Near-Infrared Photometry and Spectroscopy of L and T Dwarfs: The Effects of Temperature, Clouds, and Gravity

Abstract

We present new JHK photometry on the MKO-NIR system and JHK spectroscopy for a large sample of L and T dwarfs. Photometry has been obtained for 71 dwarfs and spectroscopy for 56. The sample comprises newly identified very red objects from the Sloan Digital Sky Survey (SDSS) and known dwarfs from the SDSS and the Two Micron All Sky Survey (2MASS). Spectral classification has been carried out using four previously defined indices (from Geballe et al. 2002, G02) that measure the strengths of the near infrared water and methane bands. We identify 9 new L8-9.5 dwarfs and 14 new T dwarfs from SDSS, including the latest yet found by SDSS, the T7 dwarf SDSS J175805.46+463311.9. We classify 2MASS J04151954−0935066 as T9, the latest and coolest dwarf found to date.We combine the new results with our previously published data to produce a sample of 59 L dwarfs and 42 T dwarfs with imaging data on a single photometric system -2and with uniform spectroscopic classification. We compare the near-infrared colors and absolute magnitudes of brown dwarfs near the L-T transition with predictions made by models of the distribution and evolution of photospheric condensates. There is some scatter in the G02 spectral indices for L dwarfs, suggesting that these indices are probing different levels of the atmosphere and are affected by the location of the condensate cloud layer. The near-infrared colors of the L dwarfs also show scatter within a given spectral type, which is likely due to variations in the altitudes, spatial distributions and thicknesses of the clouds. We have identified a small group of late L dwarfs that are relatively blue for their spectral type and that have enhanced FeH, H 2 O and K I absorption, possibly due to an unusually small amount of condensates.The scatter seen in the H − K color for late T dwarfs can be reproduced by models with a range in surface gravity. The variation is probably due to the effect on the K-band flux of pressure-induced H 2 opacity. The correlation of H − K color with gravity is supported by the observed strengths of the J-band K I doublet. Gravity is closely related to mass for field T dwarfs with ages > 10 8 yrs and the gravities implied by the H − K colors indicate that the T dwarfs in our sample have masses in the range 15 -75 M Jupiter . One of the SDSS dwarfs, SDSS J111010.01+011613.1, is possibly a very low mass object, with log g ∼ 4.2 -4.5 and mass ∼ 10 -15 M Jupiter .

Multiplicity Study of Exoplanet Host Stars: the HD 3651 AB System

“…The companion can be plotted then in a color-magnitude diagram (see Fig. 3) together with comparison brown dwarfs from Knapp et al (2004). The OMEGA-CASS photometry confirms our conclusion that the companion is a late T dwarf.…”

“…Our spectral type estimate was most recently confirmed by Luhman et al (2006) and Burgasser (2006). Knapp et al (2004). HD 3651 B is one of the faintest T dwarfs presently known.…”

Binary frequency of planet-host stars at wide separations

“…NTT/SofI low-resolution near-infrared spectra of the T4.5 dwarf co-moving with HIP 70849 (black line). Overplotted are IRTF/SpeX spectra of SDSS J000013.54+255418.6 (T4.5; Knapp et al 2004;Jameson et al 2008;Burgasser et al 2006) and the average spectrum of a T4 and T5 dwarf.…”

Photometry and Proper Motions of M, L, and T Dwarfs from the Pan-STARRS1 3 π Survey

“…Samples of Ldwarfs have revealed a surprising diversity of near-IR colors (e.g., Leggett et al 2002;Knapp et al 2004;Gizis et al 2012), which are believed to be caused by variations in surface gravity and/or dusty clouds (e.g., Kirkpatrick et al 2008; or thermochemical instabilities (Tremblin et al 2016). Objects transitioning from L to T spectral types undergo a dramatic shift to bluer near-IR colors, thought to be driven by the clearing of clouds and the formation of methane (e.g., Burgasser et al 2002;Chiu et al 2006;Saumon & Marley 2008).…”