Escaping Helium from TOI 560.01, a Young Mini-Neptune

We report helium absorption from the escaping atmosphere of TOI 560.01 (HD 73583b), a $R=2.8 R_\oplus$, $P=6.4$ d mini Neptune orbiting young ($\sim$600 Myr) K dwarf. Using Keck/NIRSPEC, we detect signal with an average depth $0.68 \pm 0.08$% in line core. The repeats during partial transit obtained month later, but is marginally stronger and bluer, perhaps reflecting changes stellar wind environment. Ingress occurs on time, egress within 12 minutes white light egress, although rises more gradually than it declines. This suggests that outflow slightly asymmetric confined to regions close planet. also exhibits slight 4 km/s redshift rather expected blueshift; this might be explained if planet has modest orbital eccentricity, radial velocity data disfavors such explanation. use XMM-Newton observations reconstruct high energy spectrum model planet's 1D 3D hydrodynamic simulations. find our models generally overpredict measured magnitude transit, size blueshift, or both. Increasing metallicity 100$\times$ solar suppresses signal, dependence predicted strength non-monotonic. Decreasing assumed EUV flux by factor 3 likewise substantially.