Cosmic-ray Induced Sputtering Of Interstellar Formaldehyde Ices

H₂CO is a ubiquitous molecule in the ISM and in the gas phase of prestellar cores, and is likely present in ice mantles, but its main desorption mechanism is unknown.

In this paper our aim is to quantify the desorption efficiency of H₂CO upon cosmic-ray impact in order to determine whether cosmic-ray induced sputtering could account for the H₂CO abundance observed in prestellar cores. Using a heavy-ion beam as a cosmic-ray analogue at the GANIL accelerator, we irradiated pure H₂CO ice films at 10 K under high vacuum conditions and monitored the ice film evolution with infrared spectroscopy and the composition of the sputtered species in the gas phase using mass spectrometry.

We derived both the effective and intact sputtering yield of pure H₂CO ices. We find that H₂CO easily polymerises under heavy-ion irradiation in the ice, and is also radiolysed into CO and CO₂. In the gas phase, the dominant sputtered species is CO and intact H₂CO is only a minor species. We determine an intact sputtering yield for pure H₂CO ices 2.5×10³ molecules ion⁻¹ for an electronic stopping power of S_e∼2830 eV (10¹⁵ molecules cm⁻²)⁻¹.

The corresponding cosmic-ray sputtering rate is Γ_CRD=1.5×10⁻¹⁸ζ molecules cm⁻² s⁻¹, where ζ is the rate of cosmic-ray ionisation of molecular hydrogen in the ISM. In the frame of a simple steady-state chemical model of freeze-out and non-thermal desorption, we find that this experimental cosmic-ray sputtering rate is too low (by an order of magnitude) to account for the observed H₂CO gas-phase abundance we derived in the prestellar core L1689B.

We find however that this abundance can be reproduced if we assume that H₂CO diluted in CO or CO₂ ices co-desorbs at the same sputtering rate as pure CO or pure CO₂ ices.

M. Faure (1), A. Bacmann (1), A. Faure (1), E. Quirico (1), P. Boduch (2), A. Domaracka (2), H. Rothard (2) ((1) IPAG, Grenoble, France, (2) GANIL, Caen, France)

Comments: accepted by Astronomy and Astrophysics. 17 pages, 17 figures. Abridged abstract
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2409.01700 [astro-ph.GA] (or arXiv:2409.01700v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2409.01700
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Submission history
From: Aurore Bacmann
[v1] Tue, 3 Sep 2024 08:37:42 UTC (5,250 KB)
https://arxiv.org/abs/2409.01700
Astrobiology, Astrochemistry,