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Molecular origin of the 15300–15800 Å absorption feature in WISEA J181006.18−101000.5

Identify the molecule(s) responsible for the absorption feature centered at 15720 Å within the 15300–15800 Å region in the near-infrared H-band spectrum of WISEA J181006.18−101000.5 and ascertain the extent to which this feature may be blended with terrestrial telluric bands or hydrogen Brackett lines.

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Background

WISEA J181006.18−101000.5 (WISE1810) is an extreme early-T subdwarf with very low metallicity. In its H-band spectrum, the authors report a sharp feature at 15720 Å within a broader 15300–15800 Å absorption region that they could not confidently attribute to any molecule in current opacity databases.

The Data Analysis Center for Exoplanets (DACE) opacity database and laboratory transmission spectra suggest HCN and H2S have high opacity in this wavelength range, and H2S has been previously detected in a T dwarf via a nearby sharp feature at 15900 Å. Despite checking for telluric contamination, the authors acknowledge possible blending with telluric bands or Brackett lines, leaving the molecular identification unresolved.

References

We identify a sharp feature at 15720\,\AA{} within a broader absorption feature at 15300--15800\,\AA{} (Fig. \ref{HCN_H2S}) that we cannot assign exactly to any molecule in the opacity database of the Data {content} Analysis Center for Exoplanets (DACE). Although we ruled out the possibility of telluric contamination by double checking the two $H$-band spectra of both the target and the telluric standard at the beginning and the end of the observations, we cannot discard that this feature is slightly blended with telluric bands or Brackett lines, which are densely populated there, hence weaker than seen.

— Detection of Methane in the Closest Extreme Metal-poor T Dwarf WISEA J181006.18-101000.5 (2503.22289 - Zhang et al., 28 Mar 2025) in Section 3, Subsection “15300–15800 Å feature”