The atmosphere of the warm Neptune GJ 436 b probed with ESPRESSO

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Abstract
Aims . We aim to identify the presence of atomic and molecular species in the upper atmosphere of the warm Neptune-sized transiting planet GJ 436 b, which has a radiative equilibrium temperature of 690 K and a mass of 25.4 M ⊕ . Methods . Using the transmission spectroscopy technique, we observed two full transits of GJ 436 b with the ESPRESSO spectrograph, covering the wavelength range from 3800 to 7880 Å. We searched for traces of atomic (H I , Li I , Na I , Mg I , V I , Cr I , Fe I , and Fe II ), along with molecular (TiO, VO) species by directly detecting planetary absorption features and by cross-correlating the planetary spectrum with theoretical spectra computed for each investigated species. Results . Our analysis reveals no strong planetary detection for any of the species, consistent with a featureless optical spectrum. We derived upper limits by combining all ESPRESSO observations. Post-transit stellar flares were detected on both nights, primarily affecting chromospheric lines. A tentative Fe I signal appears in the first transit (S/N = 3.4 ± 0.2) at a wind velocity of ~−18.6 km s −1 , which is unexpectedly large for a cool planet. This weak signal is not present in the second transit and combined with its low significance, this suggests an origin in noise. In the less probable scenario where the feature is suppressed during the second transit by the higher stellar activity state, the T1 tentative signal peaks at 1300 K, which is above the equilibrium temperature of GJ 436 b. Ultimately, this result would imply a neutral iron abundance comparable to or exceeding that of the host star.
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