A 1.46-2.48 $\mu$m Spectroscopic Atlas of a T6 Dwarf (1060 K) Atmosphere with IGRINS: First Detections of H${}_2$S and H${}_2$, and Verification of H${}_2$O, CH${}_4$, and NH${}_3$ Line Lists

Abstract

We present Gemini South/IGRINS observations of the 1060 K T6 dwarf 2MASS J08173001$-$6155158 with unprecedented resolution ($R\equiv \lambda /\Delta \lambda =45000$) and signal-to-noise ratio (SNR > 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 (H${}_2$O) line lists are overall reliable. We find the most discrepancies amongst older methane (CH${}_4$) line lists, and that the most up-to-date CH${}_4$ line lists correct many of these issues. We identify individual ammonia (NH${}_3$) lines, a hydrogen sulfide (H${}_2$S) feature at 1.5900 $\mu$m, and a molecular hydrogen (H${}_2$) feature at 2.1218 $\mu$m. These are the first unambiguous detections of H${}_2$S and H${}_2$ absorption features in an extra-solar atmosphere. With the H${}_2$ 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-SNR, high-background spectra, such as an exoplanet around a star. We demonstrate a significant enhancement in the detection of the CH${}_4$ absorption signal in this T6 dwarf with the most up-to-date line lists.

arXiv comment45 pages, 63 figures, 5 tables, Accepted to MNRAS

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