Water ice deposition and growth in molecular clouds

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Water ice deposition and growth in molecular clouds

Rawlings, JMC; Williams, DA; (2021) Water ice deposition and growth in molecular clouds. Monthly Notices of the Royal Astronomical Society , 500 (4) pp. 5117-5128. 10.1093/mnras/staa3578. (In press). Green open access

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Abstract

In interstellar clouds, the deposition of water ice on to grains only occurs at visual extinctions above some threshold value (Ath). At extinctions greater than Ath, there is a (near-linear) correlation between the inferred column density of the water ice and AV. For individual cloud complexes such as Taurus, Serpens, and ρ-Ophiuchi, Ath and the gradients of the correlation are very similar along all lines of sight. We have investigated the origin of this phenomenon, with careful consideration of the various possible mechanisms that may be involved and have applied a full chemical model to analyse the behaviours and sensitivities in quiescent molecular clouds. Our key results are as follows: (i) the ubiquity of the phenomenon points to a common cause, so that the lines-of-sight probe regions with similar, advanced, chemical, and dynamical evolution; (ii) for Taurus and Serpens Ath and the slope of the correlation can be explained as resulting from the balance of freeze-out of oxygen atoms and photodesorption of H2O molecules. No other mechanism can satisfactorily explain the phenomenon; (iii) Ath depends on the local density, suggesting that there is a correlation between local volume density and column density; (iv) the different values of Ath for Taurus and Serpens are probably due to variations in the local mean radiation field strength; (v) most ice is accreted on to grains that are initially very small (<0.01\,\mum); and (vi) the very high value of Ath observed in ρ-Ophiuchi cannot be explained in the same way, unless there is complex microstructure and/or a modification to the extinction characteristics.

Type:	Article
Title:	Water ice deposition and growth in molecular clouds
Open access status:	An open access version is available from UCL Discovery
DOI:	10.1093/mnras/staa3578
Publisher version:	http://dx.doi.org/10.1093/mnras/staa3578
Language:	English
Additional information:	This version is the version of record. For information on re-use, please refer to the publisher’s terms and conditions.
Keywords:	astrochemistry, dust, extinction, ISM: molecules
UCL classification:	UCL UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Physics and Astronomy
URI:	https://discovery.ucl.ac.uk/id/eprint/10120723