Telescopes often modify the input polarization of a source so that the measured circular or linear output state of the optical signal can be signficantly different from the input. This mixing, or polarization cross-talk, is defined by the optical sys
tem Mueller matrix. We describe here an efficient method for recovering the input polarization state of the light and the full 4 x 4 Mueller matrix of the telescope with an accuracy of a few percent without external masks or telescope hardware modification. Observations of the bright, highly polarized daytime sky using the Haleakala 3.7m AEOS telescope and a coude spectropolarimeter demonstrate the technique.
A number of binary systems present evidence of enhanced activity around periastron passage, suggesting a connection between tidal interactions and these periastron effects. The aim of this investigation is to study the time-dependent response of a st
ars surface as it is perturbed by a binary companion. We derive expressions for the rate of dissipation, $dot{E}$, of the kinetic energy by the viscous flows driven by tidal interactions on the surface layer. The method is tested by comparing the results from a grid of model calculations with the analytical predictions of Hut (1981) and the synchronization timescales of Zahn (1977, 2008). Our results for the orbital cycle averaged energy dissipation on orbital separation are consistent with those of Hut for model binaries with orbital separations at periastron >8 stellar radii. The model also reproduces the predicted pseudo-synchronization angular velocity for moderate eccentricities and the same scaling of synchronization timescales for circular orbits with separation as given by Zahn. The computations gives the distribution of $dot{E}$ over the stellar surface, and show that it is generally concentrated at the equatorial latitude, with maxima generally located around four clearly defined longitudes, corresponding to the fastest azimuthal velocity perturbations. Maximum amplitudes occur around periastron passage or slightly thereafter for supersynchronously rotating stars. In very eccentric binaries, the distribution of $dot{E}$ over the surface changes significantly as a function of orbital phase, with small spatial structures appearing after periastron. An exploratory calculation for the highly eccentric binary system delta Sco suggests that the sudden and large amplitude variations in surface properties around periastron may contribute toward the activity observed around this orbital phase.
Sensitive measurements of the linearly polarized spectra of stars can be used to deduce geometric properties of their otherwise unresolved circumstellar environments. This paper describes some of the evidence for optical pumping and absorptive linear
polarization and explores some interesting applications of linear spectropolarimetry for obtaining spatial information from imbedded stars.
Stellar spectropolarimetry is a relatively new remote sensing tool for exploring stellar atmospheres and circumstellar environments. We present the results of our HiVIS survey and a multi-wavelength ESPaDOnS follow-up campaign showing detectable line
ar polarization signatures in many lines for most obscured stars. This survey shows polarization at and below 0.1% across many lines are common in stars with often much larger H-alpha signatures. These smaller signatures are near the limit of typical systematic errors in most night-time spectropolarimeters. In an effort to increase our precision and efficiency for detecting small signals we designed and implemented the new HiVIS bi-directionally clocked detector synchronized with the new liquid-crystal polarimeter package. We can now record multiple independent polarized spectra in a single exposure on identical pixels and have demonstrated 10^-4 relative polarimetric precision. The new detector allows for the movement of charge on the device to be synchronized with phase changes in the liquid-crystal variable retarders at rates of >5Hz. It also allows for more efficient observing on bright targets by effectively increasing the pixel well depth. With the new detector, low and high resolution modes and polarization calibrations for the instrument and telescope, we substantially reduce limitations to the precision and accuracy of this new spectropolarimetric tool.
We present the results of high precision, high resolution (R~68000) optical observations of the short-period (4d) eccentric binary system Alpha Virginis (Spica) showing the photospheric line-profile variability that in this system can be attributed t
o non-radial pulsations driven by tidal effects. Although scant in orbital phase coverage, the data provide S/N>2000 line profiles at full spectral resolution in the wavelength range delta-lambda = 4000--8500 Angstroms, allowing a detailed study of the night-to-night variability as well as changes that occur on ~2 hr timescale. Using an ab initio theoretical calculation, we show that the line-profile variability can arise as a natural consequence of surface flows that are induced by the tidal interaction.
