Methane and Ammonia in the near-infrared spectra of late T dwarfs

TL;DR

This study analyzes near-infrared spectra of late T dwarfs to identify methane and ammonia features using newly computed line lists, enhancing understanding of their atmospheres and validating the new molecular data.

Contribution

It provides high-quality spectra of T8 and T9 dwarfs, identifies new methane and ammonia absorption features, and discusses their implications for atmospheric composition and molecular opacity sources.

Findings

Ammonia is the dominant opacity source between 1.233-1.266 microns in UGPS 0722-0540.

Several absorption features in T9 spectra may be due to ammonia.

Water is the main absorber in the red half of the J-band for T8 and T9 dwarfs.

Abstract

Analysis of T dwarfs using model atmospheres has been hampered by the absence of reliable line lists for methane and ammonia. Newly computed high temperature line lists for both of these important molecules are now available, so it is timely to investigate the appearance of the various absorption features in T dwarfs in order to better understand their atmospheres and validate the new line lists. We present high quality R~5000 Gemini/NIFS 1.0-2.4 microns spectra of the T8 standard 2MASS 0415-0935 and the T9 standard UGPS 0722-0540. We use these spectra to identify numerous methane and ammonia features not previously seen and we discuss the implications for our understanding of T dwarf atmospheres. Among our results, we find that ammonia is the dominant opacity source between ~1.233-1.266 microns in UGPS 0722-0540, and we tentatively identify several absorption features in this wavelength range in the T9's spectrum which may be due entirely to ammonia opacity. Our results also suggest that water rather than methane is the dominant opacity source in the red half of the J-band of the T8 dwarf. Water appears to be the main absorber in this wavelength region in the T9 dwarf until ~1.31 microns, when methane starts to dominate.