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      Mass determination of the 1:3:5 near-resonant planets transiting GJ 9827 (K2-135)

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          Abstract

          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 (d = \(30.32\pm1.62\) pc) detected by Kepler and K2 . Its brightness (V = 10.35 mag) makes the star an ideal target for detailed studies of the properties of its planets. Results. We find that GJ 9827 b has a mass of \(M_\mathrm{b}=3.74^{+0.50}_{-0.48}\) \(M_\oplus\) and a radius of \(R_\mathrm{b}=1.62^{+0.17}_{-0.16}\) \(R_\oplus\), yielding a mean density of \(\rho_\mathrm{b} = 4.81^{+1.97}_{-1.33}\) g cm\(^{-3}\). GJ 9827 c has a mass of \(M_\mathrm{c}=1.47^{+0.59}_{-0.58}\) \(M_\oplus\), radius of \(R_\mathrm{c}=1.27^{+0.13}_{-0.13}\) \(R_\oplus\), and a mean density of \(\rho_\mathrm{c}= 3.87^{+2.38}_{-1.71}\) g cm\(^{-3}\). For GJ 9827 d we derive \(M_\mathrm{d}=2.38^{+0.71}_{-0.69}\) \(M_\oplus\), \(R_\mathrm{d}=2.09^{+0.22}_{-0.21}\) \(R_\oplus\), and \(\rho_\mathrm{d}= 1.42^{+0.75}_{-0.52}\) g cm\(^{-3}\). 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 mini-Neptunes. We also find that the planetary bulk compositions are compatible with a scenario where all three planets formed with similar core/atmosphere compositions, and we speculate that while GJ 9827 b and GJ 9827 c lost their atmospheric envelopes, GJ 9827 d maintained its 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.

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          emcee: The MCMC Hammer

          We introduce a stable, well tested Python implementation of the affine-invariant ensemble sampler for Markov chain Monte Carlo (MCMC) proposed by Goodman & Weare (2010). The code is open source and has already been used in several published projects in the astrophysics literature. The algorithm behind emcee has several advantages over traditional MCMC sampling methods and it has excellent performance as measured by the autocorrelation time (or function calls per independent sample). One major advantage of the algorithm is that it requires hand-tuning of only 1 or 2 parameters compared to \(\sim N^2\) for a traditional algorithm in an N-dimensional parameter space. In this document, we describe the algorithm and the details of our implementation and API. Exploiting the parallelism of the ensemble method, emcee permits any user to take advantage of multiple CPU cores without extra effort. The code is available online at http://dan.iel.fm/emcee under the MIT License.
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            Validation of the new Hipparcos reduction

            Context.A new reduction of the astrometric data as produced by the Hipparcos mission has been published, claiming accuracies for nearly all stars brighter than magnitude Hp = 8 to be better, by up to a factor 4, than in the original catalogue. Aims.The new Hipparcos astrometric catalogue is checked for the quality of the data and the consistency of the formal errors as well as the possible presence of error correlations. The differences with the earlier publication are explained. Methods. The internal errors are followed through the reduction process, and the external errors are investigated on the basis of a comparison with radio observations of a small selection of stars, and the distribution of negative parallaxes. Error correlation levels are investigated and the reduction by more than a factor 10 as obtained in the new catalogue is explained. Results.The formal errors on the parallaxes for the new catalogue are confirmed. The presence of a small amount of additional noise, though unlikely, cannot be ruled out. Conclusions. The new reduction of the Hipparcos astrometric data provides an improvement by a factor 2.2 in the total weight compared to the catalogue published in 1997, and provides much improved data for a wide range of studies on stellar luminosities and local galactic kinematics.
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              Disk Surface Density Transitions as Protoplanet Traps

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                Author and article information

                Journal
                26 February 2018
                Article
                1802.09557
                240204e3-f44f-4757-8dc6-efd8e69c218d

                http://arxiv.org/licenses/nonexclusive-distrib/1.0/

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                Custom metadata
                15 pages, 10 Figures, 8 tables
                astro-ph.EP

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