arXiv subjects

Raphaëlle D. Haywood

Publications and source records attributed to Raphaëlle D. Haywood.

21 records · Page 2Linked to original sources

arXiv2016

A 1.9 Earth radius rocky planet and the discovery of a non-transiting planet in the Kepler-20 system

Kepler-20 is a solar-type star (V = 12.5) hosting a compact system of five transiting planets, all packed within the orbital distance of Mercury in our own Solar System. A transition from rocky to gaseous planets with a planetary transition radius of ~1.6 REarth has recently been proposed by several publications in the literature (Rogers 2015;Weiss & Marcy 2014). Kepler-20b (Rp ~ 1.9 REarth) has a size beyond this transition radius, however previous mass measurements were not sufficiently precise to allow definite conclusions to be drawn regarding its composition. We present new mass measurements of three of the planets in the Kepler-20 system facilitated by 104 radial velocity measurements from the HARPS-N spectrograph and 30 archival Keck/HIRES observations, as well as an updated photometric analysis of the Kepler data and an asteroseismic analysis of the host star (MStar = 0.948+-0.051 Msun and Rstar = 0.964+-0.018 Rsun). Kepler-20b is a 1.868+0.066-0.034 REarth planet in a 3.7 day period with a mass of 9.70+1.41-1.44 MEarth resulting in a mean density of 8.2+1.5-1.3 g/cc indicating a rocky composition with an iron to silicate ratio consistent with that of the Earth. This makes Kepler-20b the most massive planet with a rocky composition found to date. Furthermore, we report the discovery of an additional non-transiting planet with a minimum mass of 19.96+3.08-3.61 MEarth and an orbital period of ~34 days in the gap between Kepler-20f (P ~ 11 days) and Kepler-20d (P ~ 78 days).

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arXiv2015

Determining the Mass of Kepler-78b With Nonparametric Gaussian Process Estimation

Kepler-78b is a transiting planet that is 1.2 times the radius of Earth and orbits a young, active K dwarf every 8 hours. The mass of Kepler-78b has been independently reported by two teams based on radial velocity measurements using the HIRES and HARPS-N spectrographs. Due to the active nature of the host star, a stellar activity model is required to distinguish and isolate the planetary signal in radial velocity data. Whereas previous studies tested parametric stellar activity models, we modeled this system using nonparametric Gaussian process (GP) regression. We produced a GP regression of relevant Kepler photometry. We then use the posterior parameter distribution for our photometric fit as a prior for our simultaneous GP + Keplerian orbit models of the radial velocity datasets. We tested three simple kernel functions for our GP regressions. Based on a Bayesian likelihood analysis, we selected a quasi-periodic kernel model with GP hyperparameters coupled between the two RV datasets, giving a Doppler amplitude of 1.86 $\pm$ 0.25 m s$^{-1}$ and supporting our belief that the correlated noise we are modeling is astrophysical. The corresponding mass of 1.87 $^{+0.27}_{-0.26}$ M$_{\oplus}$ is consistent with that measured in previous studies, and more robust due to our nonparametric signal estimation. Based on our mass and the radius measurement from transit photometry, Kepler-78b has a bulk density of 6.0$^{+1.9}_{-1.4}$ g cm$^{-3}$. We estimate that Kepler-78b is 32$\pm$26% iron using a two-component rock-iron model. This is consistent with an Earth-like composition, with uncertainty spanning Moon-like to Mercury-like compositions.

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arXiv2013

An Earth-sized planet with an Earth-like density

Kepler-78 (KIC 8435766) was identified by Sanchis-Ojeda et al. (2013) as harbouring a transiting planet of 1.16 times the size of the Earth and an orbital period of only 8.5 hours. While the exquisite Kepler photometry was able to determine its radius and period, the mass of the planet (and thus its mean density) remained unconstrained in the absence of precise radial-velocity measurements. Here we present an accurate mass measurement of Kepler-78b using the HARPS-N spectrograph, recently installed on the Telescopio Nazionale Galileo (INAF) at the Roque de los Muchachos Observatory, La Palma, Spain. These new data yield a mass of 1.86 Earth masses. The resulting mean density of the planet is 5.57 grams per cubic centimetre, which is similar to that of the Earth and implies a composition of iron and rock. Kepler-78b, which orbits a Sun-like star called Kepler 78 located in the Cygnus constellation at a distance of about 400 light years from us, is now the smallest exoplanet for which both the mass and radius are known accurately.

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