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We present a uniform analysis of all mid-infrared $R\approx90$ spectra offield M5--T9 dwarfs obtained with the Spitzer Infrared Spectrograph (IRS). Thesample contains 113 spectra out of which 12 belong to late-M dwarfs, 69 to Ldwarfs, and 32 to T dwarfs. Sixty-eight of these spectra are presented for thefirst time. We measure strengths of the main absorption bands in the IRSspectra, 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 allspectra and strengthens with spectral type. The onset of methane and ammoniaoccurs at the L8 and T2.5 types, respectively, although ammonia can bedetectable 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. Wefind a positive correlation between the silicate absorption strength and theexcess (deviation from median) near-infrared colour at a given L subtype, whichsupports the idea that variations of silicate cloud thickness produce theobserved colour scatter in L dwarfs. We also find that variable L3--L7 dwarfsare twice more likely to have above-average silicate absorption thannon-variables. The ensemble of results solidifies the evidence for silicatecondensate clouds in the atmospheres of L dwarfs, and for the first timeobservationally establishes their emergence and sedimentation between effectivetemperatures of $\approx$2000 K and $\approx$1300 K, respectively.

monthly notices of the royal astronomical society

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