The extremely red L dwarf ULAS J222711−004547 – dominated by dust

We report the discovery of a peculiar L dwarf from the United Kingdom Infrared Deep Sky Survey Large Area Survey, ULAS J222711−004547. The very red infrared photometry (MKO J − K = 2.79 ± 0.06, WISE W1−W2 = 0.65 ± 0.05) of ULAS J222711−004547 makes it one of the reddest brown dwarfs discovered so far. We obtained a moderate resolution spectrum of this target using the XSHOOTER spectrograph on the Very Large Telescope, and we classify it as L7pec, confirming its very red nature. Comparison to theoretical models suggests that the object could be a low-gravity L dwarf with a solar or higher than solar metallicity. Nonetheless, the match of such fits to the spectral energy distribution is rather poor, and this and other less red peculiar L dwarfs pose new challenges for the modelling of ultracool atmospheres, especially to the understanding of the effects of condensates and their sensitivity to gravity and metallicity. We determined the proper motion of ULAS J222711−004547 using the data available in the literature, and we find that its kinematics do not suggest membership of any of the known young associations. We show that applying a simple de-reddening curve to its spectrum allows it to resemble the spectra of the L7 spectroscopic standards without any spectral features that distinguish it as a low-metallicity or low-gravity dwarf. Given the negligible interstellar reddening of the field containing our target, we conclude that the reddening of the spectrum is mostly due to an excess of dust in the photosphere of the target. De-reddening the spectrum using extinction curves for different dust species gives surprisingly good results and suggests a characteristic grain size of ∼0.5 μm. We show that by increasing the optical depth, the same extinction curves allow the spectrum of ULAS J222711−004547 to resemble the spectra of unusually blue L dwarfs and even slightly metal-poor L dwarfs. Grains of similar size also yield very good fits when de-reddening other unusually red L dwarfs in the L5–L7.5 range. These results suggest that the diversity in near-infrared colours and spectra seen in late L dwarfs could be due to differences in the optical thickness of the dust cloud deck.