Ammonium hydrosulfide (NH4SH): A Potential Significant Sulfur Sink In Interstellar Ices
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. We 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 and using this laboratory data to constrain NH4SH abundances in observations of 4 protostars and 2 cold dense clouds.
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. 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 NH4SH salts could account for up to 17-18% of their sulfur budgets.
Katerina Slavicinska, Adwin Boogert, Łukasz Tychoniec, Ewine F. van Dishoeck, Martijn L. van Gelder, Julia C. Santos, Pamela D. Klaassen, Patrick J. Kavanagh, Ko-Ju Chuang
Comments: Accepted for publication in A&A. 20 pages, 14 figures, and 7 tables in the main text; 15 pages, 17 figures, and 10 tables in the appendix
Subjects: Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2410.02860 [astro-ph.GA] (or arXiv:2410.02860v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2410.02860
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From: Katerina Slavicinska
[v1] Thu, 3 Oct 2024 18:00:04 UTC (6,775 KB)
https://arxiv.org/abs/2410.02860
Astrobiology