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We describe an ultra-wide-bandwidth, low-frequency receiver recently installed on the Parkes radio telescope. The receiver system provides continuous frequency coverage from 704 to 4032 MHz. For much of the band (
), the system temperature is approximately 22 K and the receiver system remains in a linear regime even in the presence of strong mobile phone transmissions. We discuss the scientific and technical aspects of the new receiver, including its astronomical objectives, as well as the feed, receiver, digitiser, and signal processor design. We describe the pipeline routines that form the archive-ready data products and how those data files can be accessed from the archives. The system performance is quantified, including the system noise and linearity, beam shape, antenna efficiency, polarisation calibration, and timing stability.
A fine-grained, up to 3-m-thick tephra bed in southwestern Saskatchewan, herein named Duncairn tephra (Dt), is derived from an early Pleistocene eruption in the Jemez Mountains volcanic field of New Mexico, requiring a trajectory of northward tephra dispersal of ~1500 km. An unusually low CaO content in its glass shards denies a source in the closer Yellowstone and Heise volcanic fields, whereas a Pleistocene tephra bed (LSMt) in the La Sal Mountains of Utah has a very similar glass chemistry to that of the Dt, supporting a more southerly source. Comprehensive characterization of these two distal tephra beds along with samples collected near the Valles caldera in New Mexico, including grain size, mineral assemblage, major- and trace-element composition of glass and minerals, paleomagnetism, and fission-track dating, justify this correlation. Two glass populations each exist in the Dt and LSMt. The proximal correlative of Dt1 is the plinian Tsankawi Pumice and co-ignimbritic ash of the first ignimbrite (Qbt1g) of the 1.24 Ma Tshirege Member of the Bandelier Tuff. The correlative of Dt2 and LSMt is the co-ignimbritic ash of Qbt2. Mixing of Dt1 and Dt2 probably occurred during northward transport in a jet stream.
We report on the analysis of virtual powder-diffraction patterns from serial femtosecond crystallography (SFX) data collected at an X-ray free-electron laser. Different approaches to binning and normalizing these patterns are discussed with respect to the microstructural characteristics which each highlights. Analysis of SFX data from a powder of Pr0.5Ca0.5MnO3 in this way finds evidence of other trace phases in its microstructure which was not detectable in a standard powder-diffraction measurement. Furthermore, a comparison between two virtual powder pattern integration strategies is shown to yield different diffraction peak broadening, indicating sensitivity to different types of microstrain. This paper is a first step in developing new data analysis methods for microstructure characterization from serial crystallography data.
While illuminating and justifying the contemporary appreciation of Edward Bulwer's early novels, Richard Henry Dana discerns in his Harvard thesis of 1837 a Bulwerian strategy that he would employ himself in Two Years Before the Mast. Both the undergraduate thesis and Two Years derive from the experiences of Dana's famous voyage, which have led him to recognise the power of literature to exercise a moral force. In the practice of Bulwer, which Dana would seek to emulate, literature becomes a means for liberating voices that the predominating social forces have been hitherto suppressing.
Short-chain oligomers of aniline are attractive semi-metallic materials for applications as organic electrodes or hole-transporting layers in organic photovoltaics. However, conventionally processed oligoanilines are often amorphous, which limits their conductivities and carrier transport mobilities. Here, we report a simple solvent-exchange method that can render a variety of oligoanilines and their derivatives into crystals of different shapes and dimensions, including 1-D fibers and wires, 2-D ribbons, and 3-D plates, hollow spheres, porous sheets, and flower-like structures. Dopant ions are also simultaneously incorporated into the crystals during self-assembly, allowing them to become conducting. Mechanistic studies suggest that the higher order crystals arise from the most primitive nanofibrillar morphology via hierarchical assembly, providing insights into a general approach to control organic crystal morphologies. Selected area electron diffraction studies reveal their single crystalline nature.
We have obtained spectra of 16 PNe in the disk of M31 and determined the abundances of He, N, O, Ne, S and Ar. Here we present the median abundances and compare them with previous M31 PN disk measurements and with PNe in the Milky Way. We also derive the radial oxygen gradient in M31, which is shallower than that in the Milky Way, even accounting for M31's larger disk scale length.
We review gas-phase abundances in PNe and describe their dual utility as archives of original progenitor metallicity via the α elements, as well as sources of processed material from nucleosynthesis during the star's evolution, i.e., C, N, and s-process elements. We describe the analysis of PN spectra to derive abundances and discuss the discrepancies that arise from different choices at each step. Abundance results for the Milky Way and Magellanic Clouds from various groups of investigators are presented; the observational results are compared with theoretical predictions of AGB stellar yields. Finally, we suggest areas where more work is needed to improve our abilities to determine abundances in PNe.
We discuss the 3D morphology, ionization structure, and kinematics of NGC 2392, the “Eskimo,” based on new and archival HST imagery and new long-slit echelle spectroscopy. High spatial resolution ionization maps of the nebula were made from HST WFPC2 imagery and compared with their velocity structure in various emission lines from echelle spectra taken with the 4m telescope at Kitt Peak. The imagery and spectra were then compared to map the kinematics of the nebula in several emission lines and decode the 3-dimensional morphology and ionization structure of the nebula, including that of C+2 from C III] 1909 Å for the first time.
We examine the abundance gradient in the Milky Way disk via homogeneously determined data for 124 Galactic planetary nebulae (PNe). We present recent results from a detailed regression analysis of the O gradient. With O, Ne, S, Cl, and Ar available and a range of galactocentric distance (Rg) from 0.9 to 21 kpc, we present additional exploration of the disk radial gradient by statistically analyzing a series of short segments of increasing average Rg.
We carefully consider numerous explanations for the sulfur abundance anomaly in planetary nebulae. No one rationale appears to be satisfactory, and we suggest that the ultimate explanation is likely to be a heretofore unidentified feature of the nebular gas which significantly impacts the sulfur ionization correction factor.