X-ray photo-desorption of NH3 and N2 from ammonia ices: Mechanisms and yields in protoplanetary disks

Abstract

Context. Ammonia is detected both in the gas phase and in the ices of protoplanetary disks. However, its gas phase abundances are still difficult to reproduce via chemical modelling when only the thermal-, UV photon-, and cosmic ray-induced processes are considered. Among other non-thermal mechanisms, X-ray photo-desorption is a relevant pathway to maintaining a budget of molecules in the gas of disks. However, no quantitative data are available to constrain its efficiency in the case of ammonia-containing ices.Aims. We studied the desorption induced by soft X-rays from ammonia ices to determine the photo-desorption yields of neutral molecules that can be applied to the conditions of protoplanetary disks. We also aim to identify the desorption mechanisms at play.Methods. Pure ammonia ices were deposited at 23 or 75 K and irradiated between 395 and 435 eV, with monochromatic synchrotron light. Desorption of neutral molecules and fragments was detected using a quadrupole mass spectrometer, calibrated to obtain des-orption yields. These values were extrapolated and the spectrum of a protoplanetary disk was used to extract average astrophysical desorption yields.Results. Photo-desorption from NH_{3 ices is dominated by the desorption of neutral NH3 and N2 molecules. The desorption mechanism mostly involves the thermalisation of Auger electrons in the ice, although resonant phenomena also contribute near the N 1s ionisation edge. The NH3 photo-desorption is independent of ice morphology and irradiation temperature. Contrary to NH3 desorption, N2 desorption is sensitive to the photon fluence received by the ice. Average photo-desorption yields derived using the TW Hya X-ray spectrum reveal that the NH3 photo-desorption would be four to six times more efficient than that of H2O. This could be at the origin of unexplained high NH3/H2O abundance ratios predicted in the disk around TW Hya. This result ought to be confirmed by the study of mixed water-ammonia ices.}