X-ray Irradiation of Ices

    During the last decade, stellar high-energy emission has been recognized as important in the formation and evolution of stars and planetary systems, as well as in the origin and evolution of life. The higher penetrability of X-rays into the hydrogen-rich interstellar gas suggests that their role in inducing chemical evolution both on ices and gas-phase molecules needs to be taken into account in computational models and for the interpretation of observational data.

    The effects of X-rays on ices start with the photon absorption by parent molecules, which experience fragmentation. In general, the fragmentation events are induced by the ionization or excitation of an inner-shell electron followed by either normal or resonant Auger decays, with the normal Auger decay active above the ionization threshold. The injection of energetic photoelectrons produces multiple ionization events generating a secondary electron cascade that dominates the chemistry. Impact ionizations of water and carbon monoxide are the major loss channels for photoelectrons with energies larger than few tens of eV. As electrons degrade their energies through subsequent ionization events, they eventually become unable to produce further ionizations and start to interact with molecules either via excitation impacts and through dissociative electron attachment, a process in which a molecule captures a low-energy electron in an excited resonant state, eventually ending up in the fragmentation of the molecular anion.

    We used an energy tunable soft X-ray synchrotron beamline provided by NSRRC to study X-ray irradiation of interstellar ice analogs. As a result of our recent X-ray irradiation of H2O:CO:NH3 ice mixtures experiments, many nitrogen-bearing molecules are identified such as, e.g., OCN⁻, NH4⁺, HNCO, CH3CN, HCONH2, and NH2COCONH2. Several infrared features of the irradiated ice mixture are compatible with the presence of glycine or its isomers. Detection of several masses in gas phase during the irradiation and warm-up revealed a number of COMs (e.g., C3H3NO, C4H5O, and C4H7N) with no clear features attributable to these species in the infrared spectra of the irradiated ice. The present results show that X-ray irradiation of the H2O:CO:NH3 ice mixture may originate a chemistry rich in organic compounds, most of them likely to be desorbed to the gas phase via photo and/or thermal desorption. Such species have been detected in many astrophysical environments and in particular in circumstellar regions. In this latter case, X-rays can permeate the disk and reach deeper regions, where less energetic radiation such as VUV is inhibited, triggering a solid phase chemistry that through photodesorption can enrich the surrounding medium with COMs.

    The physical conditions in a protoplanetary disk vary greatly, with hot and dense regions of gas and dust near the star and much colder material at greater distances from it. In protoplanetary disks, the inner edge of the region at which the temperature falls below the condensation temperature of a volatile substance is referred to as the snow line (for that species). Each volatile has a distinct location of snow line, water ice being nearest the host star, farther on CO2 and then CO. As a consequence, the ices within a disk are organized in a bi-layered structure of segregated polar (water-rich) and apolar (water-poor) components. The initial water-rich layer is thought to form early in the disk through hydrogenation of atomic oxygen. The bulk of solid CH4 and NH3 is likely also formed at this stage, through hydrogenation of carbon and nitrogen. As the disk gradually cools, free-flying molecules are removed from the gas, of which the main component (after volatile H2 not sticking to dust) is carbon monoxide. The formation of a layer of CO ice provides a feedstock for the formation of icy methanol through hydrogenation of CO. The birth of methanol marks the first generation of complex species.

    Our recent study, published in PNAS, extended a step further than previous experiments. Instead of mixing common volatile species in the ice sample, an analogue of ice mantles was prepared that took into account the more realistic ice configuration composed of two layers. X-ray irradiation of two layers led to either desorption of the ice molecules during irradiation or the destruction of molecules. During the irradiation, a negligible desorption of CH3OH was detected, whereas CO and products such as HCO, H2CO and CO2 show the most intense desorption signals, meanwhile desorption from the bottom layer species was also detected. Particular attention was paid to the desorption of molecules during the irradiation, as this condition allows comparison with recent observations of protoplanetary disks using the Atacama Large Millimeter Array, ALMA. The absence or small abundance of complex species from the cold gas in protoplanetary disks, and the presence of abundant CO, HCO and H2CO and negligible CH3OH, is compatible with our laboratory simulations of X-ray processing of realistic ice. Moreover, these experiments offer an explanation of the particularly small abundances of other COMs in the cold parts of the disk, as they are formed in the ice bulk but not ejected into the gas phase. This finding is supported by the rich chemical inventory identified in the disk around V883 Ori, a system in which a suddenly increased luminosity of the central star quickly expanded the snow lines into the disk, creating a "sublimation front".