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Issue
A&A
Volume 567, July 2014
Article Number L6
Number of page(s) 6
Section Letters
DOI https://doi.org/10.1051/0004-6361/201424339
Published online 21 July 2014

© ESO, 2014

1. Introduction

The discoveries of extrasolar planets in the past two decades have revealed a surprising variety of system architectures and planet characteristics. The wide diversity of the outcomes of the planet formation process is linked to the properties of the host stars, the characteristics of the circumstellar disks, and the effects of the environment in which the stars and their planets form and evolve (Mordasini et al. 2012).

A better understanding of the stochastic factors that affect planet formation can be achieved by observing wide binary systems with similar components. This allows some of the relevant variables (chemical composition, birth environment, age) to be the same for both components. On the other hand, the presence of stellar companions can affect the dynamical evolution of planetary systems. Kaib et al. (2013) showed that even very wide binaries (a ~ 1000 au) can have a significant impact on the survival and orbital properties of any planets they might host, because of the continuous evolution of the binaries’ orbital elements caused by interactions with passing stars and Galactic tides. Indeed, some differences in the properties of planets in binaries with respect to those orbiting single stars have been identified, such as the excess of massive close-in planets (Desidera & Barbieri 2007) and of planets on highly eccentric orbits (Kaib et al. 2013).

With a few exceptions (Desidera et al. 2011; Toyota et al. 2009), radial velocity (RV) surveys have historically excluded known binaries from their samples, or included only one component of optically resolved systems, depending on the adopted selection criteria. Furthermore, the probability to have planets transiting each component of a binary system is low, and resolving the binary is challenging for stars at the typical distance of the targets of transit surveys. Nevertheless, some candidates have been identified by the Kepler mission, with Kepler-132 being the most promising one (Lissauer et al. 2014). However, the available data only allow the conclusion that the planet candidates orbit different components. The identification of the host of each transiting object remains ambiguous, and further characterization is hampered by the faintness of the stars and their small angular separation.

Another possibility is to search for planets around the companions of known planet hosts. Stars with planets are often the subject of searches for stellar companions (e.g., Chauvin et al. 2006; Mugrauer et al. 2014), or common proper motion stars may be known well before the discovery of planets (Raghavan et al. 2006). In this letter we present the results of intensive RV monitoring of the K0 star XO-2S (TYC 3413-210-1), performed using the spectrograph HARPS-N (Cosentino et al. 2012) at the Telescopio Nazionale Galileo (TNG) as part of the programme Global Architecture of Planetary Systems (GAPS, Covino et al. 2013; Desidera et al. 2013). XO-2S (V = 11.12 mag; B − V = 0.79) is the wide (30′′, ~4000 au projected separation) companion of XO-2N, which was found to host a transiting planet of mass 0.5 MJ and orbital period 2.5 d (Burke et al. 2007). The two components are very similar (ΔR ~ 0.04 mag), and both are super-metal-rich in composition. Our observations allowed us to detect two giant planets around XO-2S, both at orbital separations larger than the hot Jupiter around XO-2N, thus making the XO-2 system the first confirmed case of a wide binary whose components both host planets, one of which is transiting. Here we report the RV measurements, stellar activity and line profile indicators, ancillary photometric observations taken at the Asiago, Valle d’Aosta and Serra La Nave Observatories supporting the Keplerian origin of the RV variations, the planet parameters, and the evaluation of the dynamical stability of the system. In a forthcoming paper (Damasso et al., in prep.) we will present a complete analysis of the XO-2 system.

2. Observations and data reduction

The system XO-2S was observed with HARPS-N at 63 individual epochs between April 2013 and May 2014. The Th-Ar simultaneous calibration was not used to avoid contamination of the stellar spectrum by the lamp lines (which might affect a proper spectral analysis; Sect. 3). Nevertheless, the drift correction with respect to the reference calibration shows a dispersion of just 0.8 m s-1, which is of very limited impact considering the typical photon-noise RV error of 2.2 m s-1.

The reduction of the spectra and the RV measurements were obtained using the latest version (Nov. 2013) of the HARPS-N instrument data reduction software pipeline and the K5 mask. The measurement of the RVs is based on the weighted cross-correlation function (CCF) method (Baranne et al. 1996; Pepe et al. 2002).

3. Stellar parameters

The extracted spectra of XO-2S were coadded to produce a merged spectrum with a peak signal-to-noise ratio of about 200 at 550 nm. We determined the stellar parameters using implementations of both the equivalenth width and the spectral synthesis methods, as described in Esposito et al. (2014) and Santos et al. (2013). We also used the infrared flux method to estimate Teff. The results agree very well with each other. The adopted parameters are the weighted averages of the individual results (Table 1). Full details will be given in Damasso et al. (in prep.).

The stellar mass, radius, and age were estimated using the Yonsei-Yale evolutionary tracks (Demarque et al. 2004) that match the effective temperature, iron abundance, and surface gravity of XO-2S (Table 1). The adopted errors include an additional 5% in mass and 3% in radius added in quadrature to the formal errors to take systematic uncertainties in stellar models into account (Southworth 2011).

The star was found to be a slow rotator (using a preliminary calibration of the full width at half maximum of the CCF and spectral synthesis) that exhibits low levels of chromospheric emission. Other age diagnostics (kinematics, lack of detectable amounts of lithium) support the old age of the system and confirm the findings by Burke et al. (2007).

Table 1

XO-2S stellar parameters.

4. RV variations and their origin

The relative RV time series is shown in Fig. 1. We report in Table 2 the full dataset. The RVs show a dispersion of 34.3 m s-1, significantly exceeding the measurement errors. A Lomb-Scargle periodogram yields the most significant power at a period of 120 d. The RV semi-amplitude resulting from a Keplerian fit is K = 58 m s-1. Moreover, the post-fit residuals show additional modulations with a period of 18.3 d1. A test based on bootstrap random permutation shows that this periodicity is highly significant, with a false-alarm probability lower than 10-4, and a corresponding semi-amplitude K ~ 20 m s-1. The period value, however, is fairly typical of rotational periods of old, solar-type dwarfs. Therefore, we performed specific checks to exclude rotational modulations and other stellar phenomena as the origin of the RV variations.

We first searched for correlations between the RVs and line profile indicators, considering the bisector velocity span as delivered by the HARPS-N pipeline and our own implementation of the diagnostics proposed by Figueira et al. (2013). We also derived indices to measure the chromospheric activity in the CaII H and K, and Hα lines. None of these indices show either significant power at the RV periods or significant correlations with the original RVs or the residuals of the one-planet fit (Fig. 3).

To further exclude stellar variability as the origin of the observed RV signals, we obtained accurate multi-band photometry at the Asiago, Serra La Nave and Aosta Valley observatories. The latter was used to monitor XO-2S during 42 nights from 2 Dec. 2013 to 8 Apr. 2014 in the I band, following the observations and data reduction procedure of the APACHE program (Sozzetti et al. 2013; Christille et al. 2013). At Serra La Nave we observed in B, V, R, I bands on four nights in 2013−2014. We also analyzed data of the XO-2 system acquired at Asiago in the context of the TASTE program (Nascimbeni et al. 2011) during ten nights in which transits of the hot-Jupiter in front of XO-2N occurred. The APACHE time series shows a scatter of 0.003 mag (nightly averages), without significant power at the period of the RV variations. The nightly averages of the Asiago and Serra La Nave datasets show a similar scatter in the magnitude difference between XO-2N and XO-2S, confirming the low level of photometric variability of the two stars. The photometric amplitude due to star-spots associated with an activity-induced RV variation with a semi-amplitude of 20 m s-1 (derived using Eq. (1) of Desort et al. 2007) is about 3%, which is one order of magnitude larger than the observed photometric variability.

In summary, the tests performed allowed us to rule out phenomena related to stellar activity and rotation as the cause of the observed RV variations with periods of ~120 and ~18 d. We conclude that these are due to Keplerian motions, which are characterized in Sect. 5.

thumbnail Fig. 1

Upper panel: relative RVs of XO-2S. The overplotted line is the best-fit Keplerian solution for two planets and a linear trend. Lower panel: residuals of XO-2S RVs after removing the contribution of the two planets XO-2Sb and XO-2Sc. The overplotted line is the best-fit linear trend.

5. Orbital parameters and dynamical stability

Orbital parameters and associated uncertainties were determined with a Bayesian differential evolution Markov chain Monte Carlo analysis of the HARPS-N data (Ter Braak 2006; Eastman et al. 2013) by maximizing a Gaussian likelihood function (e.g., Ford 2006). Our two-planet model has 13 free parameters: periastron epoch T0, orbital period P, K, e cosω\hbox{$\sqrt{e}\,\cos\omega$}, and e sinω\hbox{$\sqrt{e}\,\sin\omega$} of the two planets XO-2S b and  c, e and ω being the eccentricity and the argument of periastron, the systemic velocity γ, a slope, and a jitter term (e.g., Gregory 2005), which includes RV scatter that is possibly induced by stellar variability and/or instrumental noise that exceeds the nominal error bars. Uninformative priors were used for all parameters. Twenty-six chains were run simultaneously, and were stopped after convergence and good mixing of the chains were reached according to Ford (2006). Burn-in steps were removed following Eastman et al. (2013). The medians of the parameter posterior distributions and their 34.13% intervals are taken as final values and their 1σ uncertainties, respectively. The results are listed in Table 3 and shown in Fig. 2.

The two planets have moderately eccentric orbits: e = 0.180 ± 0.035 (XO-2S b) and 0.153 ± 0.010 (XO-2S c). The inferred minimum masses are 0.259 ± 0.014 and 1.370 ± 0.053 MJup for planets  b and  c, respectively. Interestingly, we found evidence at the 6σ level for a long-term trend of 0.053 ± 0.009 m s-1 d-1 (Fig. 1) in the residuals of the two-planet fit. This is likely due to an additional companion whose nature remains to be established. The trend cannot be due to the wide companion XO-2N because of its very large separation (a slope of the order of 10-5 m s-1 d-1 is expected). The orbital parameters of the two planetary companions are only marginally affected by the inclusion of the long-term RV slope.

Table 3

Orbital parameters.

thumbnail Fig. 2

Phased plot for the two planets orbiting XO-2S. In each panel, the RV signal of the other object and the long-term trend are subtracted. Upper panel: long-period planet (XO-2Sc). Lower panel: short-period planet (XO-2Sb).

Finally, we performed a dynamical analysis of the proposed best fit of the planetary system. We have numerically integrated the nominal system (assuming nearly coplanar orbits) over 100 Myr using the symplectic integrator SyMBA (Duncan et al. 1998) and found a stable quasi-periodic evolution. We also used the frequency map analysis, as described in Marzari et al. (2005), to explore the stability properties close to the nominal solution. We found very low diffusion values when sampling the initial conditions in a random way within the error bars of all orbital elements and of the estimated planetary and stellar masses. This indicates that the system is located in a robustly stable area of the phase space.

6. Discussion and conclusions

Based on high-precision HARPS-N Doppler measurements, we presented the discovery of a planetary system composed of a planet slightly more massive than Jupiter at 0.48 au and a Saturn-mass planet at 0.13 au around XO-2S, the wide companion of the transiting hot-Jupiter host XO-2N. The two planetary orbits are moderately eccentric and were found to be dynamically stable. There are also indications of a long-term trend in the RVs. A longer time baseline of the observations is needed to infer its origin. The two planets have periods intermediate between the hot-Jupiter pile-up and the rise in frequency at a ≥ 1 au, which means that they are located in the so-called period-valley in the distribution of known exoplanets (Wittenmyer et al. 2010). The geometric transit probabilities for the inner and outer planets are 3% and 1%, respectively. However, the available photometric data and the uncertainties on the transit epochs (Table 3) do not allow us to set any meaningful constraints on the occurrence of their transits.

This is the first confirmed case of a wide binary whose components both host planets, one of which is transiting. The two stars are almost twins, but the planetary systems around them are

not, with one component hosting a hot-Jupiter and the other one hosting a Jupiter- and a Saturn-mass planet in wider orbits. This fact makes this system a special laboratory for our understanding of planet formation processes and the influence of specific classes of planets on their parent stars for example in terms of an alteration of the chemical composition (Ramírez et al. 2010) and angular momentum (Lanza 2010; Poppenhaeger & Wolk 2014). We plan to address these questions in forthcoming publications.

Online material

Table 2

Radial velocities, bisector velocity span, and log RHK of XO-2S.

thumbnail Fig. 3

Lomb-Scargle periodogram of RV and activity indicators of XO-2S. From top to bottom: raw RVs, one-planet fit residuals, two-planet fit residuals, two-planet plus linear-trend fit residuals; H and K S index, Hα index, bisector velocity span, Vasy indicator (see Figueira et al. 2013). In all panels the periodicities of the two planetary companions are marked with vertical dashed lines.

1

During eight observing nights, the spectra were obtained twice per night with a separation of a few hours to investigate the possibility of aliases. These data corroborated the 18.3 d periodicity.

Acknowledgments

The GAPS project in Italy acknowledges support from INAF through the “Progetti Premiali” funding scheme of the Italian Ministry of Education, University, and Research. The Aosta Valley Observatory is supported by the Regional Government of the Aosta Valley, the Town Municipality of Nus and the Mont Emilius Community. J.M.C. is supported by a grant of the EU-ESF, the Autonomous Region of the Aosta Valley and the Italian Ministry of Labour and Social Policy. We thank ASI (through contracts I/037/08/0 and I/058/10/0) and the Fondazione CRT for their support to the APACHE Project. D.G. acknowledges support from the EU FP7 under grant agreement No. 267251. N.C.S. acknowledges support from Fundação para a Ciência e a Tecnologia (FCT, Portugal) through the FEDER funds in the program COMPETE, as well as through national funds, in the form of grants references RECI/FIS-AST/0176/2012 (FCOMP-01-0124-FEDER-027493), and RECI/FIS-AST/0163/2012 (FCOMP-01-0124-FEDER-027492), and in the form of the Investigador FCT contract reference IF/00169/2012 and POPH/FSE (EC) by the FEDER funding through the program Programa Operacional de Factores de Competitividade − COMPETE. N.C.S. furthermore acknowledges support from the ERC/EC under the FP7 through Starting Grant agreement No. 239953. We thank the TNG staff for help with the observations.

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All Tables

Table 1

XO-2S stellar parameters.

In the text
Table 3

Orbital parameters.

In the text
Table 2

Radial velocities, bisector velocity span, and log RHK of XO-2S.

In the text

All Figures

thumbnail Fig. 1

Upper panel: relative RVs of XO-2S. The overplotted line is the best-fit Keplerian solution for two planets and a linear trend. Lower panel: residuals of XO-2S RVs after removing the contribution of the two planets XO-2Sb and XO-2Sc. The overplotted line is the best-fit linear trend.
In the text
thumbnail Fig. 2

Phased plot for the two planets orbiting XO-2S. In each panel, the RV signal of the other object and the long-term trend are subtracted. Upper panel: long-period planet (XO-2Sc). Lower panel: short-period planet (XO-2Sb).
In the text
thumbnail Fig. 3

Lomb-Scargle periodogram of RV and activity indicators of XO-2S. From top to bottom: raw RVs, one-planet fit residuals, two-planet fit residuals, two-planet plus linear-trend fit residuals; H and K S index, Hα index, bisector velocity span, Vasy indicator (see Figueira et al. 2013). In all panels the periodicities of the two planetary companions are marked with vertical dashed lines.
In the text

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