A Sub-Neptune-sized Planet Transiting the M2.5 Dwarf G 9-40: Validation with the Habitable-zone Planet Finder
Endl, Michael; Mahadevan, Suvrath; Terrien, Ryan; Wisniewski, John; Wright, Jason T.; Ford, Eric B.; Jennings, Jeff; Monson, Andrew; Kanodia, Shubham; Ninan, Joe P.; Hebb, Leslie; Bender, Chad F.; Diddams, Scott A.; Fredrick, Connor; Halverson, Samuel; Hearty, Fred; Levi, Eric; Maney, Marissa; Metcalf, Andrew J.; Robertson, Paul; Roy, Arpita; Schwab, Christian; Sigurdsson, Steinn; Beard, Corey; Stefansson, Gudmundur; Cañas, Caleb; Brunt, Peter; Clemens, J. Christopher; Cochran, William; Huehnerhoff, Joseph; Kaplan, Kyle; Lubar, Emily; Morris, Brett; Nitroy, Colin; Ramsey, Lawrence
United States, Sweden, Russia, Australia
Abstract
We validate the discovery of a 2-Earth-radii sub-Neptune-sized planet around the nearby high-proper-motion M2.5 dwarf G 9-40 (EPIC 212048748), using high-precision, near-infrared (NIR) radial velocity (RV) observations with the Habitable-zone Planet Finder (HPF), precision diffuser-assisted ground-based photometry with a custom narrowband photometric filter, and adaptive optics imaging. At a distance of d = 27.9 $\,\mathrm{pc}$ , G 9-40b is the second-closest transiting planet discovered by K2 to date. The planet's large transit depth (∼3500 ppm), combined with the proximity and brightness of the host star at NIR wavelengths (J = 10, K = 9.2), makes G 9-40b one of the most favorable sub-Neptune-sized planets orbiting an M dwarf for transmission spectroscopy with James Webb Space Telescope, ARIEL, and the upcoming Extremely Large Telescopes. The star is relatively inactive with a rotation period of ∼29 days determined from the K2 photometry. To estimate spectroscopic stellar parameters, we describe our implementation of an empirical spectral-matching algorithm using the high-resolution NIR HPF spectra. Using this algorithm, we obtain an effective temperature of ${T}_{\mathrm{eff}}=3404\pm 73\,{\rm{K}}$ and metallicity of $[\mathrm{Fe}/{\rm{H}}]=-0.08\pm 0.13$ . Our RVs, when coupled with the orbital parameters derived from the transit photometry, exclude planet masses above 11.7M⊕ with 99.7% confidence assuming a circular orbit. From its radius, we predict a mass of $M={5.0}_{-1.9}^{+3.8}{M}_{\oplus }$ and an RV semiamplitude of $K={4.1}_{-1.6}^{+3.1}\,{\rm{m}}\ {{\rm{s}}}^{-1}$ , making its mass measurable with current RV facilities. We urge further RV follow-up observations to precisely measure its mass, to enable precise transmission spectroscopic measurements in the future.