5,133 research outputs found
Neutron Scattering and magnetization studies of BaCuCoOCl: A decorated two-dimensional antiferromagnet
BaCuOCl has two inter-penetrating square Cu sublattices, one
with square root 2 times the in-plane spacing of the other. Isotropic magnetic
interactions between the two sublattices are completely frustrated. Quantum
fluctuations resolve the intrinsic degeneracy in the ordering direction of the
more weakly coupled sublattice in favor of collinear ordering. We present
neutron scattering and magnetization studies of the magnetic structure when the
Cu ions are substituted with Co. The Co spins create new magnetic interactions
between the two sublattices. The ordering behavior of both Cu sublattices is
retained largely unmodified. Between the phase transitions of the two
sublattices spin-glass behavior is observed. Magnetization results show a
strong enhancement to the ferromagnetic aspect of the magnetic structure. The
combination of glassy behavior and large moments strongly suggest that the Co
moments induce the formation of local canted states.Comment: 4 figure
Infrared astronomical satellite (IRAS) catalogs and atlases. Volume 1: Explanatory supplement
The Infrared Astronomical Satellite (IRAS) was launched on January 26, 1983. During its 300-day mission, IRAS surveyed over 96 pct of the celestial sphere at four infrared wavelengths, centered approximately at 12, 25, 60, and 100 microns. Volume 1 describes the instrument, the mission, and data reduction
A new infrared band in the Interstellar and Circumstellar Clouds: C_4 or C_4H?
We report on the detection with the Infrared Space Observatory (ISO) of a
molecular band at 57.5 microns (174 cm^{-1}) in carbon-rich evolved stars and
in Sgr B2. Taking into account the chemistry of these objects the most
likelihood carrier is a carbon chain. We tentatively assign the band to the
nu_5 bending mode of C_4 for which a wavenumber of 170-172.4 cm^{-1} has been
derived in matrix experiments (Withey et al. 1991). An alternate carrier might
be C_4H, although the frequency of its lowest energy vibrational bending mode,
nu_7, is poorly known (130-226 cm^{-1}). If the carrier is C_4, the derived
maximum abundance is nearly similar to that found for C_3 in the interstellar
and circumstellar media by Cernicharo, Goicoechea & Caux (2000). Hence,
tetra-atomic carbon could be one of the most abundant carbon chain molecules in
these media.Comment: 11 pages, 1 figure, accepted in ApJ Letter
Colloidal templating at a cholesteric - oil interface: Assembly guided by an array of disclination lines
We simulate colloids (radius m) trapped at the interface between
a cholesteric liquid crystal and an immiscible oil, at which the helical order
(pitch p) in the bulk conflicts with the orientation induced at the interface,
stabilizing an ordered array of disclinations. For weak anchoring strength W of
the director field at the colloidal surface, this creates a template, favoring
particle positions eitheron top of or midway between defect lines, depending on
. For small , optical microscopy experiments confirm this
picture, but for larger no templating is seen. This may stem from the
emergence at moderate W of a rugged energy landscape associated with defect
reconnections.Comment: 5 pages, 4 figure
Detection of Far-Infrared Water Vapor, Hydroxyl, and Carbon Monoxide Emissions from the Supernova Remnant 3C 391
We report the detection of shock-excited far-infrared emission of H2O, OH,
and CO from the supernova remnant 3C 391, using the ISO Long-Wavelength
Spectrometer. This is the first detection of thermal H2O and OH emission from a
supernova remnant. For two other remnants, W~28 and W~44, CO emission was
detected but OH was only detected in absorption. The observed H2O and OH
emission lines arise from levels within ~400 K of the ground state, consistent
with collisional excitation in warm, dense gas created after the passage of the
shock front through the dense clumps in the pre-shock cloud. The post-shock gas
we observe has a density ~2x10^5 cm^{-3} and temperature 100-1000 K, and the
relative abundances of CO:OH:H2O in the emitting region are 100:1:7 for a
temperature of 200 K. The presence of a significant column of warm H2O suggests
that the chemistry has been significantly changed by the shock. The existence
of significant column densities of both OH and H2O, which is at odds with
models for non-dissociative shocks into dense gas, could be due to
photodissociation of H2O or a mix of fast and slow shocks through regions with
different pre-shock density.Comment: AASTeX manuscript and 4 postscript figure
Recommended from our members
Regional initiatives and the UN 2001 Conference: Building Mutual Support and Complementarity.
yesAs the agenda for the United Nations (UN) 2001 Conference on The Illicit Trade in Small Arms and Light Weapons in All Its Aspects takes shape, governments should begin to identify a set
of standards, mechanisms and specific agreements that will help consolidate, reinforce and
co-ordinate regional and national measures to address the problem of the proliferation and
misuse of small arms. An important element of this approach will be to build upon the wealth
of regional and national experiences and perspectives that illustrate the different contexts
in which efforts to combat the proliferation and misuse of small arms and light weapons have
occurred. At the same time, agreements reached at the UN 2001 Conference should be
substantial, establishing an agreed comprehensive `international action programme¿ f o r
sustained global effort on this complex problem. However there remain issues and concerns
that are common to all regions: these should be identified and addressed internationally
within the context of the UN 2001 Conference.
This briefing, the second in the Biting the Bullet series, reviews some of the current regional
e fforts on small arms and light weapons. It identifies common approaches that have been used
in different regions to counter the proliferation and misuse of small arms and light weapons,
these include: law enforcement and crime control; supplier restraint and transparency; national
legislation and regulation of arms; and arms reduction and control. The briefing analyses
initiatives using these approaches that are moving forward in West Africa, Eastern and
Southern Africa, the European Union (EU), and the development of cooperation between
EU Member States and other countries and regional organizations, including Cambodia
and the Southern African Development Community. The briefing identifies the impact and
priorities of these initiatives, suggesting ways in which the UN 2001 Conference is both
relevant to the region and what the region can contribute to the outcomes of the Conference.
The briefing concludes with recommendations on the ways in which regional processes can be
reinforced and further developed by the international community, focusing especially on the
contribution of the UN 2001 Conference. Experience is showing that much of what happens
nationally and regionally needs reinforcement and further development with assistance from
the international community. The UN 2001 Conference comes at an important time for providing
the framework ¿ through the international action programme ¿ to develop, reinforce and
c o-ordinate these national and regional processes, through developing appropriate international
norms, standards, programmes and mechanisms. Using the illustration of combating illicit arms
trafficking, this briefing outlines some of the processes that could be taken forward through
the UN 2001 Conference which would build upon and strengthen national and regional eff o r t s .
The briefing contains an annex, which provides background information on many current
regional and international initiatives, including those in Africa, the Americas, Asia, Europe
and inter-regionally, such as the Organisation for Security and Co-operation in Europe and
the North Atlantic Treaty Organisation
OH rotational lines as a diagnostic of the warm neutral gas in galaxies
We present Infrared Space Observatory (ISO) observations of several OH, CH
and H2O rotational lines toward the bright infrared galaxies NGC253 and
NGC1068. As found in the Galactic clouds in SgrB2 and Orion, the extragalactic
far-IR OH lines change from absorption to emission depending on the physical
conditions and distribution of gas and dust along the line of sight. As a
result, most of the OH rotational lines that appear in absorption toward NGC253
are observed in emission toward NGC1068. We show that the far-IR spectrum of OH
can be used as a powerful diagnostic to derive the physical conditions of
extragalactic neutral gas. In particular, we find that a warm (Tk~150 K, n(H2)<
5 10^4 cm^-3) component of molecular gas with an OH abundance of 10^{-7} from
the inner <15'' can qualitatively reproduce the OH lines toward NGC253. Similar
temperatures but higher densities (5 10^5 cm^-3) are required to explain the OH
emission in NGC1068.Comment: 5 pages, 4 figures, accepted in ApJ Part I (2004, October 6
A New Look At Carbon Abundances In Planetary Nebulae. III. DDDM1, IC 3568, IC4593, NGC 6210, NGC 6720, NGC 6826, & NGC 7009
This paper is the third in a series reporting on a study of carbon abundances
in a carefully chosen sample of planetary nebulae representing a large range in
progenitor mass and metallicity. We make use of the IUE Final Archive database
containing consistently-reduced spectra to measure line strengths of C III]
1909 along with numerous other UV lines for the planetary nebulae DDDM1, IC
3568, IC 4593, NGC 6210, NGC 6720, NGC 6826, & NGC 7009. We combine the IUE
data with line strengths from optical spectra obtained specifically to match
the IUE slit positions as closely as possible, to determine values for the
abundance ratios He/H, O/H, C/O, N/O, and Ne/O. The ratio of C III] 1909/C II
4267 is found to be effective for merging UV and optical spectra when He II
1640/4686 is unavailable. Our abundance determination method includes a 5-level
program whose results are fine-tuned by corrections derived from detailed
photoionization models constrained by the same set of emission lines. All
objects appear to have subsolar levels of O/H, and all but one show N/O levels
above solar. In addition, the seven planetary nebulae span a broad range in C/O
values. We infer that many of our objects are matter bounded, and thus the
standard ionization correction factor for N/O may be inappropriate for these
PNe. Finally, we estimate C/O using both collisionally-excited and
recombination lines associated with C+2 and find the well established result
that abundances from recombination lines usually exceed those from
collisionally-excited lines by several times.Comment: 36 pages, 7 tables, 2 figures, latex. Tables and figures supplied as
two separate postscript files. Accepted for publication in Ap
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