Ultracool Dwarfs Observed with the Spitzer Infrared Spectrograph. II. Emergence and Sedimentation of Silicate Clouds in L Dwarfs, and Analysis of the Full M5--T9 Field Dwarf Spectroscopic Sample

TL;DR

This study analyzes mid-infrared spectra of 113 M5--T9 dwarfs to investigate silicate cloud emergence and sedimentation, revealing their correlation with spectral features and variability, and establishing their temperature-dependent behavior.

Contribution

It provides the first observational evidence of silicate cloud emergence and sedimentation in L dwarf atmospheres between 2000 K and 1300 K.

Findings

Silicate absorption appears at L2 and peaks in L4--L6 dwarfs.

Water absorption is present in all spectra and increases with spectral type.

Variable L3--L7 dwarfs are more likely to have above-average silicate absorption.

Abstract

We present a uniform analysis of all mid-infrared $R\approx90$ spectra of field M5--T9 dwarfs obtained with the Spitzer Infrared Spectrograph (IRS). The sample contains 113 spectra out of which 12 belong to late-M dwarfs, 69 to L dwarfs, and 32 to T dwarfs. Sixty-eight of these spectra are presented for the first time. We measure strengths of the main absorption bands in the IRS spectra, namely H$_2$O at 6.25 $\mu$m, CH$_4$ at 7.65 $\mu$m, NH$_3$ at 10.5 $\mu$m, and silicates over 8--11 $\mu$m. Water absorption is present in all spectra and strengthens with spectral type. The onset of methane and ammonia occurs at the L8 and T2.5 types, respectively, although ammonia can be detectable as early as T1.5. Silicate absorption sets in at spectral type L2, is on average the strongest in L4--L6 dwarfs, and disappears past L8. However, silicate absorption can also be absent from the spectra at any L subtype. We find a positive correlation between the silicate absorption strength and the excess (deviation from median) near-infrared colour at a given L subtype, which supports the idea that variations of silicate cloud thickness produce the observed colour scatter in L dwarfs. We also find that variable L3--L7 dwarfs are twice more likely to have above-average silicate absorption than non-variables. The ensemble of results solidifies the evidence for silicate condensate clouds in the atmospheres of L dwarfs, and for the first time observationally establishes their emergence and sedimentation between effective temperatures of $\approx$2000 K and $\approx$1300 K, respectively.