5,863 research outputs found
Fraxinus Parryi, Nom. Nov., of NW Baja California, Mexico
The common ash of NW Baja California is usually called Fraxinus trifoliata, as in Wiggins\u27 (1980) flora of Baja California. That name is a misspelling of F. trifoliolat, which is a later homonym of F. rriJoliolara W. W. Smith (1916). The Baja Californian ash is therefore renamed Fraxinus parryi. It has also been misidentified as F. jonesii. The common name is crucecilla . This ash barely extends into San Diego County, California
The structure of the accretion disk in NGC 4258 derived from observations of its water vapor masers
A wealth of new information about the structure of the maser disk in NGC 4258
has been obtained from a series of 18 VLBA observations spanning three years,
as well as from 32 additional epochs of spectral monitoring data from 1994 to
the present, acquired with the VLA, Effelsberg, and GBT. The warp of the disk
has been defined precisely. The thickness of the maser disk has been measured
to be 12 microarcseconds (FWHM), which is slightly smaller than previously
quoted upper limits. Under the assumption that the masers trace the true
vertical distribution of material in the disk, from the condition of
hydrostatic equilibrium the sound speed is 1.5 km/s, corresponding to a thermal
temperature of 600K. The accelerations of the high velocity maser components
have been accurately measured for many features on both the blue and red side
of the spectrum. The azimuthal offsets of these masers from the midline (the
line through the disk in the plane of the sky) and derived projected offsets
from the midline based on the warp model correspond well with the measured
offsets. This result suggests that the masers are well described as discrete
clumps of masing gas, which accurately trace the Keplerian motion of the disk.
However, we have continued to search for evidence of apparent motions caused by
``phase effects.'' This work provides the foundation for refining the estimate
of the distance to NGC 4258 through measurements of feature acceleration and
proper motion. The refined estimate of this distance is expected to be
announced in the near future.Comment: 8 pages, 5 figures, to appear in proceedings of IAU Symposium 242
"Astrophysical Masers and their Environments", held in Alice Springs, March
200
How can technology make this work? Preservice teachers, off-campus learning and digital portfolios
Utilising appropriate Information Communication Technologies (ICT) as instructional tools in teacher education can be a challenging yet worthwhile endeavour. This paper reports the difficulties and benefits of a recent inter-university project requiring preservice primary teachers to construct professional digital portfolios using the support of ICT. Challenges with regard to communication and learning were numerous as 34 preservice teachers (PSTs) from three universities in NSW (situated in country towns, and in Sydney) worked together as a collaborative learning community. Meeting regularly face-to-face during the 12-month process was not always possible and so ICT resources were employed to facilitate instruction and communication. An action research methodology generated a data set of surveys, observational field notes, and student assessment artefacts. We share the findings in this paper so that others may learn from our experiences in using free access software and commonplace hardware when constructing digital portfolios with students who are some distance from campus
Infrared and radio observations of W51: Another Orion-KL at a distance of 7kpc
The bright infrared sources W51-IRS2 has at least three components with different physical and evolutionary properties. The spatial distribution and the near infrared spectra of the components in IRS2 are remarkably similar to, but more luminous than those found in Orion, where an H2 region of comparable linear size is also located close to a cluster of compact infrared sources. The characteristics of the nearby W51-NORTH H2O maser source, and the detection of 2 micro m H2 quadrupole emission in IRS2 indicate that the mass loss phenomena found in Orion-KL also exist in W51
Toward a New Distance to the Active Galaxy NGC 4258: II. Centripetal Accelerations and Investigation of Spiral Structure
We report measurements of centripetal accelerations of maser spectral
components of NGC 4258 for 51 epochs spanning 1994 to 2004. This is the second
paper of a series, in which the goal is determination of a new geometric maser
distance to NGC 4258 accurate to possibly ~3%. We measure accelerations using a
formal analysis method that involves simultaneous decomposition of maser
spectra for all epochs into multiple, Gaussian components. Components are
coupled between epochs by linear drifts (accelerations) from their centroid
velocities at a reference epoch. For high-velocity emission, accelerations lie
in the range -0.7 to +0.7 km/s/yr indicating an origin within 13 degrees of the
disk midline (the perpendicular to the line-of-sight to the black hole).
Comparison of high-velocity emission projected positions in VLBI images, with
those derived from acceleration data, provides evidence that masers trace real
gas dynamics. High-velocity emission accelerations do not support a model of
trailing shocks associated with spiral arms in the disk. However, we find
strengthened evidence for spatial periodicity in high-velocity emission, of
wavelength 0.75 mas. This supports suggestions of spiral structure due to
density waves in the nuclear accretion disk of an active galaxy. Accelerations
of low-velocity (systemic) emission lie in the range 7.7 to 8.9 km/s/yr,
consistent with emission originating from a concavity where the thin, warped
disk is tangent to the line-of-sight. A trend in accelerations of low-velocity
emission as a function of Doppler velocity may be associated with disk geometry
and orientation, or with the presence of spiral structure.Comment: Accepted to ApJ, 48 pages and 20 figure
Furthering the understanding of silicate-substitution in α-tricalcium phosphate : an X-ray diffraction, X-ray fluorescence and solid-state nuclear magnetic resonance study
High-purity (SupT) and reagent-grade (ST), stoichiometric and silicate-containing α-tricalcium phosphate (α-TCP: ST0/SupT0 and Si-TCP x = 0.10: ST10/SupT10) were prepared by solid-state reaction based on the substitution mechanism Ca3(PO4)(2-x)(SiO4)x. Samples were determined to be phase pure by X-ray diffraction (XRD), and Rietveld analysis performed on the XRD data confirmed inclusion of Si in the α-TCP structure as determined by increases in unit cell parameters; particularly marked increases in the b-axis and β-angle were observed. X-ray fluorescence (XRF) confirmed the presence of expected levels of Si in Si-TCP compositions as well as significant levels of impurities (Mg, Al and Fe) present in all ST samples; SupT samples showed both expected levels of Si and a high degree of purity. Phosphorus (31P) magic-angle-spinning solid-state nuclear magnetic resonance (MAS NMR) measurements revealed that the high-purity reagents used in the synthesis of SupT0 can resolve the 12 expected peaks in the 31P spectrum of α-TCP compared to the low-purity ST0 that showed significant spectral line broadening; line broadening was also observed with the inclusion of Si which is indicative of induced structural disorder. Silicon (29Si) MAS NMR was also performed on both Si-TCP samples which revealed Q0 species of Si with additional Si Q1/Q2 species that may indicate a potential charge-balancing mechanism involving the inclusion of disilicate groups; additional Q4 Si species were also observed, but only for ST10. Heating and cooling rates were briefly investigated by 31P MAS NMR which showed no significant line broadening other than that associated with the emergence of β-TCP which was only realised with the reagent-grade sample ST0. This study provides an insight into the structural effects of Si-substitution in α-TCP and could provide a basis for understanding how substitution affects the physicochemical properties of the material
21cm Absorption by Compact Hydrogen Disks Around Black Holes in Radio-Loud Nuclei of Galaxies
The clumpy maser disks observed in some galactic nuclei mark the outskirts of
the accretion disk that fuels the central black hole and provide a potential
site of nuclear star formation. Unfortunately, most of the gas in maser disks
is currently not being probed; large maser gains favor paths that are
characterized by a small velocity gradient and require rare edge-on
orientations of the disk. Here we propose a method for mapping the atomic
hydrogen distribution in nuclear disks through its 21cm absorption against the
radio continuum glow around the central black hole. In NGC 4258, the 21cm
optical depth may approach unity for high angular-resolution (VLBI) imaging of
coherent clumps which are dominated by thermal broadening and have the column
density inferred from X-ray absorption data, ~10^{23}/cm^2. Spreading the 21cm
absorption over the full rotation velocity width of the material in front of
the narrow radio jets gives a mean optical depth of ~0.1. Spectroscopic
searches for the 21cm absorption feature in other galaxies can be used to
identify the large population of inclined gaseous disks which are not masing in
our direction. Follow-up imaging of 21cm silhouettes of accelerating clumps
within these disks can in turn be used to measure cosmological distances.Comment: 4 page
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