K2-231 b: A sub-Neptune exoplanet transiting a solar twin in Ruprecht 147
Jason Lee Curtis, Andrew Vanderburg, Guillermo Torres, Adam L. Kraus, Daniel Huber, Andrew W. Mann, Aaron C. Rizzuto, Howard Isaacson, Andrew W. Howard, Christopher E. Henze, Benjamin J. Fulton, Jason T. Wright
DDraft version June 5, 2018
Typeset using L A TEX twocolumn style in AASTeX62
K2-231 b: A sub-Neptune exoplanet transiting a solar twin in Ruprecht 147
Jason Lee Curtis,
1, 2, 3, ∗ Andrew Vanderburg,
3, 4, † Guillermo Torres, Adam L. Kraus, Daniel Huber,
5, 6, 7, 8
Andrew W. Mann,
1, 4, ‡ Aaron C. Rizzuto, Howard Isaacson, Andrew W. Howard, Christopher E. Henze, Benjamin J. Fulton, and Jason T. Wright Department of Astronomy, Columbia University, 550 West 120th Street, New York, NY 10027, USA Center for Exoplanets and Habitable Worlds, Department of Astronomy & Astrophysics, The Pennsylvania State University,525 Davey Laboratory, University Park, PA 16802, USA Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA Institute for Astronomy, University of Hawai‘i, 2680 Woodlawn Drive, Honolulu, HI 96822, USA Sydney Institute for Astronomy (SIfA), School of Physics, University of Sydney, NSW 2006, Australia SETI Institute, 189 Bernardo Avenue, Mountain View, CA 94043, USA Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C,Denmark Astronomy Department, University of California, Berkeley, CA, USA Department of Astronomy, California Institute of Technology, Pasadena, CA, USA NASA Ames Research Center, Moffett Field, CA 94035, USA (Received November 3, 2017; Revised February 1, 2018; Accepted March 4, 2018)
Submitted to AJABSTRACTWe identify a sub-Neptune exoplanet ( R p = 2 . ± . ⊕ ) transiting a solar twin in the Ruprecht147 star cluster (3 Gyr, 300 pc, [Fe/H] = +0.1 dex). The ∼
81 day light-curve for EPIC 219800881( V = 12 .
71) from K2 Campaign 7 shows six transits with a period of 13.84 days, a depth of ∼ M (cid:63) = 1.01 ± (cid:12) , R (cid:63) = 0.95 ± (cid:12) , and T eff = 5695 ±
50 K. This star appears to be single based on our modeling of thephotometry, the low radial velocity (RV) variability measured over nearly ten years, and Keck/NIRC2adaptive optics imaging and aperture-masking interferometry. Applying a probabilistic mass–radiusrelation, we estimate that the mass of this planet is M p = 7 + 5 − ⊕ , which would cause an RVsemi-amplitude of K = 2 ± − that may be measurable with existing precise RV facilities. Afterstatistically validating this planet with BLENDER , we now designate it K2-231 b, making it the secondsubstellar object to be discovered in Ruprecht 147 and the first planet; it joins the small but growingranks of 22 other planets and 3 candidates found in open clusters.
Keywords: open clusters: individual (Ruprecht 147, NGC 6774) — stars: individual: (K2-231,EPIC 219800881, CWW 93, 2MASS J19162203 − Corresponding author: Jason Lee [email protected] ∗ NSF Astronomy and Astrophysics Postdoctoral Fellow † NASA Sagan Fellow ‡ NASA Hubble Fellow INTRODUCTIONTransit and Doppler surveys have detected thousandsof exoplanets, and modeling their rate of occurrence As of 2017 June 9, 2950 were confirmed with 2338 additional
Kepler candidates; http://exoplanets.org a r X i v : . [ a s t r o - ph . E P ] J un Curtis et al. shows that approximately one in three Sun-like starshosts at least one planet with an orbital period under29 days (Fressin et al. 2013). Stars tend to form inclusters from the gravitational collapse and fragmen-tation of molecular clouds (Lada & Lada 2003), so itis natural to expect that stars still existing in clusterslikewise host planets at a similar frequency. In fact,circumstellar disks have been observed in very youngclusters and moving groups (2.5–30 Myr; Haisch et al.2001). However, some have speculated that stars form-ing in denser cluster environments (i.e., the kind thatcan remain gravitationally bound for billions of years)will be exposed to harsher conditions than stars formedin looser associations or that join the Galactic field rel-atively quickly after formation, and this will impactthe frequency of planets formed and presently exist-ing in star clusters. For example, stars in a rich anddense cluster might experience multiple supernovae dur-ing the planet-forming period (the lifetime of a 10 M (cid:12) star is ∼
30 Myr), as well as intense FUV radiation fromtheir massive star progenitors that can photoevaporatedisks. Furthermore, stars in denser clusters ( ∼ − ) will also dynamically interact withother stars (and binary/multiple systems) at a higherfrequency than more isolated stars in the field ( ∼ − ), which might tend to disrupt disks and/oreject planets from their host star systems.These concerns have been addressed theoretically andwith observations (Scally & Clarke 2001; Smith & Bon-nell 2001; Bonnell et al. 2001; Adams et al. 2006; Fregeauet al. 2006; Malmberg et al. 2007; Spurzem et al. 2009;de Juan Ovelar et al. 2012; Vincke & Pfalzner 2016;Kraus et al. 2016; Cai et al. 2017), and all of these fac-tors were considered by Adams (2010) in evaluating thebirth environment of the solar system, but progress inthis field necessitates that we actually detect and char-acterize planets in star clusters and determine their fre-quency of occurrence.1.1. Planets Discovered in Open Clusters
Soon after the discovery of the first known exoplanetorbiting a Sun-like star (Mayor & Queloz 1995), Janes(1996) suggested open clusters as ideal targets for pho-tometric monitoring. Two decades later, we still onlyknow of a relatively small number of exoplanets existingin open clusters. One observational challenge has been Based on 528 single and binary members in M67 containedwithin 7.4 pc and 111 members within the central 1 pc (Gelleret al. 2015). that the majority of nearby star clusters are young, andtherefore their stars are rapidly rotating and magneti-cally active. Older clusters with inactive stars tend tobe more distant, and their Sun-like stars are likewise toofaint for most Doppler and ground-based transit facili-ties. The first planets discovered in open clusters withthe Doppler technique were either massive Jupiters orpotentially brown dwarfs: Lovis & Mayor (2007) foundtwo substellar objects in NGC 2423 and NGC 4349; Sato et al. (2007) detected a companion to a giant starin the Hyades; Quinn et al. (2012) discovered two hotJupiters in Praesepe (known as the “two b’s in the Bee-hive,” one of which also has a Jupiter-mass planet in along-period, eccentric orbit; Malavolta et al. 2016), andQuinn et al. (2014) discovered another in the Hyades;and, finally, nontransiting hot Jupiters have been foundin M67 around three main-sequence stars, one Jupiterwas detected around an evolved giant, and three otherplanet candidates were identified (Brucalassi et al. 2014,2016, 2017).NASA’s
Kepler mission changed this by providinghigh-precision photometry for four clusters (Meibomet al. 2011). Two sub-Neptune-sized planets were dis-covered in the 1 Gyr NGC 6811 cluster, and Meibomet al. (2013) concluded that planets occur in that denseenvironment ( N = 377 stars) at roughly the same fre-quency as in the field. After Kepler was repurposedas K2 , many more clusters were observed for ∼
80 dayseach, and as a result, many new cluster planets havebeen identified. Many of these are hosted by lower-mass stars that are intrinsically faint and difficult toreach with existing precision radial velocity (RV) facili-ties from Earth. So far, results have been reported from K2 monitoring of the following clusters, listed in orderof increasing age: Gaidos et al. (2017) reported zerodetections in the Pleiades (see also Mann et al. 2017).Mann et al. (2016a) and David et al. (2016a) indepen-dently discovered a Neptune-sized planet transiting aM4.5 dwarf in the Hyades; recently Mann et al. (2018)reported three Earth-to-Neptune-sized planets orbitinga mid-K dwarf in the Hyades (K2-136), while Ciardiet al. (2018) concurrently announced the Neptune-sizedplanet and that this K dwarf formed a binary with a late-M dwarf; the system was later reported on by Livingston The substellar objects have minimum masses of 10.6 and19.8 M
Jup , respectively. Spiegel et al. (2011) calculated thedeuterium-burning mass limits for brown dwarfs to be 11.4–14.4 M
Jup , which supports a brown dwarf classification for thelater object and places the former on the boundary betweenregimes. sub-Neptune exoplanet in Ruprecht 147 In Praesepe, Obermeier et al. (2016) an-nounced K2-95 b, a Neptune-sized planet orbiting anM dwarf, which was later studied by Libralato et al.(2016), Mann et al. (2017), and Pepper et al. (2017);adding the planets found by Pope et al. (2016), Barroset al. (2016), Libralato et al. (2016), and Mann et al.(2017), there are six confirmed planets (including K2-100 through K2-104) and one candidate that were val-idated by Mann et al. (2017). Finally, Nardiello et al.(2016) reported three planetary candidates in the M67field, although all appear to be nonmembers.Table A1 lists the 23 planets and 3 candidates thathave been discovered in clusters so far, including K2-231 b. Of these, 15 transit their host stars, and all butsix of the hosts are fainter than
V >
13, which makesprecise RV follow-up prohibitively expensive. Thesehosts are all relatively young ( ∼
650 Myr) and magnet-ically active and thus might still present a challenge toexisting Doppler facilities and techniques. Such RV ob-servations are required to measure masses and determinethe densities of these planets.1.2.
The K2 Survey of Ruprecht 147
Ruprecht 147 was also observed by K2 during Cam-paign 7. Curtis et al. (2013) demonstrated that R147is the oldest nearby star cluster, with an age of 3 Gyr ata distance of 300 pc (see also the Ph.D. dissertation ofCurtis 2016). According to Howell et al. (2014), plan-ets only a few times larger in size than Earth would bedetectable around dwarfs at least as bright as K p < M = 0 . (cid:12) in R147 . Soon after the public releaseof the Campaign 7 light-curves, we discovered a sub-stellar object transiting a solar twin in Ruprecht 147(EPIC 219388192; CWW 89A from Curtis et al. 2013),which we determined was a warm brown dwarf in aneccentric ∼ Nowak et al. (2017) inde-pendently discovered and characterized this system. As we are listing only validated exoplanets, we do not includepolluted white dwarfs, like the one in the Hyades (Zuckerman et al.2013). In this list, we have neglected exoplanets found in youngassociations and moving groups like Upper Sco (Mann et al. 2016b;David et al. 2016b), Taurus–Auriga (Donati et al. 2016, 2017; Yuet al. 2017), and Cas–Tau (David et al. 2018). J. Curtis successfully petitioned to reposition the Campaign 7field in order to accommodate R147, which would have beenlargely missed in the originally proposed pointing. https://doi.org/10.5281/zenodo.58758 Now we report the identification of an object transit-ing a different solar twin in R147 (CWW 93 from Curtiset al. 2013), which we show is a sub-Neptune exoplanet.We made this discovery while reviewing and comparinglight-curves from various groups for our stellar rotationprogram (we are measuring rotation periods for R147’sFGKM dwarfs to validate and calibrate gyrochronologyat 3 Gyr), and noticed a repeating shallow transit pat-tern spaced at ∼
14 days in the
EVEREST light-curve forEPIC 219800881 (Luger et al. 2016, 2017). In this paper, we describe our production of a light-curve, which we model to derive the properties of theexoplanet (Section 2). We also characterize the host starand check for stellar binary companionship (Section 3),and test false-positive scenarios in order to statisticallyvalidate the exoplanet (Section 4).K2-231 was also targeted by the following programs:“Statistics of Variability in Main-Sequence Stars of Ke-pler 2 Fields 6 and 7” (PI: Guzik; GO 7016), “TheMasses and Prevalence of Small Planets with K2 – Cy-cle 2” (PI: Howard; GO 7030), and “K2 follow-up of thenearby, old open cluster Ruprecht 147” (PI: Nascimbeni;GO 7056). K2 LIGHT-CURVE ANALYSISThe top panel of Figure 1 shows the
EVEREST light-curve for EPIC 219800881 that caught our attention.We then downloaded the calibrated pixel-level data fromthe Barbara A. Mikulski Archive for Space Telescopes(MAST), extracted a light-curve, and corrected for K2 systematic effects following Vanderburg & Johnson(2014). We confirmed the transits detected by eye witha Box-fitting Least-Squares (BLS) periodogram search(Kov´acs et al. 2002). The BLS periodogram iden-tified a strong signal at a 13.844 day period with atransit depth of approximately 0.06%. We then refinedthe light-curve by simultaneously fitting the K2 point-ing systematics, a low-frequency stellar activity signal(modeled with a basis spline with breakpoints spaced ev-ery 0.75 days), and transits (using Mandel & Agol 2002models), as described in Section 4 of Vanderburg et al.(2016). Deviating from our standard procedure of usingstationary apertures, we opted to use a smaller, mov-ing circular aperture with a radius of 9 (cid:48)(cid:48) (2.32 pixels)in order to exclude many nearby background stars (seeFigure 3 and Table 4). The middle panel of Figure 1 https://archive.stsci.edu/prepds/everest/ https://archive.stsci.edu/k2/ We made the original period measurement with the Pe-riodogram Service available at https://exoplanetarchive.ipac.caltech.edu
Curtis et al. shows the detrended version of our extracted light-curveusing the best-fit low-frequency model produced duringthe light-curve calibration.The determination of the physical radius of the planetcandidate and size of its orbit requires an accurate char-acterization of the host star, which we present in Sec-tion 3. In this work, we adopt the following conventionsfrom IAU 2015 Resolution B3 for the nominal radii forthe Sun and Earth, which we apply to convert the mea-sured transit quantities a/R (cid:63) and R p /R (cid:63) to physical andterrestrial units (Mamajek et al. 2015; Prˇsa et al. 2016):1 ( R ) N (cid:12) = 6 . × m and 1 ( R ) N ⊕ = 6 . × m,where this nominal terrestrial radius is Earth’s “zerotide” equatorial value.We modeled the light-curve with EXOFAST (Eastmanet al. 2013).
EXOFAST is an IDL-based transit andRV fitter for solving single-planet systems that em-ploys the Mandel & Agol (2002) analytic light-curvemodel, limb darkening parameters from Claret & Bloe-men (2011), and accounts for the long 30 minute K2 ca-dence. EXOFAST requires prior information on the timeof transit and the period of the orbit; the stellar tem-perature, metallicity, and surface gravity; and, withoutradial velocities (RVs), Eastman et al. recommendedfixing the orbit geometry to circular, as the light-curvedoes not provide adequate constraints on eccentricity orthe argument of periastron.Next, we modeled the light-curve following the proce-dure applied in the Zodiacal Exoplanets In Time (ZEIT)program, described in Mann et al. (2016a, 2017, 2018),which employs model light-curves generated with theBAsic Transit Model cAlculatioN code ( batman ; Kreid-berg 2015) and the quadratic limb-darkening law sam-pling method from Kipping (2013). We also accountedfor the 30 minute cadence and assigned a Gaussian prioron the stellar density of ρ (cid:63) = 1 . ± . ρ (cid:12) derivedfrom our estimates of the star’s mass and radius. Theposterior distributions of the various model parameterswere sampled with the affine-invariant Markov chainMonte Carlo (MCMC) code emcee (Foreman-Mackeyet al. 2013).In Table 1, we report the median values for each pa-rameter and errors as the 84.1 and 15.9 percentile values(i.e., 1 σ for a Gaussian distribution). Figure 2 plots theposterior distributions and correlations for a subset oftransit-fit parameters resulting from our MCMC analy- http://astroutils.astronomy.ohio-state.edu/exofast/ We performed preliminary modeling on a 20 hr segment ofour detrended and phase-folded light-curve centered on the ap-proximate time of transit using the web interface for
EXOFAST ,which simplified and sped up the fitting procedure. sis. Note that duration and inclination are not fit butare derived from the stellar density and impact param-eter. The eccentricity and argument of periastron areweakly constrained, which is common for long-cadencedata, especially when lacking RV data. Likewise, thestellar density posterior is essentially a reflection of theadopted prior, as it encapsulates the uncertainty in ec-centricity. Because the posteriors are not necessarilyGaussian or symmetric, it is possible that the medianvalues reported here for one set of values do not per-fectly translate to that of others. Similarly, the plottedmodel is the best fit (i.e., highest likelihood), which isnot necessarily the same as the median value.The bottom panel of Figure 1 shows this same light-curve phase-folded according to the 13.841901 day pe-riod along with the model solution from the ZEIT pro-cedure. As we will discuss later in Section 4, there isa star ∼ (cid:48)(cid:48) south of K2-231 and fainter by ∼ ≈ R p = 2 . ± . ⊕ . For comparison, EXOFAST returned R p = 2 . ± .
14 R ⊕ , which is consistent to0.3 σ . The EXOFAST uncertainty appears lower becausewe forced it to fit a circular orbit, whereas eccentricitywas allowed to float in the ZEIT procedure.
Note to readers of this preprint:
Our calibrated light-curve is included in the arXiv source file. PROPERTIES OF THE HOST STARCurtis et al. (2013) demonstrated that K2-231 is amember of Ruprecht 147, and therefore it should sharethe properties common to the cluster, including a spec-troscopic metallicity of [Fe/H] = +0.10 dex (Curtis2016), an an age of 3 Gyr, a distance of 295 pc basedon the distance modulus of m − M = 7 .
35, and an in-terstellar extinction of A V = 0 .
25 mag, derived from fit-ting Dartmouth isochrone models (Dotter et al. 2008) tothe optical and NIR color–magnitude diagrams (CMDs).We estimate the mass and radius of this star with a com-bination of spectroscopic and photometric data and thenargue that it is likely single (i.e., not a stellar binary).3.1.
Spectroscopy
On 2016 July 15, we used the MIKE spectrograph(Bernstein et al. 2003) on the 6.5 m Magellan Clay Tele-scope at Las Campanas Observatory in Chile to acquirea spectrum of K2-231 with the 0 . (cid:48)(cid:48)
70 slit, correspond-ing to a spectral resolution of R = 42 , S/N = 130 and 208 at sub-Neptune exoplanet in Ruprecht 147 N o r m a li z ed F l u x N o r m a li z ed F l u x -6 -4 -2 0 2 4 6Hours since mid-transit0.99920.99940.99960.99981.00001.00021.0004 Figure 1. K2 light-curves for EPIC 219800881: (Top) The EVEREST light-curve used to visually identify the transiting object,with the transits marked as short red vertical lines at the bottom of the figure. (Middle) Our refined and detrended light-curve,extracted with a 9 (cid:48)(cid:48) circular moving aperture while simultaneously fitting the pointing systematics, activity signal, and transitsfollowing Vanderburg et al. (2016), with the transits similarly marked. (Bottom) Our detrended light-curve, phase-foldedaccording to the 13.842 day period, along with the model for highest-likelihood solution from the ZEIT transit-fit procedure (seeTable 1), sampled at the times of observation according to the 30 minute integration cadence (red). Our calibrated light-curveand the detrended version are both available in the online journal and the arXiv posting.
Curtis et al.
Table 1.
Stellar and Planetary Properties for K2-231 bParameter Value 68.3% Confidence SourceInterval Width
Other Designations:
EPIC 219800881, NOMAD 0742–0804492, CWW 93, 2MASS J19162203 − Basic Information
R.A. [hh:mm:ss] 19:16:22.04 · · ·
Gaia DR1Decl. [dd:mm:ss] − · · · Gaia DR1Proper motion in R.A. [mas yr − ] − . − ] − . − ] 41.576 0 . ± . V magnitude 12.71 0.04 APASSDistance to R147 [pc] 295 5 C13Visual extinction ( A V ) for R147 [mag] 0.25 0.05 C13Age of R147 [Gyr] 3 0.25 C13 Stellar Properties M (cid:63) [ M (cid:12) ] 1.01 0.03 Phot+Spec+Iso R (cid:63) [ R (cid:12) ] 0.95 0.03 Phot+Spec+Isolog g (cid:63) [cgs] 4.48 0.03 Phot+Spec+Iso T eff , adopted [K] 5695 50 Phot+Spec+IsoSpectroscopic metallicity +0 .
14 0.04 SMER147 metallicity +0 .
10 0.02 SME v sin i [km s − ] 2.0 0.5 SMEMt. Wilson S HK R (cid:48) HK − .
80 0.03 Section 3.4
Planet Properties
Orbital period, P [days] 13.841901 0.001352 TransitRadius ratio, R P /R (cid:63) +0 . − . TransitScaled semimajor axis, a/R (cid:63) +4 . − . TransitTransit impact parameter, b +0 . − . TransitOrbital inclination, i [deg] 88.6 +0 . − . DerivedTransit Duration, t [hr] 2.94 +2 . − . DerivedTime of Transit T [BJD − +0 . − . TransitPlanet radius R P [R ⊕ ] 2.5 0.2 Converted Note —Coordinates are from
Gaia
DR1 (Gaia Collaboration et al. 2016a); proper motions are from HSOY(Altmann et al. 2017); the RV is the weighted mean for the six HARPS RVs and the uncertainties representthe precision and accuracy, respectively, where the accuracy is an approximation of the uncertainty in theIAU absolute velocity scale (Table 6); the V magnitude is from APASS (Henden et al. 2016); the distance,age, and extinction are from Curtis et al. (2013); the cluster metallicity was derived from SME analysis(Valenti & Piskunov 1996) of seven solar analog members of R147 (Curtis 2016); the metallicity andprojected rotational velocity were derived from SME analysis of the MIKE spectrum; the adopted stellarmass, radius, temperature, and surface gravity were derived by analyzing the available spectroscopic andphotometric data together with isochrone models (see Section 3); and the transit parameters are themedian values and the 68% interval from the posterior distributions resulting from our MCMC analysis,except for the transit duration and inclination, which are derived from from the stellar density and impactparameter. The planetary radius, measured relative to the stellar radius, is converted to terrestrial unitsusing values for the Earth and Sun radius from IAU 2015 Resolution B3. Chromospheric activity indiceswere measured from Hectochelle spectra following principles described in Wright et al. (2004). sub-Neptune exoplanet in Ruprecht 147 e b P /R * D en s i t y ( S o l a r) Figure 2.
Results of the ZEIT MCMC transit-fitting procedure. This corner plot shows posterior distributions and correlationsof a subset of the transit-fit parameters, including the ratio of planetary-to-stellar radius R p /R (cid:63) , eccentricity e , impact parameter b , and stellar density in solar units. The blue lines indicate the median values for each distribution; the red line shows the modefor the eccentricity plot. The shaded regions mark the 68%, 95%, and 99.7% contours of the MCMC posteriors. the peaks of the Mg I b and 5940–6100 ˚A orders, re-spectively. We also observed six other solar analogs inR147 at R = 42 ,
000 and 20 solar analogs in the field at R = 55 ,
000 (including 18 Sco and the Sun as seen fromthe reflection off of the dwarf planet Ceres, which we ob-served with both resolution settings). We reduced thesespectra with the Carnegie Python pipeline (“CarPy”), which performs the standard calibrations (i.e., overscan,bias, flat-field, sky-background, and scattered-light cor-rections, and mapping in wavelength using thorium–argon lamp spectra).We analyzed these spectra with version 423 of Spec-troscopy Made Easy (SME; Valenti & Piskunov 1996)following the Valenti & Fischer (2005) procedure.Adopting stellar properties for the field stars fromBrewer et al. (2016), that sample spans T eff = 5579–5960 K, log g = 4.10–4.50 dex, and [Fe/H] = − .
09 to+0 .
14 dex. We find median offsets and standard devi- http://code.obs.carnegiescience.edu/mike ations between the Brewer et al. (2016) properties andour values of T eff = −
11, 27 K, log g = − .
04, 0.035 dex,and [Fe/H] = − . . ± .
04 dex, where the uncertainty is the standard devi-ation of the sample; the standard deviation of the meanis ± S/N s andspectral resolutions of the R147 spectra (the stars are
Curtis et al. much fainter) compared to the field stars taken fromBrewer et al. (2016), and not intrinsic to the sample.For a separate project, Iv´an Ram´ırez measured stellarproperties for five of these solar analogs with the same orsimilar MIKE spectra (since his work, we have collectedhigher-quality data for particular stars for our analy-sis described here). Following Ram´ırez et al. (2013),he employed a differential analysis with respect to theSun by enforcing the excitation/ionization balance ofiron lines using the MOOG spectral synthesis code. He also fit the telluric-free regions of the wings of H α using the Barklem et al. (2002) grid. For the sameproject, Luca Casagrande measured IRFM tempera-tures for these stars following Casagrande et al. (2010).For these five stars, we find a median offset and standarddeviation for our SME values minus theirs of − ±
29 Kfor the Fe method, − ±
22 K for H α , and − ±
73 K forIRFM (I. Ram´ırez & L. Casagrande 2013, private com-munication). These differences are all within the uncer-tainties quoted and cross-validate our adopted temper-ature scale.Based on our results for the field star sample, theR147 members, and the SME statistical uncertaintiesquoted by Valenti & Fischer (2005), we adopt the fol-lowing spectroscopic parameter precisions: 50 K for T eff ,0.06 for log g , and 0.04 dex for [Fe/H]. Our error analysisassumes that our uncertainties are limited by the dataquality and our analysis technique, and not systematicsinherent in the models. As our sample is comprised ofstars quite similar to the Sun, the issues that tend toplague analyses of non-solar-type stars are assumed tobe largely mitigated. The procedure accurately repro-duces the Sun’s properties by design, as the line datawere tuned to the solar spectrum; therefore, we assumethat it can safely be applied to solar twins with the samedegree of accuracy, and we adopt our precision estimatesas our total parameter uncertainties.For K2-231, we found an effective temperature of T eff = 5697 K, surface gravity of log g = 4.453 dex,iron abundance of [Fe/H] = +0.141 dex, and rotationalbroadening of v sin i = 1.95 km s − when we adopted themacroturbulence relation from Valenti & Fischer (2005)(i.e., v mac = 3 .
87 km s − ). Adopting our preferred pa-rameters for the Dartmouth isochrone model to describethe R147 cluster (age of 3 Gyr and [Fe/H] = +0.1 dex)and querying the model at the spectroscopic tempera-ture yields an isochrone-constrained surface gravity oflog g = 4 .
483 dex, which we adopt for log g . We refitthe spectrum with metallicity fixed to the cluster value Table 2.
Photometry for K2-231
Instrument Band mag error
A/A V Gaia G · · · B V g (cid:48) g r (cid:48) r i (cid:48) i J H K S J K WISE W WISE W Note —(1) Name of instrument or survey. (2)Photometric band/filter employed. (3,4) Magni-tude and uncertainty for that observation, wherepipelines/surveys quoted errors below 0.01 mag, weset the value to 0.02 mag for analysis. (5) In-terstellar reddening coefficients computed by thePadova/PARSEC isochrone group (Bressan et al.2012) for a G2V star using the Cardelli et al. (1989)extinction law and following a procedure similar tothat described by Girardi et al. (2008). and log g fixed to this isochrone value, which returned T eff = 5672 K and v sin i = 1 . − , which is only25 K cooler than the unconstrained fit.3.2. Stellar mass and radius
We estimated the mass and radius of K2-231 bycombining our spectroscopic results with the opticaland NIR photometry provided in Table 2. We as-sembled photometry from
Gaia (Gaia Collaborationet al. 2016a,b), the AAVSO Photometric All-Sky Sur-vey (APASS; Henden et al. 2016) the CFHT’s Mega-Cam (Hora et al. 1994) presented by Curtis et al.(2013), the Two Micron All-Sky Survey (2MASS; Skrut-skie et al. 2006), the United Kingdom Infra-Red Tele-scope’s (UKIRT) Wide Field Infrared Camera (WF-CAM; Hirst et al. 2006) that was acquired by coauthorA.L. Kraus in 2011 and accessed from the WFCAM Sci-ence Archive, and NASA’s Wide-field Infrared SurveyExplorer ( WISE ; Wright et al. 2010).First, we used the PARAM 1.3 input form—the “webinterface for the Bayesian estimation of stellar param-eters” described by da Silva et al. (2006)—to estimate wsa.roe.ac.uk sub-Neptune exoplanet in Ruprecht 147 This service usesthe PARSEC stellar evolution tracks (version 1.1; Bres-san et al. 2012). The procedure requires as input theeffective temperature, metallicity, parallax, and V mag-nitude. We adopted the Curtis et al. (2013) distancemodulus and visual extinction to estimate the dered-dened magnitude ( V = V − .
25 = 12 . π = 3 .
39 mas yr − (calculated from 295 pc). Forparameter uncertainties, we adopted 50 K and 0.05 dexfor T eff and [Fe/H], and 0.05 mag for V and 0.15 masfor parallax based on the uncertainty in A V and m − M .PARAM 1.3 returned age t (cid:63) = 1 . ± . M (cid:63) = 1 . ± .
027 M (cid:12) , log g (cid:63) = 4 . ± .
029 dex (cgs),and radius R (cid:63) = 0 . ± .
029 R (cid:12) .Next, we estimated the mass and radius using thePython isochrones package (Morton 2015). Weadopted the spectroscopic T eff and log g values, the clus-ter metallicity and parallax, and the de-reddened broad-band photometry from Table 2, and ran the fit assumingthe photometry was derived from a blended and physi-cally associated binary. Only 56% of nearby field starsare single (Raghavan et al. 2010), so it is important toconsider at least binarity when characterizing this sys-tem (Raghavan et al. also found that 11% of nearbystars are in 3+ multiples). We used grid models fromthe Dartmouth Stellar Evolution Database (Dotter et al.2008) and sampled the posteriors using MultiNest (Feroz& Hobson 2008; Feroz et al. 2009, 2013) implemented inPython with the PyMultiNest package (Buchner et al.2014). Expressing uncertainties as the 68.3% (1 σ ) con-fidence intervals of the posterior distributions, we found M = 1 . ± .
016 M (cid:12) , R = 0 . ± .
021 R (cid:12) , and M = 0 . ± .
104 M (cid:12) .If the host is indeed single, then we can expect theparallax-constrained photometric analysis to return asmall secondary mass with a value at approximately thethreshold where its contributed flux is on par with thephotometric errors (i.e., consistent with no secondary).Based on this low secondary-mass estimate, there is noevidence from the photometry for a secondary compan-ion: the difference in magnitude between the result-ing primary and secondary stars is ∆ V = 7 .
29 and∆ K = 4 .
18, which is too large of a contrast to de-tect from these data (i.e., the difference between theprimary and the combined magnitude of both stars is http://stev.oapd.inaf.it/cgi-bin/param 1.3 The cluster-averaged parallax from the
Tycho–Gaia
Astro-metric Solution (TGAS; Michalik et al. 2015) from
Gaia
DR1(Gaia Collaboration et al. 2016a,b) is consistent with this at3.348 mas yr − , translating to 299 pc, based on 33 RV and AOsingle members (Curtis 2016). https://github.com/timothydmorton/isochrones V and 0.023 mag in K , the latter of whichis on par with the measurement errors). We reran thefit with isochrones assuming a single star, which re-turned M (cid:63) = 1 . ± .
017 M (cid:12) , R (cid:63) = 0 . ± .
022 R (cid:12) , d = 302 ± A V = 0 . ± .
05, and t = 2 . ± isoclassify code (Huber et al. 2017), conditioning spectroscopic T eff , log g , [Fe / H], parallax,and 2MASS
JHK photometry on a grid of interpolatedMIST isochrones (Dotter 2016). This returned M (cid:63) =1 . . − .
022 M (cid:12) , R (cid:63) = 0 . . − .
024 R (cid:12) , d = 309 + 9 − A V = 0 .
09 + 0 . − .
24 mag, and t = 2 . . − . M (cid:63) = 1 . ± .
027 M (cid:12) and R (cid:63) = 0 . ± .
027 R (cid:12) .According to the MIST model, a 3 Gyr star with mass M (cid:63) = 1 .
009 M (cid:12) and [Fe/H] = +0.1 dex has T eff =5695 K. This value is only 2 K cooler than our SMEresult, and so we adopt this value as the effective tem-perature of this star.3.3. K2-231 Is Likely Single
It is important to test K2-231 for stellar multiplic-ity. We need to know if it is a binary or higher-ordermultiple so we can confidently assume which star hoststhe transiting object and how much the light from thecompanion(s) has diluted the observed transits. Weassembled a variety of observational evidence, outlinedbelow, that collectively indicates that K2-231 is likelysingle. The various constraints derived from these dataare summarized in Figure 4, which shows the parameterspace for a range of binary scenarios with secondariesdescribed by K -band contrast (left axis) and isochrone-estimated stellar mass (right axis) as a function of pro- https://github.com/danxhuber/isoclassify Curtis et al. jected separation in angular units (bottom axis; out to1000 mas) and physical units (top axis; out to 300 AU).
Photometry:
Reiterating our result from the previoussubsection, modeling the broadband photometry withthe isochrones package suggests that K2-231 does nothave a companion with a mass M > .
34 M (cid:12) . Such asecondary would be at least ∼
321 times fainter than theprimary in V ; correcting for transit dilution would onlyincrease the transit depth by 0.3% and the planet radiusby 0.15% Basically, the effect of any binary companionallowed by the photometric modeling is negligible. Thisconstraint is illustrated in Figure 4 by the light blueshaded region at the top. Adaptive optics imaging and coronagraphy:
Weacquired natural guide star AO imaging in K (cid:48) ( λ =2 . µ m) with NIRC2 on the Keck II telescope. Wealso used the “corona600” occulting spot, which has adiameter of 600 mas and an approximate transmission of0.22% in K (cid:48) . The observations were acquired, reduced,and analyzed following Kraus et al. (2016). Table 3 liststhe K (cid:48) detection limits as a function of angular separa-tion from K2-231 ranging from 150 to 2000 mas.Table 4 lists six stars within 8 (cid:48)(cid:48) that were detected,including coordinates; angular separation, position an-gle, and K (cid:48) contrast relative to K2-231; and photometryfrom Gaia , CFHT/MegaCam, and UKIRT/WFCAM.This table also lists four stars within 10 (cid:48)(cid:48) detectedin the UKIRT imaging that were missed by NIRC2.Figure 3 shows a 30 (cid:48)(cid:48) -square K -band image fromUKIRT/WFCAM centered on the host star and high-lights the noncoronagraphic imaging footprint (magentadashed line); note that we had to offset the pointing af-ter the first image in order to get the bright neighboringstar onto the detector, which is why there is effectivelya double footprint. For reference, two circles with radiiof 5 . (cid:48)(cid:48) (cid:48)(cid:48) are also overlaid to show the approxi-mate extraction apertures used to produce light-curvesfrom the K2 data. The AO imaging and coronagra-phy yielded six detections, four of which were matchedin the UKIRT imaging (red circles), and two of whichwere apparently fainter than the UKIRT source cataloglimit (blue circles), but nevertheless show up in the im-age. Due to the placement, size, and orientation of theNIRC2 footprint, four stars within 10 (cid:48)(cid:48) of the host weremissed but show up in WFCAM (cyan circles).We calculated proper motions for the eight stars thatmatched in both Gaia and either or both MegaCam andWFCAM and found that none but the final entry appearcomoving with R147. We also inspected optical and NIRCMDs with the cluster Dartmouth model overlaid and noted that stars 1, 3, 8, and 9 are inconsistent with mem-bership, whereas 6, 7, and 10 appear near but beyondthe base of the Dartmouth isochrone. As 6 and 7 appearto be ruled out by their discrepant proper motions, thisleaves 10 as the sole candidate member in this list. Al-though too faint for
Gaia , it is conceivable that we couldmeasure its proper motion with additional NIRC2 im-ages in the future: the uncertainty on ρ is under 5 mas,whereas R147 moves at −
28 mas yr − in declination,so two observations spaced approximately by one yearshould clearly reveal any comoving stars while cancelingout the parallax effect.Only two stars are detected within 5 . (cid:48)(cid:48)
5, which is theradius of the smallest circular moving aperture that weused to extract light-curves. One star is near the edge ofthis radius and is nearly 480 times fainter than K2-231.The other, at 4 . (cid:48)(cid:48) ∼ (cid:12) are shaded gray and foundbelow the black horizontal line toward the bottom of thefigure, which the AO limit reaches at ∼
700 mas—thisdepth is not only important for searching for stellar bi-naries, but also for identifying faint, unassociated starsin the background. The lowest mass star representedin the Dartmouth isochrone model is M ≈ .
12 M (cid:12) :we also shade this region gray and label it “VLM” for“very low mass star” to distinguish it from the regionbelow the substellar boundary while highlighting thatthis represents a small region of the secondary massparameter space compared to the top-half of the figure.
Keck/NIRC2 aperture-masking interferometry:
We also acquired nonredundant aperture-masking inter-ferometry data for K2-231 on 2017 June 22 in naturalguide star mode, along with EPIC 219511354 for cali-bration. For the target and reference star, we obtainedfour (three) interferograms for a total of 80 (60) s onEPIC 219800881 (EPIC 219511354), which we analyzedfollowing Kraus et al. (2008, 2011, 2016). We report nodetections within the limits quoted in Table 5. Theseconstraints are illustrated in Figure 4 by the red shadedregion, which is drawn according to the midpoints ofthe angular separation ranges listed in Table 5.
Spectroscopy:
We observed K2-231 on 2017 June 2(near quadrature, according to the transit ephemeris)with the High Resolution Echelle Spectrometer (HIRES;Vogt et al. 1994) on the 10 m telescope at Keck Obser-vatory. No secondary spectral lines were found down sub-Neptune exoplanet in Ruprecht 147 Table 3.
Keck/NIRC2 Imaging Detection Limits
MJD Filter + Number of Total Contrast Limit (∆ K (cid:48) in mag) at Projected Separation ( ρ in mas)Coronagraph Frames Exposure (s) 150 200 250 300 400 500 700 1000 1500 200057933.42 K (cid:48) K (cid:48) +C06 4 80.00 · · · · · · · · · · · · Note —The second entry is for the coronagraphic imaging observations, which obstructs the inner 3 mas radius.
Table 4.
Keck/NIRC2 a and UKIRT/WFCAM Detected Neighbors ρ PA ∆ K (cid:48) G g (cid:48) r (cid:48) i (cid:48) J K
J2000 J2000 (mas) (deg) (mag) (mag) (mag) (mag) (mag) (mag) (mag)1 19:16:22.005 − ± ± ± − ± ± ± · · · · · · · · · − ± ± ± · · · · · · · · · · · · − ± ± ± · · · · · · · · · · · · · · · · · · − ± ± ± · · · · · · · · · · · · · · · · · · − ± ± ± − · · · · · · − · · · · · · − · · · − · · · · · · Note —The third object was only detected in the coronagraphic observation because it fell on the edge of the NIRC2 imaging footprint; seeFigure 3. The objects in the lower section were detected with UKIRT but missed by NIRC2 due to the placement, size, and orientation ofthe NIRC2 field. Star 10 is the only neighbor that appears co-moving with R147 (and therefore the planet host; stars 4 and 5 were onlydetected in NIRC2 and so lack a second astrometric epoch needed to calculate proper motions), with a CFHT − UKIRT proper motionof ( µ α cos δ, µ δ ) = (3 , −
31) mas yr − , although the baseline is relatively short at ∼ a The relative astrometry for the NIRC2 observations was computed with the plate scale and rotation adopted from Yelda et al. (2010).
Table 5.
Keck/NIRC2 aperture-masking interferometry detection limits
Confidence MJD Contrast Limit (∆ K (cid:48) in mag) at Projected Separation ( ρ in mas)Interval 10-20 20-40 40-80 80-160 160-240 240-32099.9% 57933.4 0.06 3.02 4.02 3.79 3.19 1.9699% only 57933.4 0.26 3.24 4.20 3.97 3.42 2.2 to 1% of the brightness of the primary ( ∼ (cid:12) ; al-ready ruled out by photometric modeling), excludingthe range of under ±
10 km s − separation from the pri-mary (Kolbl et al. 2015). RV variability:
We collected RVs every few years be-ginning in 2007, which show no trend due to a stel-lar companion over the baseline of nearly ten years. These include observations with the Lick/Hamilton andMMT/Hectochelle spectrographs presented in Curtiset al. (2013), the HIRES spectrum mentioned above2
Curtis et al.
15 10 5 0 -5 -10 -15 ∆ Right Ascension (arcseconds)-15-10-5051015 ∆ D e c li na t i on ( a r cs e c ond s ) Figure 3.
Image of K2-231 and neighboring stars fromUKIRT/WFCAM, taken in 2011. The solid black circleshows the 9 (cid:48)(cid:48) radius aperture used to extract the light-curve.The dashed black circle has a radius of 5 . (cid:48)(cid:48) and is the small-est aperture we tested; the transits are still visible, whichmeans that the object is either transiting K2-231 or thefainter star 4 (cid:48)(cid:48) southward. The dashed magenta line tracesout the Keck II NIRC2 footprint: six stars were detected,four of which cross-matched with the UKIRT catalog (red)and two of which were apparently too faint, though theyshow some signal in the image (blue). Four other stars aredetected in the WFCAM image within 10 (cid:48)(cid:48) but were missedby NIRC2 due to the size, placement, and orientation of thefield (cyan). Properties of these 10 neighboring stars arelisted in Table 4. (Chubak et al. 2012), and the Magellan/MIKE spectradiscussed earlier. Separately, a team led by PI Minniti targeted R147with the High Accuracy Radial velocity Planet Searcher(HARPS; Mayor et al. 2003) in 2013-2014 to look forexoplanets in R147 with masses greater than or approxi-mately equal to Neptune in relatively short-period orbitsand acquired six RV epochs with individual precisionsof ≈
10 m s − . Data were reduced and RVs extractedwith the HARPS Data Reduction Software. We down- Barycentric velocities were calculated with the IDL code
BARYCORR (Wright & Eastman 2014); see also http://astroutils.astronomy.ohio-state.edu/exofast/barycorr.html ESO program 091.C-0471(A) and 095.C-0947(A), “HuntingNeptune mass planets in the nearby old, metal rich open cluster:Ruprecht 147.”
Table 6.
RVs for K2-231
Date MJD = JD RV Uncertainty Observatory − , ,
000 (km s − ) (km s − )2007 Aug 23 54335.789 41.584 1.00 Lick2010 Jul 05 55382.264 41.397 0.30 Hecto2010 Jul 06 55383.269 41.377 0.30 Hecto2012 Sep 30 56200.644 42.112 0.70 MIKE2013 Aug 10 56514.247 41.580 0.012 HARPS2014 May 07 56784.386 41.586 0.008 HARPS2014 May 08 56785.399 41.573 0.007 HARPS2014 May 09 56786.404 41.574 0.007 HARPS2014 May 27 56804.311 41.570 0.016 HARPS2014 Jun 22 56830.298 41.577 0.008 HARPS2016 Jul 15 57584.743 41.550 0.70 MIKE2017 Jun 02 57907.075 41.760 0.30 HIRES Star B : a − .
92 0.20 HIRES2017 Aug 28 57993.804 − .
28 0.20 HIRES
Note —RV measurements collected over nearly ten years, with rms =250 m s − , consistent with K2-231 being single. See Section 3.3 fordetails. a The faint neighbor referred to as “Star B” is the first object listed inTable 4 and located 4 (cid:48)(cid:48) south of the exoplanet host at (19:16:22.319, − loaded the reduced data, including the pipeline RVs anduncertainties, from the ESO archive. We recalculated the RVs for the Lick 2007, Hecto2010, and MIKE 2016 epochs differentially relative tothe solar-twin member CWW 91 (NID 0739-0790842;EPIC 219698970). They were observed concurrently(Hectochelle) or close in time on the same night, withthe RV zero point of the reference star set to its me-dian HARPS RV of 41 . ± .
014 km s − (five visitsover 1.9 yr). For reference, Curtis et al. (2013) reporteda HIRES epoch of 41.5 km s − for this reference star.CWW 91 was not observed on the same run for theMIKE 2012 epoch, so instead we calculated the zeropoint with six other stars with HARPS RVs with concur-rent MIKE observations in order to mitigate the effectof any one of those stars being an unknown binary. Wenote that this epoch happens to be the largest outlier,although consistent within the estimated uncertainty forour MIKE RVs.The RVs are provided in Table 6. Averaging the twoHectochelle RVs, as well as the six HARPS RVs, yieldssix individual RV epochs spanning 9.8 yr with an un- Values taken from the “*ccf G2 A.fits” files. http://archive.eso.org/wdb/wdb/adp/phase3 spectral/query sub-Neptune exoplanet in Ruprecht 147 − . The HARPS RV rms is6 m s − over 10 months. RV median:
The median RV of 41 . ± .
25 km s − provides an additional stringent constraint on binarity.Consider the Hectochelle RVs: of the 50 members ob-served, selecting the 38 stars with RVs within 2 km s − of the cluster median, the two-night median and stan-dard deviation RV for R147 is 41 . ± .
70 km s − ,which is exactly equal to the Hectochelle RV for K2-231. Even if this star is single, this equality is a coin-cidence, given R147’s intrinsic velocity dispersion. TheHectochelle RV spread is likely larger than the intrinsiccluster velocity dispersion due to some binaries linger-ing in the sample and is not yet well-constrained, butit is currently estimated to be between σ R147 =0.25-0.50 km s − (see Section 3.1.2 in Curtis 2016).Assuming M = 0 . (cid:12) , RV γ =RV R147 , and σ R147 =0 . − , a hypothetical circular binary seen edge-on would require an orbital period P orb = 1175 years( ∼
118 AU) for the RV semi-amplitude ( K ) to matchthe velocity dispersion. Such binaries are ruled out bythe AO imaging and coronagraphy, except for phaseswhere the projected separation is reduced under the de-tection sensitivity curve (dark blue curve in Figure 4).For shorter-period binaries, the RV of the primary willcross the cluster’s velocity at the conjunction points butwill be larger or smaller than this value during most ofthe orbital period, neglecting dispersion. The fact thatthe RV for K2-231 is exactly equal to the cluster medianmeans that if it is a binary, we would be lucky to catchit at conjunction.For example, consider once again the hypotheticalbinary described previously: M = 0 . (cid:12) , e = 0 . i = 90 ◦ . If the semi-major axis is a = 45 AU (the ap-proximate boundary of the AO sensitivity curve), then P orb = 146 years and K = 1 km s − . The primary onlyspends 0.64% of its orbit within the ∼
10 m s − uncer-tainty of the HARPS RV data. However, the HARPSRV precision is not the appropriate limit because we donot know the intrinsic RV (or center-of-mass velocity,RV γ , if a binary) for this star. If RV γ (cid:54) = (cid:104) RV obs (cid:105) , butinstead is some other value within the cluster velocitydispersion, then it is possible that we are observing it ata quadrature point instead of conjunction, which wouldmodestly increase the probability of randomly catchingit at this orbital phase due to the longer time the starspends at the quadrature RV within the HARPS uncer-tainty. RV binary constraints:
These RVs, particularly theprecise measurements from HARPS, are useful for con- straining binary scenarios with semi-major axes closerto the primary than the region probed by AO. We esti-mated our detection sensitivity by generating simulatedRV curves with
RVLIN (Wright & Howard 2009) for bi-naries with semimajor axes a <
50 AU and secondarymasses M < . (cid:12) (rounding up the 0.34 M (cid:12) limitderived from photometric modeling). We performed asimple experiment with circular orbits seen edge-on tosketch out the approximate limits on binarity in thisregion. For each M – a combination tested, we calcu-lated the orbital period ( P orb ) and the primary’s veloc-ity semi-amplitude ( K ), then computed the RV timeseries with RVLIN . Next, we derived the optimal time ofperiastron passage that best aligns the observed RVs tothe model, which presents a best-case scenario to com-pute χ . We decided that a binary was detectable if χ ≥ χ , where the single-star model is a flatline running through the median RV.The constraints derived from this simple experimentare illustrated by green shading in Figure 4. Circular,edge-on binaries with center-of-mass RVs equal to theobserved median, RV γ = (cid:104) RV obs (cid:105) = 41 .
58 km s − , canbe ruled out for most of the remaining parameter space.Different orbital geometries and viewing perspectiveswill alter the detection sensitivity. Eccentricity can in-crease or decrease our sensitivity depending on the spe-cific orbital properties and the phase of the observedRVs. Inclination decreases sensitivity by reducing theRV semi-amplitude; however, it is improbable that thesensitivity would drop to zero, because it is unlikely thatthe binary orbital plane is exactly perpendicular to theprimary–planet plane.For now, we will conclude this discussion by statingthat the evidence suggests that K2-231 is likely single.Further progress can be made by simulating realisticbinary systems in the cluster and testing them againstthe observational constraints, which is not necessary forthis study. We already demonstrated that the allowedbinary systems would dilute the observed transits by anegligible amount. As for which component of the hypo-thetical binary hosts the transits, this is accounted forwhen statistically validating the planet with BLENDER ,discussed later in Section 4.2, by confronting the light-curve with simulations of eclipsing binaries or largerplanets transiting fainter stars that are physically asso-ciated, or in the background, to rule out these scenarios.3.4.
Activity and Rotation
We measured chromospheric Ca II H & K emis-sion indices, S and log R (cid:48) HK , from our MIKE and Hec-tochelle spectra following procedures described in Noyes4 Curtis et al. ρ (mas)1086420 C on t r a s t, ∆ K ( m ag ) Hydrogen Burning LimitVLM starsRV ?10864200 200 400 600 800 10000 100 200 300Projected Separation (AU) 0.070.120.200.300.400.500.600.700.85 S e c onda r y M a ss ( s o l a r) NRM Photometric ModelingAO
Figure 4.
Constraints on binary companionship for hypo-thetical secondaries with K (cid:48) -band contrast (left) or stellarmass (M (cid:12) ; right) as a function of projected separation inangular units (mas; bottom) or physical units (AU; top). Atseparations of ρ >
200 mas, the NIRC2 AO imaging andcoronagraphy (dark blue lined region) probe deeper thanthe very low mass stars and reaches down to the hydrogen-burning limit at ∼
700 mas (gray shaded region), which isuseful for searching for background blends; the NIRC2 non-redundant masking data reach closer to the primary star, butnot quite as deep (red shaded region). Modeling the broad-band photometry with isochrones rules out secondaries ofany separation with masses greater than M (cid:38) .
34 M (cid:12) (light blue shaded region). Combining these various con-straints leaves a small region of parameter space under 45 AU(projected) for systems with M (cid:46) .
34 M (cid:12) . The preciseHARPS RVs can rule out much of this remaining parameterspace for edge-on orbits (green shaded region); accountingfor possible inclination of the binary orbital plane relative tothe primary–planet orbit will restrict this to smaller separa-tions. et al. (1984) and Wright et al. (2004), and found S =0 . ± .
005 and log R (cid:48) HK = − . ± .
03. Figure 5shows the Hectochelle Ca II K spectrum for K2-231,along with solar spectra taken between 2006 and thepresent, which are shaded red to represent the range ofthe contemporary solar cycle. The solar spectra were ob-tained by the National Solar Observatory’s Synoptic Op-tical Long-term Investigations of the Sun (SOLIS) facil-ity with the Integrated Sunlight Spectrometer (ISS) onKitt Peak (Keller et al. 2003). The observed chromo-spheric activity level of K2-231 is somewhat higher thanthe modern solar maximum (the average maximum over http://solis.nso.edu/iss R e l a t i v e F l u x ISM
Figure 5.
The Ca II K spectral region for K2-231 as ob-served with MMT/Hectochelle in 2010 July (black line) andSOLIS/ISS spectra of the Sun taken between 2006 and thepresent (red shading) to represent the range of the modernsolar cycle. The chromospheric activity for the 3 Gyr R147star is slightly elevated above the modern solar maximum,as is typical for this cluster and expected from its age. Notethe interstellar absorption line blueward of the Ca II K linecore (for more on interstellar absorption and its impact onactivity indices, see Curtis 2017). cycles 15–24 is log R (cid:48) HK = − .
905 dex; Egeland et al.2017), which is expected because it is ∼ R (cid:48) HK = − .
80 corresponds to an age of 3.2 Gyr.An analysis of the Ca II H & K activity for thefull cluster sample is underway, and these numbers canbe considered preliminary until that study is complete.However, the solar twin status of this star simplifies thecalibration, as we can tie it directly to solar observa-tions. We tested this by differentially measuring S forK2-231 relative to the the SOLIS/ISS spectra and ap-plying the conversion from their 1 ˚A K-index to S us-ing the Egeland et al. (2017) relations. This procedureyielded S = 0 . R (cid:48) HK over our Hectochelle calibration ofonly 0.003 dex. The uncertainties are assessed by con-sidering the observed scatter for stars with multiple ob-servations and stars with overlapping spectra betweenMIKE and Hectochelle (neglecting astrophysical vari-ability) and uncertainty in the adopted ( B − V ) whentransforming S to log R (cid:48) HK .The rotation period inferred as part of the activity–rotation–age procedure (i.e., from the activity–Rossby sub-Neptune exoplanet in Ruprecht 147 P rot = 21 . ∼
21 day signal is not immediately obvi-ous. The apparent periodicity is closer to 6–7 days;this cannot be the true rotation period because thestar would correspondingly be much more active, withlog R (cid:48) HK ∼ − .
41 (Mamajek & Hillenbrand 2008). Ifthere were two major spot complexes on opposite sidesof the primary star, that would make the period of themodulation be half of the rotation period. If the rota-tional period was actually 12–14 days, we would expectlog R (cid:48) HK = − . ± .
05 dex, which is still too activecompared to the observed chromospheric emission.The EVEREST light-curve was produced with a sta-tionary aperture that encompassed many bright, neigh-boring stars. However, those same rotation signaturesare present in our 9 (cid:48)(cid:48) moving aperture light-curve (notshown, but the reader can verify this with the light-curveprovided), which means the modulation could instead beattributed to one of the neighbors in that aperture listedin Table 4. We therefore do not report a rotation periodat this time. This illustrates one of the main challengesto measuring accurate rotation periods in middle-agedclusters in crowded fields. PLANET VALIDATIONFirst, we inspected the six individual transits for vari-ations in depth, timing, and duration between the oddand even events that would indicate eccentricity or dis-similar stellar companions, under the assumption thatthese are stellar eclipsing binary (EB) transits. Figure6 shows each transit event separately, along with the
EXOFAST transit model, and they are all consistent withthe model and each other.One might think that the cluster environment wouldcreate a crowded field that would complicate the photo-metric analysis. In fact, R147 is relatively sparse due toboth the low number of (confirmed) members ( N ≈ ∼ l = 21 ◦ , b = − ◦ )means that there are quite a few background stars. Weopted for a circular moving aperture to track K2-231’smotion across its individual aperture while excluding as The conversion from rotation period to log R (cid:48) HK depends onthe the rotation period and also the adopted ( B − V ). The dered-dened APASS value is ( B − V ) = 0 .
72; applying the adoptedeffective temperature to the table of stellar data from Pecaut &Mamajek (2013) yields ( B − V ) = 0 .
67. The uncertainty in eachinput parameter contributes a similar level of uncertainty. -5 0 5Hours since Mid-transit1.0001.0021.0041.0061.0081.010 N o r m a li z ed f l u x + o ff s e t Figure 6.
Individual transit events along with the
EXOFAST transit model for K2-231 b. Their consistency, especially be-tween odd- and even-numbered events, indicates that theyare due to either the same transiting object or two with neg-ligible differences in a circular orbit (i.e., an equal-mass EB). many of the background stars shown in Figure 3 as pos-sible. The aperture used to produce the
EVEREST light-curve that we used to identify the transiting planet con-tained all the bright stars shown to the southwest ofK2-231. Our 9 (cid:48)(cid:48) circular aperture excludes all but one ofthese brighter stars. We also created apertures as smallas 5 . (cid:48)(cid:48) Star B: The Bright Neighbor Curtis et al.
The star that remains blended is located approxi-mately 4 . (cid:48)(cid:48) Gaia and CFHT/MegaCam epochs are sep-arated by ∼ µ δ = −
28 mas yr − . For K2-231, we mea-sure µ δ = − . − , and for star B, we find µ δ = − . − , which does not support clustermembership.We can also model the CFHT and UKIRT photometrywith isochrones under the assumption that it is a singledwarf star by applying a Gaussian prior on log g = 4 . ± .
5, and we find a mass M = 1 . − .
10 + 0 .
13 M (cid:12) ,radius R = 1 . − .
16 + 0 .
19 R (cid:12) , distance d = 2204 − A V = 0 . ± .
18 mag.The 3D Galactic dust map produced from 2MASS andPan-STARRS 1 (Green et al. 2015) toward K2-231quotes an interstellar reddening at 300 pc (the approxi-mate distance to R147) of E ( B − V ) = 0 .
07 + 0 . − . A V = 0 . . − .
12, which is consistent with thevalue we find from CMD isochrone fitting). According tothis map, interstellar reddening is E ( B − V ) = 0 . ± . A V = 0 . ± .
06 at 2.2 kpc, the distance we infer forstar B, and reaches a maximum value of E ( B − V ) =0 . ± .
02 at 2.28 kpc ( A V = 0 . This value is con-sistent with our result from isochrones due to the largeuncertainty, which is compounded when considering ourassumption of singularity and a dwarf luminosity class.The Schlegel et al. (1998) dust map value is marginallyless at E ( B − V ) = 0 .
146 or A V = 0 .
45, and the recal-ibrated map from Schlafly & Finkbeiner (2011) quotes E ( B − V ) = 0 .
125 or A V = 0 . BLENDER (de-scribed in the next section; Torres et al. 2011a) indi-cates that the broad features of the transit light-curvecan indeed be fit reasonably well if star B is a back-ground EB. Assuming that both the target and star Bare solar-mass stars, we find a decent fit for a compan-ion to star B of about 0.26 M (cid:12) . This EB produces asecondary eclipse, but it is very shallow ( ∼
30 ppm) andis probably not detectable in the data, given the typicalscatter of ∼
120 ppm. If we resolved star B, we expect http://argonaut.skymaps.info/query Using the 2015 version gives color excesses of 0.05 for R147,0.18 for star B, and a maximum of 0.20 at 2.44 kpc. that the undiluted transit due to this hypothetical EBwould be ∼ − ,which is also feasible to test and rule out with a fewRV observations. We acquired two RV epochs of star Bwith HIRES, which were taken 7.37 and 9.93 days frommidtransit (propagated forward according to the transitephemeris in Table 1), near the secondary eclipse andsecond quadrature points at phases of 0.53 and 0.72, re-spectively. The RVs, listed at the bottom of Table 6, areconstant to within their 0.2 km s − uncertainties. Fur-thermore, these HIRES spectra have sufficient qualityto rule out secondary spectral lines down to 1% of thebrightness of the primary, excluding ±
10 km s − separa-tion (Kolbl et al. 2015). This rules out the false-positivescenario where star B is a background EB.4.2. False-alarm Probability
Having excluded the only visible neighboring starwithin the aperture as the source of the transit signal, wethen examined the likelihood of a false positive causedby unseen stars. For this, we applied the
BLENDER sta-tistical validation technique (Torres et al. 2004, 2011b,2015) that has been used previously to validate candi-dates from the
Kepler mission (see, e.g., Torres et al.2017; Fressin et al. 2012; Borucki et al. 2013; Barclayet al. 2013; Meibom et al. 2013; Kipping et al. 2014,2016; Jenkins et al. 2015). For full details of the method-ology and additional examples of its application, we re-fer the reader to the first three sources above. Briefly,
BLENDER models the light-curve as a blend between theassumed host star and another object falling within thephotometric aperture that may be an EB or a star tran-sited by a larger planet, such that the eclipse depthsfrom these sources would be diluted by the brighter tar-get to the point where they mimic shallow planetarytransits. These contaminants may be in either the back-ground or foreground of the target or physically associ-ated with it. Fits to the K2 light-curves of a large num-ber of such simulated blend models with a broad rangeof properties allows us to rule many of them out thatresult in poor fits, and Monte Carlo simulations con-ditioned on constraints from the follow-up observations(high-resolution spectroscopy, imaging, RVs, color infor-mation) yield a probability of 99.86% that the candidateis a planet, as opposed to a false positive of one kind or sub-Neptune exoplanet in Ruprecht 147 DISCUSSIONWe have demonstrated that K2-231 is a single, solartwin member of the 3 Gyr open cluster Ruprecht 147and that it hosts a statistically validated sub-Neptuneexoplanet in a 13.84 day orbit.5.1.
Expected yield
This is the only planetary system found (as of thiswriting) out of 126 RV-confirmed members of R147 thatwere observed with K2 during Campaign 7. Neglectingthe red giants (eight stars), blue stragglers (five stars),and tight binaries (10+ stars), we searched ∼
100 FGKdwarfs. According to Table 4 in Fressin et al. (2013),the percentage of stars with at least one planet with anorbital period under 29 days is 0.93% for giant plan-ets (6–22 R ⊕ ), 0.80% for large Neptunes (4–6 R ⊕ ),10.24% for small Neptunes (2–4 R ⊕ ), 12.54% for superEarths (1.25–2 R ⊕ ), and 9.83% for Earth-sized plan-ets (0.8–1.25 R ⊕ ). If we assume a circular orbit, thetransit probability is defined as the ratio of the sum ofthe planetary and stellar radii to the semimajor axis, P tr ≡ ( R p + R (cid:63) ) /a (cid:39) ( R (cid:63) /a ). For simplicity, we as-sume that all stars are the size of the Sun (not too un-realistic). Fressin et al. (2013) quoted the occurrencerates in 11 period ranges: we focus on 0.8–2.0, 2.0–3.2, 3.2–5.9, 5.9–10, 10–17, and 17–29 days; restrict-ing the orbital periods to <
30 days ensures that atleast two transits will be present in our ∼
81 day light-curves. We calculate transit probabilities for the meanperiod for each period bin and convert these periods tosemimajor axes ( a ∝ P / ) to find transit probabilitiesin each period range. We estimate the exoplanet yieldas N planet = N star × P planet × P transit × P detect , where N planet is the number of stars observed to host planetswith periods under 30 days, N star is the number of starssurveyed (100 in this case), P planet is the percentage ofstars with at least one planet from Fressin et al. (2013), P transit is the transit probability assuming the stars are R (cid:63) = 1 R (cid:12) , and P detect is our sensitivity to detectingthese transiting planets: we assume that we can detectany planet larger than the “Earth” class with periodsunder 30 days. Based on this calculation, we expect todetect 0.05 giants, 0.04 large Neptunes, 0.45 small Nep-tunes, and 0.66 super Earths, and we would miss 0.57 In fact, Fressin et al. (2013) quoted the occurrence rates foreach period range starting at 0.8 days, so we subtract the previousbin’s value from the one under consideration. For example, theoccurrence rate for the 17–29 day bin is the value for the 0.8–29 day bin minus the value for the 0.8–17 day bin.
Earths, as we assume that our survey is not sensitive tothe Earth-sized planets (Howell et al. 2014). Basically,in this RV-vetted sample, we expect our survey to yield ∼ K2 Survey of Ruprecht 147,” we allocated apertures basedon photometric criteria and soft proper-motion cuts tostrive for completeness and ensure any actual memberthat is eventually identified and located in the Cam-paign 7 field will have a K2 light-curve. We selected1176 stars that passed our tests; however, some of thesetargets are certainly interlopers. The impending second Gaia data release (DR2) will clarify the membership sta-tus of the majority of these stars. In the meantime, weare working on a new membership catalog that will su-persede Curtis et al. (2013) and include detailed stellarproperties and multiplicity informed by AO imaging, RVmonitoring, and photometric modeling for our expandedRV-vetted membership list (Curtis 2016). Following thecompletion of the membership census, we will be able toapply our stellar properties derived from our vast photo-metric and spectroscopic database to the transit prob-ability calculations and incorporate all members withlight-curves into our occurrence analysis. Therefore, weopt to postpone a more detailed calculation of the ex-oplanet occurrence rate in R147 until these two criticalingredients, membership and sensitivity, have been ade-quately addressed.5.2.
Comparison to field stars
Fulton et al. (2017) showed that the distribution ofplanetary radii is bimodal, with a valley at about 1.8 R ⊕ and a peak at the larger side at 2.4 R ⊕ representingsub-Neptunes, which they argued are a different classof planets than the super Earths found on the smallerside of the gap (see their Figure 7). With a radius of ∼ ⊕ , K2-231 b falls on the large side of the planetradius gap (see also Rogers 2015; Weiss & Marcy 2014).Our Figure 7 presents a modified version of the bot-tom panel of Figure 8 from Fulton et al. (2017), whichshows the completeness-corrected, two-dimensional dis-tribution of planet size and orbital period derived fromthe Kepler sample. Our figure compares this distribu-tion to the properties of K2-231 b and shows that it is8
Curtis et al.
Orbital period [days] P l ane t S i z e [ E a r t h r ad ii ] lowcompleteness R e l a t i v e O cc u rr en c e Figure 7.
Two-dimensional distribution of planet size andorbital period found in the
Kepler field, adopted from Fultonet al. (2017), is shown along with the location of K2-231 b.Restricting this distribution to periods less than 40 days (i.e.,demanding the presence of two transits for a planet detec-tion) means that K2-231 b is found near a relative maximumin this distribution. found near a relative maximum. In other words, K2-231 b appears to have a fairly typical radius for a short-period (
P <
29 days) planet.5.3.
Comparison to the NGC 6811 planets
Meibom et al. (2013) concluded that the frequency ofplanets discovered in the 1 Gyr
Kepler cluster NGC 6811is approximately equal to the Fressin et al. (2013) fieldrates based on two planets found out of 377 memberssurveyed. This is about half of the raw rate found inR147 (i.e., 1 in 100 versus 2 in 377); in other words, thesame order of magnitude.The two planets found in NGC 6811 are quite simi-lar to K2-231 b: they are sub-Neptunes with radii of 2.8and 2.94 R ⊕ and periods of 17.8 and 15.7 days (Kepler-66 b and 67 b, respectively). This is unlikely to be amere coincidence, but as Figure 7 illustrates, planetswith these approximate properties are relatively moreprevalent. However, that figure shows that the relativeoccurrence of the sub-Neptunes continues, and even in-creases, to longer orbital periods. While the durationof the K2 survey of R147 was not long enough to iden-tify planets in the 40–100 day regime, presumably suchplanets could have been found in NGC 6811 during the Kepler prime mission. With this limited sample, it is unclear if any meaning should be drawn from this re-garding possible planetary architectures that can formand survive in a dense cluster, but it is at the very leastan intriguing option to consider. However, we think thisis probably due to the relatively lower
S/N light-curvesdue to NGC 6811’s large distance modulus and the re-duction in transit depth and probability with increasingorbital period.5.4.
Similar planets and estimating the mass
Considering the planets with measured masses andradii in the field, there are currently five listed onexoplanets.org with 2 . < R p / R ⊕ < . K > − ,and P >
Kepler ’s 96 b, 106 c and e, 131 b,and HIP 116454 b. The basic transit and physical prop-erties of K2-231 b and its host are similar to those ofKepler 106 c: M (cid:63) = 1 . (cid:12) , [Fe/H]= − .
12 dex, T eff = 5860 K, log g = 4 .
41 dex, V = 13, P orb = 13 days, R P = 2 . ⊕ , and a = 0 .
111 AU. Importantly, the RVsemi-amplitude for Kepler 106 c is K = 2 .
71 m s − , andthe planet mass is M p = 10 . ⊕ (Marcy et al. 2014), and this mass was measured with RV observations madewith Keck/HIRES.Applying the Wolfgang et al. (2016) mass–radius re-lation for sub-Neptune transiting planets (i.e., R P < ⊕ ), where M/ M ⊕ = 2 . R/ R ⊕ ) . , predicts a massfor K2-231 b of M p ∼ . ± . ± . ⊕ , where the un-certainties represent the standard deviation of massescomputed from a normally distributed sample of radii R p = 2 . ± . ⊕ and the normally distributed disper-sion in mass of the relation, respectively. The Chen &Kipping (2017) probabilistic mass–radius relation, im-plemented with the Forecaster
Python code, yields M p = 7 . . − . ⊕ . Assuming a circular orbit,Kepler’s Law predicts an RV semi-amplitude for K2-231of K ≈ ± − in this mass range. Querying theCPS chromospheric activity catalog (Isaacson & Fischer2010) for dwarfs with similar color and activity (i.e.,0 . < ( B − V ) < . − . < log R (cid:48) HK < − .
77, andheight above the main sequence δM V < − . This might be measurable with existingprecise RV instruments like HIRES or HARPS, as weknow the orbit ephemeris and can strategically plan re-peated observations at quadrature points to mitigate theexpected jitter. K2-231 b would then become the firstplanet with a measured mass and density in an opencluster. http://exoplanets.org/detail/Kepler-106 c sub-Neptune exoplanet in Ruprecht 147 A. PLANETS DISCOVERED IN OPEN CLUSTERSTable A1 lists the 23 planets and three candidates that have been discovered to date in open clusters. We list KICor EPIC IDs when available, whether the planet was discovered via transit or RV techniques (no cluster exoplanet hasyet been characterized with both techniques), the V magnitude and type of host, the orbital period, the planetaryradius or mass ( m sin i ), citations, and additional notes (e.g., “HJ,” referring to hot Jupiter). We assembled this listto determine how many planets are currently known in clusters, then decided that it might be of use and interest tothe reader, so we provide it here. After we submitted this manuscript, David et al. (2018) presented a list of “knownand proposed exoplanets in sub-Gyr populations detected via the transit or radial velocity method.” Their Table 1overlaps considerably with our table due to the known cluster planets mostly being found in Hyades and Praesepe. Byconstruction, their list does not include the NGC 6811 or M67 planets (and the R147 planet, since we are announcingit now), and we do not list planets found in young associations.A. Vanderburg produced the light-curve used in thiswork and corroborated the initial discovery; he is sup-ported by the NASA Sagan Fellowship. G. Torresperformed the BLENDER false-alarm analysis, and ac-knowledges partial support for this work from NASAgrant NNX14AB83G (
Kepler
Participating ScientistProgram). A.W. Howard led the acquisition of HIRESspectra of the planet host and the faint neighbor.H. Isaacson measured stellar RVs for those targets andchecked the spectra for secondary light. D. Huber pro-vided the access to Keck/HIRES needed to acquire thosespectra and also performed the isoclassify analysis;he acknowledges support by the National Aeronauticsand Space Administration under grant NNX14AB92Gissued through the
Kepler
Participating Scientist Pro-gram. A.L. Kraus, A.C. Rizzuto, and A.W. Mannacquired and analyzed the Keck adaptive optics data.A.W. Mann fit the light-curve to measure the tran-sit properties and contributed Figure 2. A.L. Krausobtained the UKIRT/WFCAM imaging. B.J. Fultoncontributed Figure 7. C. Henze ran the
BLENDER jobson the Pleiades supercomputer and pre-processed theoutput. J.T. Wright advised the Ph.D. dissertationwork of J.L. Curtis, which amassed much of the basicdata presented herein (e.g., proprietary photometry andspectroscopy), and is the submitting and administrativePI of the K2 program GO 7035.The remainder of the work was completed by J.L. Cur-tis, including the planet discovery, host star charac-terization, preliminary transit fitting, and synthesis ofthe data and contributions provided by the coauthors.He successfully led a petition for Campaign 7 to pointat Ruprecht 147 and, as science PI of GO 7035, wasawarded the program to survey the cluster while a grad-uate student at Penn State University and member ofthe Center for Exoplanets and Habitable Worlds. Hewas granted access to Magellan while serving as an SAO predoctoral fellow at the Harvard–Smithsonian Centerfor Astrophysics. The planet discovery and characteri-zation work was performed after joining Columbia Uni-versity.J.L. Curtis is supported by the National Science Foun-dation Astronomy and Astrophysics Postdoctoral Fel-lowship under award AST-1602662 and the NationalAeronautics and Space Administration under grantNNX16AE64G issued through the K2 Guest ObserverProgram (GO 7035). He thanks the referee for theirfeedback, Jason T. Wright and Marcel Ag¨ueros for serv-ing as his mentors, Luca Malavolta for commenting ona draft of this manuscript and for providing early ac-cess to the HARPS RVs acquired by the Minniti team,Iv´an Ram´ırez and Luca Casagrande for providing theirtemperature measurements, Fabienne Bastien and Ja-cob Luhn for commenting on a draft of this manuscriptand discussing RV jitter, the Harvard–Smithsonian Cen-ter for Astrophysics telescope allocation committee forgranting access to Magellan, the K2 Guest Observeroffice and Ball Aerospace for re-positioning the Cam-paign 7 field to accommodate Ruprecht 147, and thecoinvestigators of the “K2 Survey of Ruprecht 147”(GO 7035): Jason T. Wright, Fabienne Bastien, SørenMeibom, Victor Silva Aguirre, and Steve Saar.The Center for Exoplanets and Habitable Worlds issupported by the Pennsylvania State University, theEberly College of Science, and the Pennsylvania SpaceGrant Consortium.This paper includes data collected by the K2 mission.Funding for the Kepler and K2 missions is provided bythe NASA Science Mission directorate. We obtainedthese data from the Mikulski Archive for Space Tele-scopes (MAST). STScI is operated by the Associationof Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555. Support for MAST fornon-HST data is provided by the NASA Office of Space0 Curtis et al.
Table A1.
Planets in Clusters
Planet KIC/EPIC Discovery V Period Radius / Host Notes CitationsID ID Method (mag) (days) M sin i Info.
Pleiades (130 Myr): · · · C4 · · · · · · · · · · · · · · · None found 6
Hyades (650 Myr): (cid:15)
Tau b 210754593 RV 3.53 594.9 7.6 M
Jup (cid:12)
Giant 1st ever 19HD 285507 b 210495452 RV 10.47 6.09 0.917 M
Jup
K4.5 Eccentric HJ 18K2-25 b 210490365 Tr 15.88 3.485 3.43 R ⊕ M4.5 · · ·
5, 10K2-136-A b 247589423 Tr 11.20 7.98 0.99 R ⊕ K5.5 Stellar binary 4, 12K2-136-A c 247589423 Tr 11.20 17.31 2.91 R ⊕ K5.5 Stellar binary 4, 12K2-136-A d 247589423 Tr 11.20 25.58 1.45 R ⊕ K5.5 Stellar binary 4, 12HD 283869 b 248045685 Tr 10.60 ∼
106 1.96 R ⊕ K5 Candidate (1 transit) 20
Praesepe (650 Myr):
Pr0201 b 211998346 RV 10.52 4.43 0.54 M
Jup late-F HJ, “two b’s” 17Pr0211 b 211936827 RV 12.15 2.15 1.844 M
Jup late-G HJ, “two b’s” 17Pr0211 c 211936827 RV 12.15 > Jup late-G Eccentric; 1st multi 9K2-95 b 211916756 Tr 17.27 10.14 3.7 R ⊕ (cid:12) · · ·
7, 11, 14, 15K2-100 b 211990866 Tr 10.373 1.67 3.5 R ⊕ (cid:12) · · ·
1, 7, 11, 16K2-101 b 211913977 Tr 12.552 14.68 2.0 R ⊕ (cid:12) · · ·
1, 7, 11, 16K2-102 b 211970147 Tr 12.758 9.92 1.3 R ⊕ (cid:12) · · · ⊕ (cid:12) · · · ⊕ (cid:12) · · ·
7, 11EPIC 211901114 b 211901114 Tr 16.485 1.65 9.6 R ⊕ (cid:12) Candidate 11
NGC 2423 (740 Myr) a :TYC 5409-2156-1 b · · · RV 9.45 714.3 10.6 M
Jup
Giant · · · NGC 6811 (1 Gyr):
Kepler-66 b 9836149 Tr 15.3 17.82 2.80 R ⊕ (cid:12) · · · ⊕ (cid:12) · · · Ruprecht 147 (3 Gyr):
K2-231 b 219800881 Tr 12.71 13.84 2.5 R ⊕ Solar twin · · ·
This work
M67 (4 Gyr) b : YBP 401 b · · ·
RV 13.70 4.087 0.42 M
Jup
F9V HJ 2, 3YBP 1194 b 211411531 RV 14.68 6.960 0.33 M
Jup
G5V HJ 2, 3YBP 1514 b 211416296 RV 14.77 5.118 0.40 M
Jup
G5V HJ 2, 3SAND 364 b 211403356 RV 9.80 121 1.57 M
Jup
K3III · · ·
2, 3SAND 978 b c · · · RV 9.71 511 2.18 M
Jup
K4III Candidate 2, 3
References — (1) Barros et al. (2016); (2) Brucalassi et al. (2014); (3) Brucalassi et al. (2017); (4) Ciardi et al. (2018); (5) David et al. (2016a);(6) Gaidos et al. (2017); (7) Libralato et al. (2016); (8) Lovis & Mayor (2007); (9) Malavolta et al. (2016); (10) Mann et al. (2016a); (11) Mannet al. (2017); (12) Mann et al. (2018); (13) Meibom et al. (2013); (14) Obermeier et al. (2016); (15) Pepper et al. (2017); (16) Pope et al. (2016);(17) Quinn et al. (2012); (18) Quinn et al. (2014); (19) Sato et al. (2007); (20) Vanderburg et al. (2018). a Lovis & Mayor (2007) also announced a substellar object in NGC 4349, but it has a minimum mass of 19.8 M
Jup , greater than the planet–browndwarf boundary at 11.4–14.4 M
Jup , and so we do not include it here. b Nardiello et al. (2016) announced some candidates, which they concluded are likely not members of M67. c Brucalassi et al. (2017) referred to this detection as a planet candidate and stated that YBP 778 and YBP 2018 are also promising candidates. sub-Neptune exoplanet in Ruprecht 147
Gaia , processed bythe
Gaia
Data Processing and Analysis Consortium(DPAC). Funding for the DPAC has been provided by national institutions, in particular the institutionsparticipating in the
Gaia
Multilateral Agreement.This publication makes use of data products fromthe Wide-field Infrared Survey Explorer, which is oper-ated by the Jet Propulsion Laboratory, California Insti-tute of Technology, under contract with or with fundingfrom the National Aeronautics and Space Administra-tion. This research has also made use of NASA’s As-trophysics Data System, and the VizieR and SIMBADdatabases, operated at CDS, Strasbourg, France.This research has made use of the ESO ScienceArchive Facility to access data collected for ESO pro-grammes 091.C-0471(A) and 095.C-0947(A).Some of the data presented herein were obtained atthe W.M. Keck Observatory, which is operated as ascientific partnership among the California Institute ofTechnology, the University of California, and the Na-tional Aeronautics and Space Administration. The ob-servatory was made possible by the generous financialsupport of the W.M. Keck Foundation. We wish to rec-ognize and acknowledge the very significant cultural roleand reverence that the summit of Maunakea has alwayshad within the indigenous Hawai‘ian community. Weare most fortunate to have the opportunity to conductobservations from this mountain.This work also utilized SOLIS data obtained by theNSO Integrated Synoptic Program (NISP), managed bythe National Solar Observatory, which is operated by theAssociation of Universities for Research in Astronomy(AURA), Inc., under a cooperative agreement with theNational Science Foundation.
Facilities:
Kepler (K2), CFHT (MegaCam), ESO:3.6m(HARPS), Keck:I (HIRES), Keck:II (NIRC2), Magel-lan:Clay (MIKE), MMT (Hectochelle), Shane (Hamil-ton), SOLIS (ISS), UKIRT (WFCAM)
Software:
BARYCORR (Wright & Eastman 2014),batman (Kreidberg 2015), EXOFAST (Eastman et al.2013), forecaster (Chen & Kipping 2017), isochrones (Mor-ton 2015), isoclassify (Huber et al. 2017), RVLIN (Wright& Howard 2009), SME (Valenti & Piskunov 1996)REFERENCES
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