2,557 research outputs found
Infrared Helium-Hydrogen Line Ratios as a Measure of Stellar Effective Temperature
We have observed a large sample of compact planetary nebulae in the
near-infrared to determine how the 2^1P-2^1S HeI line at 2.058um varies as a
function of stellar effective temperature, Teff. The ratio of this line with HI
Br g at 2.166um has often been used as a measure of the highest Teff present in
a stellar cluster, and hence on whether there is a cut-off in the stellar
initial mass function at high masses. However, recent photoionisation modelling
has revealed that the behaviour of this line is more complex than previously
anticipated. Our work shows that in most aspects the photoionisation models are
correct. In particular, we confirm the weakening of the 2^1P-2^1S as Teff
increases beyond 40000K. However, in many cases the model underpredicts the
observed ratio when we consider the detailed physical conditions in the
individual planetary nebulae. Furthermore, there is evidence that there is
still significant 2^1P-2^1S HeI line emission even in the planetary nebulae
with very hot (Teff>100000K) central stars. It is clear from our work that this
ratio cannot be considered as a reliable measure of effective temperature on
its own.Comment: 24 pages 11 figures (in 62 separate postscript files) Accepted for
publication in Monthly Notices of the Royal Astronomical Societ
Near Infrared Spectra of Compact Planetary Nebulae
This paper continues our study of the behaviour of near infrared helium
recombination lines in planetary nebula. We find that the 1.7007um 4^3D-3^3P
HeI line is a good measure of the HeI recombination rate, since it varies
smoothly with the effective temperature of the central star. We were unable to
reproduce the observed data using detailed photoionisation models at both low
and high effective temperatures, but plausible explanations for the difference
exist for both. We therefore conclude that this line could be used as an
indicator of the effective temperature in obscured nebula. We also
characterised the nature of the molecular hydrogen emission present in a
smaller subset of our sample. The results are consistent with previous data
indicating that ultraviolet excitation rather than shocks is the main cause of
the molecular hydrogen emission in planetary nebulae.Comment: Accepted for publication in MNRA
On partial order semantics for SAT/SMT-based symbolic encodings of weak memory concurrency
Concurrent systems are notoriously difficult to analyze, and technological
advances such as weak memory architectures greatly compound this problem. This
has renewed interest in partial order semantics as a theoretical foundation for
formal verification techniques. Among these, symbolic techniques have been
shown to be particularly effective at finding concurrency-related bugs because
they can leverage highly optimized decision procedures such as SAT/SMT solvers.
This paper gives new fundamental results on partial order semantics for
SAT/SMT-based symbolic encodings of weak memory concurrency. In particular, we
give the theoretical basis for a decision procedure that can handle a fragment
of concurrent programs endowed with least fixed point operators. In addition,
we show that a certain partial order semantics of relaxed sequential
consistency is equivalent to the conjunction of three extensively studied weak
memory axioms by Alglave et al. An important consequence of this equivalence is
an asymptotically smaller symbolic encoding for bounded model checking which
has only a quadratic number of partial order constraints compared to the
state-of-the-art cubic-size encoding.Comment: 15 pages, 3 figure
Some Further Results for the Stationary Points and Dynamics of Supercooled Liquids
We present some new theoretical and computational results for the stationary
points of bulk systems. First we demonstrate how the potential energy surface
can be partitioned into catchment basins associated with every stationary point
using a combination of Newton-Raphson and eigenvector-following techniques.
Numerical results are presented for a 256-atom supercell representation of a
binary Lennard-Jones system. We then derive analytical formulae for the number
of stationary points as a function of both system size and the Hessian index,
using a framework based upon weakly interacting subsystems. This analysis
reveals a simple relation between the total number of stationary points, the
number of local minima, and the number of transition states connected on
average to each minimum. Finally we calculate two measures of localisation for
the displacements corresponding to Hessian eigenvectors in samples of
stationary points obtained from the Newton-Raphson-based geometry optimisation
scheme. Systematic differences are found between the properties of eigenvectors
corresponding to positive and negative Hessian eigenvalues, and localised
character is most pronounced for stationary points with low values of the
Hessian index.Comment: 16 pages, 2 figure
Helium and Hydrogen Line Ratios and The Stellar Content of Compact HII Regions
We present observations and models of the behaviour of the HI and HeI lines
between 1.6 and 2.2um in a small sample of compact HII regions. As in our
previous papers on planetary nebulae, we find that the `pure' 1.7007um
4^3D-3^3P and 2.16475um 7^(3,1)G-4^(3,1)F HeI recombination lines behave
approximately as expected as the effective temperature of the central exciting
star(s) increases. However, the 2.058um 2^1P-2^1S HeI line does not behave as
the model predicts, or as seen in planetary nebulae. Both models and planetary
nebulae showed a decrease in the HeI 2^1P-2^1S/HI Br gamma ratio above an
effective temperature of 40000K. The compact HII regions do not show any such
decrease. The problem with this line ratio is probably due to the fact that the
photoionisation model does not account correctly for the high densities seen in
these HII regions, and that we are therefore seeing more collisional excitation
of the 2^1P level than the model predicts. It may also reflect some deeper
problem in the assumed model stellar atmospheres. In any event, although the
normal HeI recombination lines can be used to place constraints on the
temperature of the hottest star present, the HeI 2^1P-2^1S/HI Br gamma ratio
should not be used for this purpose in either Galactic HII regions or in
starburst galaxies, and conclusions from previous work using this ratio should
be regarded with extreme caution. We also show that the combination of the near
infrared `pure' recombination line ratios with mid-infrared forbidden line data
provides a good discriminant of the form of the far ultraviolet spectral energy
distribution of the exciting star(s). From this we conclude that CoStar models
are a poor match to the available data for our sources, though the more recent
WM-basic models are a better fit.Comment: Accepted for publication in MNRA
IR Dust Bubbles: Probing the Detailed Structure and Young Massive Stellar Populations of Galactic HII Regions
We present an analysis of wind-blown, parsec-sized, mid-infrared bubbles and
associated star-formation using GLIMPSE/IRAC, MIPSGAL/MIPS and MAGPIS/VLA
surveys. Three bubbles from the Churchwell et al. (2006) catalog were selected.
The relative distribution of the ionized gas (based on 20 cm emission), PAH
emission (based on 8 um, 5.8 um and lack of 4.5 um emission) and hot dust (24
um emission) are compared. At the center of each bubble there is a region
containing ionized gas and hot dust, surrounded by PAHs. We identify the likely
source(s) of the stellar wind and ionizing flux producing each bubble based
upon SED fitting to numerical hot stellar photosphere models. Candidate YSOs
are also identified using SED fitting, including several sites of possible
triggered star formation.Comment: 37 pages, 17 figure
Global Optimization by Basin-Hopping and the Lowest Energy Structures of Lennard-Jones Clusters Containing up to 110 Atoms
We describe a global optimization technique using `basin-hopping' in which
the potential energy surface is transformed into a collection of
interpenetrating staircases. This method has been designed to exploit the
features which recent work suggests must be present in an energy landscape for
efficient relaxation to the global minimum. The transformation associates any
point in configuration space with the local minimum obtained by a geometry
optimization started from that point, effectively removing transition state
regions from the problem. However, unlike other methods based upon hypersurface
deformation, this transformation does not change the global minimum. The lowest
known structures are located for all Lennard-Jones clusters up to 110 atoms,
including a number that have never been found before in unbiased searches.Comment: 8 pages, 3 figures, revte
The RMS Survey: The Bolometric Fluxes and Luminosity Distributions of Young Massive Stars
Context: The Red MSX Source (RMS) survey is returning a large sample of
massive young stellar objects (MYSOs) and ultra-compact (UC) \HII{} regions
using follow-up observations of colour-selected candidates from the MSX point
source catalogue. Aims: To obtain the bolometric fluxes and, using kinematic
distance information, the luminosities for young RMS sources with far-infrared
fluxes. Methods: We use a model spectral energy distribution (SED) fitter to
obtain the bolometric flux for our sources, given flux data from our work and
the literature. The inputs to the model fitter were optimised by a series of
investigations designed to reveal the effect varying these inputs had on the
resulting bolometric flux. Kinematic distances derived from molecular line
observations were then used to calculate the luminosity of each source.
Results: Bolometric fluxes are obtained for 1173 young RMS sources, of which
1069 have uniquely constrained kinematic distances and good SED fits. A
comparison of the bolometric fluxes obtained using SED fitting with trapezium
rule integration and two component greybody fits was also undertaken, and
showed that both produce considerable scatter compared to the method used here.
Conclusions: The bolometric flux results allowed us to obtain the luminosity
distributions of YSOs and UC\HII{} regions in the RMS sample, which we find to
be different. We also find that there are few MYSOs with L
10\lsol{}, despite finding many MYSOs with 10\lsol{} L
10\lsol{}.Comment: 12 pages, 12 figures, 3 tables, accepted to A&A. The full versions of
tables 1 and 2 will be available via the CDS upon publicatio
Concurrent Kleene Algebra: Free Model and Completeness
Concurrent Kleene Algebra (CKA) was introduced by Hoare, Moeller, Struth and
Wehrman in 2009 as a framework to reason about concurrent programs. We prove
that the axioms for CKA with bounded parallelism are complete for the semantics
proposed in the original paper; consequently, these semantics are the free
model for this fragment. This result settles a conjecture of Hoare and
collaborators. Moreover, the techniques developed along the way are reusable;
in particular, they allow us to establish pomset automata as an operational
model for CKA.Comment: Version 2 includes an overview section that outlines the completeness
proof, as well as some extra discussion of the interpolation lemma. It also
includes better typography and a number of minor fixes. Version 3
incorporates the changes by comments from the anonymous referees at ESOP.
Among other things, these include a worked example of computing the syntactic
closure by han
Birds of the Rawa Aopa Watumohai National Park, South-East Sulawesi
The Rawa Aopa Watumohai National Park was visited between 9 September and 9 November 1995 and surveys conducted from a range of localities within the park. This constitutes the first study of the avi fauna of this large protected area. A total of 157 different species were recorded (155 to species level) including Mycteria cimerea, Macrocephalon maleo, Cacatua sulphurea and Zoosterops consobrinorum. An annotated list of selected species is presented and a full species list appears as an. appendix
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