1,845 research outputs found
Searching for evidence of jet-cloud interaction in radio galaxies. First results for 3C 381
We present results of Gemini spectroscopy and Hubble Space Telescope imaging
of the 3C~381 radio galaxy. Possible ionising mechanisms for the Extended
Emission-Line Region were studied through state-of-the-art diagnostic analysis
employing line-ratios. Photoionisation from the central engine as well as
mixed-medium photoionisation models fail in reproducing both the strengths and
the behaviour of the highest-excitation lines, such as [NeV]3424, HeII, and
[OIII}]5007, which are measured at very large distances from the AGN.
Shock-ionisation models provide a better fit to the observation. Expanding
shocks with velocities higher than 500 km/s are capable of reaching the
observed intensity ratios for lines with different ionisation states and
excitation degrees. This model also provide a direct explanation of the
mechanical energy input needed to explain the high-velocity line-splitting
observed in the velocity field.Comment: 21 pages, 11 figures; to be published in Monthly Notices of the Royal
Astronomical Societ
Penrose Limits, the Colliding Plane Wave Problem and the Classical String Backgrounds
We show how the Szekeres form of the line element is naturally adapted to
study Penrose limits in classical string backgrounds. Relating the "old"
colliding wave problem to the Penrose limiting procedure as employed in string
theory we discuss how two orthogonal Penrose limits uniquely determine the
underlying target space when certain symmetry is imposed. We construct a
conformally deformed background with two distinct, yet exactly solvable in
terms of the string theory on R-R backgrounds, Penrose limits. Exploiting
further the similarities between the two problems we find that the Penrose
limit of the gauged WZW Nappi-Witten universe is itself a gauged WZW plane wave
solution of Sfetsos and Tseytlin. Finally, we discuss some issues related to
singularity, show the existence of a large class of non-Hausdorff solutions
with Killing Cauchy Horizons and indicate a possible resolution of the problem
of the definition of quantum vacuum in string theory on these time-dependent
backgrounds.Comment: Some misprints corrected. Matches the version in print. To appear in
Classical & Quantum Gravit
Initial Conditions and the Structure of the Singularity in Pre-Big-Bang Cosmology
We propose a picture, within the pre-big-bang approach, in which the universe
emerges from a bath of plane gravitational and dilatonic waves. The waves
interact gravitationally breaking the exact plane symmetry and lead generically
to gravitational collapse resulting in a singularity with the Kasner-like
structure. The analytic relations between the Kasner exponents and the initial
data are explicitly evaluated and it is shown that pre-big-bang inflation may
occur within a dense set of initial data. Finally, we argue that plane waves
carry zero gravitational entropy and thus are, from a thermodynamical point of
view, good candidates for the universe to emerge from.Comment: 18 pages, LaTeX, epsfig. 3 figures included. Minor changes; paragraph
added in the introduction, references added and typos corrected. Final
version published in Classical and Quantum Gravit
Long-Term Optical Monitoring of Eta Carinae. Multiband light curves for a complete orbital period
The periodicity of 5.5 years for some observational events occurring in Eta
Carinae manifests itself across a large wavelength range and has been
associated with its binary nature. These events are supposed to occur when the
binary components are close to periastron. To detect the previous periastron
passage of Eta Car in 2003, we started an intensive, ground-based, optical,
photometric observing campaign. We continued observing the object to monitor
its photometric behavior and variability across the entire orbital cycle. Our
observation program consisted of daily differential photometry from CCD images,
which were acquired using a 0.8 m telescope and a standard BVRI filter set at
La Plata Observatory. The photometry includes the central object and the
surrounding Homunculus nebula. We present up-to-date results of our observing
program, including homogeneous photometric data collected between 2003 and
2008. Our observations demonstrated that Eta Car has continued increasing in
brightness at a constant rate since 1998. In 2006, it reached its brightest
magnitude (V ~ 4.7) since about 1860s. The object then suddenly reverted its
brightening trend, fading to V = 5.0 at the beginning of 2007, and has
maintained a quite steady state since then. We continue the photometric
monitoring of Eta Car in anticipation of the next "periastron passage",
predicted to occur at the beginning of 2009.Comment: Accepted by A&A. The paper contains 3 figures and 2 table
The stability of Killing-Cauchy horizons in colliding plane wave space-times
It is confirmed rigorously that the Killing-Cauchy horizons, which sometimes
occur in space-times representing the collision and subsequent interaction of
plane gravitational waves in a Minkowski background, are unstable with respect
to bounded perturbations of the initial waves, at least for the case in which
the initial waves have constant aligned polarizations.Comment: 8 pages. To appear in Gen. Rel. Gra
Exponential-Potential Scalar Field Universes I: The Bianchi I Models
We obtain a general exact solution of the Einstein field equations for the
anisotropic Bianchi type I universes filled with an exponential-potential
scalar field and study their dynamics. It is shown, in agreement with previous
studies, that for a wide range of initial conditions the late-time behaviour of
the models is that of a power-law inflating FRW universe. This property, does
not hold, in contrast, when some degree of inhomogeneity is introduced, as
discussed in our following paper II.Comment: 16 pages, Plain LaTeX, 1 Figure to be sent on request, to appear in
Phys. Rev.
Gravitational Optics: Self-phase modulation and harmonic cascades
Nonlinear wave interaction of low amplitude gravitational waves in flat
space-time is considered. Analogy with optics is established. It is shown that
the flat metric space-time is equivalent to a centro-symmetric optical medium,
with no second order susceptibility. The lowest order nonlinear effects are
those due to the third order nonlinearity and include self-phase modulation and
high harmonic generation. These processes lead to an efficient energy dilution
of the gravitational wave energy over an increasingly large spectral range.Comment: 12 pages, REVTEX
On the evolution of a large class of inhomogeneous scalar field cosmologies
The asymptotic behaviour of a family of inhomogeneous scalar field
cosmologies with exponential potential is studied. By introducing new variables
we can perform an almost complete analysis of the evolution of these
cosmologies. Unlike the homogeneous case (Bianchi type solutions), when k^2<2
the models do not isotropize due to the presence of the inhomogeneitiesComment: 23 pages, 1 figure. Submitted to Classical and Quantum Gravit
The Evolving Role of Faculty: Traditional Scholarship, Instructional Scholarship and Service Scholarship
Faculty workload decisions made by a departmental unit often create a conflict for faculty because promotion/tenure decisions usually focus primarily on individual scholarly achievements. This paper describes an approach to faculty evaluation that considers both departmental and individual needs by expanding the view of scholarship to include Research, Instruction, and Service
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