Physical Parameters of the Multi-Planet Systems HD 106315 and GJ 9827

Abstract

HD 106315 and GJ 9827 are two bright, nearby stars that host multiple super-Earths and sub-Neptunes discovered by K2 that are well suited for atmospheric characterization. We refined the planets' ephemerides through Spitzer transits, enabling accurate transit prediction required for future atmospheric characterization through transmission spectroscopy. Through a multi-year high-cadence observing campaign with Keck/HIRES and Magellan/PFS, we improved the planets' mass measurements in anticipation of HST transmission spectroscopy. For GJ 9827, we modeled activity-induced radial velocity signals with a Gaussian process informed from the Calcium II H&K lines in order to more accurately model the effect of stellar noise on our data. We found planet masses of M${}_b$=$4.87\pm 0.37$ M${}_{\oplus }$, M${}_c$=$1.92\pm 0.49$ M${}_{\oplus }$, and M${}_d$=$3.42\pm 0.62$ M${}_{\oplus }$. For HD 106315, we found that such activity-radial velocity decorrelation was not effective due to the reduced presence of spots and speculate that this may extend to other hot stars as well (T${}_{\mathrm{eff}}>6200$ K). We found planet masses of M${}_b$=$10.5\pm 3.1$ M${}_{\oplus }$ and M${}_c$=$12.0\pm 3.8$ M${}_{\oplus }$. We investigated all of the planets' compositions through comparing their masses and radii to a range of interior models. GJ 9827 b and GJ 9827 c are both consistent with an Earth-like rocky composition, GJ 9827 d and HD 106315 b both require additional volatiles and are consistent with moderate amounts of water or hydrogen/helium, and HD 106315 c is consistent with 10% hydrogen/helium surrounding an Earth-like rock and iron core.

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Cited by in corpus (10)

• JWST/NIRISS reveals the water-rich "steam world" atmosphere of GJ 9827 d
• Analysis of Early Science observations with the CHaracterising ExOPlanets Satellite (CHEOPS) using pycheops
• DREAM I. Orbital architecture orrery
• Evidence for the volatile-rich composition of a 1.5-$R_{\oplus }$ planet
• Water absorption in the transmission spectrum of the water-world candidate GJ9827d
• A multi-wavelength look at the GJ 9827 system -- No evidence of extended atmospheres in GJ 9827 b and d from HST and CARMENES data
• Connection between planetary He I $\lambda$10830 Å absorption and extreme-ultraviolet emission of planet-host stars
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