Photodestruction and Growth of Interstellar Polycyclic Aromatic Hydrocarbons

Thierry Allain

Dissertation, Technische Universität Berlin, 1996

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Abstract:

The discovery of a series of emission bands in the infrared spectra of various sources characterized by a strong ultraviolet radiation field (active galaxies, reflexion nebulae, planetary nebulae, HII regions, ...) has recently given rise to one of the most exciting puzzle of modern Astrophysics (Gillet et al. 1973). Until 1984 there were indeed no convincing explanation to account for the complete set of emission bands and they have therefore become famous in Astrophysics under the appellation of "Unidentified InfraRed" (UIR) Bands. Léger & Puget (1984) and Allamandola et al. (1985) first noticed the good match between the infrared vibrational modes of polycyclic aromatic hydrocarbons (PAHs) and the UIR bands and, thus, concluded that free PAHs could be the carriers of the UIR bands. With this assumption these authors deduced a large abundance of PAHs in the interstellar medium (ISM). However, the assignement of the UIR bands to the vibrational modes of free PAHs is still
questioned and some other valuable proposals have been made to account for the UIR bands. In other words, the presence of large amounts of PAHs in space is still a matter of debate in the astrophysical community.

The philosophy of the present thesis is to get new information about interstellar PAHs independently of astronomical observations. Only experimental results and theoretical calculations are used with the purpose to determine the characteristical properties of the PAHs capable to survive under the conditions of the ISM. The calculations are carried out, in a first part, in the regions where the UIR bands are seen, and in a second part, in the circumstellar envelopes (CSE) of C-stars where it is known since 1983 that PAHs favourably form and grow (Keller & Sedlmayr 1983).

In the first part PAH photodestruction rates in the ISM are calculated by use of the experimental results of Jochims et al. (1994). As a result, astrophysically relevant rates --- that is to say, integrated over the interstellar radiation field --- are derived for the photoejection of hydrogen and acetylene from PAH molecules.

Since it is usually argued that PAHs are probably ionized and partially dehydrogenated under the conditions of the ISM, a method for evaluating simultaneously the hydrogen coverage and the degree of ionization of interstellar PAHs is developed and applied to the astrophysical media where the UIR bands are observed.

In order to study the photoejection rates of these ionized and partially dehydrogenated PAHs, a model is developed by extending the approach used for PAH molecules to PAH cations and to PAH with low hydrogen coverage. The rates for photoejection of hydrogen and carbon particles from ionized and partially dehydrogenated PAHs are proved to be higher that those of their parent molecules, especially for the partially dehydrogenated PAHs which are very unstable against photodestruction. Comparing the rate for photoejection of acetylene to that of the reverse process, accretion of carbon particles, leads to the conclusion that only PAHs containing more than 50 carbon atoms are able to resist the ultraviolet radiation field in the regions showing the UIR bands.

Since PAHs with more than 50 carbon atoms can possibly survive in the ISM, it is worth studying the regions where they form and grow. It is therefore the aim of the second part of this thesis to deal with PAHs in circumstellar environments. For this purpose, the stationary models for dust forming circumstellar shells of C-giants developed at the Institute of Astronomy and Astrophysics of the Technical University Berlin are used, especially the model for the shell of IRC+10216, which is the nearest and most studied carbon-rich red giant (Winters et al. 1994, Winters 1994).

The physical processes which could favour PAH formation in the CSEs of carbon-rich red giants are analyzed. On the basis of these results, the density of small PAHs are evaluated. Thereby, it is also confirmed that PAHs rapidly growth by succesive hydrogen abstraction and addition of acetylene molecules, up to macroscopic sizes. The link between PAHs and dust particles is studied numerically, with the result that PAHs could possibly be the seeds for dust grains.

The processes undergone by PAHs when they reach the outer part of the circumstellar envelopes are also investigated. To allow for reliable calculations, new values are calculated for the dust and molecular optical depths in the envelopes of C-stars. Using these results, the PAH ionization and hydrogenation states are estimated in the envelope of IRC+10216. The growth via C^+ accretion and the photodestruction of PAHs are also studied with the aim of estimating the size distribution of the PAHs ejected into the ISM. Although it is demonstrated that the growth and destruction processes are rapid enough to modify the PAH size distribution along their way from the star outwards to the ISM, no definite conclusions can be drawn because of the lack of data concerning the yields of the processes which occur after the collision between PAHs and C^+.

A direct application of the present work is the theoretical estimate of the density of PAHs in the ISM. This simply results from the balance of all the processes contributing to the input of PAHs (formation and growth in situ, contribution of C-stars, of shock waves) and to the disappearance of PAHs (photodestruction, collision with dust particles, sputtering). This work and further experimental investigation will make such a study possible and, thus, will allow to test whether PAHs are as ubiquitous and abundant as observations imply.