Astrochemistry

Ammonium hydrosulfide (NH4SH): A Potential Significant Sulfur Sink In Interstellar Ices

By Keith Cowing
Status Report
astro-ph.GA
October 7, 2024
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Ammonium hydrosulfide (NH4SH): A Potential Significant Sulfur Sink In Interstellar Ices
Abundances of the ammonium cation w.r.t. H2O ice plotted against the τintC4/τintC3 ratios of ices toward isolated dense clouds (left, blue) and protostars (right, red). To facilitate comparison, the protostellar and cloud values are also plotted in the left and right plots, respectively, in light gray. A higher τintC4/τintC3 is indicative of a greater redshift in the peak position of the 6.85 μm feature. The plotted values are taken from Boogert et al. (2011) (clouds) and Boogert et al. (2008) (protostars). The Spearman’s rank correlation coefficient ρ and the p-value indicating probability of non-correlation are provided in the top right corner. The plotted NH4 + abundances have been multiplied by a correction factor of 4.4/3.6 to account for the difference between NH4+ ν4 mode band strength used by Boogert et al. (2008) and Boogert et al. (2011) (4.4×10-17 cm molec–1 from Schutte & Khanna 2003) and the band strengths calculated in this work to enable a direct comparison to our NH4+ ice upper limits in Table 5. Upper limits have been excluded from this plot, as well as data from the sources 2MASS J19214480+1121203 (due to the high error of its τintC4/τintC3 ratio) and IRAS 03301+3111 (due to its τintC4/τintC3 ratio being undefined because its τintC3 = 0). — astro-ph.GA

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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Submission history
From: Katerina Slavicinska
[v1] Thu, 3 Oct 2024 18:00:04 UTC (6,775 KB)
https://arxiv.org/abs/2410.02860

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