Abstract
Formed at an early stage of gas-phase ion-molecule chemistry, hydrides – molecules containing a heavy element covalently bonded to one or more hydrogen atoms – play an important role in interstellar chemistry as they are the progenitors of larger and more complex species in the interstellar medium. In recent years, the careful analysis of the spectral signatures of hydrides have led to their use as tracers of different constituents, and phases of the interstellar medium and in particular the more diffuse environments. Diffuse clouds form an essential link in the stellar gas life-cycle as they connect both the late and early stages of stellar evolution. As a result, diffuse clouds are continuously replenished by material which makes them reservoirs for heavy elements and hence ideal laboratories for the study of astrochemistry. This review will journey through a renaissance of hydride observations detailing puzzling hydride discoveries and chemical mysteries with special focus carbon-bearing hydrides to demonstrate the big impact of these small molecules and ending with remarks on the future of their studies.
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Data Availability
All data generated or analysed during this study are included in this published article (and its supplementary information files).
Notes
Now accessible with the Mid-Infrared Instrument on the James Webb Space Telescope (JWST) (Rieke et al. 2015) after the end of the Spitzer mission.
The optical depth, \(\tau \), can be estimated from the ratio between the observed line brightness temperature (\(T_{\mathrm{l}}\)) and the background continuum temperature (\(T_{ \mathrm{c}}\)) following the radiative transfer (RT) equation. In the case of absorption line spectroscopy, the RT equation can be approximated to \(T_{\mathrm{l}} = T_{c}{\mathrm{e}}^{-\tau}\).
SOFIA Science Mission Operations is jointly operated by the Universities Space Research Association, Inc., under NASA contract NAS2-97001, and the Deutsches SOFIA Institut under DLR contract 50 OK 0901 and 50 OK 1301 to the University of Stuttgart. upGREAT is financed by resources from the participating institutes, and by the Deutsche Forschungsgemeinschaft (DFG) within the grant for the Collaborative Research Centre 956, as well as by the Federal Ministry of Economics and Energy (BMWI) via the German Space Agency (DLR) under Grants 50 OK 1102, 50 OK 1103 and 50 OK 1104.
The National Radio Astronomy Observatory (NRAO) is operated by Associated Universities Inc., under a collaborative agreement with the US National Science Foundation.
Observations carried out under the umbrella of the JWST ERS program PDRS4all (Berné et al. 2022).
IRAS 16293–2422 is a low-mass protostar located within the Ophiuchus star-forming complex and hosts a rich chemistry, owing to which it has been extensively studied, for example under the ALMA Protostellar interferometric line survey (PILS; Coutens et al. 2016).
The beam filling factor is given by \(({\theta _{\normalfont{\mathrm{S}}}^{2} + \theta _{\normalfont{\mathrm{B}}}^{2})/\theta _{\normalfont{\mathrm{S}}}^{2}}\), where \(\theta _{\normalfont{\mathrm{S}}}\) and \(\theta _{\normalfont{\mathrm{B}}}\) are the FWHM source, and beam sizes, respectively.
The nominal position toward Orion KL, corresponds to the peak of the CO molecular line emission and is located at R.A., Dec. = \({05^{\mathrm{h}}35^{\mathrm{m}}14_{\boldsymbol{\cdot}}^{\mathrm{s}}10}, {-05^{\circ}22^{\prime}26 _{\boldsymbol{\cdot}}^{\prime \prime}54}\).
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Acknowledgements
A.M.J is grateful to the anonymous referee, for a careful review of the manuscript of this article and their insightful comments which have helped to improved this manuscript. A.M.J would like to thank Karl M. Menten and David Neufeld for their support and guidance and Paule Sonnentrucker, Michael Busch and Michael R. Rugel for their comments on specific sections of this summary. Part of this research was carried out when A.M.J. was a member of the International Max Planck Research School (IMPRS) for Astronomy and Astrophysics at the Universities of Bonn and Cologne. A.M.J. was generously supported by USRA through a grant for SOFIA Program 08-0038.
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A.M.J. was generously supported by USRA through a grant for SOFIA Program 08-0038.
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Appendix: Spectroscopic parameters
Appendix: Spectroscopic parameters
This appendix presents the frequencies and relevant spectroscopic parameters of the different transitions discussed in this work for CH and its isotopologues in Tables A1 and A2, and \(\mathrm{CH}_{2}\) in Table A3, respectively.
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Jacob, A.M. Small molecules, big impact: a tale of hydrides past, present, and future. Astrophys Space Sci 368, 76 (2023). https://doi.org/10.1007/s10509-023-04229-8
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DOI: https://doi.org/10.1007/s10509-023-04229-8