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Kepler-21b: A rocky planet around a V=8.25 magnitude star

M. Lůpez-Morales, R. D. Haywood, J. L. Coughlin, L. Zeng, L. A. Buchhave, H. A. C. Giles, L. Affer, A. S. Bonomo, D. Charbonneau, A. C. Cameron, R. Cosentino, C. D. Dressing, X. Dumusque, P. Figueira, A. F. M. Fiorenzano, A. Harutyunyan, J. Johnson, D. W. Latham, E. Lopez, C. Lovis, L. Malavolta, M. Mayor, G. Micela, E. Molinari, A. Mortier, F. Motalebi, V. Nascimbeni, F. Pepe, D. F. Phillips, G. Piotto, D. Pollacco, D. Queloz, K. Rice, D. Sasselov, D. Segransan, A. Sozzetti, S. Udry, A. Vanderburg, C. A. Watson

Abstract
HD 179070, aka Kepler-21, is a V = 8.25 F6IV star and the brightest exoplanet host discovered by Kepler. An early detailed analysis by Howell et al. of the first 13 months (Q0–Q5) of Kepler light curves revealed transits of a planetary companion, Kepler-21b, with a radius of about 1.60 ± 0.04 R and an orbital period of about 2.7857 days. However, they could not determine the mass of the planet from the initial radial velocity (RV) observations with Keck-HIRES, and were only able to impose a 2σ upper limit of 10 M . Here, we present results from the analysis of 82 new RV observations of this system obtained with HARPS-N, together with the existing 14 HIRES data points. We detect the Doppler signal of Kepler-21b with a RV semiamplitude K = 2.00 ± 0.65 m s-1, which corresponds to a planetary mass of 5.1 ± 1.7 M. We also measure an improved radius for the planet of 1.639 +0.019/−0.015 R, in agreement with the radius reported by Howell et al. We conclude that Kepler-21b, with a density of 6.4 ± 2.1 g cm-3, belongs to the population of small, ≲6 M planets with iron and magnesium silicate interiors, which have lost the majority of their envelope volatiles via stellar winds or gravitational escape. The RV analysis presented in this paper serves as an example of the type of analysis that will be necessary to confirm the masses of TESS small planet candidates.

Keywords
planets and satellites: formation, planets and satellites: individual: Kepler-21b, stars: individual: HD 179070, techniques: photometric, techniques: radial velocities, techniques: spectroscopic

The Astronomical Journal
Volume 152, Number 6
2016 December

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Instituto de Astrof√≠sica e Ci√™ncias do Espa√ßo Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia