Spectropolarimetry with the DAO 1.8-m telescope

The fast-switching DAO spectropolarimeter mounted on the 1.8-m Plaskett telescope started operation in 2007. Almost 14,000 medium-resolution (R=15,000) polarimetric spectra of 65 O - F type stars have been obtained since then in the course of three ongoing projects: the DAO Magnetic Field Survey, supporting observations for the CFHT MiMeS survey, and an investigation of the systematic differences between the observed longitudinal field measured with the Hbeta line and metallic lines. The projects are briefly described here. The current status as well as some results are presented.Comment: "Magnetic Stars", Proceedings of the International Conference, Nizhny Arkhyz, 27 August - 1 September 201

Least squares DOA estimation with an informed phase unwrapping and full bandwidth robustness

The weighted least-squares (WLS) direction-of-arrival estimator that minimizes an error based on interchannel phase differences is both computationally simple and flexible. However, the approach has several limitations, including an inability to cope with spatial aliasing and a sensitivity to phase wrapping. The recently proposed phase wrapping robust (PWR)-WLS estimator addresses the latter of these issues, but requires solving a nonconvex optimization problem. In this contribution, we focus on both of the described shortcomings. First, a conceptually simpler alternative to PWR is presented that performs comparably given a good initial estimate. This newly proposed method relies on an unwrapping of the phase differences vector. Secondly, it is demonstrated that all microphone pairs can be utilized at all frequencies with both estimators. When incorporating information from other frequency bins, this permits a localization above the spatial aliasing frequency of the array. Experimental results show that a considerable performance improvement is possible, particularly for arrays with a large microphone spacing

An Inexpensive Liquid Crystal Spectropolarimeter for the Dominion Astrophysical Observatory Plaskett Telescope

A new, inexpensive polarimetric unit has been constructed for the Dominion Astrophysical Observatory (DAO) 1.8-m Plaskett telescope. It is implemented as a plug-in module for the telescope's existing Cassegrain spectrograph, and enables medium resolution (R~10,000) circular spectropolarimetry of point sources. A dual-beam design together with fast switching of the wave plate at rates up to 100Hz, and synchronized with charge shuffling on the CCD, is used to significantly reduce instrumental effects and achieve high-precision spectropolarimetric measurements for a very low cost. The instrument is optimized to work in the wavelength range 4700 - 5300A to simultaneously detect polarization signals in the H beta line as well as nearby metallic lines. In this paper we describe the technical details of the instrument, our observing strategy and data reduction techniques, and present tests of its scientific performance.Comment: 32 pages, 15 figures. Accepted for publication in PAS

Photoreactivation of Lethal Damage Induced in Hamster X Xenopus Hybrid Cells and Their Parentals by UV Light

A85 Xenopus cells that exhibited a high level of photoreactivation (PR) and V79B2 hamster cells that exhibited little PR were fused to produce the V79B2 x A85 cell line — a hybrid line which possessed a relatively stable karyotype, with most cells containing the entire V79B2 and A85 genomes. UV and UV plus PR fluence-survival relations were then determined and compared for the hybrid and parental lines in a first attempt to elucidate interactions of the parental PR mechanisms in the hybrid. It was anticipated that the A85 genome in the hybrid would produce PR enzyme in sufficient concentration and of such a nature as to efficiently PR UV-induced lethal damage in both A85 and V79B2 DNA, and little difference would be observed in the levels of PR exhibited by the V79B2 x A85 and A85 lines. To the contrary, the level of PR observed for the hybrid was substantially below that observed for the A85 line. To assist in the interpretation of this unexpected observation, three additional preliminary studies were carried out: 1) Comparison of the optimum PR schemes for the A85 and hybrid lines, 2) examination of relations between the PR and dark UV repair mechanisms possessed by these lines, and 3) comparison of the levels of PR of chromatid deletions induced by UV in selected V79B2 and A85 chromosomes of the hybrid. The results suggested that the relatively low level of PR manifested by the hybrid cells was a consequence of their inability to efficiently PR pyrimidine dimers induced by UV in V79B2 DNA

Abundance And Magnetic Field Geometries Of The Helium-strong Stars

High signal-to-noise coude reticon spectra of five helium-strong stars have been obtained in two spectral regions ({dollar}\lambda\lambda{dollar}3980-4160 and {dollar}\lambda\lambda{dollar}4400-4580) at the Canada-France-Hawaii Telescope, Mauna Kea, Hawaii. Phase-resolved sets of spectra have been obtained for the magnetic-variable stars {dollar}\sigma{dollar} Ori E, HD 37776, and HD 64740. Additional observations have also been made of the sharp-lined helium-strong stars HD 58260 and {dollar}\delta{dollar} Ori C. These data, and additional Zeeman analyzer observations, have been used in an attempt to determine the magnetic field geometries and abundance distributions of the program stars. The models have been obtained by use of a line synthesis program that incorporates the effects of an assumed magnetic field and abundance distribution into the calculation of line profiles.;Helium line profile variations of HD 64740 may be modelled with two spots with anomalously high helium abundances near the magnetic poles of the star. Silicon, appears to have a large abundance in a high latitude band around the star. The surface magnetic field of HD 64740 is not strong enough to cause magnetic intensification effects in the Si III multiplet 2 line profiles.;The peculiar, broad helium line profiles of {dollar}\delta{dollar} Ori C suggest two possible helium abundance geometries for the star. A helium-rich spot near the visible magnetic pole of an otherwise extremely helium-poor star, and a vertical helium abundance gradient are both able to reproduce the observed profiles of the star. The second non-variable program star, HD 58260, also shows marginal evidence for such an abundance geometry.;The helium line profiles of {dollar}\sigma{dollar} Ori E are reproduced with two spots of large helium abundance ({dollar}\epsilon\sb{lcub}\rm He{rcub}{dollar} {dollar}\u3e{dollar} 0.5) near the intersections of the magnetic and rotation equators of the star. Silicon, magnesium, nitrogen, and oxygen all appear to be overabundant in a high latitude band on the surface of the star. Observations of the variable H{dollar}\alpha{dollar} emission of the star are presented.;A theoretical model for the peculiar longitudinal magnetic field variations of HD 37776 is proposed. The model consists of colinear dipole, quadrupole, and octupole field components that result in a magnetic field that varies between 20 and 60 kG over the surface of the star. Helium and silicon abundance geometries consistent with equatorial and polar distributions of the respective elements support the field model, as do the pronounced differential magnetic intensification effects seen in the silicon lines.;Several suggestions for interesting future observational and theoretical work are discussed

Doctor of Philosophy

dissertationRecent advances in the study of archaic hominin DNA have resulted in the rapid development and application of new methods designed to test our relationship to our nearest relatives. These methods have been applied with archaic samples in contexts with ghost admixture, sparse sampling, ascertainment bias, and poorly understood historical events. They have also been applied to modern samples with complex relationships which will exacerbate the same problems. Here, we introduce a new method for estimating the admixture fraction, and test it and several previous methods to determine their sensitivity to the above problems. Finally, we apply these methods to Single Nucleotide Polymorphism (SNP) microarray and whole genome data, and compare our estimates to those published previously

Doppler imaging of the helium-variable star a Cen

The helium-peculiar star a Cen exhibits line profile variations of elements such as iron, nitrogen and oxygen in addition to its well-known extreme helium variability. New high S/N, high-resolution spectra are used to perform a quantitative measurement of the abundances of the star and determine the relation of the concentrations of the heavier elements on the surface of the star to the helium concentration and the magnetic field orientation. Doppler images have been created using programs described in earlier papers by Rice and others. An alternative surface abundance mapping code has been used to model the helium line variations after our Doppler imaging of certain individual helium lines produced mediocre results. We confirm the long-known existence of helium-rich and helium-poor hemispheres on a Cen and we measure a difference of more than two orders of magnitude in helium abundance from one side of the star to the other. Helium is overabundant by a factor of about 5 over much of the helium-rich hemisphere. Of particular note is our discovery that the helium-poor hemisphere has a very high abundance of helium-3, approximately equal to the helium-4 abundance. a Cen is therefore a new member of the small group of helium-3 stars and the first well-established magnetic member of the class. For the three metals investigated here, there are two strong concentrations of abundance near the equator consistent with the positive magnetic maximum and two somewhat weaker concentrations of abundance where the helium concentration is centered and roughly where the negative peak of the magnetic field would be found. Another strong concentration is found near the equator and this is not explainable in terms of any simple symmetry with the helium abundance or the apparent magnetic field main polar locations.Comment: 9 pages, 9 figure