A&A 618, A116 (2018) https://doi.org/10.1051/0004-6361/201832872 Astronomy & © ESO 2018 Astrophysics Mass determination of the 1:3:5 near-resonant planets transiting GJ 9827 (K2-135)? J. Prieto-Arranz1,2, E. Palle1,2, D. Gandolfi3, O. Barragán3, E. W. Guenther1,4, F. Dai5,6, M. Fridlund7,8, T. Hirano9, J. Livingston10, R. Luque1,2, P. Niraula11, C. M. Persson 8, S. Redfield11, S. Albrecht12, R. Alonso1,2, G. Antoniciello3, J. Cabrera13, W. D. Cochran14, Sz. Csizmadia13, H. Deeg1,2, Ph. Eigmüller13, M. Endl14, A. Erikson13, M. E. Everett15, A. Fukui16, S. Grziwa17, A. P. Hatzes 4, D. Hidalgo1,2, M. Hjorth12, J. Korth17, D. Lorenzo-Oliveira18, F. Murgas1,2, N. Narita10,19,20, D. Nespral 1,2, G. Nowak1,2, M. Pätzold17, P. Montañez Rodríguez1,2, H. Rauer13,21, I. Ribas22,23, A. M. S. Smith13, T. Trifonov24, V. Van Eylen7, and J. N. Winn5 (Affiliations can be found after the references) Received 22 February 2018 / Accepted 14 July 2018 ABSTRACT Context. Multiplanet systems are excellent laboratories to test planet formation models as all planets are formed under the same initial conditions. In this context, systems transiting bright stars can play a key role, since planetary masses, radii, and bulk densities can be measured. Aims. GJ 9827 (K2-135) has recently been found to host a tightly packed system consisting of three transiting small planets whose orbital periods of 1.2, 3.6, and 6.2 days are near the 1:3:5 ratio. GJ 9827 hosts the nearest planetary system (∼30 pc) detected by NASA’s Kepler or K2 space mission. Its brightness (V = 10.35 mag) makes the star an ideal target for detailed studies of the properties of its planets. Methods. Combining the K2 photometry with high-precision radial-velocity measurements gathered with the FIES, HARPS, and HARPS-N spectrographs we revised the system parameters and derive the masses of the three planets. +0:48 +0:14 Results. We find that GJ 9827 b has a mass of Mb = 3:69−0:46 M⊕ and a radius of Rb = 1:58−0:13 R⊕ , yielding a mean density of ρb = +1:74 −3 +0:58 +0:11 +2:31 −3 5:11−1:27 g cm . GJ 9827 c has a mass of Mc = 1:45−0:57 M⊕ , radius of Rc = 1:24−0:11 R⊕ , and a mean density of ρc = 4:13−1:77 g cm . +0:58 +0:11 +0:71 −3 For GJ 9827 d, we derive Md = 1:45−0:57 M⊕ , Rd = 1:24−0:11 R⊕ , and ρd = 1:51−0:53 g cm . Conclusions. GJ 9827 is one of the few known transiting planetary systems for which the masses of all planets have been determined with a precision better than 30%. This system is particularly interesting because all three planets are close to the limit between super- Earths and sub-Neptunes. The planetary bulk compositions are compatible with a scenario where all three planets formed with similar core and atmosphere compositions, and we speculate that while GJ 9827 b and GJ 9827 c lost their atmospheric envelopes, GJ 9827 d maintained its primordial atmosphere, owing to the much lower stellar irradiation. This makes GJ 9827 one of the very few systems where the dynamical evolution and the atmospheric escape can be studied in detail for all planets, helping us to understand how compact systems form and evolve. Key words. planetary systems – techniques: high angular resolution – techniques: photometric – techniques: radial velocities – stars: abundances – stars: individual: GJ 9827 1. Introduction mechanisms, since all planets have formed under the same ini- tial conditions. The short orbital period increases the geometric Systems containing multiple planets have drawn much atten- probability to see the planets transiting their host stars, allow- tion because they have frequently been seen as potential solar ing us to measure the planetary radii. The Doppler reflex motion system analogues. However, none of the systems discovered is larger, enabling the mass determination via radial velocity so far resemble ours. The vast majority of multiplanet sys- (RV) measurements using state-of-the-art, high-precision spec- tems identified by the NASA’s Kepler space mission contains trographs. However, although more than 200 systems with three super-Earths (1 ≤ Rp ≤ 2 R⊕) and sub-Neptunes (2 ≤ Rp ≤ 4 R⊕) or more planets have been discovered so far, many questions in tightly packed configurations, with orbits smaller than the remain unanswered. orbit of Mercury (Winn & Fabrycky 2015). How do compact planetary systems form? It has been pro- Compact systems containing planets of different sizes and posed that planets with short orbital periods might have either masses are the best test beds to constrain planetary formation formed in situ (Chiang & Laughlin 2013), or at much larger dis- ? tance from their host star and then moved inwards via type I Based on observations made with (a) the ESO-3.6 m telescope at or type II migration mechanisms (for a review see Baruteau et La Silla Observatory under program ID 099.C-0491 and 0100.C-0808; al. 2014). Once the disk has been dispersed, planets could also (b) the Italian Telescopio Nazionale Galileo operated on the island of La Palma by the Fundación Galileo Galilei of the Istituto Nazionale di migrate through planet–planet scattering (see, e.g., Marzari & Astrofisica; (c) the Nordic Optical Telescope, operated by the Nordic Weidenschilling 2002). Explaining the formation of compact Optical Telescope Scientific Association at the Observatorio del Roque systems with in situ formation is however not easy because a lot de los Muchachos. of material in the inner disk is required in order to form planets. Article published by EDP Sciences A116, page 1 of 15 A&A 618, A116 (2018) Using an in situ formation model, Hansen & Murray(2013) Table 1. Equatorial coordinates, optical and near-infrared magnitude, found that there are roughly 50% more single-planet candidates and stellar parameters of GJ 9827. observed than those produced by any model population. How can we observationally distinguish between different GJ 9827 scenarios? In order to gain insights into the formation of com- pact systems, we have to understand whether the planets formed RAa (J2000.0) 23:27:04.83647 at large distance (e.g., beyond the snow-line), or close-in to Deca (J2000.0) −01:17:10.5816 their host star. It is now well accepted that the composition of Distancea (pc) 29:6864 ± 0:1033 a pre-main sequence disk – where planet formation takes place – V-band magnitudeb (mag) 10:35 ± 0:10 depends on the radial distance from the host star. The chemical J-band magnitudec (mag) 7:984 ± 0:020 abundance of planets can thus be used to trace their formation. Spectral typed K6 V e Thiabaud et al.(2015) showed that the C/O is a good tracer to Effective temperature Teff (K) 4219 ± 70 e assess whether a given planet formed in situ or not. The Mg/Si Surface gravity log g? (cgs) 4:657 ± 0:042 and Fe/Si bulk composition ratios are also interesting tracers. Iron abundancee [Fe/H] (dex) −0:29 ± 0:12 e In this respect, the discovery that the ultra-short period planet Mass M? (M ) 0:637 ± 0:051 e K2-106 b (Guenther et al. 2017) has an iron core containing Radius R? (R ) 0:622 ± 0:051 +20 e −1 80−30% of its mass supports the notion that this planet might Projected rot. velocity v sin i? ( km s ) 1:5 ± 1:0 f −1 have formed in a metal rich environment – typically close to the Microturbulent velocity vmic ( km s ) 0:9 (fixed) g −1 host star, where photophoresis process can separate iron from sil- Macroturbulent velocity vmac ( km s ) 0:5 (fixed) e icates in the early phase of planet formation (Wurm et al. 2013). Interstellar reddening Av (mag) 0:04 ± 0:08 On the contrary, if a close-in planet (a / 0.1 AU) were found to have a high quantity of water, this would imply that the planet Notes. (a)Gaia DR2 (Gaia Collaboration et al. 2018). The quoted uncer- formed beyond the snow-line and then migrated inwards to its tainty takes into account possible Gaia systematics following Luri current position (Raymond et al. 2008; Lopez 2017). et al.(2018). (b)(Mumford 1956). (c)2MASS (Skrutskie et al. 2006). As pointed out by Izidoro et al.(2017), the period ratio distri- (d)Houdebine et al.(2017). (e)This work. ( f )Bruntt et al.(2010). (g)Gray bution of planets in multiplanet systems can also provide some (2008). clues about the formation mechanisms involved. Using N-body simulations together with a model of gaseous disk, Izidoro et al. Gandolfi et al. 2017; Barragán et al. 2017; Dai et al. 2017; (2017) found that only 50–60% of resonant chains became unsta- Guenther et al. 2017). ble whereas to match observations at least 75% (and probably Teske et al.(2018) recently reported on the mass determina- 90–95% according to Kepler results) must be expected. tions of the three planets based on a ∼7-year-long RV monitoring In order to address these questions, a well characterized carried out with the Planet Finder Spectrograph on the Magellan sample of multiplanet systems transiting relatively bright stars II telescope. We compare their results with ours in Sect.7. for which planetary radii, masses, and orbital parameters have been determined with high accuracy is needed. The three 2. Ground based follow-up observations brightest systems known to host three or more planets for which 2.1.