标题： 一颗海王星大小的行星在M2.5矮星G 9-40上凌日：使用宜居带行星探测器验证 显示英文标题

标题： A sub-Neptune sized planet transiting the M2.5-dwarf G 9-40: Validation with the Habitable-zone Planet Finder

摘要： 我们验证了在附近的高视向速度M2.5矮星G 9-40 (EPIC 212048748)周围发现了一颗直径为2个地球半径的亚海王星大小行星。使用高精度近红外（NIR）径向速度（RV）观测数据，通过宜居带行星探测器（HPF），使用定制窄带光电滤光片的高精度扩散辅助地面光电测量，以及自适应光学成像进行了验证。 距离为$d=27.9\mathrm{pc}$处，G 9-40b是迄今为止由K2发现的第二颗最接近的凌日行星。 该行星的大凌日深度（$\sim$3500ppm），加上其宿主恒星在近红外波段的邻近性和亮度（J=10，K=9.2），使G 9-40b成为使用JWST、ARIEL和即将出现的极大望远镜进行传输光谱学研究的最有利的亚海王星大小的围绕M型矮星运转的行星之一。 这颗恒星相对不活跃，从K2测光数据确定的旋转周期为$\sim$29天。 为了估计光谱恒星参数，我们描述了使用高分辨率近红外HPF光谱的实证光谱匹配算法的实现方法。 使用这种方法，我们得到了有效温度为$T_{\mathrm{eff}}=3404\pm73$K，金属丰度为$\mathrm{[Fe/H]}=-0.08\pm0.13$。 我们的径向速度（RVs），当与通过凌星测光得到的轨道参数结合时，排除了行星质量超过$11.7 M_\oplus$的可能性。根据99.7\%的半径，我们预测该行星的质量为$M=5.0^{+3.8}_{-1.9} M_\oplus$，并且径向速度半振幅为$K=4.1^{+3.1}_{-1.6}\mathrm{m\:s^{-1}}$，这使得使用当前的径向速度设施可以测量其质量。我们敦促进一步的径向速度后续观测，以精确测量其质量，从而在未来能够进行精确的传输光谱测量。 显示英文摘要

摘要： We validate the discovery of a 2 Earth radii sub-Neptune-size 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 narrow-band 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 ($\sim$3500ppm), 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 planet orbiting an M-dwarf for transmission spectroscopy with JWST, ARIEL, and the upcoming Extremely Large Telescopes. The star is relatively inactive with a rotation period of $\sim$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 $T_{\mathrm{eff}}=3404\pm73$K, and metallicity of $\mathrm{[Fe/H]}=-0.08\pm0.13$. Our RVs, when coupled with the orbital parameters derived from the transit photometry, exclude planet masses above $11.7 M_\oplus$ with 99.7% confidence assuming a circular orbit. From its radius, we predict a mass of $M=5.0^{+3.8}_{-1.9} M_\oplus$ and an RV semi-amplitude of $K=4.1^{+3.1}_{-1.6}\mathrm{m\:s^{-1}}$, 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.