Conference Summary: HI Science in the Next Decade

The atomic hydrogen (HI) 21cm line measures the gas content within and around galaxies, traces the dark matter potential and probes volumes and objects that other surveys do not. Over the next decade, 21cm line science will exploit new technologies, especially focal plane and aperture arrays, and will see the deployment of Epoch of Reionization/Dark Age detection experiments and Square Kilometer Array (SKA) precursor instruments. Several experiments designed to detect and eventually to characterize the reionization history of the intergalactic medium should deliver first results within two-three years time. Although "precision cosmology" surveys of HI in galaxies at z ~ 1 to 3 require the full collecting area of the SKA, a coherent program of HI line science making use of the unique capabilities of both the existing facilities and the novel ones demonstrated by the SKA precursors will teach us how many gas rich galaxies there really are and where they reside and will yield fundamental insight into how galaxies accrete gas, form stars and interact with their environment.Comment: To appear in AIP Conference Proceedings, "The Evolution of Galaxies through the Neutral Hydrogen Window", Feb 1-3 2008, Arecibo, Puerto Rico, eds. R. Minchin & E. Momjian. 8 page

Redundancy Calibration of Phased Array Stations

Our aim is to assess the benefits and limitations of using the redundant visibility information in regular phased array systems for improving the calibration. Regular arrays offer the possibility to use redundant visibility information to constrain the calibration of the array independent of a sky model and a beam models of the station elements. It requires a regular arrangement in the configuration of array elements and identical beam patterns. We revised a calibration method for phased array stations using the redundant visibility information in the system and applied it successfully to a LOFAR station. The performance and limitations of the method were demonstrated by comparing its use on real and simulated data. The main limitation is the mutual coupling between the station elements, which leads to non-identical beams and stronger baseline dependent noise. Comparing the variance of the estimated complex gains with the Cramer-Rao Bound (CRB) indicates that redundancy is a stable and optimum method for calibrating the complex gains of the system. Our study shows that the use of the redundant visibility does improve the quality of the calibration in phased array systems. In addition it provides a powerful tool for system diagnostics. Our results demonstrate that designing redundancy in both the station layout and the array configuration of future aperture arrays is strongly recommended. In particular in the case of the Square Kilometre Array with its dynamic range requirement which surpasses any existing array by an order of magnitude.Comment: 16 pages, 15 figures, accepted for publication in the A&A in Section 13, acceptance date: 1st May 2012. NOTE: Please contact the first author for high resolution figure

Synthesis Minimizations and Mesh Algorithm Selection: An Extension of the Ultrasonic 3D Camera

Statement of Purpose The purpose of this Capstone project was to perform synthesis minimizations and optimal mesh algorithm selection for some of the digital components of a prototype Ultrasonic 3D Camera, the subject of my group senior design project for computer engineering. Both of the high-level design tasks that I performed were unnecessary for the scope of the senior design class, whose focus was simply to perform a proof-of-concept or create a basic, functioning prototype. The steps I took in performing synthesis optimizations and mesh algorithm selection went beyond the scope of the senior project, by doing the polishing that would be most suited for a project that was eventually going to be turned into an actual product. Design Methodology Synthesis optimizations were performed using the Altera Quartus II software, available to me as a student of the L.C. Smith College of Engineering and Computer Science. With the assistance of the Quartus II sofware, I synthesized the behavioral VHDL code written for the project, and inspected the resulting netlists for places where additional logic was being unnecessarily included. In the places where unnecessary logic was found, I eliminated it, by rewriting behavioral code as structural, or by simply clarifying the definition of components. Mesh algorithm selection was performed using the Microsoft Visual Studio IDE (Integrated Design Environment) in conjunction with the OGRE 3D graphics engine. Using Visual Studio and OGRE, I was able to experiment with different mesh-forming algorithms and determine which method was best suited for the Camera, given the nature of the incoming data. Outcome The VHDL code for the project is now optimized for synthesis, such that if my group were to take steps to have the VHDL code synthesized into a netlist, and then eventually into a mask, and then turned into an actual integrated circuit, that circuit would be almost minimally small, while still performing all its necessary functions. Likewise, the mesh algorithm selected, the naïve method, works perfectly well with the nature of the data that the Camera obtains

Rotation, spectral variability, magnetic geometry and magnetosphere of the Of?p star CPD -28 2561

We report magnetic and spectroscopic observations and modeling of the Of?p star CPD -28 2561. Using more than 75 new spectra, we have measured the equivalent width variations and examined the dynamic spectra of photospheric and wind-sensitive spectral lines. A period search results in an unambiguous 73.41 d variability period. High resolution spectropolarimetric data analyzed using Least-Squares Deconvolution yield a Zeeman signature detected in the mean Stokes V profile corresponding to phase 0.5 of the spectral ephemeris. Interpreting the 73.41 d period as the stellar rotational period, we have phased the equivalent widths and inferred longitudinal field measurements. The phased magnetic data exhibit a weak sinusoidal variation, with maximum of about 565 G at phase 0.5, and a minimum of about -335 G at phase 0.0, with extrema approximately in phase with the (double-wave) Halpha equivalent width variation. Modeling of the Halpha equivalent width variation assuming a quasi-3D magnetospheric model produces a unique solution for the ambiguous couplet of inclination and magnetic obliquity angles: (i, beta) or (beta, i)=(35 deg,90 deg). Adopting either geometry, the longitudinal field variation yields a dipole polar intensity Bd=2.6\pm 0.9~kG, consistent with that obtained from direct modelling of the Stokes V profiles. We derive a wind magnetic confinement parameter eta*\simeq 100, leading to an Alfv\'en radius RA\simeq 3-5~R*, and a Kepler radius RK\simeq 20~R*. This supports a physical scenario in which the Halpha emission and other line variability have their origin in an oblique, co-rotating 'dynamical magnetosphere' structure resulting from a magnetically channeled wind. Nevertheless, the details of the formation of spectral lines and their variability within this framework remain generally poorly understood.Comment: 18 pages, accepted by MNRAS Replaced 28 March 2015 to include corrected figure 10 (see MNRAS erratum to this effect

Abundances of lithium, sodium, and potassium in Vega

Vega's photospheric abundances of Li, Na, and K were determined by using considerably weak lines measured on the very high-S/N spectrum, while the non-LTE correction and the gravity-darkening correction were adequately taken into account. It was confirmed that these alkali elements are mildly underabundant ([Li/H] ~ -0.6, [Na/H] ~ -0.3, and [K/H] ~ -0.2) compared to the solar system values, as generally seen also in other metals. Since the tendency of Li being more deficient than Na and K is qualitatively similar to what is seen in typical interstellar cloud, the process of interstellar gas accretion may be related with the abundance anomaly of Vega, as suspected in the case of lambda Boo stars.Comment: Accepted for publication in MNRAS; 8 pages, 9 figure

Dispersive Fourier Transformation for Versatile Microwave Photonics Applications

Abstract: Dispersive Fourier transformation (DFT) maps the broadband spectrum of an ultrashort optical pulse into a time stretched waveform with its intensity profile mirroring the spectrum using chromatic dispersion. Owing to its capability of continuous pulse-by-pulse spectroscopic measurement and manipulation, DFT has become an emerging technique for ultrafast signal generation and processing, and high-throughput real-time measurements, where the speed of traditional optical instruments falls short. In this paper, the principle and implementation methods of DFT are first introduced and the recent development in employing DFT technique for widespread microwave photonics applications are presented, with emphasis on real-time spectroscopy, microwave arbitrary waveform generation, and microwave spectrum sensing. Finally, possible future research directions for DFT-based microwave photonics techniques are discussed as well

Magnetic fields and chemical peculiarities of the very young intermediate-mass binary system HD 72106

The recently discovered magnetic Herbig Ae and Be stars may provide qualitatively new information about the formation and evolution of magnetic Ap and Bp stars. We have performed a detailed investigation of one particularly interesting binary system with a Herbig Ae secondary and a late B-type primary possessing a strong, globally ordered magnetic field. Twenty high-resolution Stokes V spectra of the system were obtained with the ESPaDOnS instrument mounted on the CFHT. In these observations we see clear evidence for a magnetic field in the primary, but no evidence for a magnetic field in the secondary. A detailed abundance analysis was performed for both stars, revealing strong chemical peculiarities in the primary and normal chemical abundances in the secondary. The primary is strongly overabundant in Si, Cr, and other iron-peak elements, as well as Nd, and underabundant in He. The primary therefore appears to be a very young Bp star. In this context, line profile variations of the primary suggest non-uniform lateral distributions of surface abundances. Interpreting the 0.63995 +/- 0.00009 day variation period of the Stokes I and V profiles as the rotational period of the star, we have modeled the magnetic field geometry and the surface abundance distributions of Si, Ti, Cr and Fe using Magnetic Doppler Imaging. We derive a dipolar geometry of the surface magnetic field, with a polar strength of 1230 G and an obliquity of 57 degrees. The distributions Ti, Cr and Fe are all qualitatively similar, with an elongated patch of enhanced abundance situated near the positive magnetic pole. The Si distribution is somewhat different, and its relationship to the magnetic field geometry less clear.Comment: Accepted by Monthly Notices of the Royal Astronomical Society, September 2008. 15 pages, 10 figure