Ammonium hydrosulfide (NH₄SH) as a potentially significant sulfur sink in interstellar ices

Abstract

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.