Constraining Theories of SiO Maser Polarization: Analysis of a pipi/2 EVPA Change

The full theory of polarized SiO maser emission from the near-circumstellar environment of Asymptotic Giant Branch stars has been the subject of debate, with theories ranging from classical Zeeman origins to predominantly non-Zeeman anisotropic excitation or propagation effects. Features with an internal electric vector position angle (EVPA) rotation of $\sim \pi /2$ offer unique constraints on theoretical models. In this work, results are presented for one such feature that persisted across five epochs of SiO $\nu=1, J=1-0$ VLBA observations of TX Cam. We examine the fit to the predicted dependence of linear polarization and EVPA on angle ($\theta$) between the line of sight and the magnetic field against theoretical models. We also present results on the dependence of $m_c$ on $\theta$ and their theoretical implications. Finally, we discuss potential causes of the observed differences, and continuing work

Radio Supernovae in the Great Survey Era

Radio properties of supernova outbursts remain poorly understood despite longstanding campaigns following events discovered at other wavelengths. After ~ 30 years of observations, only ~ 50 supernovae have been detected at radio wavelengths, none of which are Type Ia. Even the most radio-loud events are ~ 10^4 fainter in the radio than in the optical; to date, such intrinsically dim objects have only been visible in the very local universe. The detection and study of radio supernovae (RSNe) will be fundamentally altered and dramatically improved as the next generation of radio telescopes comes online, including EVLA, ASKAP, and MeerKAT, and culminating in the Square Kilometer Array (SKA); the latter should be > 50 times more sensitive than present facilities. SKA can repeatedly scan large (> 1 deg^2) areas of the sky, and thus will discover RSNe and other transient sources in a new, automatic, untargeted, and unbiased way. We estimate SKA will be able to detect core-collapse RSNe out to redshift z ~ 5, with an all-redshift rate ~ 620 events yr^-1 deg^-2, assuming a survey sensitivity of 50 nJy and radio lightcurves like those of SN 1993J. Hence SKA should provide a complete core-collapse RSN sample that is sufficient for statistical studies of radio properties of core-collapse supernovae. EVLA should find ~ 160 events yr^-1 deg^-2 out to redshift z ~ 3, and other SKA precursors should have similar detection rates. We also provided recommendations of the survey strategy to maximize the RSN detections of SKA. This new radio core-collapse supernovae sample will complement the detections from the optical searches, such as the LSST, and together provide crucial information on massive star evolution, supernova physics, and the circumstellar medium, out to high redshift. Additionally, SKA may yield the first radio Type Ia detection via follow-up of nearby events discovered at other wavelengths.Comment: 21 pages, 5 figures, accepted for publication in Ap

Data reduction techniques for Very Long Baseline Interferometric spectropolarimetry

This thesis reports the results of an investigation into techniques for the calibration and imaging of spectral line polarization observations in Very Long Baseline Interferometry (VLBI). A review is given of the instrumental and propagation effects which need to be removed in the course of calibrating such obervations, with particular reference to their polarization dependence. The removal of amplitude and phase errors and the determination of the instrumental feed response is described. The polarization imaging of such data is discussed with particular reference to the case of poorly sampled cross-polarization data. The software implementation of the algorithms within the Astronomical Image Processing System (AlPS) is discussed and the specific case of spectral line polarization reduction for data observed using the MK3 VLBI system is considered in detail. VLBI observations at two separate epochs of the 1612 MHz OH masers towards the source IRC+ 10420 are reduced as part of this work. Spectral line polarization maps of the source structure are presented, including a discussion of source morphology and variability. The source is sigmficantly circularly polarized at VLBI resolution, but does not display appreciable linear polarization. A proper motion study of the circumstellar envelope is presented, which supports an ellipsoidal kinematic model with anisotropic radial outflow. Kinematic modelling of the measured proper motions suggests a distance to the source of ~ 3 kpc. The cirumstellar magnetic field strength in the masing regions is determined as 1-3 mG, assuming Zeeman splitting as the polarization mechanism

Bootstrap resampling as a tool for radio-interferometric imaging fidelity assessment

We report on a numerical evaluation of the statistical bootstrap as a technique for radio-interferometric imaging fidelity assessment. The development of a fidelity assessment technique is an important scientific prerequisite for automated pipeline reduction of data from modern radio interferometers. We evaluate the statistical performance of two bootstrap methods, the model-based bootstrap and subsample bootstrap, against a Monte Carlo parametric simulation, using interferometric polarization calibration and imaging as the representative problem under study. We find both statistical resampling techniques to be viable approaches to radio-interferometric imaging fidelity assessment which merit further investigation. We also report on the development and implementation of a new self-calibration algorithm for radio-interferometric polarimetry which makes no approximations for the polarization source model.Comment: Accepted by AJ; 41 pages, 13 figure

Stellar masers, circumstellar envelopes, and supernova remnants

This paper reviews recent advances in the study or circumstellar masers and masers found toward supernova remnants. The review is organized by science focus area, including the astrophysics of extended stellar atmospheres, stellar mass-loss processes and outflows, late-type evolved stellar evolution, stellar maser excitation and chemistry, and the use of stellar masers as independent distance estimators. Masers toward supernova remnants are covered separately. Recent advances and open future questions in this field are explored.Comment: 16 pages, 2 figures, 2 tables: based on a review talk presented at IAU Symposium 242: Astrophysical masers and their environments, held at Alice Springs (Australia), March 12-16, 200

Multi-epoch imaging polarimetry of the SiO masers in the extended atmosphere of the Mira variable TX Cam

We present a time series of synoptic images of the linearly-polarized v=1, J=1-0 SiO maser emission toward the Mira variable, TX Cam. These data comprise 43 individual epochs at an approximate biweekly sampling over an optical pulsation phase range of 0.68 to 1.82. The images have an angular resolution of ~500 microarcsec and were obtained using the Very Long Baseline Array (VLBA), operating in the 43 GHz band in spectral-line, polarization mode. We have previously published the total intensity time series for this pulsation phase range; this paper serves to present the linearly-polarized image sequence and an associated animation representing the evolution of the linear polarization morphology over time. We find a predominantly tangential polarization morphology, a high degree of persistence in linear polarization properties over individual component lifetimes, and stronger linear polarization in the inner projected shell than at larger projected shell radii. We present an initial polarization proper motion analysis examining the possible dynamical influence of magnetic fields in component motions in the extended atmospheres of late-type, evolved stars.Comment: 40 pages, 16 figure