2,095 research outputs found
Linear independence of cables in the knot concordance group
We produce infinite families of knots for which the set of cables is linearly independent in the knot concordance group. We arrange that these examples lie arbitrarily deep in the solvable and bipolar filtrations of the knot concordance group, denoted by and respectively. As a consequence, this result cannot be reached by any combination of algebraic concordance invariants, Casson-Gordon invariants, and Heegaard-Floer invariants such as tau, epsilon, and Upsilon. We give two applications of this result. First, for any n>=0, there exists an infinite family such that for each fixed i, is a basis for an infinite rank summand of and is linearly independent in . Second, for any n>=1, we give filtered counterexamples to Kauffman's conjecture on slice knots by constructing smoothly slice knots with genus one Seifert surfaces where one derivative curve has nontrivial Arf invariant and the other is nontrivial in both and . We also give examples of smoothly slice knots with genus one Seifert surfaces such that one derivative has nontrivial Arf invariant and the other is topologically slice but not smoothly slice
Semi-Empirical Bound on the Chlorinr-37 Solar Neutrino Experiment
The Kamiokande measurement of energetic Boron-8 neutrinos from the sun is
used to set a lower bound on the contribution of the same neutrinos to the
signal in the \Chlorine\ experiment. Implications for Beryllium-7 neutrinos are
discussed.Comment: Latex, 6 pages + 1 postscript figure (included). UTAPHY-HEP-
Improved definition of crustal magnetic anomalies for MAGSAT data
The routine correction of MAGSAT vector magnetometer data for external field effects such as the ring current and the daily variation by filtering long wavelength harmonics from the data is described. Separation of fields due to low altitude sources from those caused by high altitude sources is affected by means of dual harmonic expansions in the solution of Dirichlet's problem. This regression/harmonic filter procedure is applied on an orbit by orbit basis, and initial tests on MAGSAT data from orbit 1176 show reduction in external field residuals by 24.33 nT RMS in the horizontal component, and 10.95 nT RMS in the radial component
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Climate adaptation by crop migration.
Many studies have estimated the adverse effects of climate change on crop yields, however, this literature almost universally assumes a constant geographic distribution of crops in the future. Movement of growing areas to limit exposure to adverse climate conditions has been discussed as a theoretical adaptive response but has not previously been quantified or demonstrated at a global scale. Here, we assess how changes in rainfed crop area have already mediated growing season temperature trends for rainfed maize, wheat, rice, and soybean using spatially-explicit climate and crop area data from 1973 to 2012. Our results suggest that the most damaging impacts of warming on rainfed maize, wheat, and rice have been substantially moderated by the migration of these crops over time and the expansion of irrigation. However, continued migration may incur substantial environmental costs and will depend on socio-economic and political factors in addition to land suitability and climate
Inverting Ray-Knight identity
We provide a short proof of the Ray-Knight second generalized Theorem, using
a martingale which can be seen (on the positive quadrant) as the Radon-Nikodym
derivative of the reversed vertex-reinforced jump process measure with respect
to the Markov jump process with the same conductances. Next we show that a
variant of this process provides an inversion of that Ray-Knight identity. We
give a similar result for the Ray-Knight first generalized Theorem.Comment: 18 page
Investigating the transport of angular momentum from young stellar objects: do H2 jets from Class I YSOs rotate?
In this pilot study, we examine molecular jets from the embedded Class I
sources, HH 26 and HH 72, to search, for the first time, for kinematic
signatures of jet rotation from young embedded sources.High resolution
long-slit spectroscopy of the H2 1-0 S(1) transition was obtained using
VLT/ISAAC, position-velocity (PV) diagrams constructed and intensity-weighted
radial velocities transverse to the jet flow measured. Mean intensity-weighted
velocities vary between vLSR ~ -90 and -65 km/s for HH 26, and -60 and -10 km/s
for HH 72; maxima occur close to the intensity peak and decrease toward the jet
borders. Velocity dispersions are ~ 45 and ~ 80 km/s for HH 26 and HH 72,
respectively, with gas motions as fast as -100 km/s present. Asymmetric PV
diagrams are seen for both objects which a simple empirical model of a
cylindrical jet section shows could in principle be reproduced by jet rotation
alone. Assuming magneto-centrifugal launching, the observed HH 26 flow may
originate at a disk radius of 2-4 AU from the star with the toroidal component
of the magnetic field dominant at the observed location, in agreement with
magnetic collimation models. We estimate that the kinetic angular momentum
transported by the HH 26 jet is ~ 2E5 M_sun/yr AU km/s. This value (a lower
limit to the total angular momentum transported by the flow) already amounts to
70% of the angular momentum that has to be extracted from the disk for the
accretion to proceed at the observed rate.
The results of this pilot study suggest that jet rotation may also be present
at early evolutionary phases and supports the hypothesis that they carry away
excess angular momentum, thus allowing the central protostar to increase its
mass.Comment: 11 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
Recipes for stellar jets: results of combined optical/infrared diagnostics
We examine the conditions of the plasma along a sample of 'classical'
Herbig-Haro jets located in the Orion and Vela star forming regions, through
combined optical-infrared spectral diagnostics. Our sample includes HH 111, HH
34, HH 83, HH 73, HH 24 C/E, HH 24 J, observed at moderate spatial/spectral
resolution. The obtained spectra cover a wide wavelength range from 0.6-2.5 um,
including many transitions from regions of different excitation conditions.
This allows us to probe the density and temperature stratification which
characterises the cooling zones behind the shock fronts along the jet. The
derived physical parameters (such as the extinction, the electron density and
temperature, the ionisation fraction, and the total density) are used to
estimate the depletion onto dust grains of Calcium and Iron with respect to
solar abundances. This turns out to be between 70% and 0% for Ca and ~90% for
Fe, suggesting that the weak shocks present in the beams are not capable of
completely destroying the dust grains. We then derive the mass flux rates
(Mdot_jet is on average 5 10^-8 M_solar yr^-1) and the associated linear
momentum fluxes. The latter are higher than, or of the same order as, those
measured in the coaxial molecular flows, suggesting that the flows are jet
driven. Finally, we discuss differences between jets in our sample.Comment: 19 pages, 15 figures, accepted by A&
A combined optical/infrared spectral diagnostic analysis of the HH1 jet
Complete flux-calibrated spectra covering the spectral range from 6000 A to
2.5 um have been obtained along the HH1 jet and analysed in order to explore
the potential of a combined optical/near-IR diagnostic applied to jets from
young stellar objects. Important physical parameters have been derived along
the jet using various diagnostic line ratios. This multi-line analysis shows,
in each spatially unresolved knot, the presence of zones at different
excitation conditions, as expected from the cooling layers behind a shock
front. In particular, a density stratification in the jet is evident from
ratios of various lines of different critical density. In particular, [FeII]
lines originate in a cooling layer located at larger distances from the shock
front than that generating the optical lines, where the compression is higher
and the temperature is declining. The derived parameters were used to measure
the mass flux along the jet, adopting different procedures, the advantages and
limitations of which are discussed. dM/dt is high in the initial part of the
flow but decreases by about an order of magnitude further out. Conversely, the
mass flux associated with the warm molecular material is low and does not show
appreciable variations along the jet. We suggest that part of the mass flux in
the external regions is not revealed in optical and IR lines because it is
associated with a colder atomic component, which may be traced by the far-IR [O
I]63 um line.
Finally, we find that the gas-phase abundance of refractory species is lower
than the solar value suggesting that a significant fraction of dust grains may
still be present in the jet beam.Comment: Accepted on Astronomy & Astrophysic
Proper Motions and Variability of the H Emission in the HH~46/47system
We report here on the first proper motion measurements of molecular hydrogen
emission features in the Herbig-Haro 46/47 outflow. Assuming a distance of 350
pc to this flow, the inferred tangential velocities range from a few tens to
almost 500 km/s . The highest velocities are observed for H2 knots either in,
or close to, the jet/counterjet axes. Knots constituting the wings of the large
scale H2 bow (see, for example, Eisl\"offel et al. 1994) are found to move much
more slowly. These results appear to be in agreement with recent numerical
simulations of H2 emission from pulsed jets. We also report the first detection
of variability in H2 features for a young stellar object (YSO) outflow. It was
found that several H2 knots significantly changed their luminosity over the 4
year timebase used to conduct our study. This is in line with current estimates
for the cooling time of gas radiating shocked H2 emission in YSO environments.Comment: 2 figure
Cosmological expansion and local physics
The interplay between cosmological expansion and local attraction in a
gravitationally bound system is revisited in various regimes. First, weakly
gravitating Newtonian systems are considered, followed by various exact
solutions describing a relativistic central object embedded in a Friedmann
universe. It is shown that the ``all or nothing'' behaviour recently discovered
(i.e., weakly coupled systems are comoving while strongly coupled ones resist
the cosmic expansion) is limited to the de Sitter background. New exact
solutions are presented which describe black holes perfectly comoving with a
generic Friedmann universe. The possibility of violating cosmic censorship for
a black hole approaching the Big Rip is also discussed.Comment: 17 pages, LaTeX, to appear in Phys. Rev.
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