A 1.46-2.48 μm spectroscopic atlas of a T6 dwarf (1060 K) atmosphere with IGRINS: first detections of H2S and H2, and verification of H2O, CH4, and NH3 line lists

We present Gemini South/IGRINS observations of the 1060 K T6 dwarf 2MASS J08173001-6155158 with unprecedented resolution ($R\equiv \lambda /\Delta \lambda =45\, 000$) and signal-to-noise ratio (S/N > 200) for a late-type T dwarf. We use this benchmark observation to test the reliability of molecular line lists used up-to-date atmospheric models. We determine which spectroscopic regions should be used to estimate the parameters of cold brown dwarfs and, by extension, exoplanets. We present a detailed spectroscopic atlas with molecular identifications across the H and K bands of the near-infrared. We find that water (H₂O) line lists are overall reliable. We find the most discrepancies amongst older methane (CH₄) line lists, and that the most up-to-date CH₄ line lists correct many of these issues. We identify individual ammonia (NH₃) lines, a hydrogen sulfide (H₂S) feature at 1.5900 $\mu$m, and a molecular hydrogen (H₂) feature at 2.1218 $\mu$m. These are the first unambiguous detections of H₂S and H₂ absorption features in an extra-solar atmosphere. With the H₂ detection, we place an upper limit on the atmospheric dust concentration of this T6 dwarf: at least 500 times less than the interstellar value, implying that the atmosphere is effectively dust-free. We additionally identify several features that do not appear in the model spectra. Our assessment of the line lists is valuable for atmospheric model applications to high-dispersion, low-S/N, high-background spectra, such as an exoplanet around a star. We demonstrate a significant enhancement in the detection of the CH₄ absorption signal in this T6 dwarf with the most up-to-date line lists.