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

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 NH 4 + cation remains tentative. Aims . We aim to spectroscopically investigate the plausibility of NH 4 SH 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 NH 4 SH abundances in observations of interstellar ices. Methods . Laboratory transmission IR spectra of NH 3 :H 2 S ice mixtures both with and without H 2 O were collected. The apparent band strengths of the NH 4 + asymmetric bending ( ν 4 ) mode and the SH − stretching mode in H 2 O-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 NH 4 + ν 4 mode at ~6.8 µm/1470 cm −1 and the SH − stretching mode at ~3.9 µm/2560 cm −1 , respectively, in NH 4 SH:H 2 O mixtures. The peak position of the NH 4 + ν 4 mode redshifts with increasing temperature but also with increasing salt concentration with respect to matrix species H 2 O and NH 3 . The observed 6.85 µm feature is fit well with the laboratory NH 4 SH:H 2 O ice spectra. NH 4 + column densities obtained from the 6.85 µm band range from 8–23% with respect to H 2 O 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 NH 4 + ν 4 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 NH 4 + column densities toward three of the protostars. Conclusions . The redshift of the 6.85 µm feature correlates with higher abundances of NH 4 + with respect to H 2 O 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 NH 4 + ν 4 + SH − R combination mode may be an alternative means of detection. Its tentative assignment adds to mounting evidence supporting the presence of NH 4 + salts in ices and is the first tentative observation of the SH − anion toward interstellar ices. If the majority (≳80–85%) of the NH 4 + cations quantified toward the investigated sources in this work are bound to SH − anions, then NH 4 SH salts could account for up to 17–18% of their sulfur budgets.