527 research outputs found
Solid-phase C60 in the peculiar binary XX Oph?
We present infrared spectra of the binary XX Oph obtained with the Infrared Spectrograph on the Spitzer Space Telescope. The data show some evidence for the presence of solid C60– the first detection of C60 in the solid phase – together with the well-known ‘unidentified infrared’ emission features. We suggest that, in the case of XX Oph, the C60 is located close to the hot component, and that in general it is preferentially excited by stars having effective temperatures in the range 15 000–30 000 K. C60 may be common in circumstellar environments, but unnoticed in the absence of a suitable exciting source
Magnetic structure of Yb2Pt2Pb: Ising moments on the Shastry-Sutherland lattice.
Neutron diffraction measurements were carried out on single crystals and powders of Yb2Pt2Pb, where Yb moments form two interpenetrating planar sublattices of orthogonal dimers, a geometry known as Shastry-Sutherland lattice, and are stacked along the c axis in a ladder geometry. Yb2Pt2Pb orders antiferromagnetically at TN=2.07K, and the magnetic structure determined from these measurements features the interleaving of two orthogonal sublattices into a 5×5×1 magnetic supercell that is based on stripes with moments perpendicular to the dimer bonds, which are along (110) and (−110). Magnetic fields applied along (110) or (−110) suppress the antiferromagnetic peaks from an individual sublattice, but leave the orthogonal sublattice unaffected, evidence for the Ising character of the Yb moments in Yb2Pt2Pb that is supported by point charge calculations. Specific heat, magnetic susceptibility, and electrical resistivity measurements concur with neutron elastic scattering results that the longitudinal critical fluctuations are gapped with ΔE≃0.07meV
Influence of landscape attributes on Virginia opossum density
TheVirginia opossum (Didelphis virginiana), North America\u27s only marsupial, has a range extending from southern Ontario, Canada, to the Yucatan Peninsula, Mexico, and from the Atlantic seaboard to the Pacific. Despite the Virginia opossum\u27s taxonomic uniqueness in relation to other mammals in North America and rapidly expanding distribution, its ecology remains relatively understudied. Our poor understanding of the ecology of this important mesopredator is especially pronounced in the rural southeastern United States. Our goal was to estimate effects of habitat on opossum density within an extensive multiyear spatial capture‐recapture study. Additionally, we compared the results of this spatial capture‐recapture analysis with a simple relative abundance index. Opossum densities in the relatively underdeveloped regions of the southeastern United States were lower compared to the more human‐dominated landscapes of the Northeast and Midwest. In the southeastern United States, Virginia opossums occurred at a higher density in bottomland swamp and riparian hardwood forest compared to upland pine (Pinus spp.) plantations and isolated wetlands. These results reinforce the notion that the Virginia opossum is commonly associated with land cover types adjacent to permanent water (bottomland swamps, riparian hardwood). The relatively low density of opossums at isolated wetland sites suggests that the large spatial scale of selection demonstrated by opossums gives the species access to preferable cover types within the same landscape
A Cu2+ (S = 1/2) Kagom\'e Antiferromagnet: MgxCu4-x(OH)6Cl2
Spin-frustrated systems are one avenue for inducing macroscopic quantum
states in materials. However, experimental realization of this goal has been
difficult because of the lack of simple materials and, if available, the
separation of the unusual magnetic properties arising from exotic magnetic
states from behavior associated with chemical disorder, such as site mixing.
Here we report the synthesis and magnetic properties of a new series of
magnetically frustrated materials, MgxCu4-x(OH)6Cl2. Because of the
substantially different ligand-field chemistry of Mg2+ and Cu2+, site disorder
within the kagom\'e layers is minimized, as directly measured by X-ray
diffraction. Our results reveal that many of the properties of these materials
and related systems are not due to disorder of the magnetic lattice but rather
reflect an unusual ground state.Comment: Accepted for publication in J. Am. Chem. Soc
Lunar Lander Offloading Operations Using a Heavy-Lift Lunar Surface Manipulator System
This study investigates the feasibility of using a heavy-lift variant of the Lunar Surface Manipulator System (LSMS-H) to lift and handle a 12 metric ton payload. Design challenges and requirements particular to handling heavy cargo were examined. Differences between the previously developed first-generation LSMS and the heavy-lift version are highlighted. An in-depth evaluation of the tip-over risk during LSMS-H operations has been conducted using the Synergistic Engineering Environment and potential methods to mitigate that risk are identified. The study investigated three specific offloading scenarios pertinent to current Lunar Campaign studies. The first involved offloading a large element, such as a habitat or logistics module, onto a mobility chassis with a lander-mounted LSMS-H and offloading that payload from the chassis onto the lunar surface with a surface-mounted LSMS-H. The second scenario involved offloading small pressurized rovers with a lander-mounted LSMS-H. The third scenario involved offloading cargo from a third-party lander, such as the proposed ESA cargo lander, with a chassis-mounted LSMS-H. In all cases, the analyses show that the LSMS-H can perform the required operations safely. However, Chariot-mounted operations require the addition of stabilizing outriggers, and when operating from the Lunar surface, LSMS-H functionality is enhanced by adding a simple ground anchoring system
Spitzer and ground-based infrared observations of the 2006 eruption of RS Ophiuchi
We present Spitzer Space Telescope and complementary ground-based infrared
observations of the recurrent nova RS Ophiuchi, obtained over the period 64-111
days after the 2006 eruption. The Spitzer IRS data show a rich emission line
spectrum superimposed on a free-free continuum. The presence of fine structure
and coronal infrared lines lead us to deduce that there are at least two
temperatures (1.5e5K and 9e5K) in the ejecta/wind environment, and that the
electron density in the `cooler' region is 2.2e5 cm-3. The determination of
elemental abundances is not straightforward but on the assumption that the Ne
and O fine structure lines arise in the same volume of the ejecta, the O/Ne
ratio is >~0.6 by number.Comment: 13 pages, 4 figures, accepted for publication in ApJ Letter
Can Quantum de Sitter Space Have Finite Entropy?
If one tries to view de Sitter as a true (as opposed to a meta-stable)
vacuum, there is a tension between the finiteness of its entropy and the
infinite-dimensionality of its Hilbert space. We invetsigate the viability of
one proposal to reconcile this tension using -deformation. After defining a
differential geometry on the quantum de Sitter space, we try to constrain the
value of the deformation parameter by imposing the condition that in the
undeformed limit, we want the real form of the (inherently complex) quantum
group to reduce to the usual SO(4,1) of de Sitter. We find that this forces
to be a real number. Since it is known that quantum groups have
finite-dimensional representations only for root of unity, this suggests
that standard -deformations cannot give rise to finite dimensional Hilbert
spaces, ruling out finite entropy for q-deformed de Sitter.Comment: 10 pages, v2: references added, v3: minor corrections, abstract and
title made more in-line with the result, v4: published versio
Silicate dust in the environment of RS Ophiuchi following the 2006 eruption
We present further Spitzer Space Telescope observations of the recurrent nova
RS Ophiuchi, obtained over the period 208-430 days after the 2006 eruption. The
later Spitzer IRS data show that the line emission and free-free continuum
emission reported earlier is declining, revealing incontrovertible evidence for
the presence of silicate emission features at 9.7 and 18microns. We conclude
that the silicate dust survives the hard radiation impulse and shock blast wave
from the eruption. The existence of the extant dust may have significant
implications for understanding the propagation of shocks through the red giant
wind and likely wind geometry.Comment: 12 pages, 4 figures, accepted for publication in ApJ (Letters
The Early Spectrophotometric Evolution of V1186 Scorpii (Nova Scorpii 2004 #1)
We report optical photometry and optical through mid-infrared spectroscopy of
the classical nova V1186 Sco. This slowly developing nova had an complex light
curve with multiple secondary peaks similar to those seen in PW Vul. The time
to decline 2 magnitudes, t, was 20 days but the erratic nature of the light
curve makes determination of intrinsic properties based on the decline time
(e.g., luminosity) problematic, and the often cited MMRD relationship of Della
Valle and Livio (1995) fails to yield a plausible distance. Spectra covering
0.35 to 35 m were obtained in two separate epochs during the first year of
outburst. The first set of spectra, taken about 2 months after visible maximum,
are typical of a CO-type nova with narrow line emission from \ion{H}{1},
\ion{Fe}{2}, \ion{O}{1} and \ion{He}{1}. Later data, obtained between 260 and
380 days after maximum, reveal an emerging nebular spectrum. \textit{Spitzer}
spectra show weakening hydrogen recombination emission with the emergence of
[\ion{Ne}{2}] (12.81 m) as the strongest line. Strong emission from
[\ion{Ne}{3}] (15.56 m) is also detected. Photoionization models with low
effective temperature sources and only marginal neon enhancement (Ne 1.3
Ne) are consistent with these IR fine-structure neon lines indicating
that V1186 Sco did not occur on a ONeMg white dwarf. In contrast, the slow and
erratic light curve evolution, spectral development, and photoionization
analysis of the ejecta imply the outburst occurred on a low mass CO white
dwarf. We note that this is the first time strong [\ion{Ne}{2}] lines have been
detected so early in the outburst of a CO nova and suggests that the presence
of mid-infrared neon lines is not directly indicative of a ONeMg nova event.Comment: 7 figures, 37 pages. Astronimocal Journal accepte
Quantum Criticality
This is a review of the basic theoretical ideas of quantum criticality, and
of their connection to numerous experiments on correlated electron compounds. A
shortened, modified, and edited version appeared in Physics Today. This arxiv
version has additional citations to the literature.Comment: 17 pages, 7 figures; (v2) added ref
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