2020同步年報

022 ACTIVITY REPORT 2020 Fig. 1 : Sketch of the bilayer ice experiment. (a) The bottom H 2 O:CH 4 :NH 3 mixture covered by a layer of CO:CH 3 OH. (b) X-ray irradiation of the ice induc- es a rapid destruction of CH 3 OH, leading to the formation of new species rather than its photodesorption. During the irradiation, a negligible desorption of CH 3 OH was detected, whereas CO and products such as HCO, H 2 CO and CO 2 show the most intense desorption signals. Desorption from the bottom layer species was also detected. [Reproduced from Ref. 1] The cold gas in protoplanetary discs seems to be devoid of complicated organic species (known as COMs in the astron- omy jargon). Whereas CO, CO 2 , HCO and H 2 CO are typically abundant molecules in the cold zones of the disc, methanol or acetonitrile are found in only a few regions, and more complicated organic species are not observed. This effect is to some extent unexpected, as chemical and physical conditions in discs appear not so drastically different from those in other interstellar regions, in striking contrast with the analysis of meteorites and comets that are instead rich in complicated organic species. Laboratory experiments using the right ingredients The work, 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 con- figuration composed of two layers. Indeed, hydrogenation of O, C and N on the bare dust surface produced a first layer of H 2 O, CH 4 , NH 3 and other reduced species. On top there- of, a second layer of species was formed in the gaseous phase and required lower temperatures to stick onto the dust, dominated by CO and CH 3 OH, the latter presumably formed by hydrogenation of CO in the ice. The source of radiation, soft X-rays, was provided by TLS 08B1 in NSRRC. These X-rays irradiating two layers in experiments led to either desorption of the ice molecules during irradiation or the destruction of molecules ( Fig. 1 ). As a result of this breakage, more COMs were formed. Comparison with the observed molecular distribution Particular attention was paid to the desorption of mole- cules during the irradiation, as this condition allows com- parison with recent observations of protoplanetary disks using the Atacama Large Millimeter Array, ALMA. The absence or small abundance of complicated species from the cold gas in protoplanetary disks, and the presence of abundant CO, HCO and H 2 CO and negligible CH 3 OH, is compatible with this laboratory simulations of X-ray pro- cessing 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 gaseous phase. This find- ing is supported by the rich chemical inventory identified in the disc around V883 Ori, a system in which a suddenly increased luminosity of the central star quickly expanded the snow lines into the disc, creating a “sublimation front”. As the chemical implications of X-ray-rich environments are, so far, relatively unexplored, much more work remains to be done, and NSRRC will generate a fundamental and precious contribution. (Reported by Yu-Jung Chen and his collabora- tors, National Central University) This report features the work of Yu-Jung Chen, Angela Ciaravella and their collaborators, published in PNAS 117 , 16149 (2020). TLS 08B1 BM – AGM • XPS, XAS • Materials Science, Chemistry, Surface, Astro-Physics Reference 1. A. Ciaravella, G. M. Munoz Caro, A. Jimenez-Escobar, C. Cecchi-Pestellini, L.-C. Hsiao, C.-H. Huang, Y.-J. Chen, Proc. Natl. Acad. Sci. USA 117 , 16149 (2020). (a) (b)