Diving Beneath the Sea of Stellar Activity: Chromatic Radial Velocities of the Young AU Mic Planetary System

We present updated radial-velocity (RV) analyses of the AU Mic system. AU Mic is a young (22 Myr) early-M dwarf known to host two transiting planets-P <SUB> b </SUB> ~ 8.46 days, ${R}_{b}={4.38}_{-0.18}^{+0.18}\ {R}_{\oplus }$ , P <SUB> c </SUB> ~ 18.86 days, ${R}_{c}={3.51}_{-0.16}^{+0.16}\ {R}_{\oplus }$ . With visible RVs from Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical echelle Spectrographs (CARMENES)-VIS, CHIRON, HARPS, HIRES, MINERVA-Australis, and Tillinghast Reflector Echelle Spectrograph, as well as near-infrared (NIR) RVs from CARMENES-NIR, CSHELL, IRD, iSHELL, NIRSPEC, and SPIRou, we provide a 5σ upper limit to the mass of AU Mic c of M <SUB> c </SUB> ≤ 20.13 M <SUB>⊕</SUB> and present a refined mass of AU Mic b of ${M}_{b}={20.12}_{-1.57}^{+1.72}\ {M}_{\oplus }$ . Used in our analyses is a new RV modeling toolkit to exploit the wavelength dependence of stellar activity present in our RVs via wavelength-dependent Gaussian processes. By obtaining near-simultaneous visible and near-infrared RVs, we also compute the temporal evolution of RV "color" and introduce a regressional method to aid in isolating Keplerian from stellar activity signals when modeling RVs in future works. Using a multiwavelength Gaussian process model, we demonstrate the ability to recover injected planets at 5σ significance with semi-amplitudes down to ≍10 m s<SUP>-1</SUP> with a known ephemeris, more than an order of magnitude below the stellar activity amplitude. However, we find that the accuracy of the recovered semi-amplitudes is ~50% for such signals with our model.