39,047 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
Galactic porosity and a star formation threshold for the escape of ionising radiation from galaxies
The spatial distribution of star formation within galaxies strongly affects
the resulting feedback processes. Previous work has considered the case of a
single, concentrated nuclear starburst, and also that of distributed single
supernovae (SNe). Here, we consider ISM structuring by SNe originating in
spatially distributed clusters having a cluster membership spectrum given by
the observed HII region luminosity function. We show that in this case, the
volume of HI cleared per SN is considerably greater than in either of the two
cases considered hitherto.
We derive a simple relationship between the ``porosity'' of the ISM and the
star formation rate (SFR), and deduce a critical SFR_crit, at which the ISM
porosity is unity. This critical value describes the case in which the SN
mechanical energy output over a timescale t_e is comparable with the ISM
``thermal'' energy contained in random motions; t_e is the duration of SN
mechanical input per superbubble. This condition also defines a critical gas
consumption timescale t_exh, which for a Salpeter IMF and random velocities of
\simeq 10 km s-1 is roughly 10e10 years.
We draw a link between porosity and the escape of ionising radiation from
galaxies, arguing that high escape fractions are expected if SFR >~ SFR_crit.
The Lyman Break Galaxies, which are presumably subject to infall on a timescale
< t_exh, meet this criterion, as is consistent with the significant leakage of
ionising photons inferred in these systems. We suggest the utility of this
simple parameterisation of escape fraction in terms of SFR for semi-empirical
models of galaxy formation and evolution and for modeling mechanical and
chemical feedback effects.Comment: Accepted to MNRAS. 11 pages, 1 figure; uses mn2e.cls (included
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
Strong spin fluctuations in -FeSe observed by neutron spectroscopy
We have performed powder inelastic neutron scattering measurements on the
unconventional superconductor -FeSe ().
The spectra reveal highly dispersive paramagnetic fluctuations emerging from
the square-lattice wave vector extending beyond 80 meV in energy.
Measurements as a function of temperature at an energy of did not show any variation from to
. The results show that FeSe is close to an instability
towards antiferromagnetism characteristic of the parent phases of the
high- iron arsenide superconductors, and that the iron paramagnetic
moment is neither affected by the orthorhombic-to-tetragonal structural
transition at nor does it undergo a change in
spin state over the temperature range studied.Comment: Revised version, includes Supplementary Materia
An extension theorem for conformal gauge singularities
We analyse conformal gauge, or isotropic, singularities in cosmological
models in general relativity. Using the calculus of tractors, we find
conditions in terms of tractor curvature for a local extension of the conformal
structure through a cosmological singularity and prove a local extension
theorem.Comment: 43 pages, no figures, version as published in JMP, small changes,
updated reference
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
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