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

Abstract

We report the discovery of a peculiar L dwarf from the UKIDSS Large Area Survey (LAS), ULAS J222711−004547. The very red infrared photometry (MKO J − K = 2.79±0.06, WISE W 1−W 2 = 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 VLT/XSHOOTER, and 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 red L dwarfs pose new challenges for the modeling 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 low metallicity or low gravity. 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 dereddening other unusually red L dwarfs in the L5 to 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.

First light of the VLT planet finder SPHERE

“…And grains of intermediate size have optical column of C/H = 55.8 ppm (in log-normal size distributions) and 42.6 ppm respectively. This result is consistent with the results of Marocco et al (2014), who show that the spectra of red L6-L8 dwarfs could match partially the 0.7−2.5 µm spectra of mid to late-L field-dwarfs once dereddened by interstellar extinction laws (Cardelli et al 1989;Fitzpatrick 1999). Marocco et al (2014) showed that the near-infrared spectra of red L dwarfs dereddened by corundum (Al 2 O 3 ), iron (Fe), or enstatite (MgSiO 3 ) extinctions could also provide a good match to standard field dwarfs spectra.…”

“…This result is consistent with the results of Marocco et al (2014), who show that the spectra of red L6-L8 dwarfs could match partially the 0.7−2.5 µm spectra of mid to late-L field-dwarfs once dereddened by interstellar extinction laws (Cardelli et al 1989;Fitzpatrick 1999). Marocco et al (2014) showed that the near-infrared spectra of red L dwarfs dereddened by corundum (Al 2 O 3 ), iron (Fe), or enstatite (MgSiO 3 ) extinctions could also provide a good match to standard field dwarfs spectra. These three grains species along with forsterite (Mg 2 SiO 4 ) are expected to play a dominant role onto the chemistry and radiative transfer into the atmosphere of brown dwarfs and warm young giant planets (Fegley & Lodders 1996;Tsuji et al 1996;Allard et al 2001;Lodders & Fegley 2002;Visscher et al 2010).…”

“…The red spectra, colors and luminosity problem of these extreme objets are also observed for a growing population of isolated dusty L dwarfs identified in the field (e.g. Liu et al 2013;Schneider et al 2014;Marocco et al 2014;Gizis et al 2015).…”

“…These three grains species along with forsterite (Mg 2 SiO 4 ) are expected to play a dominant role onto the chemistry and radiative transfer into the atmosphere of brown dwarfs and warm young giant planets (Fegley & Lodders 1996;Tsuji et al 1996;Allard et al 2001;Lodders & Fegley 2002;Visscher et al 2010). Therefore, we used the extinctions curves of corundum, enstatite, and iron computed by Marocco et al (2014) for a range of characteristic grain radii (hereafter r) from 0.05 to 1.00 µm, in steps of 0.05 µm for the analysis of HR8799 planet properties. For each mean grain size, a Gaussian size distribution of width √ 2σ = 0.1 × r is adopted.…”

Spectral energy distribution simulations of a possible ring structure around the young, red brown dwarf G 196-3 B

“…Martín et al 2001;Lodieu et al 2008;Peña Ramírez et al 2015), and not all very red L dwarfs display the full list of spectroscopic signposts indicative of low gravity (e.g. Marocco et al 2014;Kellogg et al 2015). In Zapatero Osorio et al (2014), the authors derived ages in the interval ≈ 10-500 Myr for a sample of field, highly reddened L dwarfs with clear spectroscopic evidence for low-tointermediate gravity atmospheres.…”