377 2400 −11.941 ± 0.030 
2454465.342 2400 −11.003 ± 0.018 
2454466.335 2400 −11.804 ± 0.021 
2454490.329 2400 −11.013 ± 0.143 
2454490.358 2400 −10.990 ± 0.120 
2454491.340 2400 −11.955 ± 0.024 
SOPHIE 
2454340.569 3300 −11.657 ± 0.008 
2454342.505 3600 −11.575 ± 0.011 
2454508.262 1680 −11.027 ± 0.014 
2454509.268 1680 −11.336 ± 0.017 
2454510.276 1680 −11.990 ± 0.020 
2454511.262 1680 −11.135 ± 0.016 
2454512.262 1680 −11.244 ± 0.016 

2.2.1 NOT and FIES

Spectroscopic observations were obtained using the new FIES spectrograph mounted on the NOT Telescope. A total of seven RV points were obtained during 2007 December 2, 28–31 and 2008 January 24–25. WASP-10 required observations with an exposure time of 2400 s due its relative faintness (V= 12.7) yielding a peak signal-to-noise ratio per resolution element of ≈60–70 in the Hα region. FIES was used in medium resolution mode with R= 46 000 with simultaneous ThAr calibration. We used the bespoke data reduction package fiestool3 to extract the spectra and a specially developed idl line-fitting code to obtain RVs with a precision of 15–25 m s−1 (except for the poor night of 2008 January 24, JD 2454490).

2.2.2 OHP 1.9 m and SOPHIE

Additional RV measurements were taken for WASP-10 on 29 and 30 August 2007, and again between 2008 Feb 11 and 15 with the OHP 1.93 m telescope and the SOPHIE spectrograph (Bouchy et al. 2006), a total of seven usable spectra were acquired. We used SOPHIE in its high-efficiency mode, acquiring simultaneous star and sky spectra through separate fibers with a resolution of R= 40 000. Thorium–Argon calibration images were taken at the start and end of each night, and at 2- to 3-h intervals throughout the night. The RV drift never exceeded 2 to 3 m s−1, even on a night-to-night basis. Although errors for each RV measurement are limited by the photon noise. Thus, the average RV error is ≈14 m s−1 and includes the 2 to 3 m s−1 systematic error and the contribution from the photon noise. Typical signal-to-noise ratio estimates for each spectra were ≈30 (near 5500 Å). The SOPHIE WASP-10 spectra were cross-correlated against a K5V template provided by the SOPHIE control and reduction software. Typical full width at half-maximum (FWHM) and contrasts for these spectra were ≈10.2–10.4 km s−1 and 30–31 per cent, respectively. The cross-correlation techniques and derivation of errors in the RV measurements are presented in Pollacco et al. (2008).

3 RESULTS AND ANALYSIS

3.1 Stellar parameters

We merged all available WASP-10 FIES spectra into one high-quality spectrum in order to perform a detailed spectroscopic analysis of the stellar atmospheric properties. Radial velocity signatures were carefully removed during the process. This merged spectrum was then continuum normalized with a low-order polynomial to retain the shape of the broadest spectral features. The total signal-to-noise ratio of the combined spectrum was ≈180 per pixel. We were not able to include the SOPHIE spectra in this analysis, because these spectra were obtained in the high-efficiency (HE) mode, which is known to suffer from problems with removal of the blaze function.

As previously undertaken for our analysis of WASP-1 (Stempels et al. 2007) and WASP-3 (Pollacco et al. 2008), we employed the methodology of Valenti & Fischer (2005), using the same tools, techniques and model atmosphere grid. We used the package Spectroscopy Made Easy (sme) (Valenti & Piskunov 1996), which combines spectral synthesis with multidimensional χ2 minimization to determine which atmospheric parameters best reproduce the observed spectrum of WASP-10 (effective temperature Teff, surface gravity log g, metallicity [M/H], projected RV v sin i, systemic RV vrad, microturbulence vmic and the macroturbulence vmac).

The four spectral regions we used in our analysis are: (i) 5160–5190 Å, covering the gravity-sensitive Mg b triplet; (ii) 5850–5950 Å, with the temperature and gravity-sensitive Na i D doublet; (iii) 6000–6210 Å, containing a wealth of different metal lines, providing leverage on the metallicity, and (iv) 6520–6600 Å, covering the strongly temperature-sensitive Hα line. In addition, we analysed a small region around the Li i 6708 line to possibly derive a lithium abundance, but no Li i 6708 was detected for WASP-10. The parameters we obtained from this analysis are listed in Table 2. In addition to the spectral analysis, we also use available photometry [from The Amateur Sky Survey (NOMAD), Naval Observatory Merged Astrometric Dataset (TASS4) and Carlsberg Meridian Catalogue 14 (CMC14) catalogues] plus Two-Micron All-Sky Survey to estimate the effective temperature using the infrared flux method (Blackwell & Shallis 1977). This yields Teff= 4650 ± 120 K, which is in agreement with the spectroscopic analysis and a spectral type of K5. The characteristics of WASP-10 are also given in Table 2.

Table 2

Stellar parameters for WASP-10. The last five parameters were derived from the SME analysis of the FIES spectroscopy.

Parameter WASP-10 
RA (J2000) 23 15 58.3 
Dec. (J2000) +31 27 46.4 
V 12.7 
Distance 90 ± 20 pc 
Teff 4675 ± 100 K 
log g 4.40 ± 0.20 
[M/H] 0.03 ± 0.20 
v sin i <6 km s−1 
vrad −11.44 ± 0.03 km s−1 
Parameter WASP-10 
RA (J2000) 23 15 58.3 
Dec. (J2000) +31 27 46.4 
V 12.7 
Distance 90 ± 20 pc 
Teff 4675 ± 100 K 
log g 4.40 ± 0.20 
[M/H] 0.03 ± 0.20 
v sin i <6 km s−1 
vrad −11.44 ± 0.03 km s−1 

3.2 Markov Chain Monte Carlo analysis

Transit timing and the RV measurements provide detailed information about the orbit. We modelled WASP-10b's transit photometry and the reflex motion of the host star simultaneously using the Markov Chain Monte Carlo (MCMC) algorithm described in detail by Collier Cameron et al. (2007a), and the same techniques that were applied to WASP-3 by Pollacco et al. (2008) to which we refer the reader for more details.

We find WASP-10b to have a radius 1.28+0.08− 0.09RJ, mass of 2.96+0.22−0.17MJ and a significant non-zero eccentricity of 0.059+0.014−0.004. The best-fitting solution for the MCMC model for a circular orbit (e= 0) has a χ2 55 higher than the solution with a non-zero eccentricity, and thus, the eccentricity is significant at >99.6 per cent confidence level using the F test. The values of the parameters of the optimal solution are given, together with their associated 1σ confidence intervals, in Table 3. The FIES + SOPHIE RV data measurements are plotted in Fig. 2 together with the best-fitting global fit to the SuperWASP-N, MERCATOR and Tenagra transit photometry.

Table 3

WASP-10 system parameters and 1σ error limits derived from MCMC analysis.

Parameter Symbol Value 
Transit epoch (BJD) T0 2454357.85808+0.00041−0.00036
Orbital period P 3.0927636+0.0000094−0.000021
Planet/star area ratio (Rp/Rs)2 0.029+0.001−0.001 
Transit duration tT 0.098181+0.0019−0.0015
Impact parameter b 0.568+0.054−0.084R* 
Stellar reflex velocity K1 0.5201+0.0084−0.010 km s−1 
Centre-of-mass velocity γ −11.4854+0.0012−0.0034 km s−1 
Orbital semimajor axis a 0.0369+0.0012−0.0014 AU 
Orbital inclination I 86.9+0.6−0.5 degrees 
Orbital eccentricity ε 0.059+0.014−0.004 
Arg. periastron ω 2.917+0.222−0.245 (rad) 
Stellar mass M* 0.703+0.068−0.080 M 
Stellar radius R* 0.775+0.043−0.040 R 
Stellar surface gravity log g* 4.51+0.06−0.05 (CGS) 
Stellar density ρ* 1.51+0.25−0.20ρ 
Planet radius Rp 1.28+0.077−0.091RJ 
Planet mass Mp 2.96+0.22−0.17MJ 
Planetary surface gravity log gp 3.62 ± 0.06 (CGS) 
Planet density ρp 1.43+0.31−0.29ρJ 
Planet temp (A= 0) Teql 1119+26−28
χ2ν (photometric) χ2phot 4145 
Photometric data points Nphot 4151 
χ2ν (spectroscopic) χ2spec 17.2 
Spectroscopic data points Nspec 14 
Parameter Symbol Value 
Transit epoch (BJD) T0 2454357.85808+0.00041−0.00036
Orbital period P 3.0927636+0.0000094−0.000021
Planet/star area ratio (Rp/Rs)2 0.029+0.001−0.001 
Transit duration tT 0.098181+0.0019−0.0015
Impact parameter b 0.568+0.054−0.084R* 
Stellar reflex velocity K1 0.5201+0.0084−0.010 km s−1 
Centre-of-mass velocity γ −11.4854+0.0012−0.0034 km s−1 
Orbital semimajor axis a 0.0369+0.0012−0.0014 AU 
Orbital inclination I 86.9+0.6−0.5 degrees 
Orbital eccentricity ε 0.059+0.014−0.004 
Arg. periastron ω 2.917+0.222−0.245 (rad) 
Stellar mass M* 0.703+0.068−0.080 M 
Stellar radius R* 0.775+0.043−0.040 R 
Stellar surface gravity log g* 4.51+0.06−0.05 (CGS) 
Stellar density ρ* 1.51+0.25−0.20ρ 
Planet radius Rp 1.28+0.077−0.091RJ 
Planet mass Mp 2.96+0.22−0.17MJ 
Planetary surface gravity log gp 3.62 ± 0.06 (CGS) 
Planet density ρp 1.43+0.31−0.29ρJ 
Planet temp (A= 0) Teql 1119+26−28
χ2ν (photometric) χ2phot 4145 
Photometric data points Nphot 4151 
χ2ν (spectroscopic) χ2spec 17.2 
Spectroscopic data points Nspec 14 
Figure 2

Simultaneous MCMC solutions to the WASP-10 photometry and RV data. (a) The top panels show the MCMC solutions to the combined SuperWASP-N, MERCATOR V and Tenagra I band photometry. All the data (apart from that from SuperWASP-N) were averaged in 300s bins). (b) The lower panel shows the MCMC solution to the FIES + SOPHIE RV data. (FIES RVs are shown as filled circles and SOPHIE as filled squares). The model fit to the RV data includes orbital eccentricity (solid line), and for a circular orbit (dashed line). Both RV models also include the Rossiter–McLaughlin effect, which is small for this system given the low v sin i of the host star (<6 km s−1).

Figure 2

Simultaneous MCMC solutions to the WASP-10 photometry and RV data. (a) The top panels show the MCMC solutions to the combined SuperWASP-N, MERCATOR V and Tenagra I band photometry. All the data (apart from that from SuperWASP-N) were averaged in 300s bins). (b) The lower panel shows the MCMC solution to the FIES + SOPHIE RV data. (FIES RVs are shown as filled circles and SOPHIE as filled squares). The model fit to the RV data includes orbital eccentricity (solid line), and for a circular orbit (dashed line). Both RV models also include the Rossiter–McLaughlin effect, which is small for this system given the low v sin i of the host star (<6 km s−1).

3.3 Line-bisector variation

Line bisectors have been shown to be a powerful diagnostic in distinguishing true ESPs from blended and eclipsing stellar systems chromospheric activity (Queloz et al. 2001). Torres et al. (2004) showed that for OGLE-TR-33 line asymmetries which changed with a 1.95-d period, it was a blended system. From the cross-correlation function (CCF), we obtained the line bisectors and these are plotted, as a function of RV, in Fig. 3.

Figure 3

Line bisectors as a function of RV for WASP-10. Plot symbols are the same as Fig. 2. Analysis of these line bisectors for WASP-10 does not show a correlation between the bisector velocity (Vspan) and stellar RV (see the text).

Figure 3

Line bisectors as a function of RV for WASP-10. Plot symbols are the same as Fig. 2. Analysis of these line bisectors for WASP-10 does not show a correlation between the bisector velocity (Vspan) and stellar RV (see the text).

We quantified the significance of the bisector variation as follows. We determined the inverse-variance-weighted averages of the RV and bisector span as  

formula
where the vi and bi are the RV and span bisector values, respectively, and the weights wi are the inverse variances of the individual bisector measurements. The uncertainty in the span bisector is assumed to be 2.5 times the uncertainty on the RV in our data. If we define forumla and forumla, then the slope is determined as  
formula
The value of the scaling factor forumla is determined with a signal-to-noise ratio  
formula
We obtain SNR = 1.16 indicating a non-significant correlation between the bisector span and RV variations. This demonstrates that the CCF remains symmetric, and that the RV variations are not likely to be caused by line-of-site binarity or stellar activity and indicate WASP-10b is an exoplanet.

4 DISCUSSION

Photometric surveys have now provided a large sample of transiting ESP that can be used to determine their mass–radius relation and provide constraints on their compositions. Here, we presented the discovery of a new ESP with a mass of 2.96MJ, 1.28RJ radius and a significant eccentricity of 0.059+0.014−0.004. We now discuss the properties of WASP-10b in relation to the current sample of transiting ESP, starting with its non-zero eccentricity.

Most of the current sample of published transiting ESP have orbits consistent with being circular and are fit with models using zero eccentricity as is expected for short-period planets in orbits with semimajor axes <0.2 au. Recent work (Jackson, Greenberg & Barnes 2008; Mardling 2007, and references therein) has investigated the effects of tidal dissipation on the orbits of short-period ESP. The evolution of the orbital eccentricity appears to be driven primarily by tidal dissipation within the planet, giving a circularization time-scale substantially less than 1 Gyr for typical tidal dissipation parameter, Qp= 105 to 106. WASP-10 is a K dwarf with a spin period of 12 d and JK= 0.62 and is rotating more slowly than stars of comparable colour in the Hyades (Terndrup et al. 2000). This suggests a rotational age between 600 Myr and 1 Gyr. Thus, the persistence of substantial orbital eccentricity in WASP-10b is therefore surprising.

One plausible mechanism for maintaining the high eccentricity is secular interaction with an additional planet in the system. Adams & Laughlin (2006) explore the effects of dynamical interactions among planets in extrasolar planetary systems and conclude that the outer planets can cause the inner planet to move through a range of eccentricities over time-scales that are short when compared to the lifetime of the system, but very long when compared to the current observational baseline. However, recently Matsumura, Takeda & Rasio (2008) have argued that an unseen companion driving short-period systems is unlikely. They present an upper limit of 1 MNeptune for a possible unseen companion in the GJ 436 system and exclude this based on the current RV upper limits of ≤5 m s−1. Matsumura et al. (2008) also present a range of tidal quality Qp time-scales that could be as large as 109 yr, and argue that this new class of eccentric, short-period ESP is simply still in the process of circularizing. WASP-10b has not been extensively studied to rule out a putative outer plane that may be driving its eccentricity. Thus, the ≈6 per cent eccentricity of WASP-10b makes it an attractive target for future transit-timing variation studies, and for longer term RV monitoring to establish the mass and period of the putative outer planet.

The majority of transiting ESP found have masses below 1.5MJ, although there are a few more massive ESP. HD 17156 and COROT-Exo-2 have similar masses to WASP-10b and although there are two more massive ESP, the nearly 9 MJ HAT P-2 (HD 147506b) (Bakos et al. 2007) and 7.3 MJ WASP-14b (Joshi et al. 2008), this higher mass region has been poorly explored. Additional transiting objects in the mass range are important for completing the current ESP mass–radius relations and constraining their compositions. The current sample of transiting ESP reveals a large range of densities. We derive a mean density for WASP-10b of ≈1.89 g cm−3(1.42 ρJ) and it would lie along the higher density contour in a mass–radius plot (Pollacco et al. 2008; Sozzetti et al. 2007).

One ultimate goal of our transit-search programme is to provide the observational grist that will stimulate and advance refined models for the formation and evolution of the hot and very hot Jupiters (e.g. Burrows et al. 1997; Fortney, Marley & Barnes 2007; Seager et al. 2007). By thus constraining the underlying physics, we will have a richer context for the interpretation of the lower mass planets expected from missions such as COROT and Kepler.

The SuperWASP Consortium consists of astronomers primarily from the Queen's University Belfast, St Andrews, Keele, Leicester, The Open University, Isaac Newton Group La Palma and Instituto de Astrofísica de Canarias. The SuperWASP Cameras were constructed and operated with funds made available from Consortium Universities and the UK's Science and Technology Facilities Council. SOPHIE observations have been funded by the Optical Infrared Coordination Network. The data from the Mercator and NOT telescopes were obtained under the auspices of the International Time of the Canary Islands. We extend our thanks to the staff of the ING and OHP for their continued support of SuperWASP-N and SOPHIE instruments. FPK is grateful to AWE Aldermaston for the award of a William Penney Fellowship.

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