SDSS J141624.08+134826.7: Blue L dwarfs and non-equilibrium chemistry

We present an analysis of the recently discovered blue L dwarf SDSS J141624.08+134826.7. We extend the spectral coverage of its published spectrum to ∼4 μm by obtaining a low-resolution L-band spectrum with SpeX on the NASA IRTF. The spectrum exhibits a tentative weak CH₄ absorption feature at 3.3 μm but is otherwise featureless. We derive the atmospheric parameters of SDSS J141624.08+134826.7 by comparing its 0.7-4.0 μm spectrum to the atmospheric models of Marley and Saumon which include the effects of both condensate cloud formation and nonequilibrium chemistry due to vertical mixing and find the best-fitting model has T_eff= 1700 K, log g= 5.5 (cm s^-2), f_sed= 4, and K_zz= 10⁴ cm ² s^-1. The derived effective temperature is significantly cooler than previously estimated but we confirm the suggestion by Bowler et al. that the peculiar spectrum of SDSS J141624.08+134826.7 is primarily a result of thin condensate clouds. In addition, we find strong evidence of vertical mixing in the atmosphere of SDSS J141624.08+134826.7 based on the absence of the deep 3.3 μm CH₄ absorption band predicted by models computed in chemical equilibrium. Finally, this result suggests that observations of blue L dwarfs are an appealing way to quantitatively estimate the vigor of mixing in the atmospheres of L dwarfs because of the dramatic impact such mixing has on the strength of the 3.3 μm CH₄ band in the emergent spectra of L dwarfs with thin condensate clouds.