Hydrogen-line profiles from accreting gas giants and their CPDs

Far fewer gas giants have been caught in their accretion phase than mature ones are known. Extremely Large Telescope (ELT) instruments will have a higher sensitivity and a smaller inner working angle than instruments up to now, which should allow more productive searches and detailed characterisation. We study the observability of accreting gas giants with METIS, the first-generation ELT spectrograph. We focus on the accretion-tracing hydrogen recombination lines accessible at a resolution R=1e5, mainly Brackett alpha and Pfund-series lines. Our approach is general but we take PDS 70 b as a fiducial case. To calculate high-resolution line profiles, we combine a semianalytical multidimensional description of the flow onto an accreting planet and its circumplanetary disc (CPD) with local non-equilibrium shock-emission models. We assume the limiting scenario of no extinction appropriate for gas giants in gaps and negligible contribution from magnetospheric accretion columns. We use simulated detector sensitivities to compute required observing times. Both the planet surface and the CPD surface shocks contribute to the total line profile, which is non-Gaussian and much narrower than the free-fall velocity. For the adopted baseline accretion rate onto PDS 70 b, the Br alpha line peak is equal to the photospheric continuum modulated mostly by water features. However, the rotation of the planet broadens the features, helping the shock excess stand out. At Br alpha, already the continuum of PDS 70 b should yield SNR=12 in 4 h. The peak excess should require only about 15 min to reach SNR=3. Br alpha is a potent planet formation tracer accessible to METIS in little integration time. Resolved line profiles will place independent constraints especially on the mass and radius of an accreting planet, and help identify the accretion mechanism(s) at work.