The transiting planet WASP-10b

WASP-10b: a 3M, gas-giant planet transiting a late-type K star

Abstract

We report the discovery of WASP-10b, a new transiting extrasolar planet (ESP) discovered by the WASP Consortium and confirmed using NOT FIES and SOPHIE radial velocity data. A 3.09 day period, 29 mmag transit depth, and 2.36 hour 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 1.28R, a mass of 2.96M 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 2M 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.

keywords:
methods: data analysis – stars: planetary systems – techniques: radial velocities – techniques: photometric
12

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 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 5 confirmed ESP, all of which have periods of less than 3 days (Anderson et al., 2008; Collier Cameron et al., 2007b; Pollacco et al., 2008; Wilson et al., 2008), and reported an additional 103 (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 () for high-precision radial velocity 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 radial velocity observations with the Nordic Optical Telescope new FIbre-fed Echelle Spectrograph (FIES) and the OHP SOPHIE collaboration. These observations lead to the discovery and confirmation of a new, relatively high mass, gas-giant exoplanet, WASP-10b.

Figure 1: (a.) The top panel shows the SuperWASP light curve for WASP-10b (1SWASP J231558.30+312746.4). All the data (apart from that from SuperWASP-N) were averaged in 300 second bins. The data were phased using the ephemeris, days.

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 x 2048 pixel back illuminated CCDs. This combination of lens and camera yields a field of view of 7.8 7.8 with an angular size of 14.2 per pixel. During 2004, SuperWASP was run with 4 or 5 cameras as the operations moved from commissioning to routine automated observing. We show the WASP-10b SuperWASP light curve in Figure 1.

2.1 Higher Precision Photometry

We obtained photometry of WASP-10 with the MEROPE instrument on the 1.2m MERCATOR Telescope in V-band on 1 September 2007. 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 22 binning over 2.9 hours. Despite clear conditions, exposure times were varied from 25-30s 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 5 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 Network4 on the Tenagra II, 0.81m F7 Ritchey-Chretien telescope sited in the Sonora desert in S. Arizona, USA. The science camera contains a 1k 1k SITe CCD with a pixel scale of 0.87 arcsec/pixel and a field of view of 14.8. 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 was then performed using daophot (Stetson, 2007) within IRAF. Differential photometry was derived from a selection of typically 5 to 10 comparison stars within the frame.

These confirmed the object had a sharp egress with an amplitude of 0.0330.001mag. 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 radial velocity (RV) follow-up observations of WASP-10 with the 2.5m Nordic Optical Telescope (NOT) new FIbre-fed Echelle Spectrograph (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.

BJD
(s) km s
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

Table 1: Journal of radial-velocity 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 radial-velocity measurements were secured. The quoted uncertainties in the radial velocity errors include components due to photon noise (Section 2.2) and 10 m s of jitter (Section 3.2) added in quadrature.

NOT and FIES

Spectroscopic observations were obtained using the new FIES spectrograph mounted on the NOT Telescope. A total of seven radial velocity points were obtained during 2 December 2007, 28–31 December 2007 and 24–25 January 2008. WASP-10 required observations with an exposure time of 2400s due its relative faintness (V=12.7) yielding a peak signal-to-noise ratio per resolution element of 60–70 in the H region. FIES was used in medium resolution mode with R=46000 with simultaneous ThAr calibration. We used the bespoke data reduction package FIEStool5 to extract the spectra and a specially developed IDL line-fitting code to obtain radial velocities with a precision of 15–25 m s (except for the poor night of 24 January 2008, JD 2454490).

OHP 1.9 m and SOPHIE

Additional radial velocity measurements were taken for WASP-10 on 2007 August 29 and 30, 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 7 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=40000. Thorium-Argon calibration images were taken at the start and end of each night, and at 2- to 3-hourly intervals throughout the night. The radial-velocity drift never exceeded 2-3 m s, even on a night-to-night basis. Although errors for each radial velocity measurement are limited by the photon-noise. Thus, the average radial velocity error is 14 m s and includes the 2-3 m s 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 FWHM and contrasts for these spectra were 10.2–10.4 km s and 30–31%, respectively. The cross-correlation techniques and derivation of errors in the radial velocity measurements are presented in Pollacco et al. (2008).

3 Results and Analysis

3.1 Stellar parameters

Parameter WASP-10
RA (J2000) 23 15 58.3
Dec (J2000) +31 27 46.4
V 12.7
distance 9020 pc
K
log 
M/H
 sin  6 km s
-11.44 0.03 km s
Table 2: Stellar parameters for WASP-10. The last 5 parameters were derived from the SME analysis of the FIES spectroscopy.

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 HE (high-efficiency) 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 minimization to determine which atmospheric parameters best reproduce the observed spectrum of WASP-10 (effective temperature , surface gravity , metallicity [M/H], projected radial velocity , systemic radial velocity , microturbulence and the macroturbulence ).

The four spectral regions we used in our analysis are: (1) 5160–5190 Å, covering the gravity-sensitive Mg b triplet (2) 5850–5950 Å, with the temperature and gravity-sensitive Na ı D doublet; (3) 6000-6210 Å, containing a wealth of different metal lines, providing leverage on the metallicity, and (4) 6520–6600 Å, covering the strongly temperature-sensitive H-alpha line. In addition we analyzed 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 NOMAD, TASS4 and CMC14 catalogues), plus 2MASS to estimate the effective temperature using the Infrared Flux Method (Blackwell & Shallis, 1977). This yields T = 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.

Figure 2: Simultaneous Markov-chain Monte Carlo (MCMC) solutions to the WASP-10 photometry and radial velocity data. a. Top panels show the MCMC solutions to the combined SuperWASP-N, MERCATOR , and Tenagra band photometry. b. The lower panel shows the MCMC solution to the FIES + SOPHIE radial velocity (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 of the host star (6 km s).

3.2 Markov-chain Monte Carlo analysis

Transit timing and the radial-velocity 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 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 , mass of and a significant non-zero eccentricity of . The best fit solution for the MCMC model for a circular orbit (e = 0) has a 55 higher than the solution with non-zero eccentricity, and thus, the eccentricity is significant at 99.6% confidence level using the test. The values of the parameters of the optimal solution are given, together with their associated 1- confidence intervals, in Table 3. The FIES+SOPHIE radial-velocity data measurements are plotted in Figure 2 together with the best-fitting global fit to the SuperWASP-N, MERCATOR, and Tenagra transit photometry.

Figure 3: Line bisectors as a function of radial velocity (RV) for WASP-10. Plot symbols are the same as Figure 2. Analysis of these line-bisectors for WASP-10 does not show a correlation between the bisector velocity (V) and stellar radial-velocity (see text).

3.3 Line-bisector variation

Line bisectors have been shown to be a powerful diagnostic in distinguishing true extra-solar planets 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 day 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 Figure 3.

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

where the and are the RV and span bisector values respectively and the weights 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 and , then the slope is determined as

The value of the scaling factor is determined with signal-to-noise ratio

We obtain = 1.16, indicating a non-significant correlation between the bisector span and radial velocity variations. This demonstrates that the cross-correlation function remains symmetric, and that the radial-velocity variations are not likely to be caused by line-of-site binarity or stellar activity and indicate WASP-10b is an exoplanet.

Parameter Symbol Value
Transit epoch (BJD) days
Orbital period days
Planet/star area ratio
Transit duration days
Impact parameter
Stellar reflex velocity km s
Centre-of-mass velocity km s
Orbital semimajor axis AU
Orbital inclination degrees
Orb. eccentricity
Arg. periastron (rad)
Stellar mass
Stellar radius
Stellar surface gravity (CGS)
Stellar density
Planet radius
Planet mass
Planetary surface gravity (CGS)
Planet density
Planet temp () K
(photometric) 4145
Photometric data points N 4151
(spectroscopic) 17.2
Spectroscopic data points N 14
Table 3: WASP-10 system parameters and 1- error limits derived from MCMC analysis.

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 2.96M, 1.28R radius, and a significant eccentricity of . 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 semi-major axes 0.2 AU. Recent work (Jackson et al., 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 circularisation timescale substantially less than 1 Gyr for typical tidal dissipation parameter, Q = 10 to 10. WASP-10 is a K dwarf with a spin period of 12 days 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 outer planets can cause the inner planet to move through a range of eccentricities over timescales that are short when compared to the lifetime of the system, but very long when compared to the current observational baseline. However, recently Matsumura et al. (2008) have argued that an unseen companion driving short-Period systems is unlikely. They present an upper limit of 1 M for a possible unseen companion in the GJ 436 system and exclude this based on the current radial velocity upper limits of 5 m/s. Matsumura et al. (2008) also present a range of tidal quality Q timescales that could be as large as 10 years, and argue that this new class of eccentric, short period ESP are 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% 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 1.5M, 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 M HAT P-2 (HD 149026b) (Bakos et al., 2007) and 7.3 M 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 extrasolar giant planets (ESP) reveals a large range of densities. We derive a mean density for WASP-10b of 1.89 g cm (1.42 ) 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 et al., 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.

Acknowledgments

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 was 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.

Footnotes

  1. pagerange: WASP-10b: a 3M, gas-giant planet transiting a late-type K starWASP-10b: a 3M, gas-giant planet transiting a late-type K star
  2. pubyear: 2008
  3. http://www.inscience.ch/transits/
  4. www.lcogt.net
  5. http://www.not.iac.es/instruments/fies/fiestool/FIEStool.html

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