tokens: (Hindmarsh 1983 ). We regularly reconverge the PT structure with the radiative/convective code during the time integration until we reach a steady-state for both the chemistry and energy conservation/hydrostatic equilibrium. We then get a coherent atmospheric structure with out-of-equilibrium chemical abundances. As in Tremblin et al. ( 2016 ), we propose that a process similar to fingering convection is responsible for this turbulent mixing and leads to the out-of-equilibrium abundances. Fingering convection can be triggered in the atmospheres of brown dwarfs and exoplanets because of the gradient of mean molecular weight induced by the chemical transitions CO/CH 4 and N 2 /NH 3 at chemical equilibrium. Similar to convective motions that lead toadiabatic PT structure in the deep atmosphere, fingering convection could impact the PT structure of the atmosphere. As in Tremblin et al. ( 2016 ), we propose that fingering-convective motions can induce a temperature gradient reduction, leading to a reddening in the modeled spectrum. This temperature gradient reduction in the atmosphere is modeled using an artificially reduced adiabatic index γ in between two pressure levels on top of the convective zone. In Figure 1 , we show the influence of various model parameters at effective temperatures of 1700 and 1300 K, representative of the objects studied in this study. At 1700 K we can see a degeneracy between gravity and metallicity on the spectral shape of the H and K bands. Decreasing gravity or increasing metallicity tends to form a triangular H band and a strong and flatter K band. Decreasing the temperature gradient in the atmosphere by decreasing the effective adiabatic index γ increases the reddening in J − K . Increasing the surface gravity at an effective temperature of 1300 K has a strong effect on the out-of-equilibrium chemistry and the quenching of CO in the deep atmosphere. At higher gravities, the CO/CH 4 transition is deeper in the atmosphere, hence it is more difficult to prevent the formation of CH 4 (strongly appearing at log g = 4.5) for a given mixing coefficient K zz . Thus, varying K zz would approximately lead to the same effect as varying gravity for T eff = 1300 K. For the fitting of the observations we have probed a similar parameter space, log g in [3.5, 4.5], [M/H] in [0, 0.5], T eff in [1200, 1800], and γ in [1.01, 1.1]. Figure 1. Effects of varying different parameters on the spectral models obtained with the ATMO code. 2.1. DENIS J1425279-365023 DENIS J1425279-365023 was first identified as an L5 dwarf (Kendall et al. 2004 ) with a trigonometric distance of ∼11.57 ± 0.11 pc (Dieterich et al. 2014 ). A SpeX Prism obtained by Gagné et al. ( 2015a ) indicates that the object is a L4 INT-G object based on the index-based classification of Allers Liu ( 2013 ). Based on the galactic position and space velocity of the object, Gagné et al. ( 2015a , 2015b ) also concluded that DENIS J1425-36 is a bona fide member of the
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