2,127 research outputs found
Are young stars always associated with cold massive disks? A CO and millimeter interferometric continuum survey
The results of a combined millimeter-spectral-line and continuum survey of cold far-infrared sources selected to favor embedded young stars in the Galaxy are presented. The spectral-line observations were performed with the 5 meter antenna of the University of Texas Millimeter-Wave Observatory. High resolution continuum observations were obtained with the Owens Valley (OVRO) Millimeter-Wave Interferometer. The goal of the survey was to gain insight into the mass, temperature, and distribution of cold dust which envelopes stars during the earliest stages of their evolution. The first phase of our survey involved 1.2 arcmin resolution observations of CO-12 and CO-13 emission lines toward each source. All but two sources had detectable CO emission. We found that 40% of the sources appear to be associated with star formation as evidenced by the presence of enhanced CO-12 line widths or broad wings. At least five of these objects are associated with bipolar molecular outflows. The second phase of our survey involves high resolution 2.7 mm continuum observations with 3 interferometer baselines ranging from 15 to 55 m in length. Preliminary results indicate that about 25% of the sources in our sample have detectable continuum emission on scales less than 30 arcsec. The high percentage of sources with enhanced CO-12 line widths or broad wings indicates that a significant fraction of our samples, 40%, are likely to be young stars. The lower detection percentage in the continuum observations, 25%, suggest that such objects are not always surrounded by large concentrations of gas and dust. The continuum detection percentage for actual dust emission could be lower than that given above since emission from ionized gas could be responsible for the observed 2.7 mm emission in some objects. To get an understanding of the type of object detected in our survey, a map of one of the survey sources, L1689N, has been made using the OVRO mm interferometer
The impact of shocks on the chemistry of molecular clouds: high resolution images of chemical differentiation along the NGC1333-IRAS2A outflow
This paper presents a detailed study of the chemistry in the outflow
associated with the low-mass protostar NGC1333-IRAS2A down to 3" (650 AU)
scales. Millimeter-wavelength aperture-synthesis observations from the OVRO and
BIMA interferometers and (sub)millimeter single-dish observations from the
Onsala 20m telescope and CSO are presented. The interaction of the highly
collimated protostellar outflow with a molecular condensation ~15000 AU from
the central protostar is clearly traced by molecular species such as HCN, SiO,
SO, CS, and CH3OH. Especially SiO traces a narrow high velocity component at
the interface between the outflow and the molecular condensation.
Multi-transition single-dish observations are used to distinguish the chemistry
of the shock from that of the molecular condensation and to address the
physical conditions therein. Statistical equilibrium calculations reveal
temperatures of 20 and 70 K for the quiescent and shocked components,
respectively, and densities near 10^6 cm^{-3}. Significant abundance
enhancements of two to four orders of magnitude are found in the shocked region
for molecules such as CH3OH, SiO and the sulfur-bearing molecules. HCO+ is seen
only in the aftermath of the shock consistent with models where it is destroyed
through release of H2O from grain mantles in the shock. N2H+ shows narrow
lines, not affected by the outflow but rather probing the ambient cloud.
Differences in abundances of HCN, H2CO and CS are seen between different
outflow regions and are suggested to be related to differences in the atomic
carbon abundance. Compared to the warm inner parts of protostellar envelopes,
higher abundances of in particular CH3OH and SiO are found in the outflows,
which may be related to density differences between the regions.Comment: 18 pages, 13 figures. Accepted for publication in A&
A λ = 1.3 Millimeter Aperture Synthesis Molecular Line Survey of Orion Kleinmann-Low
We present a 1".3 spatial resolution interferometric spectral line survey of the core of the Orion molecular cloud, obtained with the OVRO millimeter array. Covering 4 GHz bandwidth in total, the survey contains ~100 emission lines from 18 chemical species. The spatial distributions of a number of molecules point to source I near the IRc2 complex as the dominant energy source in the region but do not rule out the presence of additional lower luminosity objects. At arcsecond resolution, the offsets between dust emission and various molecular tracers suggest that the spectacular "hot core" emission in the Orion core arises via the heating and ablation of material from the surfaces of very high density clumps located ≳500 AU from source I and traced by the dust emission. We find no evidence for a strong internal heating source within the hot core condensation(s)
Scanning Electron Microscopy in Bone Pathology: Review of Methods, Potential and Applications
This article reviews the applications of SEM methods to human bone pathologies referring to studies made at UCL. We consider the methods which may be most suitable; these prove to be not routine in the context of most bio-medical applications of SEM.
Valuable information can be obtained from a bone sample if its edges are ground flat, before making either (a) a matrix surface preparation by washing away all the cells or (b) a mineralising front preparation, by also dissolving the osteoid -for which hydrogen peroxide is recommended to produce a robust specimen. BSE contrast from a cut block surface can be used to measure bone phase volume. SE contrasts from natural surfaces (trabeculae, canals and lacunae) can be used to study forming, resting and resorbing* surfaces both qualitatively and quantitatively (*except in the case of histological osteomalacia, where the existence of osteoid will go undetected and reversal lines will be difficult to distinguish from recently resorbed surfaces).
We also recommend the use of PMMA embedded bone blocks, which can be used as obtained from the pathologist, but are better embedded by a more rigorous procedure. BSE image analysis can be used to quantitate bone density fractions opening up a completely new investigative method for the future. Osteoid can be measured automatically using CL if the bone sample is block stained with brilliant sulphaflavine before embedding or if a scintillant is added to the embeddant. We give examples of observations made from a number of bone diseases: vitamin D resistant rickets, ostegenesis imperfecta; osteomalacia; osteoporosis; hyperparathyroidism; fluorosis; Paget \u27s disease; tumour metastasis to bone
Epitaxy of hexagonal ABO quantum materials
Hexagonal O oxides (, = cation) are a rich materials class for
realizing novel quantum phenomena. Their hexagonal symmetry, oxygen trigonal
bipyramid coordination and quasi-two dimensional layering give rise to
properties distinct from those of the cubic O perovskites. As bulk
materials, most of the focus in this materials class has been on the rare earth
manganites, MnO ( = rare earth); these materials display coupled
ferroelectricity and antiferromagnetic order. In this review, we focus on the
thin film manifestations of the hexagonal O oxides. We cover the
stability of the hexagonal oxides and substrates which can be used to template
the hexagonal structure. We show how the thin film geometry not only allows for
further tuning of the bulk-stable manganites but also the realization of
metastable hexagonal oxides such as the FeO that combine
ferroelectricity with weak ferromagnetic order. The thin film geometry is a
promising platform to stabilize additional metastable hexagonal oxides to
search for predicted high-temperature superconductivity and topological phases
in this materials class.Comment: The following article has been accepted by Applied Physics Review
Simian immunodeficiency virus infection in wild-caught chimpanzees from Cameroon
Simian immunodeficiency viruses (SIVcpz) infecting chimpanzees (Pan troglodytes) in west central Africa are the closest relatives to all major variants of human immunodeficiency virus type 1 ([HIV-1]; groups M, N and O), and have thus been implicated as the source of the human infections; however, information concerning the prevalence, geographic distribution, and subspecies association of SIVcpz still remains limited. In this study, we tested 71 wild-caught chimpanzees from Cameroon for evidence of SIVcpz infection. Thirty-nine of these were of the central subspecies (Pan troglodytes troglodytes), and 32 were of the Nigerian subspecies (Pan troglodytes vellerosus), as determined by mitochondrial DNA analysis. Serological analysis determined that one P. t. troglodytes ape (CAM13) harbored serum antibodies that cross-reacted strongly with HIV-1 antigens; all other apes were seronegative. To characterize the newly identified virus, 14 partially overlapping viral fragments were amplified from fecal virion RNA and concatenated to yield a complete SIVcpz genome (9,284 bp). Phylogenetic analyses revealed that SIVcpzCAM13 fell well within the radiation of the SIVcpzPtt group of viruses, as part of a clade including all other SIVcpzPtt strains as well as HIV-1 groups M and N. However, SIVcpzCAM13 clustered most closely with SIVcpzGAB1 from Gabon rather than with SIVcpzCAM3 and SIVcpzCAM5 from Cameroon, indicating the existence of divergent SIVcpzPtt lineages within the same geographic region. These data, together with evidence of recombination among ancestral SIVcpzPtt lineages, indicate long-standing endemic infection of central chimpanzees and reaffirm a west central African origin of HIV-1. Whether P. t. vellerosus apes are naturally infected with SIVcpz requires further study
Girls’ and women’s education within Unesco and the World Bank, 1945–2000
By 2000, girls’ and women’s education was a priority for international development organisations. While studies have examined the impact of recent campaigns and programmes, there has been less exploration of ideas about girls’ and women’s education within development thought in the immediate post?colonial period, and the political mechanisms through which this came to be a global concern. Through a study of policy documents, this paper investigates how the education of girls and women came to be prioritised within the two principle UN agencies involved with education since 1945, the World Bank and Unesco. A shift in priorities is evident, from ensuring formal rights and improving the status of women, to expanding the productive capacities of women, fertility control and poverty reduction. While the ascendance of human capital theory provided a space for a new perception of the role of women’s education in development, in other policy arenas women’s education was central to exploring more substantive, rights?based notions of gender equality. Ultimately, the goal of improving girls’ and women’s education fitted into diverse development agendas, paving the way for it to become a global development priority
Bulgac-Kusnezov-Nos\'e-Hoover thermostats
In this paper we formulate Bulgac-Kusnezov constant temperature dynamics in
phase space by means of non-Hamiltonian brackets. Two generalized versions of
the dynamics are similarly defined: one where the Bulgac-Kusnezov demons are
globally controlled by means of a single additional Nos\'e variable, and
another where each demon is coupled to an independent Nos\'e-Hoover thermostat.
Numerically stable and efficient measure-preserving time-reversible algorithms
are derived in a systematic way for each case. The chaotic properties of the
different phase space flows are numerically illustrated through the
paradigmatic example of the one-dimensional harmonic oscillator. It is found
that, while the simple Bulgac-Kusnezov thermostat is apparently not ergodic,
both of the Nos\'e-Hoover controlled dynamics sample the canonical distribution
correctly
H_2 Emission From Disks Around Herbig Ae and T Tauri Stars
We present the initial results of a deep ISO-SWS survey
for the low J pure rotational emission lines of H2
toward a number of Herbig Ae and T Tauri stars.
The objects are selected to be as isolated as possible
from molecular clouds, with a spectral energy distribution
characteristic of a circumstellar disk. For most
of them the presence of a disk has been established
directly by millimeter interferometry. The S (1) line is
detected in most sources with a peak flux of 0.3-1 Jy.
The S(0) line is definitely seen in 2 objects: GG Tau
and HD 163296. The observations suggest the presence
of "warm" gas at T_(kin) ≈ 100 K with a mass of a
few % of the total gas+ dust mass, derived assuming
a gas-to-dust ratio of 100:1. The S(1) peak flux does
not show a strong correlation with spectral type of
the central star or continuum flux at 1.3 millimeter.
Possible origins for the warm gas seen in H_2 are discussed,
and comparisons with model calculations are
made
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