Cosmic-ray induced sputtering of interstellar formaldehyde ices

Context. In the cold and dense regions of the interstellar medium (ISM), for example in prestellar cores, gas-phase chemical abundances undergo a steep decrease due to the freeze-out of molecules onto the dust grain surfaces. While the depletion of many species would bring molecular abundances to undetected levels within short timescales, non-thermal desorption mechanisms such as UV photodesorption or cosmic-ray sputtering allows the return of a fraction of the ice mantle species back to the gas phase and prevents a complete freeze-out in the densest regions. In the last decade much effort has been devoted to understanding the microphysics of desorption and quantifying molecular desorption yields. Aims. 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. Methods. Using a heavy-ion beam as a cosmic-ray analogue at the Grand Acc & eacute;l & eacute;rateur National d'Ions Lourds (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. In addition, using IRAM millimetre observations, we also determined the H2CO gas-phase abundance in the prestellar core L1689B. Results. 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 of 2.5 x 10(3) molecules ion(-1) for an electronic stopping power of S-e similar to 2830 eV (10(15) molecules cm(-2))(-1). The corresponding cosmic-ray sputtering rate is Gamma(CRD) = 1.5 x 10(18)zeta molecules cm(-2) s(-1), where zeta 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.