Detection of He I 10830 Å absorption during the transit of a warm Neptune around the M-dwarf GJ 3470 with the Habitable-zone Planet Finder

Understanding the dynamics and kinematics of out-flowing atmospheres of hot and warm exoplanets is crucial to understanding the origins and evolutionary history of the exoplanets near the evaporation desert. Recently, ground based measurements of the meta-stable Helium atom's resonant absorption at 10830 A has become a powerful probe of the base environment which is driving the outflow of exoplanet atmospheres. We report the detection of He I 10830 A in absorption (equivalent width ∼ 0.012±0.002 A) in the exosphere of a warm Neptune orbiting the M-dwarf, GJ 3470, during three transits using the Habitable Zone Planet Finder (HPF) near infrared spectrograph. This marks the first reported detection of He I 10830 A, atmospheric absorption for a planet orbiting a M-dwarf. Our detected absorption is broad and its blueshifted wing extends to -36 km/sec, the largest reported in the literature to date. We modelled the state of Helium atoms in the exosphere of GJ3470b based on assumptions on the UV and X-ray flux of GJ 3470, and found our measurement of flux-weighted column density of meta-stable state He = 2.4×10¹⁰cm⁻², derived from our transit observations, to be consistent with model, within its uncertainties. The methodology developed here will be useful to study and constrain the atmosphere outflow models of other exo-planets, like GJ 3470b, which are near the edge of the evaporation desert.