2,644 research outputs found
A Spitzer IRAC Census of the Asymptotic Giant Branch Populations in Local Group Dwarfs. II. IC 1613
We present Spitzer Space Telescope IRAC photometry of the Local Group dwarf
irregular galaxy IC 1613. We compare our 3.6, 4.5, 5.8, and 8.0 micron
photometry with broadband optical photometry and find that the optical data do
not detect 43% and misidentify an additional 11% of the total AGB population,
likely because of extinction caused by circumstellar material. Further, we find
that a narrowband optical carbon star study of IC 1613 detects 50% of the total
AGB population and only considers 18% of this population in calculating the
carbon to M-type AGB ratio. We derive an integrated mass-loss rate from the AGB
stars of 0.2-1.0 x 10^(-3) solar masses per year and find that the distribution
of bolometric luminosities and mass-loss rates are consistent with those for
other nearby metal-poor galaxies. Both the optical completeness fractions and
mass-loss rates in IC 1613 are very similar to those in the Local Group dwarf
irregular, WLM, which is expected given their similar characteristics and
evolutionary histories.Comment: Accepted by ApJ, 26 pages, 10 figures, version with high-resolution
figures available at: http://webusers.astro.umn.edu/~djackson
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
Self-driven lattice-model Monte Carlo simulations of alloy thermodynamic
Monte Carlo (MC) simulations of lattice models are a widely used way to
compute thermodynamic properties of substitutional alloys. A limitation to
their more widespread use is the difficulty of driving a MC simulation in order
to obtain the desired quantities. To address this problem, we have devised a
variety of high-level algorithms that serve as an interface between the user
and a traditional MC code. The user specifies the goals sought in a high-level
form that our algorithms convert into elementary tasks to be performed by a
standard MC code. For instance, our algorithms permit the determination of the
free energy of an alloy phase over its entire region of stability within a
specified accuracy, without requiring any user intervention during the
calculations. Our algorithms also enable the direct determination of
composition-temperature phase boundaries without requiring the calculation of
the whole free energy surface of the alloy system
Enteric helminths promote Salmonella co-infection by altering the intestinal metabolome
Intestinal helminth infections occur pre dominantly in regions where exposure to enteric bacterial pathogens is also common. Helminth infections inhibit host immunity against microbial pathogens, which has largely been attributed to the induction of regulatory or type 2 (Th2) immune responses. Here we demonstrate an additional three-way interaction in which helminth infection alters the metabolic environment of the host intestine to enhance bacterial pathogenicity. We show that an ongoing helminth infection increased colonization by Salmonella independently of T regulatory or Th2 cells. Instead, helminth infection altered the metabolic profile of the intestine, which directly enhanced bacterial expression of Salmonella pathogenicity island 1 (SPI-1) genes and increased intracellular invasion. These data reveal a novel mechanism by which a helminth-modified metabolome promotes susceptibility to bacterial co-infection
New infrared star clusters in the Northern and Equatorial Milky Way with 2MASS
We carried out a survey of infrared star clusters and stellar groups on the
2MASS J, H and K_s all-sky release Atlas in the Northern and Equatorial Milky
Way (350 < l < 360, 0 < l < 230). The search in this zone complements that in
the Southern Milky Way (Dutra et al. 2003a). The method concentrates efforts on
the directions of known optical and radio nebulae. The present study provides
167 new infrared clusters, stellar groups and candidates. Combining the two
studies for the whole Milky Way, 346 infrared clusters, stellar groups and
candidates were discovered, whereas 315 objects were previously known. They
constitute an important new sample for future detailed studies.Comment: Accepted to Astronomy and Astrophysic
The Search for Invariance: Repeated Positive Testing Serves the Goals of Causal Learning
Positive testing is characteristic of exploratory behavior, yet it seems to be at odds with the aim of information seeking. After all, repeated demonstrations of one’s current hypothesis often produce the same evidence and fail to distinguish it from potential alternatives. Research on the development of scientific reasoning and adult rule learning have both documented and attempted to explain this behavior. The current chapter reviews this prior work and introduces a novel theoretical account—the Search for Invariance (SI) hypothesis—which suggests that producing multiple positive examples serves the goals of causal learning. This hypothesis draws on the interventionist framework of causal reasoning, which suggests that causal learners are concerned with the invariance of candidate hypotheses. In a probabilistic and interdependent causal world, our primary goal is to determine whether, and in what contexts, our causal hypotheses provide accurate foundations for inference and intervention—not to disconfirm their alternatives. By recognizing the central role of invariance in causal learning, the phenomenon of positive testing may be reinterpreted as a rational information-seeking strategy
Degree of explanation
Partial explanations are everywhere. That is, explanations citing causes that explain some but not all of an effect are ubiquitous across science, and these in turn rely on the notion of degree of explanation. I argue that current accounts are seriously deficient. In particular, they do not incorporate adequately the way in which a cause’s explanatory importance varies with choice of explanandum. Using influential recent contrastive theories, I develop quantitative definitions that remedy this lacuna, and relate it to existing measures of degree of causation. Among other things, this reveals the precise role here of chance, as well as bearing on the relation between causal explanation and causation itself
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Effect of temperature and strain rate on the compressive behaviour of supramolecular polyurethane
Supramolecular polyurethanes (SPUs) possess thermoresponsive and thermoreversible properties, and those characteristics are highly desirable in both bulk commodity and value-added applications such as adhesives, shape-memory materials, healable coatings and lightweight, impact-resistant structures (e.g. protection for mobile electronics). A better understanding of the mechanical properties, especially the rate and temperature sensitivity, of these materials are required to assess their suitability for different applications. In this paper, a newly developed SPU with tuneable thermal properties was studied, and the response of this SPU to compressive loading over strain rates from 10−3 to 104 s−1 was presented. Furthermore, the effect of temperature on the mechanical response was also demonstrated. The sample was tested using an Instron mechanical testing machine for quasi-static loading, a home-made hydraulic system for moderate rates and a traditional split Hopkinson pressure bars (SHPBs) for high strain rates. Results showed that the compression stress-strain behaviour was affected significantly by the thermoresponsive nature of SPU, but that, as expected for polymeric materials, the general trends of the temperature and the rate dependence mirror each other. However, this behaviour is more complicated than observed for many other polymeric materials, as a result of the richer range of transitions that influence the behaviour over the range of temperatures and strain rates tested
Olive phenology as a sensitive indicator of future climatic warming in the Mediterranean
Experimental and modelling work suggests a strong dependence of olive flowering date on spring temperatures. Since airborne pollen concentrations reflect the flowering phenology of olive populations within a radius of 50 km, they may be a sensitive regional indicator of climatic warming. We assessed this potential sensitivity with phenology models fitted to flowering dates inferred from maximum airborne pollen data. Of four models tested, a thermal time model gave the best fit for Montpellier, France, and was the most effective at the regional scale, providing reasonable predictions for 10 sites in the western Mediterranean. This model was forced with replicated future temperature simulations for the western Mediterranean from a coupled ocean-atmosphere general circulation model (GCM). The GCM temperatures rose by 4·5 °C between 1990 and 2099 with a 1% per year increase in greenhouse gases, and modelled flowering date advanced at a rate of 6·2 d per °C. The results indicated that this long-term regional trend in phenology might be statistically significant as early as 2030, but with marked spatial variation in magnitude, with the calculated flowering date between the 1990s and 2030s advancing by 3–23 d. Future monitoring of airborne olive pollen may therefore provide an early biological indicator of climatic warming in the Mediterranean
Topological mechanochemistry of graphene
In view of a formal topology, two common terms, namely, connectivity and
adjacency, determine the quality of C-C bonds of sp2 nanocarbons. The feature
is the most sensitive point of the inherent topology of the species so that
such external action as mechanical deformation should obviously change it and
result in particular topological effects. The current paper describes the
effects caused by uniaxial tension of a graphene molecule in due course of a
mechanochemical reaction. Basing on the molecular theory of graphene, the
effects are attributed to both mechanical loading and chemical modification of
edge atoms of the molecule. The mechanical behavior is shown to be not only
highly anisotropic with respect to the direction of the load application, but
greatly dependent on the chemical modification of the molecule edge atoms thus
revealing topological character of the graphene deformation.Comment: 9 pages, 10 figures, 1 table. arXiv admin note: text overlap with
arXiv:1301.094
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