32,738 research outputs found
The Superbubble Size Distribution in the Interstellar Medium of Galaxies
We use the standard, adiabatic shell evolution to predict the size
distribution N(R) for populations of OB superbubbles in a uniform ISM. We
derive N(R) for simple cases of superbubble creation rate and mechanical
luminosity function (MLF). For R < the characteristic radius R_e, N(R) is
dominated by stalled objects, while for R>R_e it is dominated by growing
objects. We also briefly investigate N(R) resulting from momentum-conserving
shell evolution. We predict a peak in N(R) corresponding to individual SNRs. To
estimate the MLF, we also examine evolutionary effects on the HII region
luminosity function (HII LF), finding that for nebular luminosity fading as a
power law in time, there is a minimum observed slope for the HII LFs.
Comparison with the largely complete HI hole catalog for the SMC shows
surprising agreement in the predicted and observed slope of N(R), suggesting
that no other fundamental process is needed to explain the size distribution of
shells in the SMC. Further comparison with largely incomplete HI data for M31,
M33, and Holmberg II is also encouraging. We present expressions for the ISM
porosity parameters, and estimate that they are substantially <1 for all of the
galaxies except Holmberg II. Most of these galaxies therefore may not be
strongly dominated by a hot interstellar component. However, porosity results
for the Galaxy remain inconclusive.Comment: 25 pages, MN latex, 4 figures. MNRAS accepted. Complete abstract and
preprint also available at http://ast.cam.ac.uk/~oey/oeypubs.htm
The Bell-Szekeres Solution and Related Solutions of the Einstein-Maxwell Equations
A novel technique for solving some head-on collisions of plane homogeneous
light-like signals in Einstein-Maxwell theory is described. The technique is a
by-product of a re-examination of the fundamental Bell-Szekeres solution in
this field of study. Extensions of the Bell-Szekeres collision problem to
include light-like shells and gravitational waves are described and a family of
solutions having geometrical and topological properties in common with the
Bell-Szekeres solution is derived.Comment: 18 pages, Latex fil
Statistical Confirmation of a Stellar Upper Mass Limit
We derive the expectation value for the maximum stellar mass (m_max) in an
ensemble of N stars, as a function of the IMF upper-mass cutoff (m_up) and N.
We statistically demonstrate that the upper IMF of the local massive star
census observed thus far in the Milky Way and Magellanic Clouds clearly
exhibits a universal upper mass cutoff around 120 - 200 M_sun for a Salpeter
IMF, although the result is more ambiguous for a steeper IMF.Comment: PDF, 5 pages, 4 figures. Accepted to the Astrophysical Journal
Letter
Meeting of the MINDS: an information retrieval research agenda
Since its inception in the late 1950s, the field of Information Retrieval (IR) has developed tools that help people find, organize, and analyze information. The key early influences on the field are well-known. Among them are H. P. Luhn's pioneering work, the development of the vector space retrieval model by Salton and his students, Cleverdon's development of the Cranfield experimental methodology, Spärck Jones' development of idf, and a series of probabilistic retrieval models by Robertson and Croft. Until the development of the WorldWideWeb (Web), IR was of greatest interest to professional information analysts such as librarians, intelligence analysts, the legal community, and the pharmaceutical industry
Strengths of singularities in spherical symmetry
Covariant equations characterizing the strength of a singularity in spherical
symmetry are derived and several models are investigated. The difference
between central and non-central singularities is emphasised. A slight
modification to the definition of singularity strength is suggested. The
gravitational weakness of shell crossing singularities in collapsing spherical
dust is proven for timelike geodesics, closing a gap in the proof.Comment: 16 pages, revtex. V2. Classification of irregular singular points
completed, Comments and references on singularities with a continuous metric
amende
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