4,726 research outputs found
A double main sequence turn-off in the rich star cluster NGC 1846 in the Large Magellanic Cloud
We report on HST/ACS photometry of the rich intermediate-age star cluster NGC
1846 in the Large Magellanic Cloud, which clearly reveals the presence of a
double main sequence turn-off in this object. Despite this, the main sequence,
sub-giant branch, and red giant branch are all narrow and well-defined, and the
red clump is compact. We examine the spatial distribution of turn-off stars and
demonstrate that all belong to NGC 1846 rather than to any field star
population. In addition, the spatial distributions of the two sets of turn-off
stars may exhibit different central concentrations and some asymmetries. By
fitting isochrones, we show that the properties of the colour-magnitude diagram
can be explained if there are two stellar populations of equivalent metal
abundance in NGC 1846, differing in age by approximately 300 Myr. The absolute
ages of the two populations are ~1.9 and ~2.2 Gyr, although there may be a
systematic error of up to +/-0.4 Gyr in these values. The metal abundance
inferred from isochrone fitting is [M/H] ~ -0.40, consistent with spectroscopic
measurements of [Fe/H]. We propose that the observed properties of NGC 1846 can
be explained if this object originated via the tidal capture of two star
clusters formed separately in a star cluster group in a single giant molecular
cloud. This scenario accounts naturally for the age difference and uniform
metallicity of the two member populations, as well as the differences in their
spatial distributions.Comment: 9 pages, 8 figures, accepted for publication in MNRAS. A version with
full resolution figures may be obtained at
http://www.roe.ac.uk/~dmy/papers/MN-07-0441-MJ_rv.ps.gz (postscript) or at
http://www.roe.ac.uk/~dmy/papers/MN-07-0441-MJ_rv.pdf (PDF
Oscillations in a maturation model of blood cell production.
We present a mathematical model of blood cell production which describes both the development of cells through the cell cycle, and the maturation of these cells as they differentiate to form the various mature blood cell types. The model differs from earlier similar ones by considering primitive stem cells as a separate population from the differentiating cells, and this formulation removes an apparent inconsistency in these earlier models. Three different controls are included in the model: proliferative control of stem cells, proliferative control of differentiating cells, and peripheral control of stem cell committal rate. It is shown that an increase in sensitivity of these controls can cause oscillations to occur through their interaction with time delays associated with proliferation and differentiation, respectively. We show that the characters of these oscillations are quite distinct and suggest that the model may explain an apparent superposition of fast and slow oscillations which can occur in cyclical neutropenia. © 2006 Society for Industrial and Applied Mathematics
On the Exponentials of Some Structured Matrices
In this note explicit algorithms for calculating the exponentials of
important structured 4 x 4 matrices are provided. These lead to closed form
formulae for these exponentials. The techniques rely on one particular Clifford
Algebra isomorphism and basic Lie theory. When used in conjunction with
structure preserving similarities, such as Givens rotations, these techniques
extend to dimensions bigger than four.Comment: 19 page
Asymmetric supernova remnants generated by Galactic, massive runaway stars
After the death of a runaway massive star, its supernova shock wave interacts
with the bow shocks produced by its defunct progenitor, and may lose energy,
momentum, and its spherical symmetry before expanding into the local
interstellar medium (ISM). We investigate whether the initial mass and space
velocity of these progenitors can be associated with asymmetric supernova
remnants. We run hydrodynamical models of supernovae exploding in the
pre-shaped medium of moving Galactic core-collapse progenitors. We find that
bow shocks that accumulate more than about 1.5 Mo generate asymmetric remnants.
The shock wave first collides with these bow shocks 160-750 yr after the
supernova, and the collision lasts until 830-4900 yr. The shock wave is then
located 1.35-5 pc from the center of the explosion, and it expands freely into
the ISM, whereas in the opposite direction it is channelled into the region of
undisturbed wind material. This applies to an initially 20 Mo progenitor moving
with velocity 20 km/s and to our initially 40 Mo progenitor. These remnants
generate mixing of ISM gas, stellar wind and supernova ejecta that is
particularly important upstream from the center of the explosion. Their
lightcurves are dominated by emission from optically-thin cooling and by X-ray
emission of the shocked ISM gas. We find that these remnants are likely to be
observed in the [OIII] lambda 5007 spectral line emission or in the soft
energy-band of X-rays. Finally, we discuss our results in the context of
observed Galactic supernova remnants such as 3C391 and the Cygnus Loop.Comment: 21 pages, 16 figure
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