TKS V. Twin sub-Neptunes Transiting the Nearby G Star HD 63935

Abstract: We present the discovery of two nearly identically-sized sub-Neptune transiting planets orbiting HD 63935, a bright ($V=8.6$ mag), sun-like ($T_{eff}=5560K$) star at 49 pc. TESS identified the first planet, HD 63935 b (TOI-509.01), in Sectors 7 and 34. We identified the second signal (HD 63935 c) in Keck HIRES and Lick APF radial velocity data as part of our followup campaign. It was subsequently confirmed with TESS photometry in Sector 34 as TOI-509.02. Our analysis of the photometric and radial velocity data yields a robust detection of both planets with periods of $9.0600 \pm 0.007$ and $21.40 \pm 0.0019$ days, radii of $2.99 \pm 0.14$ and $2.90 \pm 0.13$ $R_\oplus$, and masses of $10.8 \pm 1.8$ and $11.1 \pm 2.4$ $M_\oplus$. We calculate densities for planets b and c consistent with a few percent of the planet mass in hydrogen/helium envelopes. We also describe our survey's efforts to choose the best targets for JWST atmospheric followup. These efforts suggest that HD 63935 b will have the most clearly visible atmosphere of its class. It is the best target for transmission spectroscopy (ranked by Transmission Spectroscopy Metric, a proxy for atmospheric observability) in the so-far uncharacterized parameter space comprising sub-Neptune-sized (2.6 $R_\oplus$ $<$ $R_p$ $<$ 4 $R_\oplus$), moderately-irradiated (100 $F_\oplus$ $<$ $F_p$ $<$ 1000 $F_\oplus$) planets around G-stars. Planet c is also a viable target for transmission spectroscopy, and given the indistinguishable masses and radii of the two planets, the system serves as a natural laboratory for examining the processes that shape the evolution of sub-Neptune planets.