Two planetary systems with transiting Earth-size and super-Earth planets orbiting late-type dwarf stars
E. Diez Alonso, J.I. Gonzalez Hernandez, S.L. Suarez Gomez, D.S. Aguado, C. Gonzalez Gutierrez, A. Suarez Mascareno, A. Cabrera-Lavers, J. Gonzalez-Nuevo, B. Toledo Padron, J. Gracia, F.J. de Cos Juez, R. Rebolo
aa r X i v : . [ a s t r o - ph . E P ] J un MNRAS , 1–6 (2018) Preprint 6 June 2018 Compiled using MNRAS L A TEX style file v3.0
Two planetary systems with transiting Earth-size andsuper-Earth planets orbiting late-type dwarf stars
E. D´ıez Alonso , J. I. Gonz´alez Hern´andez , , S. L. Su´arez G´omez , D. S. Aguado ,C. Gonz´alez Guti´errez , A. Su´arez Mascare˜no , A. Cabrera-Lavers , ,J. Gonz´alez-Nuevo , B. Toledo–Padr´on , , J. Gracia , F. J. de Cos Juez ⋆ , R. Rebolo , , Department of Exploitation and Exploration of Mines, University of Oviedo, Oviedo, Spain Instituto de Astrof´ısica de Canarias, E–38205 La Laguna, Tenerife, Spain Universidad de La Laguna, Dpto. Astrof´ısica, E–38206 La Laguna, Tenerife, Spain Departamento de F´ısica, Universidad de Oviedo, C. Federico Garc´ıa Lorca 18, E-33007, Oviedo, Spain Observatoire Astronomique de l ' Universit´e de G`eneve, 1290 Versoix, Switzerland GRANTECAN, Cuesta de San Jos´e s/n, E-38712, Bre˜na Baja, La Palma, Spain Department of Construction and Manufacturing Engineering, University of Oviedo, Oviedo, Spain Consejo Superior de Investigaciones Cient´ıficas, Spain
Accepted 2018 June 04. Received 2018 April 25
ABSTRACT
We present two new planetary systems found around cool dwarf stars with datafrom the K2 mission. The first system was found in K2-239 (EPIC 248545986), charac-terized in this work as M3.0V and observed in the th campaign of K2. It consists ofthree Earth-size transiting planets with radii of 1.1, 1.0 and 1.1 R ⊕ , showing a compactconfiguration with orbital periods of 5.24, 7.78 and 10.1 days, close to 2:3:4 resonance.The second was found in K2-240 (EPIC 249801827), characterized in this work asM0.5V and observed in the th campaign. It consists of two transiting super-Earthswith radii 2.0 and 1.8 R ⊕ and orbital periods of 6.03 and 20.5 days. The equilibriumtemperatures of the atmospheres of these planets are estimated to be in the range of380-600 K and the amplitudes of signals in transmission spectroscopy are estimatedat ∼
10 ppm.
Key words: planets and satellites: detection – techniques: photometric – techniques:spectroscopic – stars: low mass – stars: individual: K2-239, K2-240
Low-mass stars are primary targets in the search for Earth-size planets and in the study of their properties. Low-massstars (0.1 M ⊙ < M < M ⊙ ) account for 70% of the stellarpopulation in the Milky Way (Henry et al. 1994), meaningthey have a hugely significant impact in the overall statis-tics of planets in the Galaxy. Exoplanets with close-in orbitstend to be terrestrial when the mass of the star decreases(Howard et al. 2012), with an average of ∼ P orb <
50 days around each low-mass star(Dressing & Charbonneau 2015).Transiting Earth-size planets induce deeper dimmingsin the light-curve of low mass stars and stronger radial ve-locity signals than in more massive stars. Temperate plan- ⋆ E-mail: [email protected] ets orbit closer and have shorter orbital periods, so itis easier to detect planets in the habitable zone (orbitalrange in which a planet’s atmosphere can warm the sur-face to allow surface liquid water) (Anglada-Escud´e et al.2016; Gillon et al. 2017). Signals in transit transmissionspectroscopy (Charbonneau et al. 2002) are also stronger forstars with a small radius, so planets orbiting near bright low-mass stars are also suitable for atmospheric characterization(Kreidberg et al. 2014).Detecting transiting planetary systems is of great valuein terms of estimating the mass and density of their plan-ets measuring transit timing variations (Gillon et al. 2017),which are stronger for compact systems in resonances. Thesesystems are also suitable for testing the formation scenariosfrom the study of resonances that could be the result of mi-grations (Papaloizou & Szuszkiewicz 2005).Until now, the Kepler mission (Borucki et al. 2010) has © E.D. Alonso et al. been the most successful facility detecting exoplanets by thetransit method. Since the beginning of 2014, Kepler has beenon its second mission (K2) (Howell et al. 2014), monitoringdifferent fields near the ecliptic plane for ∼
80 days. K2 hasfound many exoplanet candidates (Vanderburg et al. 2015;Crossfield et al. 2016; D´ıez Alonso et al. 2018; Hirano et al.2018) in each observation campaign.Campaign 14 was conducted between May 31st and Au-gust th α =10:42:44, δ =+06:51:06). Campaign 15ran between August 23rd and November th α =15:34:28, δ =-20:04:44).In this study we present the detection of two plane-tary system during these campaings. The first consists ofthree Earth-size transiting planets orbiting K2-239 (EPIC248545986) ( α =10:42:22.633, δ =+04:26:28.86), observed inlong cadence mode during campaign 14. The second con-sists of two transiting super-Earths orbiting K2-240 (EPIC249801827 ( α =15:11:23.907, δ =-17:52:30.78), observed inlong cadence mode during campaign 15. On March th λ / δλ ∼ ) ineach of the BVRI bands were reduced in the standard man-ner, flux calibrated, telluric corrected, and finally combinedinto a single spectrum (see Fig. 1).The spectrum was compared with SDSS/BOSS ref-erence spectra of M-type stars from Kesseli et al. (2017).The comparison was made with the HAMMER code(Covey et al. 2007), obtaining the best fit for a M3V starwith [ Fe / H ] ∼ . The relative intensity of the NaI lines at5890 and 8180 ˚A rule out the possibility of the star being gi-ant, while the relative depth of the strong molecular bands ofTiO at 7000-7300 ˚A points to [ Fe / H ] ∼ . Maldonado et al.(2015), working with measurements of spectral index fromHARPS spectra, conclude T eff ∼ ±
50 K for M3V stars,which is in agreement with our estimates of the stellar pa-rameters. Figure 1 plots our comparison of the OSIRIS spec-trum of K2-239 with reference spectra from M2.0V to M4.0Vstars.We computed the stellar parameters from J,H,V,Kmagnitudes listed in Table 1, applying the empirical re-lationships established by Mann et al. (2013, 2015) andPecaut & Mamajek (2013), using the tabulated stellar pa-rameters from Pecaut & Mamajek (2013) and the Mass- Luminosity relation for Main-sequence M dwarfs fromBenedict et al. (2016). All the parameters are listed in Ta-ble 1.Taking m V = . ± . (Table 1) and M V = . ± . from Pecaut & Mamajek (2013) tabulated parameters,we estimate a distance to K2-239 of ± pc.We measured a radial velocity from the OSIRIS spec- Figure 1.
OSIRIS spectrum of K2-239 compared with referencespectra of M2.0V - M4.0V stars. Best fit is obtained for M3V starwith [ Fe / H ] ∼ . All spectra are normalized at λ = 7575 ˚A. trum v r = − . ± . kms − , which combined with the es-timated distance and the proper motions µ α = − . ± . [mas/yr] and µ δ = . ± . [mas/yr], results in the velocitycomponents listed in Table 1. From the probability distri-butions of Reddy et al. (2006), we derive that K2-239 is amember of the Galactic thin disk. K2-240 has been observed by the Radial Velocity Ex-periment (RAVE) (Steinmetz et al. 2006). RAVE’s DR5(Kunder et al. 2017) presents data from medium-resolutionspectra (R ∼ T eff = ± K and log g = . ± . ,confirming that K2-240 is a cool dwarf star.We repeated exactly the same analysis followed for K2-239 to derive the stellar parameters accurately, obtainingthe parameters listed in Table 1. These parameters are con-sistent with K2-240 being a M0.5V star.We also note that a very clear rotation signal is presentin the light curve from K2. From a Lomb-Scargle (Scargle1982) analysis we estimate P rot = . ± . d.From RAVE’s radial velocity v r = . ± . kms − ,our estimated distance of d = ± pc, and proper motions µ α = − . ± . [mas/yr] and µ δ = − . ± . [mas/yr],we compute velocity components listed in Table 1. From theprobability distributions of Reddy et al. (2006), we derivethat K2-240 is a member of the Galactic thin disk. We followed the work of Vanderburg & Johnson (2014) toanalyze the K2 corrected photometry of our target stars,detrending stellar variability with a spline fit and search-ing for periodic signals using the Box Least Squares (BLS)method (Kov´acs et al. 2002) on attained data. This anal-ysis shows three transit signals with periods 5.240 ± ± ± ± ± MNRAS000
OSIRIS spectrum of K2-239 compared with referencespectra of M2.0V - M4.0V stars. Best fit is obtained for M3V starwith [ Fe / H ] ∼ . All spectra are normalized at λ = 7575 ˚A. trum v r = − . ± . kms − , which combined with the es-timated distance and the proper motions µ α = − . ± . [mas/yr] and µ δ = . ± . [mas/yr], results in the velocitycomponents listed in Table 1. From the probability distri-butions of Reddy et al. (2006), we derive that K2-239 is amember of the Galactic thin disk. K2-240 has been observed by the Radial Velocity Ex-periment (RAVE) (Steinmetz et al. 2006). RAVE’s DR5(Kunder et al. 2017) presents data from medium-resolutionspectra (R ∼ T eff = ± K and log g = . ± . ,confirming that K2-240 is a cool dwarf star.We repeated exactly the same analysis followed for K2-239 to derive the stellar parameters accurately, obtainingthe parameters listed in Table 1. These parameters are con-sistent with K2-240 being a M0.5V star.We also note that a very clear rotation signal is presentin the light curve from K2. From a Lomb-Scargle (Scargle1982) analysis we estimate P rot = . ± . d.From RAVE’s radial velocity v r = . ± . kms − ,our estimated distance of d = ± pc, and proper motions µ α = − . ± . [mas/yr] and µ δ = − . ± . [mas/yr],we compute velocity components listed in Table 1. From theprobability distributions of Reddy et al. (2006), we derivethat K2-240 is a member of the Galactic thin disk. We followed the work of Vanderburg & Johnson (2014) toanalyze the K2 corrected photometry of our target stars,detrending stellar variability with a spline fit and search-ing for periodic signals using the Box Least Squares (BLS)method (Kov´acs et al. 2002) on attained data. This anal-ysis shows three transit signals with periods 5.240 ± ± ± ± ± MNRAS000 , 1–6 (2018) wo planetary systems with transiting Earth-size and super-Earth planets orbiting late-type dwarf stars Figure 2.
K2 detrended (top) and normalized (bottom) light curves for K2-239 (left) and K2-240 (right). Characters b, c and d showtimes of observed transits for planets in each system.
Table 1.
Stellar parameters for K2-239 and K2-240
Parameter K2-239 K2-240 SourceV [mag] . ± .
040 13 . ± . (1)R [mag] . ± .
020 12 . ± . (1)I [mag] . ± .
030 11 . ± . (1)J [mag] . ± .
026 10 . ± . (2)H [mag] . ± .
021 9 . ± . (2)K [mag] . ± .
021 9 . ± . (2) T eff [K] ±
18 3810 ± (3) [ Fe / H ] − . ± . − . ± . (3)Radius [ R ⊙ ] . ± .
01 0 . ± . (3)Mass [ M ⊙ ] . ± .
01 0 . ± . (3)Luminosity [ L ⊙ ] . ± .
001 0 . ± . (3) log g [cgs] . ± . . ± . (3) P rot [d] – . ± . (3)Distance [pc] ± ± (3) µ α [mas/yr] − . ± . − . ± . (1) µ δ [mas/yr] . ± . − . ± . (1)U, V, W [km/s] -6.8, 4.2, -10.2 -5.4, -23.6, -1.7 (3)(1) UCAC4 (Zacharias et al. 2013).(2) 2MASS (Cutri et al. 2003).(3) This work. We performed MCMC analysis on each phase-foldedtransit (Figs. 3 and 4) to estimate the planetary param-eters, fitting models from Mandel & Agol (2002) with theExofast package (Eastman et al. 2013). For each data point,the light curve was resampled 10 times uniformly spacedover the 29.5-minute long cadence of K2 and averaged, fol-lowing Kipping (2010). For the calculations we set the valuesof T eff , log g , and [ Fe / H ] listed in Table 1, and orbital periodslisted above. We also worked with the assumption of eccen-tricity e = , valid for transiting planets in a multi-planetarysystem (Van Eylen & Albrecht 2015).The planets in the K2-239 system have estimated radii . ± . R ⊕ (b), . ± . R ⊕ (c) and . ± . R ⊕ (d), orbitalperiods of 5.242 ± ± ± . ± . AU (b), . ± . AU (c) and . ± . AU (d).The planets in the K2-240 system have estimatedradii . + . − . R ⊕ (b) and . + . − . R ⊕ (c), orbital periods of6.034 ± ± . ± . AU (b), . ± . AU (c).Table 2 summarizes all the parameters obtained for the plan-ets.
Figure 3.
Phase-folded light curves corresponding to planets b(top), c (middle), and d (bottom) in the K2-239 system. Solidcurves represent best model fits obtained by MCMC.
We acquired images of K2-239 with the OSIRIS camera-spectrograph on March th δ mag < δ mag < MNRAS , 1–6 (2018)
E.D. Alonso et al.
Figure 4.
Phase-folded light curves corresponding to planets b(top) and c (bottom) in the K2-240 system. Solid curves representbest model fits obtained by MCMC.
At ExoFOP–K2 an AO image of K2-240 is available,acquired with the NIRC2 instrument at the 10 m Keck 2 tele-scope (Maunakea, Hawaii). The image excludes companionsat 0.2 arc seconds with δ mag < δ mag < Assuming the planet radii listed in Table 2, and themean density for planets satisfying R p ≤ . R ⊕ fromWeiss & Marcy (2014), we obtain M b = . ± . M ⊕ , M c = . ± . M ⊕ , M d = . ± . M ⊕ for planets b, c, and d, respec-tively in the K2-239 system. Adopting M p ≪ M ∗ , circularorbits and sin i ∼
1, we computed induced semi-amplitudes instellar velocity variations of 0.9 ms − for planet b, 0.5 ms − for planet c and 0.7 ms − for planet d, well-suited for radialvelocity monitoring with ultra-stable spectrographs suchas ESPRESSO (Pepe et al. 2014; Gonz´alez Hern´andez et al.2017) at the VLT.The amplitude of the signal in transit transmission spec-troscopy can be estimated as R p · h eff ( R ∗ ) (Gillon et al. 2016)with h eff the effective atmospheric height. h eff is relatedto the atmospheric scale height H = K · T/ µ · g (K Boltz-mann’s constant, T atmospheric temperature, µ atmosphericmean molecular mass, g surface gravity). Assuming h eff =7 · H (Miller-Ricci & Fortney 2010) for a transparent volatiledominated atmosphere ( µ = 20) with 0.3 Bond albedo, wefound amplitudes in transit transmission spectroscopy of1.2 · − (b), 1.1 · − (c) and − (d).We used the Mercury package (Chambers 1999) to sim-ulate and test the evolution and stability of the system for years. We simulated using Bulirsch – Stoer integrator, https://exofop.ipac.caltech.edu/k2/ Figure 5.
Top panel: OSIRIS/GTC image taken of K2-239 fieldwith seeing 0.6 arc seconds in the i-band Sloan filter, superim-posed on the 2MASS image of the field. Bottom panel: contrastcurve and AO image of K2-240 acquired with the NIRC2 instru-ment at the 10 m Keck-2 telescope. adopting circular orbits and masses from the mass-radius re-lation. We do not find significant changes in the eccentricityor in the inclination of the orbits, showing a dynamicallystable system.To estimate the masses for the planets of theK2-240 system we used the mass-radius relation fromWeiss & Marcy (2014) for planets satisfying . ≤ R p / R ⊕ ≤ , obtaining M b = . + . − . M ⊕ , M c = . + . − . M ⊕ . Under theassumption of M p ≪ M ∗ , circular orbits and sin i ∼
1, wecomputed induced semi-amplitudes in stellar velocity vari-ations of 2.5 ms − for planet b and 1.5 ms − for planet c.With the same assumptions as in the previous section, weestimate amplitudes in transit transmission spectroscopy of1.2 · − (b) and 6.6 · − (c).We also tested the stability of K2-240 system with theMercury package as described in the previous section. Againour simulations point towards a dynamically stable system.The planetary systems presented in this work, withequilibrium temperatures estimated in the range 380-600 K,are suitable targets for incoming facilities; Plato, monitoringin shorter cadence mode, could reveal transit timing varia-tions that allow accurate planetary masses to be estimated.The James Webb Telescope could find signs of planetary at-mospheres. Ultra-stable spectrographs such as ESPRESSOat VLT, could also carry out radial velocity follow-up, sothese are promising targets to improve our understanding ofcompact Earth-sized planetary systems (K2-239) and super- MNRAS , 1–6 (2018) wo planetary systems with transiting Earth-size and super-Earth planets orbiting late-type dwarf stars Table 2.
Parameters for planets in the K2-239 and K2-240 systems.
Planet Parameters K2-239 b K2-239 c K2-239 d K2-240 b K2-240 cOrbital period (P) [d] . ± .
001 7 . ± .
001 10 . ± .
001 6 . ± .
001 20 . ± . Semi-major axis (a) [AU] . ± . . ± . . ± . . ± . . ± . Radius ( R p ) [ R ⊕ ] . ± . . ± . . ± . . + . − . . + . − . Mass ( M p ) [ M ⊕ ] . ± . . ± . . ± . . + . − . . + . − . Equilibrium Temperature ( T eq ) [K] + − + − + − + − + − Transit Parameters K2-239 b K2-239 c K2-239 d K2-240 b K2-240 cEpoch (BKJD) [days] 3075.191 3083.860 3075.381 3163.825 3172.722Radius of planet in stellar radii ( R p / R ∗ ) . + . − . . + . − . . ± . . ± . . ± . Semi major axis in stellar radii (a/ R ∗ ) . + . − . . + . − . . + . − . . + . − . . ± . Linear limb-darkening coeff ( u ) . + . − . . + . − . . ± .
056 0 . + . − . . + . − . Quadratic limb-darkening coeff ( u ) . ± .
053 0 . + . − . . + . − . . + . − . . + . − . Inclination (i) [deg] . + . − . . + . − . . + . − . . + . − . . + . − . Impact Parameter (b) . + . − . . + . − . . ± .
25 0 . + . − . . + . − . Transit depth ( δ ) . + . − . . + . − . . ± . . ± . . + . − . Total duration ( T ) [d] . + . − . . + . − . . + . − . . + . − . . + . − . Earth systems on the rocky-gaseous boundary (EPIC K2-240).
ACKNOWLEDGEMENTS
EDA, CGG and JCJ acknowledge Spanish ministry projectAYA2017-89121-P. JIGH, BTP, DSA and RRL acknowledgethe Spanish ministry project MINECO AYA2014- 56359-P.JIGH also acknowledges financial support from the Span-ish Ministry of Economy and Competitiveness (MINECO)under the 2013 Ram´on y Cajal program MINECO RYC-2013-14875. ASM acknowledges financial support from theSwiss National Science Foundation (SNSF). JGN and SLSGacknowledge financial support from the I+D 2015 projectAYA2015- 65887-P (MINECO/FEDER). JGN also acknowl-edges financial support from the Spanish MINECO for aRam´on y Cajal fellowship (RYC-2013-13256).Based on observations made with the Gran TelescopioCanarias (GTC), installed in the Spanish Observatorio delRoque de los Muchachos of the Instituto de Astrof´ısica deCanarias, in the island of La Palma.
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