Atmospheres from very low-mass stars to extrasolar planets

Within the next few years, several instruments aiming at imaging extrasolar planets will see first light. In parallel, low mass planets are being searched around red dwarfs which offer more favorable conditions, both for radial velocity detection and transit studies, than solar-type stars. We review recent advancements in modeling the stellar to substellar transition. The revised solar oxygen abundances and cloud models allow to reproduce the photometric and spectroscopic properties of this transition to a degree never achieved before, but problems remain in the important M-L transition characteristic of the effective temperature range of characterizable exoplanets.Comment: submitted to Memorie della Societa Astronomica Italian

Computations on the infra-red spectra of triatomic molecules

"Computations on the Infra-red Spectra of Triatomic Molecules" is a write-up of two projects in the field of computational spectroscopy. The first is the calculation of the opacity of hot water vapour. The water molecule is one of the most important absorbers in the infra red spectrum and it has been studied at length. Perhaps surprisingly, there is little reliable data on hot water (above - 1000 K) available. The major source of experimental data is around twenty-five years old and is flawed. It is both inaccurate and imprecise. Another source for molecular data is the HITRAN database. The HITRAN data gives very accurate results for water at low temperatures but is unreliable at the higher temperatures encountered, for example, in the atmospheres of cool stars. It was clear that the available data for hot water was somewhat lacking. Since there is interest in modelling cool stellar atmospheres it was necessary to calculate a more comprehensive list of molecular data. The results of those calculations are presented including a comparison with the previously available sources of data. Subsequent developments in this area and possible future work are discussed. The second project is the calculation of an effective potential energy surface for the ground electronic state of nitrogen dioxide. Nitrogen dioxide has a somewhat unusual electronic structure which makes it a particularly interesting molecule. This also creates difficulties peculiar to nitrogen dioxide. Another reason for working on this molecule is that it is a significant atmospheric pollutant and absorber. There have been a number of attempts to determine an accurate ground state potential energy surface for nitrogen dioxide. These attempts are described and their deficiencies discussed. The method with which the new surface was determined is presented. Rovibrational calculations on the surface are compared with observations and possible future work is considered

Fitting of a PES for rovibrational calculations on NO2NO_{2}

$^{a}$S.A. Tashkun and P. Jensen, J. Mol. Spec., 165, 173-184 (1994). $^{b}$O.L. Polyansky, P. Jensen and J. Tennyson, J. Chem. Phys., 101, 7651-7657 (1994)Author Institution: Department of Physics and Astronomy, University College London; Physikaliach-Chemisches Institut, Justus-Lieting-Universit\""{a}t GiessenWe took the potential energy surface for the ground electronic state of $NO_{2}$ constructed by Tashkun and $Jensen^{a}$ using the MORBID Hamiltonian and computer program. We wished to fit this experimental date using the exact kinetic energy (EKE) operator. Initially, EKE calculations were not not possible due to the presence of holes in the surface. A better starting point for the fit was constructed using the method in Polyansky $et al^{b}$ whereby MORBID and EKE calculations are combined. We then improved the surface by fitting it to available experimental data using an iterative least-squares fitting procedure. In the low-energy region ($<10,000$ $cm^{-1}$) the fitting was successful. Above apporximately $10,000 cm^{-1}$ the higher electronic state begins to peturb the vibrational energy levels of the ground state. We were unable to fit the surface well to the experimental date above $10,000 cm^{-1}$. We aim to provide a surface which will give accurate rovibrational levels of $NO_{2}$