Ammonium hydrosulfide (NH_4SH) as a potentially significant sulfur sink in interstellar ices

• Published in: Astronomy and astrophysics (Berlin) 2024-11
• Main Authors: Slavicinska, K., Boogert, A.C.A., Tychoniec, L, van Dishoeck, E.F., van Gelder, M.L., Navarro, M.G., Santos, J.C., Klaassen, P.D., Kavanagh, P.J., Chuang, K. -J.
• Format: Article
• Language: English
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/202451383

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 sim 6.85 mu m is observed, but its long-standing assignment to the NH$_ $ cation remains tentative. We aim to spectroscopically investigate the plausibility of NH$_ $SH salt serving as a sulfur reservoir and a carrier of the 6.85 mu m band in interstellar ices by characterizing its IR signatures and apparent band strengths in water-rich laboratory ice mixtures. We then use this laboratory data to constrain NH$_ $SH abundances in observations of interstellar ices. Laboratory transmission IR spectra of NH$_ $:H$_ $S ice mixtures both with and without H$_ $O were collected. The apparent band strengths of the NH$_ $ asymmetric bending ($ $) mode and the SH$^ $ stretching mode in H$_ $O-containing mixtures were calculated with Beer's law plots. The IR features of the laboratory salts were compared to those observed toward a sample of four protostars (two low-mass, two high-mass) and two cold dense clouds without star formation. Apparent band strengths ranging from 3.2(pm 0.3)-3.6(pm 0.4)times 10$^ $ cm molec$^ $ and 3.1(pm 0.4)-3.7(pm 0.5)times 10$^ $ cm molec$^ $ are calculated for the NH$_ $ mode at sim 6.8 mu m/1470 cm$^ $ and the SH$^ $ stretching mode at sim 3.9 mu m/2560 cm$^ $, respectively, in NH$_ $SH:H$_ $O mixtures. The peak position of the NH$_ $ mode redshifts with increasing temperature but also with increasing salt concentration with respect to matrix species H$_ $O and NH$_ $. The observed 6.85 mu m feature is fit well with the laboratory NH$_ $SH:H$_ $O ice spectra. NH$_ $ column densities obtained from the 6.85 mu m band range from 8-23 with respect to H$_ $O toward the sample of protostars and dense clouds. These column densities are consistent with the optical depths observed at 3.9 mu m (the SH$^ $ stretching mode spectral region). A weak and broad feature observed at sim 5.3 mu m/1890 cm$^ $ is tentatively assigned to the combination mode of the NH$_ $ mode and the SH$^ $ libration. The combined upper limits of four other counter-anion candidates, OCN$^ $, CN$^ $, HCOO$^ $, and Cl$^ $, are determined to be lesssim 15-20 of the total NH$_ $ co