A Statistical Survey of Peculiar L and T Dwarfs in SDSS, 2MASS, and WISE

Abstract

We present the final results from a targeted search for brown dwarfs with unusual near-infrared colors. From a positional cross-match of the Sloan Digital Sky Survey (SDSS), 2-Micron All-Sky Survey (2MASS), and WideField Infrared Survey Explorer (WISE) catalogs, we have identified 144 candidate peculiar L and T dwarfs. Spectroscopy confirms that 20 of the objects are peculiar or are candidate binaries. Of the 420 objects in our full sample 9 are young (200 Myr; 2.1%) and another 8 (1.9%) are unusually red, with no signatures of youth. With a spectroscopic -J K s color of 2.58±0.11mag, one of the new objects, the L6 dwarf 2MASS J03530419 +0418193, is among the reddest field dwarfs currently known and is one of the reddest objects with no signatures of youth known to date. We have also discovered another potentially very-low-gravity object, the L1 dwarf 2MASS J00133470+1109403, and independently identified the young L7 dwarf 2MASS J00440332+0228112, which was first reported by Schneider and collaborators. Our results confirm that signatures of low gravity are no longer discernible in low to moderate resolution spectra of objects older than ∼200 Myr. The 1.9% of unusually red L dwarfs that do not show other signatures of youth could be slightly older, up to ∼400 Myr. In this case a red -J K s color may be more diagnostic of moderate youth than individual spectral features. However, its is also possible that these objects are relatively metal-rich, and thus have enhanced atmospheric dust content.

Aerosols in Exoplanet Atmospheres

“…It is important to note, however, that these studies do not consider the kinetics of cloud formation, and thus how cloud opacity varies with gravity on directly imaged exoplanets is still uncertain. Furthermore, interpreting spectra of the reddest objects still requires the inclusion of high altitude submicron aerosols and/or highly vertically extended cloud layers (Allart et al., 2020; Burningham et al., 2017; Hiranaka et al., 2016; Kellogg et al., 2017; Lew et al., 2016; Manjavacas et al., 2018; Schlawin et al., 2017; Stone et al., 2020).…”

Gemini Planet Imager Spectroscopy of the Dusty Substellar Companion HD 206893 B

“…In particular, young L dwarfs associated with moving groups have redder NIR colors than field counterparts of the same spectral type, an effect attributed to lower surface gravities that retain clouds at high altitude (Marley et al 2012). From a large survey of 420 ultracool dwarfs, Kellogg et al (2017) found similar (∼2%) fractions of unusually red objects in both younger (<200 Myr) and older (200 Myr) populations, an age delineation corresponding to the halting of gravitational contraction within ultracool evolutionary models (e.g., Burrows et al 2001;Baraffe et al 2015) and roughly corresponding to the demarcation between very low and intermediate gravity indicators from Allers & Liu (2013). While the age and peculiarity of ultracool objects are difficult to precisely quantify, a significant population of reportedly older red L dwarfs exists (see Looper et al 2008).…”

Detection of new strongly variable brown dwarfs in the L/T transition

“…J0013−1143, a T3 dwarf recently discovered by Kellogg et al (2017), was added to the list for our second epoch observations. The authors classify J0013 as a likely unresolved binary with a T3.5 + T4.5 composite spectra, as it better matches the spectral fit and satisfies two of the six binary index criteria described by Burgasser et al (2010a).…”