# Dust/ice mixing in cold regions and solid-state water in the diffuse interstellar medium

## Abstract

Whether ice in cold cosmic environments is physically separated from the silicate dust or mixed with individual silicate moieties is not known. However, different grain models give very different compositions and temperatures of grains. The aim of the present study is to compare the mid-infrared spectra of laboratory silicate grain/water ice mixtures with astronomical observations to evaluate the presence of dust/ice mixtures in interstellar and circumstellar environments. The laboratory data can explain the observations, assuming reasonable mass-averaged temperatures for the protostellar envelopes and protoplanetary disks, demonstrating that a substantial fraction of water ice may be mixed with silicate grains. On the basis of the combination of laboratory data and infrared observations, we provide evidence of the presence of solid-state water in the diffuse interstellar medium. Our results have implications for future laboratory studies investigating cosmic dust grain analogues and for future observations trying to identify the structure, composition and temperature of grains in different astrophysical environments.

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## Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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## Acknowledgements

We thank E. Gibb and H. Terada for providing the observational data. This work was supported by the Research Unit FOR 2285 ‘Debris Disks in Planetary Systems’ of the Deutsche Forschungsgemeinschaft (grant JA 2107/3–2). T.H. acknowledges support from the European Research Council under the Horizon 2020 Framework Programme via the ERC Advanced Grant Origins 83 24 28.

## Author information

Authors

### Contributions

A.P. led the project. A.P. and J.B. designed the research and wrote the paper. A.P. performed the laboratory experiments. J.B. performed the analysis of the observational data. C.J. and T.H. participated in data interpretation and discussion.

### Corresponding author

Correspondence to Alexey Potapov.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature Astronomy thanks Herma Cuppen, Erika Gibb and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data

### Extended Data Fig. 1 The normalized 150 K spectra of the silicate/ice mixtures.

MgSiO3/H2O 7.5, Mg2SiO4/H2O 10.5, and MgFeSiO4/H2O 9.0 samples (left) and the MgSiO3/H2O samples with the mass ratios of 2.7 and 7.5 (right).

### Extended Data Fig. 2 Amount of trapped water.

Dependence of the volume percentage of remaining water in the silicate/H2O samples at 200 K on the silicate/H2O mass ratio.

### Extended Data Fig. 3 ISO-SWS observations of Cyg OB2 12.

The top panel shows the three ISO observations of Cyg OB 2 12 (TDT 33504130, 03602226, and 13901048). The two lower spectra are offset by 5, respectively 10 Jy for clarity. Also, show in this panel are the fitted polynomial continua for the three observations. The lower panel shows the derived optical depth for the three observations (gray curves) and the re-binned average (black curve).

### Extended Data Fig. 4 Spitzer IRS observations of Cyg OB2 12.

The top panel shows the data taken in the 3 spectral orders of the short-wavelength module of the low-resolution spectrograph (green, red and blue data points) the short-wavelength module of the high-resolution spectrograph (magenta data points) and the second order of the long-wavelength module of the low-resolution spectrograph (brown data points). Also plotted in his panel is the fitted power-law continuum (black dashed line). The inset figure shows a zoom of the spectral region between 7 and 8 microns. The grey shaded area indicates the position of a blend of HI emission lines. The lower panel shows the derived apparent optical depth profile. The inset panel shows a zoom of the 5.5 to 8 microns spectral region. All plotted error bars represent a 1-sigma uncertainty limit.

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Potapov, A., Bouwman, J., Jäger, C. et al. Dust/ice mixing in cold regions and solid-state water in the diffuse interstellar medium. Nat Astron (2020). https://doi.org/10.1038/s41550-020-01214-x