Physical Parameters of the Multiplanet 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±0.37 M ⊕ , M c =1.92 ± 0.49 M ⊕ , and M d =3.42 ± 0.62 M ⊕ . 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 eff > 6200 K). We found planet masses of M b =10.5 ± 3.1 M ⊕ and M c =12.0 ± 3.8 M ⊕ . 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 by mass.

Analysis of Early Science observations with the CHaracterising ExOPlanets Satellite (CHEOPS) usingpycheops

“…The rotation of HD 106315 measured from spectral line broadening is moderately fast (vrot sin i ≈ 13 km s −1 ) but the K2 light curve and ground-based photometry show that the intrinsic variability of this star is < ≈ 0.2% at optical wavelengths (Crossfield et al 2017;Kosiarek et al 2021). There are several published estimates for the mass and radius of this star based on a variety of methods -these are summarised in Table 4 together with our own estimates based on the methods described in Section 3.3.…”

“…The inner planet (b) is a super-Earth with a radius of 2.44 R⊕ and an orbital period of 9.55 days; the outer planet (c) is a Neptune-size planet with a radius of 4.35 R⊕ and a period of 21.06 days (Barros et al 2017). Kosiarek et al (2021) have measured accurate masses for these planets based on extensive multi-year radial velocity measurements for these planets together with transits observed with Spitzer. That study was motivated by on-going and planned observing programmes with Hubble Space Telescope (HST) and James Webb Space Telescope (JWST) to characterise the atmospheres of these planets.…”

“…We first analysed both transits individually using Dataset.lmfit_transit in order to identify which decor- relation parameters are needed for each visit. We fixed the orbital period at the value P = 9.552105 d and assumed that the orbital eccentricity is e = 0 (Kosiarek et al 2021).…”

“…The values of the planet mass (Mp) and radius (Rp) given in Tables 3, 5, 7 and 9 Trifonov et al (2018). 2: Kosiarek et al (2021). 3.…”

“…The values of sin i and Rp/R in Table 11 come from combining our results in Table 5 based on the analysis of the CHEOPS and K2 light curves with those from Kosiarek et al (2021) based on the analysis of two transits of HD 106315 b observed with Spitzer at 4.5 µm. We have not used the values of a/R from Kosiarek et al because they are either inconsistent with the mean stellar density measured independently by several authors shown in Table 4, or not precise enough to be useful.…”