ejected from a nova explosion has been developed. Consisting of two parts, one to describe the chem istry from ju st after o utburst through until approximately fifty days later followed by one of two models describing chemistry to generate nucléation sites about which dust grains can form, this work is able to propose viable pathways th a t might lead to dust condensation on a scale commensurate with observation.
In chapter 3 a model is built with chemistry specific to the nova environment including, most im portantly, a realistic photochemistry. The use of synthetic spectra in calculating photorate coefficients, instead of a blackbody approximation to the nova radiation field, is one of the crucial changes th a t make this chemistry so rich as compared with previous work. The addition of a thorough nitrogen chemistry has also proved im portant as nitrogen bearing species are found to play a key rôle in the chemistry of CO. The chemistry is further enriched with an expanded oxygen chemistry and the inclusion of sodium as a representative low ionisation potential metal.
From these models we make estim ates of the column densities of two key, observed species: CO and CN. For both we find a broad range of possibilities depending on the ejecta geometry; in constraining the models with observations of CO in nova Cas 1993, we find th a t CO must form in a relatively thin shell, possibly a localised region occupying approxim ately one tenth of the principal ejectum. This falls in line with previous work. For CN, we find the abundance to be very much less sensitive to the ejectum density than in the case of CO. In light of this, we propose an observational diagnostic to estim ate the density of the C O /C N region by measuring the abundance ratio of these two species.
182 CHAPTER 6. CONCLUSIONS AND FUTURE PROJECTS
It hcLS long been assumed th a t in an astro physical environment, such as a nova outflow, where both carbon and oxygen are present, CO will satu rate thus locking away the least abundant of the two species. In this work it is found th a t CO remains unsaturated at all times thus leaving open the possibilities of carbon and oxygen chemistry coexisting. It is possible th a t this could lead to new ideas of how both silicon and carbon based dust can appear to form from the same ejecta. Furtherm ore, the presence of free oxygen to produce other molecules such as water may help explain recent infrared observations of w ater lines in the planetary nebula NGC 7027.
The exploration of pathways to the formation of dust nucléation sites in chapter 4 finds th a t stable abundances of a wide variety of large molecules may be formed within the principal ejectum. Two scenarios are explored: one an isomer independent hydro carbon model and a modified version of a PAH reaction network adapted to the nova environment. The la tte r is found to generate a smaller number of large molecules al though still approxim ately 1 0^ times the minimum number of nucléation sites required
to produce the required visual optical depth of unity. Thus, no m atter w hat model we choose, sufficient nucléation sites may be generated to result in a large number of small dust grains. W hilst the isomer independent hydrocarbon is found to be relatively efficient over a wide region of param eter space (tem perature and density), the PAH scheme places strong constraints on the tem perature and density of the ejectum material, requiring th a t the tem perature ju st after maximum light be in the range 4000-5800 K and the density be between 1 0^ and cm“ ^ within approximately 1 0 days prior to dust nucléation at
least.
Finally, we consider a fluid dynamic model. In this work it was necessary to use only a simple chemistry but, unlike the chemical models before, it is not necessary to ‘impose’ an artificial evolution of physical param eters such as the tem perature and density. In this way we find th a t, as a result of dynamics alone, a narrow region is formed within the principal ejectum within which CO may exist and a take p art in a rich chemistry. Outside of this region the ejectum is heated to a point where CO is dissociated. In this work we find th a t strong constraints are placed on the tem perature of the principal ejectum which must be no hotter than approxim ately 4000 K. From this model we calculate column densities for CO which are approxim ately an order of magnitude too high with respect to estim ates of the CO column in nova V705 Cas 1993, but with a chemistry th a t is over simplified and lacking realistic photoreactions this may be considered an upper limit. We also find
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the modelled ratio of CO:C may be pulled into agreement with observations of V705 Cas provided a more complete chemistry is employed.
Refining the model with the inclusion of a simple ‘diffuse enhanced’ wind com ponent ejected after the principal ejectum we develop a plausible model to explain th e tem poral evolution of both the CO lines observed in V705 Cas and the diffuse enhanced spectral com ponents observed in many novae. The predicted column density of CO is also brought closer to the observed value for V7005 Cas.
It is proposed th a t this work marks a considerable advance in the field of nova chem istry, in particular with respect to the use of realistic radiation fields, a broad chem istry and, latterly, sophisticated fluid dynamic modelling techniques. To progress in this field it would be highly desirable to incorporate more experimental d a ta into the reaction net works. For this we require lab work to measure, for example, the UV photocross-sections of molecules found within the early nova. Detailed studies of reactions involving species in highly excited electronic, vibrational and rotational states would also be desirable, in particular for modelling the early ejectum and understanding the chemistry of regions very close to shocks.
The fluid dynamic models of chapter 5 show considerable promise and there is little doubt th a t considerable progress may be made in understanding the stru ctu re of nova outflows and developing chemical diagnostics in the near future. It will be necessary properly to model a radiation field and to extend modelling to two and possibly three dimensions, but the com putational resources now exist to handle such complexities. W ith such sophisticated dynamic modelling it should be possible to study the effect of complex stru cture within outflows and then consider how such structure might arise as a result of the dynamics.
As with all research, this thesis inevitably suggests yet more questions and further research, but the future holds a great deal of promise for this young though fast m aturing field of investigation. Particularly promising is the ever closer integration of chemistry with highly realistic physics. From chemistry oriented kinetics models to CFD codes, the variety and diagnostic power of the tools available promise an exciting and fruitful future.