Molecules in harsh astronomical environments: electron driven processes

There is a growing realisation that small molecules exist not just in quiescent cool clouds but also in much more active astrophysical regions such as planetary nebulae and diffuse interstellar clouds. These regions often contain significant numbers of free, quasi-thermal electrons, up to 10-4 compared to molecular hydrogen. These electrons can effect chemical change and drive observable spectroscopy processes (see A.J. Lim, I. Rabadan and J. Tennyson, MNRAS, 306, 473 (1999) for example); the cross sections between electrons and molecular ions are particularly large. Additionally electron molecule collisions are important elsewhere, for example they are the main driver behind planetary aurorae and many molecular masers. Models of all these regimes require data which is largely unknown and, in many cases, cannot be determined from laboratory based measurements. Over the last two decades the UCL group has developed the UK molecular R-matrix codes to provide a first principles, quantum mechanical treatment of the collision between low energy electrons and small molecules. This code has been used to treat collisions leading to rotational excitation involving important astrophysical ions (see for example A. Faure and J. Tennyson, MNRAS, 325, 443 (2001)) and the strongly dipolar water molecule (A. Faure, J.D. Gorfinkiel and J. Tennyson, MNRAS 347, 323 (2004)). However these treatments are still very limited in their scope. Thus, for example, calculations on electron collisions with water which are important for models of water masers and cometary emissions, and will undoubtedly be needed to interpret observations from ESA's forthcoming Herschel mission, need to be extended to treat both much higher rotational levels and vibrational motion. Recent observations of molecular emissions from C-shocked regions of the ISM (Jimenez-Serra et al, ApJ 650, L135 (2007)) showed that it is possible to recover local electron densities by using electron molecule collisions calculations (this work used ones performed by the proposer). The present proposal is for a PhD student who will use the QuantemolN implementation of the UK polyatomic R- matrix code to study electron collisions with molecules of astrophysical interest such as OH and SiO. Similar electron collisions with C2, important in cometary tails and elsewhere, will also be attempted. The QuantemolN code, which will be provided by the company, is very suitable for these studies since it is an expert system which greatly increases the ease and speed with which a user can perform very technically demanding electron collision calculations. In return the student will assist the company in adding further features to this code, for example to treat rotational and vibrational excitation. Adding to the functionality of the code is a strategic aim of Quantemol. The student will be provided training in performing electron molecule collision calculations, interpreting the results and using them in astronomical models and to interpret astronomical spectra. S/he will interact with people directly observing the processes, several of whom (for example Dr J Rawlings and Dr S Viti) are at UCL. S/he will also experience working with a small start up company which gives the opportunity to be involved both in the software development and in the interaction with other users of the code. This proposal follows a highly successful CASE studentship award (now in its final year) to Mr HN Varambhia who has both QuantemolN to do studies on HCN, HNC, CO and other astrophysically important systems (Eg Varambhia et al, Electron-impact rotational excitation of HCN, HNC, DCN and DNC, MNRAS in press) which has been of immense benefit to the company by raising its scientific profile which led to new orders for the existing Quantemol-N package and interest in both Quantemol-N and Quantemol-P from both the UK and abroad.