423 research outputs found
Feasibility of Interstellar Travel
The feasibility of interstellar flight is discussed. Mathematical equations for single-stage and multistage rocket propulsion are developed; velocity data and transit times are presented. The conclusions indicate that interstellar travel is theoretically feasible by utilizing known staged nuclear-energy systems
Resolving the nucleus of Centaurus A at mid-IR wavelengths
We have observed Centaurus A with the MID-infrared Interferometric instrument
(MIDI) at the Very Large Telescope Interferometer (VLTI) at resolutions of 7 -
15 mas (at 12.5 micron) and filled gaps in the (u,v) coverage in comparison to
earlier measurements. We are now able to describe the nuclear emission in terms
of geometric components and derive their parameters by fitting models to the
interferometric data. With simple geometrical models, the best fit is achieved
for an elongated disk with flat intensity profile with diameter 76 +/- 9 mas x
35 +/- 2 mas (1.41 +/- 0.17 pc x 0.65 +/- 0.03 pc) whose major axis is oriented
at a position angle (PA) of 10.1 +/- 2.2 degrees east of north. A point source
contributes 47 +/- 11 % of the nuclear emission at 12.5 micron. There is also
evidence that neither such a uniform nor a Gaussian disk are good fits to the
data. This indicates that we are resolving more complicated small-scale
structure in AGNs with MIDI, as has been seen in Seyfert galaxies previously
observed with MIDI. The PA and inferred inclination i = 62.6 +2.1/-2.6 degrees
of the dust emission are compared with observations of gas and dust at larger
scales.Comment: Accepted for the PASA special issue on Centaurus
Chiral-Odd Fragmentation Functions in Single Pion Inclusive Electroproduction
We consider a sub-leading twist chiral-odd pion fragmentation function and
explore its contribution in single pion semi-inclusive electroproduction. We
evaluate the single beam-spin azimuthal asymmetry and the double spin
asymmetry in polarized electroproduction of pions from an unpolarized
and transversely polarized nucleon respectively. The beam asymmetry is
expressed as the product of chiral-odd, and -odd and even distribution and
fragmentation functions. The double spin asymmetry contains information on the
quark's transversity distribution. In a quark diquark-spectator framework we
estimate these asymmetries at , , and energies.Comment: 10 pages, 4 eps figures, elsart.cls, typos corrected. To be published
in Physics Letters
A diversity of dusty AGN tori: Data release for the VLTI/MIDI AGN Large Program and first results for 23 galaxies
The AGN-heated dust distribution (the "torus") is increasingly recognized not
only as the absorber required in unifying models, but as a tracer for the
reservoir that feeds the nuclear Super-Massive Black Hole. Yet, even its most
basic structural properties (such as its extent, geometry and elongation) are
unknown for all but a few archetypal objects. Since most AGNs are unresolved in
the mid-infrared, we utilize the MID-infrared interferometric Instrument (MIDI)
at the Very Large Telescope Interferometer (VLTI) that is sensitive to
structures as small as a few milli-arcseconds (mas). We present here an
extensive amount of new interferometric observations from the MIDI AGN Large
Program (2009 - 2011) and add data from the archive to give a complete view of
the existing MIDI observations of AGNs. Additionally, we have obtained
high-quality mid-infrared spectra from VLT/VISIR. We present correlated and
total flux spectra for 23 AGNs and derive flux and size estimates at 12 micron
using simple axisymmetric geometrical models. Perhaps the most surprising
result is the relatively high level of unresolved flux and its large scatter:
The median "point source fraction" is 70 % for type 1 and 47 % for type 2 AGNs
meaning that a large part of the flux is concentrated on scales smaller than
about 5 mas (0.1 - 10 pc). Among sources observed with similar spatial
resolution, it varies from 20 % - 100 %. For 18 of the sources, two nuclear
components can be distinguished in the radial fits. While these models provide
good fits to all but the brightest sources, significant elongations are
detected in eight sources. The half-light radii of the fainter sources are
smaller than expected from the size ~ L^0.5 scaling of the bright sources and
show a large scatter, especially when compared to the relatively tight
size--luminosity relation in the near-infrared.Comment: A&A in press; 93 pages, 63 figures, 39 tables; data available only
via CD
Coupled channel study of a0 resonances
The coupled channel model of the a0(980) and a0(1450) resonances has been
constructed using the separable pion-eta and K-anti K interactions. We have
shown that two S-matrix poles corresponding to the a0(980) meson have
significantly different widths in the complex energy plane. The K-anti K to
pion-eta branching ratio, predicted in our model near the a0(1450) mass, is in
agreement with the result of the Crystal Barrel Collaboration. The K-anti K
interaction in the S-wave isovector state is not sufficiently attractive to
create a bound a0(980) meson.Comment: 15 pages, latex, 4 eps figures, 2 tables, minor corrections, version
to appear in Phys. Lett.
A mechanism for the T-odd pion fragmentation function
We consider a simple rescattering mechanism to calculate a leading twist
-odd pion fragmentation function, a favored candidate for filtering the
transversity properties of the nucleon. We evaluate the single spin azimuthal
asymmetry for a transversely polarized target in semi-inclusive deep inelastic
scattering (for HERMES kinematics). Additionally, we calculate the double
-odd asymmetry in this framework.Comment: 6 pages revtex, 7 eps figures, references added and updated in this
published versio
Novel Transversity Properties in Semi-Inclusive Deep Inelastic Scattering
The -odd distribution functions contributing to transversity properties of
the nucleon and their role in fueling nontrivial contributions to azimuthal
asymmetries in semi-inclusive deep inelastic scattering are investigated. We
use a dynamical model to evaluate these quantities in terms of HERMES
kinematics.Comment: 5 pages revtex; 5 eps figures. References added. To appear as a Rapid
Communication in Physical Review
Final state interactions and the transverse structure of the pion using non-perturbative eikonal methods
In the factorized picture of semi-inclusive hadronic processes the naive time
reversal-odd parton distributions exist by virtue of the gauge link which
renders it color gauge invariant. The link characterizes the dynamical effect
of initial/final-state interactions of the active parton due soft gluon
exchanges with the target remnant. Though these interactions are
non-perturbative, studies of final-state interaction have been approximated by
perturbative one-gluon approximation in Abelian models. We include higher-order
contributions by applying non-perturbative eikonal methods incorporating color
degrees of freedom in a calculation of the Boer-Mulders function of the pion.
Using this framework we explore under what conditions the Boer Mulders function
can be described in terms of factorization of final state interactions and a
spatial distribution in impact parameter space.Comment: To appear in Phys.Lett.B, 9 pages, 5 figures, added refs. and
discussio
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