Ammonium hydrosulfide (NH4SH) as a potentially significant sulfur sink in interstellar ices
Authors: Slavicinska K. , Boogert A. C. A. , Tychoniec Ł. , van Dishoeck E. F. , van Gelder M. L. , Navarro M. G. , Santos J. C. , Klaassen P. D. , Kavanagh P. J. , Chuang K.-J. 
Published: November 2024

Context. Sulfur is depleted with respect to its cosmic standard abundance in dense star-forming regions. It has been suggested that this depletion is caused by the freeze-out of sulfur on interstellar dust grains, but the observed abundances and upper limits of sulfur-bearing ices remain too low to account for all of the missing sulfur. Toward the same environments, a strong absorption feature at ~6.85 µm is observed, but its long-standing assignment to the NH4+ cation remains tentative.
Aims. We aim to spectroscopically investigate the plausibility of NH4SH salt serving as a sulfur reservoir and a carrier of the 6.85 µm band in interstellar ices by characterizing its IR signatures and apparent band strengths in water-rich laboratory ice mixtures. We then use this laboratory data to constrain NH4SH abundances in observations of interstellar ices.
Methods. Laboratory transmission IR spectra of NH3:H2S ice mixtures both with and without H2O were collected. The apparent band strengths of the NH4+ asymmetric bending (ν4) mode and the SH− stretching mode in H2O-containing mixtures were calculated with Beer’s law plots. The IR features of the laboratory salts were compared to those observed toward a sample of four protostars (two low-mass, two high-mass) and two cold dense clouds without star formation.
Results. Apparent band strengths ranging from 3.2(±0.3)-3.6(±0.4)×10−17 cm molec−1 and 3.1(±0.4)-3.7(±0.5)×10−19 cm molec−1 are calculated for the NH4+ ν4 mode at ~6.8 µm/1470 cm−1 and the SH− stretching mode at ~3.9 µm/2560 cm−1, respectively, in NH4SH:H2O mixtures. The peak position of the NH4+ ν4 mode redshifts with increasing temperature but also with increasing salt concentration with respect to matrix species H2O and NH3. The observed 6.85 µm feature is fit well with the laboratory NH4SH:H2O ice spectra. NH4+ column densities obtained from the 6.85 µm band range from 8–23% with respect to H2O toward the sample of protostars and dense clouds. These column densities are consistent with the optical depths observed at 3.9 µm (the SH− stretching mode spectral region). A weak and broad feature observed at ~5.3 µm/1890 cm−1 is tentatively assigned to the combination mode of the NH4+ ν4 mode and the SH− libration. The combined upper limits of four other counter-anion candidates, OCN−, CN−, HCOO−, and Cl−, are determined to be ≲ 15–20% of the total NH4+ column densities toward three of the protostars.
Conclusions. The redshift of the 6.85 µm feature correlates with higher abundances of NH4+ with respect to H2O in both the laboratory data presented here and observational data of dense clouds and protostars. The apparent band strength of the SH− feature is likely too low for the feature to be detectable in the spectrally busy 3.9 µm region, but the 5.3 µm NH4+ ν4 + SH− R combination mode may be an alternative means of detection. Its tentative assignment adds to mounting evidence supporting the presence of NH4+ salts in ices and is the first tentative observation of the SH− anion toward interstellar ices. If the majority (≳80–85%) of the NH4+ cations quantified toward the investigated sources in this work are bound to SH− anions, then NH4 SH salts could account for up to 17–18% of their sulfur budgets.