In this first of three articles on the optical absorption of electrons in half-filled Peierls-distorted chains we present analytical results for non-interacting tightbinding electrons. We carefully derive explicit expressions for the current operator, the dipole transition matrix elements, and the optical absorption for electrons with a cosine dispersion relation of band width W and dimerization parameter δ. New correction ("η"-)terms to the current operator are identified. A broad band-to-band transition is found in the frequency range W δ < ω < W whose shape is determined by the joint density of states for the upper and lower Peierls subbands and the strong momentum dependence of the transition matrix elements.

We describe the HIgh Precision Polarimetric Instrument (HIPPI), a polarimeter built at UNSW Australia and used on the Anglo-Australian Telescope (AAT). HIPPI is an aperture polarimeter using a ferro-electric liquid crystal modulator. HIPPI measures the linear polarization of starlight with a sensitivity in fractional polarization of ∼4 × 10 −6 on low polarization objects and a precision of better than 0.01% on highly polarized stars. The detectors have a high dynamic range allowing observations of the brightest stars in the sky as well as much fainter objects. The telescope polarization of the AAT is found to be 48 ± 5 × 10 −6 in the g ′ band.

No abstract

We present linear polarization measurements of nearby FGK dwarfs to parts-permillion (ppm) precision. Before making any allowance for interstellar polarization, we found that the active stars within the sample have a mean polarization of 28.5 ± 2.2 ppm while the inactive stars have a mean of 9.6 ± 1.5 ppm. Amongst inactive stars we initially found no difference between debris disk host stars (9.1 ± 2.5 ppm) and the other FGK dwarfs (9.9 ± 1.9 ppm). We develop a model for the magnitude and direction of interstellar polarization for nearby stars. When we correct the observations for the estimated interstellar polarization we obtain 23.0 ± 2.2 ppm for the active stars, 7.8 ± 2.9 ppm for the inactive debris disk host stars and 2.9 ± 1.9 ppm for the other inactive stars. The data indicates that whilst some debris disk host stars are intrinsically polarized most inactive FGK dwarfs have negligible intrinsic polarization, but that active dwarfs have intrinsic polarization at levels ranging up to ∼ 45 ppm. We briefly consider a number of mechanisms, and suggest differential saturation of spectral lines in the presence of magnetic fields is the best able to explain the polarization seen in active dwarfs. The results have implications for current attempts to detect polarized reflected light from hot Jupiters by looking at the combined light of the star and planet.