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Detection of the elusive dangling OH ice features at ~2.7 μm in Chamaeleon I with JWST NIRCam

Abstract

Ascertaining the morphology and composition of the icy mantles covering dust grains in dense, cold regions of the interstellar medium is essential to developing accurate astrochemical models, determining conditions for ice formation, constraining chemical interactions in and on icy grains and understanding how ices withstand space radiation. The widely observed infrared spectroscopic signature of H2O ice at ~3 μm discriminates crystalline from amorphous structures in interstellar ices. Weaker bands seen only in laboratory ice spectra at ~2.7 μm, termed ‘dangling OH’ (dOH), are attributed to water molecules not fully bound to neighbouring water molecules and are often considered as tracing the degree of ice compaction. We exploit the high sensitivity of JWST NIRCam to detect two dOH features at 2.703 and 2.753 μm along multiple lines of sight probing the dense cloud Chamaeleon I, attributing these signatures to unbound dOH in cold water ice and dOH in interaction with other molecular species. These detections open a path to using the dOH features as tracers of the formation, composition, morphology and evolution of icy grains during the star and planet formation process.

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Fig. 1: A selection of eight lines of sight towards Cha I exhibiting absorption features at ~2.7 μm.
Fig. 2: Locally baselined spectra of the dOH absorption features around 2.7 μm on an optical depth scale for the eight objects in Cha I.
Fig. 3: Observed spectra in Cha I compared with experimental spectra of absorption features around 2.7 μm in pure water ices.
Fig. 4: Observed spectra in Cha I compared with experimental spectra of absorption features around 2.7 μm in amorphous binary ice mixtures.
Fig. 5: Illustration of the various OH bonding scenarios observed in Cha I.

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Data availability

Observational data are available to download from MAST. Extracted 2.5–5 μm spectra will be published in Smith et al. B (in prep) and will be made available on Zenodo (https://zenodo.org/communities/jwst-iceage-ers/records)65.

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Acknowledgements

J.A.N. and E.D. acknowledge support from French Programme National ‘Physique et Chimie du Milieu Interstellaire’ (PCMI) of the CNRS/INSU with the INC/INP, co-funded by the CEA and the CNES. Z.L.S. acknowledges financial support from the Royal Astronomical Society through the E. A. Milne Travelling Fellowship. A.C.A.B. and J.E. acknowledge support from the Space Telescope Science Institute for programme JWST-ERS-01309.019. B.M. and V.J.H. acknowledge funding from the Spanish MIC grant PID2020-113084GB-I00/AEI/10.13039/501100011033. M.K.M. acknowledges financial support from the Dutch Research Council (NWO; grant VI.Veni.192.241). T.S. acknowledges support from JSPS KAKENHI (grant nos. JP20H05845 and JP21H01145) and Leading Initiative for Excellent Young Researchers, MEXT, Japan. F.S. acknowledges funding from JWST/NIRCam contract to the University of Arizona, NAS5-02105. M.N.D. acknowledges the Holcim Foundation Stipend, the Swiss National Science Foundation (SNSF) Ambizione grant number 180079, the Center for Space and Habitability (CSH) Fellowship and the IAU Gruber Foundation Fellowship. S.I. and H.L. acknowledge support from the Danish National Research Foundation through the Center of Excellence ‘InterCat’ (grant agreement number DNRF150). I.J-.S acknowledges funding from grant nos. PID2019-105552RB-C41 and PID2022-136814NB-I00 from the Spanish Ministry of Science and Innovation/State Agency of Research MCIN/AEI/10.13039/501100011033 and by ‘ERDF A way of making Europe’. This work was supported by a grant from the Simons Foundation (686302, KIÖ) and an award from the Simons Foundation (321183FY19, KIÖ). W.R.M.R. acknowledges support from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 101019751 MOLDISK). N. Pirzkal is acknowledged for sharing the NIRCam Grism V6 sensitivity curves for spectral calibration. A. J. Galaviz III (SwRI) is acknowledged for graphic design. N. Munch Mikkelsen is acknowledged for sharing his simulations of mixed water and carbon dioxide ices. This research has made use of data from the Herschel Gould Belt survey (HGBS) project. The HGBS is a Herschel Key Programme jointly carried out by SPIRE Specialist Astronomy Group 3 (SAG 3), scientists of several institutes in the PACS Consortium (CEA Saclay, INAF-IFSI Rome and INAF-Arcetri, KU Leuven, MPIA Heidelberg) and scientists of the Herschel Science Center (HSC).

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Authorship is defined according to the CRediT (https://credit.niso.org/) taxonomy: J.A.N. (conceptualization, methodology, software, validation, formal analysis, investigation, resources, writing—original draft, writing—review and editing, visualization, supervision, project administration and funding acquisition); H.J.F. (conceptualization, methodology, validation, resources, writing—original draft, writing—review and editing, supervision, project administration and funding acquisition); Z.L.S. (conceptualization, methodology, software, validation, formal analysis, investigation, resources, data curation, writing—original draft, writing—review and editing, and visualization); E.D. (conceptualization, methodology, software, validation, formal analysis, investigation, resources, writing—original draft, writing—review and editing, and project administration); A.C.A.B. (conceptualization, methodology, investigation, resources, writing—review and editing, supervision, project administration and funding acquisition); H.M.C. (conceptualization, methodology, validation, formal analysis, investigation, writing—original draft, and writing—review and editing); H.J.D. (conceptualization, methodology, software, validation, formal analysis, investigation and supervision); F.D. (conceptualization, methodology, validation, formal analysis, investigation, resources, writing—original draft, writing—review and editing, and supervision); E.E. (conceptualization, methodology, software, validation, formal analysis, investigation and supervision); J.E. (conceptualization, methodology, software, validation, formal analysis and investigation); B.M.G. (conceptualization, methodology, validation, formal analysis, investigation, resources and writing—review and editing); B.H. (conceptualization, methodology, validation, formal analysis and investigation); T.L. (conceptualization, methodology, validation, formal analysis, investigation, writing—original draft, writing—review and editing, and supervision); B.M. (conceptualization, methodology, validation, formal analysis, investigation, writing—original draft, and writing—review and editing); M.K.M. (conceptualization, methodology, validation, formal analysis, investigation, resources, data curation, writing—original draft, writing—review and editing, project administration and funding acquisition); M.E.P. (conceptualization, methodology, validation, formal analysis, investigation, writing—original draft, writing—review and editing, and supervision); T.S. (conceptualization, methodology, software, validation, formal analysis, investigation, writing—original draft, and writing—review and editing); F.S. (conceptualization, methodology, software, validation, formal analysis and investigation); J.B.B. (writing—review and editing); W.A.B. (validation, and writing—review and editing); P.C. (writing—review and editing); E.C. (investigation, and writing—review and editing); M.N.D. (writing—review and editing); V.J.H. (investigation, and writing—review and editing); S.I. (writing—review and editing); I.J.-S. (writing—review and editing); H.L. (writing—review and editing); G.J.M. (validation, and writing—review and editing); B.A.M. (writing—review and editing); K.I.O. (writing—review and editing); G.P. (writing—review and editing); D.Q. (visualization, and writing—review and editing); W.R.M.R. (writing—review and editing); R.G.U. (investigation, and writing—review and editing).

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Correspondence to J. A. Noble.

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Extended data

Extended Data Fig. 1 The positions of the eight objects where dOH was observed in Cha I in this work.

The object positions are overplotted on the NIRCam imaging frame F200W in orange, and H2 column density contours derived from Herschel far-IR observations52,53,54 in grey. This non-exhaustive list of detections traces lines of sight probing the densest region of the Cha I cloud where the most ice and dust was observed in the NIRCam WFSS observations as part of the Ice Age program8. (Smith et al. B, in prep). The two yellow stars represent the positions of Class I (NE) and Class 0 (SW) protostars targeted with pointed observations in the Ice Age program (PID 1309). The two background stars already described in refs. 8,25 are shown in cyan circles, while the remaining six objects presented here are in black circles. The distance between the two objects marked in cyan is ~ 10,000 au.

Extended Data Fig. 2 The process of data analysis for the dOH absorption features.

Upper panel: Local baseline (orange) fitted to convert spectral data on a flux scale (black) to an optical depth scale (shown in Fig. 2). Here, the spectrum in blue is the normalised flux, that is measured flux/continuum flux. Lower panel: photospheric determination was performed as described in the Methods, with fitting weighted to the lowest wavelength range of the data, as highlighted in the panel. The best-fit PHOENIX simulation (green) well reproduces the narrow photospheric absorption features that dominate the observed normalised flux spectrum (blue).

Extended Data Fig. 3 Illustration of effects of NIRCam wavelength and sensitivity calibration updates on the resulting extracted spectrum of NIR38 (object 077).

The first extraction with the optimal box method (original wavelength and sensitivity calibration) is presented offset in grey8 while the updated extraction is presented in black. Spectra are extracted and locally baselined as described in the Methods section. Positions of absorption features of pure CO2 combination modes (orange) and dOH from pure H2O (blue) are shown as dotted lines. The area under the ~ 2.7 μm band in the spectrum presented in ref. 8 (grey) that could potentially be attributed to pure CO2 combination modes was ~ 18% (orange fitted peak) and dOH in water ice was ~ 82% (blue fitted peak). In the spectrum presented here, extracted with the updated wavelength calibration, sensitivity curve and extraction method (black curve), the contribution of pure CO2 combination modes is only < 4%. These values are derived from fits, as described in the Methods section, and without taking into account photospheric features which potentially contribute in this region, as can be seen by inspection of the photospheric model spectrum (green).

Extended Data Fig. 4 The observed spectra compared to laboratory data for pure and mixed ices, normalised to the peak of the ~ 3 μm absorption band of H2O, with no local baseline correction.

Lower panel: observed optical depth spectrum (black) derived by stacking the spectra of the four highest flux sources (object 077 in green, object 282 in dark blue, object 268 in red, and object 092 in light blue). Upper panel: Water ices deposited onto substrates held at increasing temperatures to form increasingly compact amorphous ices; ices prepared at 10 K (light blue), 60 K (pink), 90 K (orange) and normalised to 3.05 μm. Measured in transmission at 10 K. Second panel from top: water ices prepared by increasingly energetic processes to form increasingly compact amorphous ices (background deposition at 10 K (dark blue), background deposition at 110 K (orange), formed on surface by radical reactions at 10 K (green)), normalised to 2.95 μm and measured by RAIRS at 10 K. Third panel from top: laboratory binary ice mixtures of H2O with CO2 (dark blue), CO (orange), NH3 (light blue), CH4 (green), and an example ‘disruptive’ organic molecule (in this case pyrene, yellow dash-dot) measured in transmission. A pure H2O spectrum deposited at 60 K is shown in pink. Laboratory spectra were selected such that the abundance of the second molecular species in the H2O ice was similar to that determined for NIR38 (object 077) in refs. 8,25.

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Noble, J.A., Fraser, H.J., Smith, Z.L. et al. Detection of the elusive dangling OH ice features at ~2.7 μm in Chamaeleon I with JWST NIRCam. Nat Astron (2024). https://doi.org/10.1038/s41550-024-02307-7

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