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Register a new userSingle and Double Photoionization and Photodissociation of Toluene by Soft X-rays in Circumstellar Environment
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T. Monfredini
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The formation of polycyclic aromatic hydrocarbons (PAHs) and their methyl derivatives occurs mainly in the dust shells of asymptotic giant branch (AGB) stars. The bands at 3.3 and 3.4 $mu$m, observed in infrared emission spectra of several objects, are attributed C-H vibrational modes in aromatic and aliphatic structures, respectively. In general, the feature at 3.3 $mu$m is more intense than the 3.4 $mu$m. Photoionization and photodissociation processes of toluene, the precursor of methylated PAHs, were studied using synchrotron radiation at soft X-ray energies around the carbon K edge with time-of-flight mass spectrometry. Partial ion yields of a large number of ionic fragments were extracted from single and 2D-spectra, where electron-ion coincidences have revealed the doubly charged parent-molecule and several doubly charged fragments containing seven carbon atoms with considerable abundance. textit{Ab initio} calculations based on density functional theory were performed to elucidate the chemical structure of these stable dicationic species. The survival of the dications subjected to hard inner shell ionization suggests that they could be observed in the interstellar medium, especially in regions where PAHs are detected. The ionization and destruction of toluene induced by X-rays were examined in the T Dra conditions, a carbon-rich AGB star. In this context, a minimum photodissociation radius and the half-life of toluene subjected to the incidence of the soft X-ray flux emitted from a companion white dwarf star were determined.
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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.
We present large scale 9 x 27 (25 pc x 70 pc) far-IR observations of the Sgr B2 complex using the spectrometers on board the Infrared Space Observatory (ISO). The far-IR spectra are dominated by the strong continuum emission of dust and by the fine structure lines of high excitation potential ions (NII, NIII and OIII) and those of neutral or weakly ionized atoms (OI and CII). The line emission has revealed a very extended component of ionized gas. The study of the NIII 57 microns/NII 122 microns and OIII 52/88 microns line intensity ratios show that the ionized gas has a density of n_e~10^{2-3} cm^-3 while the ionizing radiation can be characterized by a diluted but hard continuum, with effective temperatures of ~35000 K. Photoionization models show that the total number of Lyman photons needed to explain such an extended component is approximately equal to that of the HII regions in Sgr B2(N) and (M) condensations. We propose that the inhomogeneous and clumpy structure of the cloud allows the radiation to reach large distances through the envelope. Therefore, photodissociation regions (PDRs) can be numerous at the interface of the ionized and the neutral gas. The analysis of the OI (63 and 145 microns) and CII (158 microns) lines indicates an incident far-UV field (G_0, in units of the local interstellar radiation field) of 10^{3-4} and a H density of 10^{3-4} cm^{-3} in such PDRs. We conclude that extended photoionization and photodissociation are also taking place in Sgr B2 in addition to more established phenomena such as widespread low--velocity shocks.
Optical interferometry is a powerful tool to investigate the close environment of AGB stars. With a spatial resolution of a few milli-arcseconds, it is even possible to image directly the surface of angularly large objects. This is of special interest forMira stars and red supergiants for which the dust-wind is initiated from or very close to the photosphere by an interplay between pulsation and convection. Based on two-epoch interferometric observations of the Mira star X Hya, we present how the variation of the angular size with wavelength challenges pulsation models and how reconstructed images can reveal the evolution of the object shape and of its asymmetric structures.
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.
Photodissociation is the dominant removal process of molecules in any region exposed to intense ultraviolet (UV) radiation. This includes diffuse and translucent interstellar clouds, dense photon-dominated regions, high velocity shocks, the surface layers of protoplanetary disks, and cometary and exoplanetary atmospheres. The rate of photodissociation depends on the cross sections for absorption into a range of excited electronic states, as well as on the intensity and shape of the radiation field at each position into the region of interest. Thus, an acccurate determination of the photodissociation rate of even a simple molecule like water or carbon monoxide involves many detailed considerations ranging from its electronic structure to its dissociation dynamics and the specifics of the radiation field that the molecule is exposed to. In this review chapter, each of these steps in determining photodissociation rates is discussed systematically and examples are provided.
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