This work investigates molecular opacities under conditions appropriate to the atmospheres of late-type stars. Given a specified initial abundance of the chemical elements, the equations of atomic ionization and molecular dissociation are solved by iteration for various temperatures and densities. With the abundances of the atomic and molecular species so determined at a particular temperature and density, the continuous absorption coefficient due to a number of processes is first computed, then the molecular band absorption coefficient is computed line-by-line superimposed for diatomic and triatomic molecules considered separately. The band calculations for the diatomic molecules include all possible isotopic variants, but computing resources did not permit this degree of detail for triatomic molecules, nor diatomic and triatomic molecules to be considered together. These calculations are performed for a number of different temperatures and densities. The theory for calculating the spectral lines due to molecules is discussed in some detail, with various methods being investigated for computing opacities including the contributions from these spectral lines. With the computing resources available, the best method for the calculation of the opacity due to the molecular absorption is found to be that of opacity sampling, sampling is adequate, provided the frequency of sampling is adequate. From the results presented in this thesis, a number of important conclusions can be drawn. At low temperatures and high densities, pressure-induced absorption due to molecular hydrogen is an important source of continuous opacity. Isotopic variants of at least diatomic molecules pave to be considered in any calculations, as they can significantly affect the contribution that diatomic molecules make to the Rosseland mean opacities. Also at low temperatures, water is a major source of absorption. Finally, tables of some thermodynamic quantities for the molecular gas are given for a number of temperatures and densities.

The Upsilon Andromedae system was the first multi-planet system discovered orbiting a main-sequence star. We describe the detection of water vapor in the atmosphere of the innermost non-transiting gas giant ups. And. b by treating the star-planet system as a spectroscopic binary with high-resolution, ground-based spectroscopy. We resolve the signal of the planet's motion and break the mass-inclination degeneracy for this non-transiting planet via deep combined flux observations of the star and the planet. In total, seven epochs of Keck NIRSPEC L band observations, three epochs of Keck NIRSPEC short-wavelength K band observations, and three epochs of Keck NIRSPEC long wavelength K band observations of the ups. And. system were obtained. We perform a multi-epoch cross-correlation of the full data set with an atmospheric model. We measure the radial projection of the Keplerian velocity (K-P = 55 +/- 9 km s(-1)), true mass (M-b = 1.7(-0.24)(+0.33)M(J)), and orbital inclination (i(b) 24 degrees +/- 4 degrees), and determine that the planet's opacity structure is dominated by water vapor at the probed wavelengths. Dynamical simulations of the planets in the ups. And. system with these orbital elements for ups. And. b show that stable, long-term (100 Myr) orbital configurations exist. These measurements will inform future studies of the stability and evolution of the ups. And. system, as well as the atmospheric structure and composition of the hot Jupiter.

To calculate the emergent radiation field, a realistic atmospheric model and algorithm must be developed. The radiation field may be characterized by the emergent intensities of scattered light. This is possible only if the algorithm determines these intensities as dependent upon atmospheric and angular parameters.

<p> Unsworth and Monteith's (1972) aerosol attenuation coefficient TA was calculated with hourly cloudless data at four North American and four European stations for varying time periods. Monthly and seasonal turbidity trends were examined. Annual cycles were observed with summer maximums and winter minimums. The North American stations were less turbid and had more pronounced trends than the European stations. Both air mass origin and local weather affect the turbidity. Local sources of pollution have a significant effect on turbidity most notably in large urban centres.</p> / Thesis / Candidate in Philosophy

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This PhD thesis presents work on the atmospheres of both brown dwarfs and exoplanets from an observers viewpoint. The composition and weather of these worlds are explored starting with M-type brown dwarfs and continuing through the L, T and Y spectral sequence, before entering the planetary regime of hot-Jupiters and super-Earths. The similarities and differences between these objects such as their radii, surface gravities, pressures, temperatures and composition are discussed. This thesis presents new results from an extensive near-infrared monitoring survey of a uniform and unbiased sample of 69 L &amp; T dwarfs spanning the L0 to T8 spectral range. Results show that amongst 14 identified variables, nine of them newly identified, variable brown dwarfs are not concentrated at the L - T transition, nor are they observed in a specific colour, or preferentially in binary systems. The thesis also presents narrow-band photometric measurements of the hot-Jupiter HAT-P-1b and the super-Earth GJ~1214b using the 10.4~m Gran Telescopio Canarias (GTC) and the OSIRIS instrument. Results for HAT-P-1b show a strong presence of potassium in the atmosphere caused by a large scale height, possibly due to higher than anticipated temperatures in the upper atmosphere or the dissociation of molecular hydrogen caused by the UV flux from the host star. Results for GJ 1214b, which constitute the first tunable filter measurements of a super-Earth, find no evidence for the presence of methane showing a featureless transmission spectrum consistent with previous studies.

The scattered radiation fields in spherical planetary atmospheres have been considered by a new method which is called the Quasi-Spherical method. This method is applicable to planets with radii which are much larger than the height of their atmosheres. The scattering of 0.5 (mu)m radiation in a conservative and vertically inhomogeneous atmosphere has been discussed. Results comparing the emerging radiation from plane-parallel and spherical models for the earth's atmosphere have been presented for four different aerosol distributions in addition to the normal molecular composition. These results indicate measurable differences on the order of 10 to 300% as the angle of observation and/or the angle of incident sun falls within 10(DEGREES) from the horizon. Also, the obtained results in the spherical atmosphere show that additional layers of aerosols in either the stratosphere or the troposphere can be detected by satellite or aircraft radiometric measurements, while the plane-parallel atmosphere does not permit such a detection. The accuracy of the obtained results by the present method can be increased by increasing the number of spherical shells in the spherical atmosphere. The emerging radiation in homogeneous and inhomogeneous Rayleigh atmospheres as computed by the Quasi-Spherical and the Monte Carlo methods compare quite well. By applying the divergence theorem it was shown by the present method that the total flux in the spherical atmosphere is conserved within 1.351%.

Precise measurements of transit lightcurves can be used to constrain the composition and structure of exoplanet atmospheres. Unfortunately, efforts to extract this information are usually hampered by the presence of correlated noise that is degenerate with the astrophysical signal of interest. A major theme of this thesis is the application of robust analysis methods to properly account for such degeneracies. In particular, I advocate the use of Bayesian inference for lightcurve fitting. For this study, the Bayesian framework is exploited by modelling lightcurves as Gaussian processes (GPs), which offer numerous advantages over traditional decorrelation methods. The main advantage is that GPs do not require a functional form to be specified for the poorly understood lightcurve systematics. Instead, the high-level properties of the signal covariance are parameterised, allowing complex correlations to be marginalised over relatively low-dimension parameter spaces. I use GP models to analyse transit and eclipse lightcurves for the hot Jupiters HD189733b, HD209458b, and HAT-P-32b. The work is spread over three separate projects. Firstly, I re-analyse the majority of the transits and eclipses that have been observed using the Spitzer Space Telescope Infrared Array Camera (IRAC) for HD189733b and HD20945b. The GP analyses generally produce uncertainties for inferred planet parameters that are factors of ~1-5 larger than those quoted in the literature. In a number of cases, I obtain results that are fundamentally different to those published previously, with significant implications for the understanding of the atmospheres. Secondly, I report an eclipse observation for HD189733b that was made using the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) over the 290-570nm wavelength range. Geometric albedos of Ag=0.37⁺¹²<sub style='position: relative; left: -1.3em;'>-13</sub> and Ag=0.37⁺¹³<sub style='position: relative; left: -1.3em;'>-12</sub> are measured in the wavelength ranges 290-450nm and 450-570nm, respectively. This represents the first ever multi-wavelength eclipse measurement made for an exoplanet at visible wavelengths. The nonzero albedo in the 290-450nm wavelength channel provides evidence for scattering in the atmosphere, possibly by haze/clouds or H2 molecules. The relatively low albedo in the 450-570nm wavelength channel is interpreted as being due to absorption by the wings of the Na 589nm doublet. Thirdly, I present two transit observations for HAT-P-32b made using the Nordic Optical Telescope (NOT) Andalucia Faint Object Spectrograph (ALFOSC) in multi-object spectroscopy mode over the 414-702nm wavelength range. A joint GP model is fit to the two white lightcurves produced by integrating the spectra over the full wavelength range. Spectroscopic lightcurves are also generated by binning into 32nm-wide wavelength channels, and preliminary lightcurve analyses are performed. The extracted transmission spectrum shows some evidence for absorption features, but this interpretation is currently very tentative. Further refinements to the data reduction and lightcurve analysis are suggested, which will allow the transmission spectrum to be evaluated more definitively.

HCN is an important constituent in Titan's upper atmosphere, serving as the main coolant in the local energy budget. In this study, we derive the HCN abundance at the altitude range of 960-1400 km, combining the Ion-Neutral Mass Spectrometer data acquired during a large number of Cassini flybys with Titan. Typically, the HCN abundance declines modestly with increasing altitude and flattens to a near constant level above 1200 km. The data reveal a tendency for dayside depletion of HCN, which is clearly visible below 1000 km but weakens with increasing altitude. Despite the absence of convincing anti-correlation between HCN volume mixing ratio and neutral temperature, we argue that the variability in HCN abundance makes an important contribution to the large temperature variability observed in Titan's upper atmosphere.

Graduation date: 2002