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Register a new userX-ray induced desorption and photochemistry in CO ice
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We report an investigation of X-ray induced desorption of neutrals, cations and anions from CO ice. The desorption of neutral CO, by far the most abundant, is quantified and discussed within the context of its application to astrochemistry. The desorption of many different cations, including large cations up to the mass limit of the spectrometer, are observed. In contrast, the only desorbing anions detected are O$^-$ and C$^-$. The desorption mechanisms of all these species are discussed with the aid of their photodesorption spectrum. The evolution of the X-ray absorption spectrum shows significant chemical modifications of the ice upon irradiation, which along with the desorption of large cations gives a new insight into X-ray induced photochemistry in CO ice.
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Millimeter and centimeter observations are discovering an increasing number of interstellar complex organic molecules (iCOMs) in a large variety of star forming sites, from the earliest stages of star formation to protoplanetary disks and in comets. In this context it is pivotal to understand how the solid phase interactions between iCOMs and grain surfaces influence the thermal desorption process and, therefore, the presence of molecular species in the gas phase. In laboratory, it is possible to simulate the thermal desorption process deriving important parameters such as the desorption temperatures and energies. We report new laboratory results on temperature-programmed desorption (TPD) from olivine dust of astrophysical relevant ice mixtures of water, acetonitrile, and acetaldehyde. We found that in the presence of grains, only a fraction of acetaldehyde and acetonitrile desorbs at about 100 K and 120 K respectively, while 40% of the molecules are retained by fluffy grains of the order of 100 {mu}m up to temperatures of 190-210 K. In contrast with the typical assumption that all molecules are desorbed in regions with temperatures higher than 100 K, this result implies that about 40% of the molecules can survive on the grains enabling the delivery of volatiles towards regions with temperatures as high as 200 K and shifting inwards the position of the snowlines in protoplanetary disks. These studies offer a necessary support to interpret observational data and may help our understanding of iCOMs formation providing an estimate of the fraction of molecules released at various temperatures.
Electronic excitations near the surface of water ice lead to the desorption of adsorbed molecules, through a so far debated mechanism. A systematic study of photon-induced indirect desorption, revealed by the spectral dependence of the desorption (7 to 13 eV), is conducted for Ar, Kr, N2, and CO adsorbed on H2O or D2O amorphous ices. The mass and isotopic dependence and the increase of intrinsic desorption efficiency with photon energy all point to a mechanism of desorption induced by collisions between adsorbates and energetic H or D atoms, produced by photodissociation of water. This constitutes a direct and unambiguous experimental demonstration of the mechanism of indirect desorption of weakly adsorbed species on water ice, and sheds new light on the possibility of this mechanism in other systems. It also has implications for the description of photon-induced desorption in astrochemical models.
Solid O2 has been proposed as a possible reservoir for oxygen in dense clouds through freeze-out processes. The aim of this work is to characterize quantitatively the physical processes that are involved in the desorption kinetics of CO-O2 ices by interpreting laboratory temperature programmed desorption (TPD) data. This information is used to simulate the behavior of CO-O2 ices under astrophysical conditions. The TPD spectra have been recorded under ultra high vacuum conditions for pure, layered and mixed morphologies for different thicknesses, temperatures and mixing ratios. An empirical kinetic model is used to interpret the results and to provide input parameters for astrophysical models. Binding energies are determined for different ice morphologies. Independent of the ice morphology, the desorption of O2 is found to follow 0th-order kinetics. Binding energies and temperature-dependent sticking probabilities for CO-CO, O2-O2 and CO-O2 are determined. O2 is slightly less volatile than CO, with binding energies of 912+-15 versus 858+-15 K for pure ices. In mixed and layered ices, CO does not co-desorb with O2 but its binding energies are slightly increased compared with pure ice whereas those for O2 are slightly decreased. Lower limits to the sticking probabilities of CO and O2 are 0.9 and 0.85, respectively, at temperatures below 20K. The balance between accretion and desorption is studied for O2 and CO in astrophysically relevant scenarios. Only minor differences are found between the two species, i.e., both desorb between 16 and 18K in typical environments around young stars. Thus, clouds with significant abundances of gaseous CO are unlikely to have large amounts of solid O2.
{CircumStellar Envelopes (CSEs) of stars are complex chemical objects for which theoretical models encounter difficulties in elaborating a comprehensive overview of the occurring chemical processes. Along with photodissociation, ion-neutral reactions and dissociative recombination might play an important role in controlling molecular growth in outer CSEs. The aim of this work is to provide experimental insights into pathways of photochemistry-driven molecular growth within outer CSEs to draw a more complete picture of the chemical processes occurring within these molecule-rich environments. A simplified CSE environment was therefore reproduced in the laboratory through gas-phase experiments exposing relevant gas mixtures to an Extreme UltraViolet (EUV) photon source. This photochemical reactor should ultimately allow us to investigate chemical processes and their resulting products occurring under conditions akin to outer CSEs. We used a recently developed EUV lamp coupled to the APSIS photochemical cell to irradiate CSE relevant gas mixtures of H$_2$, CO and N$_2$, at one wavelength, 73.6 nm. The detection and identification of chemical species in the photochemical reactor was achieved through in-situ mass spectrometry analysis of neutral and cationic molecules. We find that exposing CO-N$_2$-H$_2$ gas mixtures to EUV photons at 73.6 nm induces photochemical reactions that yield the formation of complex, neutral and ionic species. Our work shows that N$_2$H$^+$ can be formed through photochemistry along with highly oxygenated ion molecules like HCO$^+$ in CSE environments. We also observe neutral N-rich organic species including triazole and aromatic molecules. These results confirm the suitability of our experimental setting to investigate photochemical reactions and provide fundamental insights into the mechanisms of molecular growth in the outer CSEs.
UV photochemistry in the surface layers of protoplanetary disks dramatically alters their composition relative to previous stages of star formation. The abundance ratio CN/HCN has long been proposed to trace the UV field in various astrophysical objects, however to date the relationship between CN, HCN, and the UV field in disks remains ambiguous. As part of the ALMA Large Program MAPS (Molecules with ALMA at Planet-forming Scales), we present observations of CN N=1-0 transitions at 0.3 resolution towards five disk systems. All disks show bright CN emission within $sim$50-150 au, along with a diffuse emission shelf extending up to 600 au. In all sources we find that the CN/HCN column density ratio increases with disk radius from about unity to 100, likely tracing increased UV penetration that enhances selective HCN photodissociation in the outer disk. Additionally, multiple millimeter dust gaps and rings coincide with peaks and troughs, respectively, in the CN/HCN ratio, implying that some millimeter substructures are accompanied by changes to the UV penetration in more elevated disk layers. That the CN/HCN ratio is generally high (>1) points to a robust photochemistry shaping disk chemical compositions, and also means that CN is the dominant carrier of the prebiotically interesting nitrile group at most disk radii. We also find that the local column densities of CN and HCN are positively correlated despite emitting from vertically stratified disk regions, indicating that different disk layers are chemically linked. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.
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