Abstract
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 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 . 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 , which corresponds to a planetary mass of 5.1 ± 1.7 . We also measure an improved radius for the planet of 1.639 +0.019/−0.015 , in agreement with the radius reported by Howell et al. We conclude that Kepler-21b, with a density of 6.4 ± 2.1 , belongs to the population of small, ≲6 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.
Highlights
Results from NASA’s Kepler Satellite Mission have revealed an abundance of planets smaller than 2 RÅ with orbital periods less than 100 days (Howard et al 2012; Dressing & Charbonneau 2013; Fressin et al 2013; Petigura et al 2013a, 2013b; Foreman-Mackey et al 2014; Dressing & Charbonneau 2015; Silburt et al 2015)
Almost all of the confirmed rocky planets are on highly irradiated orbits, where they are bombarded by large amounts of ionizing EUV and X-ray radiation, which can drive a photoevaporative wind from the atmosphere of the planet and over a planet’s lifetime can remove a significant amount of mass from planets with volatile envelopes (e.g., Owen & Jackson 2012)
While it is possible that the short-period rocky planets formed with their current Earth-like compositions, their low masses and highly irradiated orbits mean that they could be the remnant cores of volatile-rich hot Neptunes, which have lost their envelopes
Summary
Results from NASA’s Kepler Satellite Mission have revealed an abundance of planets smaller than 2 RÅ with orbital periods less than 100 days (Howard et al 2012; Dressing & Charbonneau 2013; Fressin et al 2013; Petigura et al 2013a, 2013b; Foreman-Mackey et al 2014; Dressing & Charbonneau 2015; Silburt et al 2015). While it is possible that the short-period rocky planets formed with their current Earth-like compositions, their low masses and highly irradiated orbits mean that they could be the remnant cores of volatile-rich hot Neptunes, which have lost their envelopes Even considering all these scenarios, it is not clear why a transition between bare cores and planets with significant volatiles would occur at 1.6 RÅ. These components correspond to the systematic trends belonging to the specific target being analyzed, which arise due to motion on the detector, as well as instrumental variation and cosmic ray impacts These basis vectors are removed from the time series photometry, which is normalized by dividing by the median flux level in each quarter. Since charge is conserved on the Kepler CCDs to a very high degree, accurate differential photometry is achievable for saturated objects, as long as enough pixels capture all the saturated regions and a significant amount of the star’s pointspread function (Koch et al 2010)
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