Evidence for He i 10830 Å Absorption during the Transit of a Warm Neptune around the M-dwarf GJ 3470 with the Habitable-zone Planet Finder

Abstract

Understanding the dynamics and kinematics of outflowing atmospheres of hot and warm exoplanets is crucial to understanding the origins and evolutionary history of the exoplanets near the evaporation desert. Recently, groundbased measurements of the meta-stable helium atom's resonant absorption at 10830 Å has become a powerful probe of the base environment which is driving the outflow of exoplanet atmospheres. We report evidence for the He I 10830 Å in absorption (equivalent width ∼0.012±0.002 Å) in the exosphere of a warm Neptune orbiting the M-dwarf GJ 3470, during three transits using the Habitable Zone Planet Finder near-infrared spectrograph. This marks the first reported evidence for He I 10830 Å atmospheric absorption for a planet orbiting an M-dwarf. Our detected absorption is broad and its blueshifted wing extends to −36 km s −1 , the largest reported in the literature to date. We modeled 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 helium () =´-N 2.4 10 cm He S 10 2 3 2 , derived from our transit observations, to be consistent with the model, within its uncertainties. The methodology developed here will be useful to study and constrain the atmospheric outflow models of other exoplanets like GJ 3470b, which are near the edge of the evaporation desert. Unified Astronomy Thesaurus concepts: Exoplanet atmospheric composition (2021); Exoplanet atmospheres (487); Exosphere (499); High resolution spectroscopy (2096); Near infrared astronomy (1093); Exoplanet astronomy (486)

Global 3D hydrodynamic modeling of absorption in Lyα and He 10830 A lines at transits of GJ3470b

“…Palle et al 2020 also simulated the absorption at 10830 Å line by 1D isothermal hydrodynamic model with non-equilibrium calculation of He(2 3 S) population. Similar to 1D modeling of Ninan et al 2019, they found that He(2 3 S) distribution extends to at least 10R p . The best fit to the observational data was achieved with the mass loss rate of 3•10 10 g/s at the thermosphere temperature of 6000 K and net blue shift of the line of 3.2 km/s.…”

“…The spectrally resolved absorption at transit shows the depth of 1.5% around the line center restricted by the interval [-30; 20] km/s, while the half width interval of absorption is [-22; 10] km/s. Thus, the second measurement of HeI absorption more or less confirms the first one of (Ninan et al 2019), albeit much more clearly. Palle et al 2020 also simulated the absorption at 10830 Å line by 1D isothermal hydrodynamic model with non-equilibrium calculation of He(2 3 S) population.…”

“…Despite relatively low S/N ratio, the data indicate that absorption takes place mostly at the blue wing [-36; 9] km/s of the line. Ninan et al 2019 inferred column density of He(2 3 S) atoms at the level of 2.4•10 10 cm -2 . The simulation, based on 1D profiles of helium atoms and ions simulated in (Salz et al 2016) assuming standard abundance He/H=0.1, yielded the column density one order of magnitude larger.…”

“…In this case the thermal dispersion of the SW protons and that of the generated ENAs is large enough to generate sufficient thermal broadening of the Lyα line. In regard of He 10830 Å line, the calculation at parameters, for which the best fit for the Lyα data was obtained, revealed that in order to reproduce the absorption measured in Ninan et al 2019 andPalle et al 2020, the helium abundance in the upper atmosphere of GJ 3470b should be of about 7 times less than the typical Solar value.…”

“…It is not affected by the interstellar medium and can be observed by ground telescopes. Ninan et al 2019 reported about 1% absorption in three transits of GJ 3470b, calculated by averaging over the line width of 1.2 Å (or 33 km/s). Despite relatively low S/N ratio, the data indicate that absorption takes place mostly at the blue wing [-36; 9] km/s of the line.…”