18,646 research outputs found
Herschel/HIFI Observations of Hydrogen Fluoride Toward Sagittarius B2(M)
Herschel/HIFI observations have revealed the presence of widespread
absorption by hydrogen fluoride (HF) J = 1-0 rotational transition, toward a
number of Galactic sources. We present observations of HF J = 1-0 toward the
high-mass star-forming region Sagittarius B2(M). The spectrum obtained shows a
complex pattern of absorption, with numerous features covering a wide range of
local standard of rest velocities (-130 to 100 km -1). An analysis of this
absorption yields HF abundances relative to H2 of ~1.3 {\times}10-8, in most
velocity intervals. This result is in good agreement with estimates from
chemical models, which predict that HF should be the main reservoir of
gas-phase fluorine under a wide variety of interstellar conditions.
Interestingly, we also find velocity intervals in which the HF spectrum shows
strong absorption features that are not present, or are very weak, in spectra
of other molecules, such as 13CO (1-0) and CS (2-1). HF absorption reveals
components of diffuse clouds with small extinction that can be studied for the
first time. Another interesting observation is that water is significantly more
abundant than hydrogen fluoride over a wide range of velocities toward
Sagittarius B2(M), in contrast to the remarkably constant H2O/HF abundance
ratio with average value close to unity measured toward other Galactic sources
Collisional excitation of far-infrared line emissions from warm interstellar carbon monoxide (CO)
Motivated by recent observations with Herschel/PACS, and the availability of
new rate coefficients for the collisional excitation of CO (Yang et al. 2010),
the excitation of warm astrophysical CO is revisited with the use of numerical
and analytic methods. For the case of an isothermal medium, results have been
obtained for a wide range of gas temperatures (100 to 5000 K) and H2 densities
(1E+3 to 1E+9 cm-3), and presented in the form of rotational diagrams, in which
the logarithm of the column density per magnetic substate, log (N[J]/g[J]), is
plotted for each state, as a function of its energy, E[J]. For rotational
transitions in the wavelength range accessible to Herschel/PACS, such diagrams
are nearly linear when n(H2) > 1E+8 cm-3. When log10(n[H2]) = 6.8 to 8, they
exhibit significant negative curvature, whereas when log10(n[H2]) < 4.8 the
curvature is uniformly positive throughout the PACS-accessible range. Thus, the
observation of a positively-curved CO rotational diagram does not NECESSARILY
require the presence of multiple temperature components. Indeed, for some
sources observed with Herschel/PACS, the CO rotational diagrams show a modest
positive curvature that can be explained by a single isothermal component.
Typically, the required physical parameters are H2 densities in the 1E+4 to
1E+5 cm-3 range and temperatures, T, close to the maximum at which CO can
survive. Other sources exhibit rotational diagrams with more curvature than can
be accounted for by a single temperature component. For the case of a medium
with a power-law distribution of gas temperatures, with dN/dT proportional to T
to the power -b, results have been obtained for H2 densities 1E+3 to 1E+9 cm-3
and power-law indices, b, in the range 1 to 5; such a medium can account for a
CO rotational diagram that is more positively curved than any resulting from an
isothermal medium.Comment: Accepted for publication in the Astrophysical Journa
Filament bifurcations in a one-dimensional model of reacting excitable fluid flow
Recently, it has been shown that properties of excitable media stirred by
two-dimensional chaotic flows can be properly studied in a one-dimensional
framework \cite{excitablePRL,excitablePRE}, describing the transverse profile
of the filament-like structures observed in the system. Here, we perform a
bifurcation analysis of this one-dimensional approximation as a function of the
{\it Damk{\"o}hler} number, the ratio between the chemical and the strain
rates. Different branches of stable solutions are calculated, and a Hopf
bifurcation, leading to an oscillating filament, identified.Comment: 9 pages, 4 figures; elsart.cls styl
An Essentialist Theory of the Meaning of Slurs
In this paper, I develop an essentialist model of the semantics of slurs. I defend the view that slurs are a species of kind terms: Slur concepts encode mini-theories which represent an essence-like element that is causally connected to a set of negatively-valenced stereotypical features of a social group. The truth-conditional contribution of slur nouns can then be captured by the following schema: For a given slur S of a social group G and a person P, S is true of P iff P bears the “essence” of G—whatever this essence is—which is causally responsible for stereotypical negative features associated with G and predicted of P. Since there is no essence that is causally responsible for stereotypical negative features of a social group, slurs have null-extension, and consequently, many sentences containing them are either meaningless or false. After giving a detailed outline of my theory, I show that it receives strong linguistic support. In particular, it can account for a wide range of linguistic cases that are regarded as challenging, central data for any theory of slurs. Finally, I show that my theory also receives convergent support from cognitive psychology and psycholinguistics
DIP: Disruption-Tolerance for IP
Disruption Tolerant Networks (DTN) have been a popular subject of recent
research and development. These networks are characterized by frequent, lengthy
outages and a lack of contemporaneous end-to-end paths. In this work we discuss
techniques for extending IP to operate more effectively in DTN scenarios. Our
scheme, Disruption Tolerant IP (DIP) uses existing IP packet headers, uses the
existing socket API for applications, is compatible with IPsec, and uses
familiar Policy-Based Routing techniques for network management
Buy-and-Hold Property for Fully Incomplete Markets when Super-replicating Markovian Claims
We show that when the price process represents a fully incomplete market,
the optimal super-replication of any Markovian claim with
being nonnegative and lower semicontinuous is of buy-and-hold type. Since both
(unbounded) stochastic volatility models and rough volatility models are
examples of fully incomplete markets, one can interpret the buy-and-hold
property when super-replicating Markovian claims as a natural phenomenon in
incomplete markets
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