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Register a new userThe photodissociation of CO in circumstellar envelopes
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M.A.T. Groenewegen
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Carbon monoxide is the most abundant molecule after H$_2$ and is important for chemistry in circumstellar envelopes around late-type stars. The size of the envelope is important when modelling low-J transition lines and deriving mass-loss rates from such lines. Now that ALMA is coming to full power the extent of the CO emitting region can be measured directly for nearby asymptotic giant branch (AGB) stars. In parallel, it has become obvious in the past few years that the strength of the interstellar radiation field (ISRF) can have a significant impact on the interpretation of the emission lines. In this paper an update and extension of the classical Mamon et al. (1988; ApJ 328, 797) paper is presented; these authors provided the CO abundance profile, described by two parameters, as a function of mass-loss rate and expansion velocity. Following recent work an improved numerical method and updated H$_2$ and CO shielding functions are used and a larger grid is calculated that covers more parameter space, including the strength of the ISRF. The effect of changing the photodissociation radius on the low-J CO line intensities is illustrated in two cases.
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Carbon monoxide (CO) is the most abundant molecule after molecular hydrogen and is important for the chemistry in circumstellar envelopes around evolved stars. When modelling the strength and shape of molecular lines, the size of the CO envelope is an input parameter and influences the derived mass-loss rates. In particular the low-J transition CO lines are sensitive to the CO photodissociation radius. Recently, new CO photodissociation radii have been published using different formalisms that differ considerably. One set of calculations is based on an escape-probability formalisms that uses numerical approximations derived in the early-eighties. The accuracy of these approximations is investigated and it is shown that they are less accurate than claimed. Improved formalism are derived. Nevertheless, the changes in CO envelope size are small to moderate, less than 2% for models with $10^{-7}< dot{M}< 10^{-4}$ msolyr and at most 7% for model with $dot{M} = 10^{-8}$ msolyr.
Studies of the CO and HI radio emission of some evolved stars are presented using data collected by the IRAM Plateau de Bure interferometer and Pico Veleta telescope, the Nanc{c}ay Radio Telescope and the JVLA and ALMA arrays. Approximate axial symmetry of the physical and kinematic properties of the circumstellar envelope (CSE) are observed in CO emission, in particular, from RS Cnc, EP Aqr and the Red Rectangle. A common feature is the presence of a bipolar outflow causing an enhanced wind velocity in the polar directions. HI emission extends to larger radial distances than probed by CO emission and displays features related to the interaction between the stellar outflow and interstellar matter. With its unprecedented sensitivity, FAST will open a new window on such studies. Its potential in this domain is briefly illustrated.
Ultraviolet (UV) photodissociation of CO controls the abundances and distribution of CO and its photodissociation products. This significantly influences the gas-phase chemistry in the circumstellar envelope (CSE) around evolved stars. A better understanding of CO photodissociation in outflows also provides a more precise estimate of mass-loss rates. We aim to update the CO photodissociation rate in an expanding spherical envelope assuming that the interstellar radiation field (ISRF) photons penetrate through the envelope. This will allow us to precisely estimate the CO abundance distributions in CSEs around evolved stars. We used the most recent CO spectroscopic data to precisely calculate the depth dependency of the photodissociation rate of each CO dissociating line. We calculated the CO self- and mutual-shielding functions in an expanding envelope. We investigated the dependence of the CO profile on the five fundamental parameters mass-loss rate, the expansion velocity, the CO initial abundance, the CO excitation temperature, and the strength of the ISRF. Our derived CO envelope size is smaller than the commonly used radius derived by Mamon et al. 1988. The difference between results varies from 1% to 39% and depends on the H2 and CO densities of the envelope. We list two fitting parameters for a large grid of models to estimate the CO abundance distribution. We demonstrate that the CO envelope size can differ between outflows with the same effective content of CO, but different CO abundance, mass-loss rate, and the expansion velocity as a consequence of differing amounts of shielding by H2 and CO. We find that the abundance of CO close to the star and the outflow density both can have a significant effect on the size of the molecular envelope. We also demonstrate that modest variations in the ISRF can cause measurable differences in the envelope extent.
Aims. Photodissociation by UV light is an important destruction mechanism for 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 C17O and 13C17O. 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 2.6e-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(12CO)/N(13CO) ratios seen in diffuse clouds. The photodissociation of C17O and 13C17O shows almost exactly the same depth dependence as that of C18O and 13C18O, respectively, so 17O and 18O are equally fractionated with respect to 16O. This supports the recent hypothesis that CO photodissociation in the solar nebula is responsible for the anomalous 17O and 18O abundances in meteorites.
The recent detection of fullerene (C$_{60}$) in space and the positive assignment of five diffuse interstellar bands to C$_{60}^+$ reinforce the notion that fullerene-related compounds can be efficiently formed in circumstellar envelopes and be present in significant quantities in the interstellar medium. Experimental studies have shown that C$_{60}$ can be readily hydrogenated, raising the possibility that hydrogenated fullerenes (or fulleranes, C$_{60}$H$_m$, $m=1-60$) may be abundant in space. In this paper, we present theoretical studies of the vibrational modes of isomers of C$_{60}$H$_m$. Our results show that the four mid-infrared bands from the C$_{60}$ skeletal vibrations remain prominent in slightly hydrogenated C$_{60}$, but their strengths diminish in different degrees with increasing hydrogenation. It is therefore possible that the observed infrared bands assigned to C$_{60}$ could be due to a mixture of fullerenes and fulleranes. This provides a potential explanation for the observed scatter of the C$_{60}$ band ratios. Our calculations suggest that a feature around 15$mu$m due to the breathing mode of heavily hydrogenated C$_{60}$ may be detectable astronomically. A preliminary search for this feature in 35 C$_{60}$ sources is reported.
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