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 Mb=4.87±0.37 M⊕, Mc=1.92±0.49 M⊕, and Md=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 (Teff>6200 K). We found planet masses of Mb=10.5±3.1 M⊕ and Mc=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 surrounding an Earth-like rock and iron core.
@article{arxiv.2009.03398,
title = {Physical Parameters of the Multi-Planet Systems HD 106315 and GJ 9827},
author = {Molly R. Kosiarek and David A. Berardo and Ian J. M. Crossfield and Cesar Laguna and Caroline Piaulet and Joseph M. Akana Murphy and Steve B. Howell and Gregory W. Henry and Howard Isaacson and Benjamin Fulton and Lauren M. Weiss and Erik A. Petigura and Aida Behmard and Lea A. Hirsch and Johanna Teske and Jennifer A. Burt and Sean M. Mills and Ashley Chontos and Teo Mocnik and Andrew W. Howard and Michael Werner and John H. Livingston and Jessica Krick and Charles Beichman and Varoujan Gorjian and Laura Kreidberg and Caroline Morley and Jessie L. Christiansen and Farisa Y. Morales and Nicholas J. Scott and Jeffrey D. Crane and Sharon Xuesong Wang and Stephen A. Shectman and Lee J. Rosenthal and Samuel K. Grunblatt and Ryan A. Rubenzahl and Paul A. Dalba and Steven Giacalone and Chiara Dane Villanueva and Qingtian Liu and Fei Dai and Michelle L. Hill and Malena Rice and Stephen R. Kane and Andrew W. Mayo},
journal= {arXiv preprint arXiv:2009.03398},
year = {2021}
}