Constraining atmospheric composition from the outflow: helium observations reveal the fundamental properties of two planets straddling the radius gap

TOI-836 is a $\sim2-3$ Gyr K dwarf with an inner super Earth ($R=1.7\,R_\oplus$, $P=3.8\,d$) and an outer mini Neptune ($R=2.6\,R_\oplus$, $P=8.6\,d$). Recent JWST/NIRSpec 2.8--5.2 $\mu$m observations have revealed flat transmission spectra for both planets. We present Keck/NIRSPEC observations of escaping helium from this system. While planet b shows no absorption in the 1083 nm line to deep limits ($<0.2$\%), 836c shows strong (0.7\%) absorption in both visits. These results demonstrate that the inner super-Earth has lost its primordial atmosphere while the outer mini-Neptune has not. Self-consistent 1D radiative-hydrodynamic models of c using pyTPCI, an updated version of The PLUTO-CLOUDY Interface, reveal that the helium signal is highly sensitive to metallicity: its equivalent width collapses by a factor of 13 as metallicity increases from 10x to 100x solar, and by a further factor of 12 as it increases to 200x solar. The observed equivalent width is 88\% of the model prediction for 100x metallicity, suggesting that c may have an atmospheric metallicity close to 100x solar. This is similar to K2-18b and TOI-270d, the first two mini-Neptunes with detected absorption features in JWST transmission spectra. We highlight the helium triplet as a potentially powerful probe of atmospheric composition, with complementary strengths and weaknesses to atmospheric retrievals. The main strength is its extreme sensitivity to metallicity in the scientifically significant range of 10--200x solar, and the main weakness is the enormous model uncertainties in outflow suppression and confinement mechanisms, such as magnetic fields and stellar winds.