The photodissociation and chemistry of CO isotopologues: applications to interstellar clouds and circumstellar disks

Abstract

Aims. Photodissociation by UV light is an important destruction mechanism for carbon monoxide (CO) in many astrophysical environments, ranging from interstellar clouds to protoplanetary disks. The aim of this work is to gain a better understanding of the depth dependence and isotope-selective nature of this process.Methods. We present a photodissociation model based on recent spectroscopic data from the literature, which allows us to compute depth-dependent and isotope-selective photodissociation rates at higher accuracy than in previous work. The model includes self-shielding, mutual shielding and shielding by atomic and molecular hydrogen, and it is the first such model to include the rare isotopologues C 17 O and 13 C 17 O. We couple it to a simple chemical network to analyse CO abundances in diffuse and translucent clouds, photon-dominated regions, and circumstellar disks. Results. The photodissociation rate in the unattenuated interstellar radiation field is 2.6 × 10 −10 s −1 , 30% higher than currently adopted values. Increasing the excitation temperature or the Doppler width can reduce the photodissociation rates and the isotopic selectivity by as much as a factor of three for temperatures above 100 K. The model reproduces column densities observed towards diffuse clouds and PDRs, and it offers an explanation for both the enhanced and the reduced N( 12 CO)/N( 13 CO) ratios seen in diffuse clouds.

Branching Ratio Measurements for Vacuum Ultraviolet Photodissociation of 12C16O

“…This observation is in agreement with previous studies 8,11,24 that both 1 Π− 1 Σ + and 1 Π− 1 Π interactions play important roles in the predissociation of CO into the singlet channel and that the repulsive 1 Σ + state has been identified as the D′ 1 Σ + state, but the 1 Π state has not yet been identified. 2,17 According to the perfect linear dependence of the 1/R value on J′(J′ + 1), we should also be able to conclude that k T in eq 7 does not depend on the rotational quantum number J′. This explains why Okazaki et al 11 can use k 0 T , which is equivalent to k T here, as a parameter (independent of the rotational quantum number J′) Σ + (v′ = 0) state, the percentage into the triplet channel is much smaller than that into the singlet channel; thus, the ratio R = k T /k S ≈ k T /(k T + k S ) = P T .…”

Computational Modeling of High Temperature Gas-Grain Chemistry in Interstellar Medium (ISM)

“…The amount of carbon locked in the form of CO as opposed to C or C + will determine the abundance of small and large carbon-chain molecules. 36,56 There was a minor decrease in CO in the gas phase which can be attributed to it accreting onto the grain surface in a chemisorption site. There was also a slight increase in abundance of atomic carbon in the gas phase past 400 K which can be attributed to the chemisorbed species desorbing into the gas phase in this regime.…”

“…For CO self-shielding and cross-shielding the method of Visser et al was adopted and coupled to the network. 36 These regions can also be subjected to high fluxes of X-rays. X-rays can either ionize atoms directly and produce photo-electrons, or the photo-electrons can cause secondary ionization.…”

“…The decrease in abundance of water seen at high temperatures is due to an increase in destruction pathways, for example an increase in H and C + at 800 K will increase the rates of the following reactions: Carbon monoxide (CO) is one of the most abundant molecules in astrophysical environments, second only in abundance to molecular hydrogen. 36 It is the main gas phase reservoir of interstellar carbon and therefore controls much of the gas phase and grain surface chemistry in ISM, especially in the Fischer-Tropsch process which is a focus here. 55 For this reason we thought it pertitent to explore it's fractional abundance when considering the new chemisorption processes added.…”

“…The amount of carbon locked in the form of CO as opposed to C or C + will determine the abundance of small and large carbon-chain molecules. 36,56 The slight decrease in gas phase abundance is simply due to CO accreting into a chemisorption site on the grain surface with a binding energy of 40,258 K. CO then dissociates on the grain into atomic carbon and oxygen which are bound in chemisorption sites on the grain surface. Atomic carbon begins to be enhanced in the gas phase by more then an order of magnitude past 400 K when comparing these results to models ran without considering chemisorption.…”