4,365 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
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
On the stability of bow shocks generated by red supergiants: the case of IRC-10414
In this Letter, we explore the hypothesis that the smooth appearance of bow
shocks around some red supergiants (RSGs) might be caused by the ionization of
their winds by external sources of radiation. Our numerical simulations of the
bow shock generated by IRC-10414 (the first-ever RSG with an optically detected
bow shock) show that the ionization of the wind results in its acceleration by
a factor of two, which reduces the difference between the wind and space
velocities of the star and makes the contact discontinuity of the bow shock
stable for a range of stellar space velocities and mass-loss rates. Our best
fit model reproduces the overall shape and surface brightness of the observed
bow shock and suggests that the space velocity and mass-loss rate of IRC-10414
are 50 and , respectively, and that the number density of the local ISM is
3 . It also shows that the bow shock emission comes
mainly from the shocked stellar wind. This naturally explains the enhanced
nitrogen abundance in the line-emitting material, derived from the spectroscopy
of the bow shock. We found that photoionized bow shocks are 1550
times brighter in optical line emission than their neutral counterparts, from
which we conclude that the bow shock of IRC-10414 must be photoionized.Comment: 5 pages, 5 figures. Accepted for publication in MNRAS Letter
What measurable zero point fluctuations can(not) tell us about dark energy
We show that laboratory experiments cannot measure the absolute value of dark
energy. All known experiments rely on electromagnetic interactions. They are
thus insensitive to particles and fields that interact only weakly with
ordinary matter. In addition, Josephson junction experiments only measure
differences in vacuum energy similar to Casimir force measurements. Gravity,
however, couples to the absolute value. Finally we note that Casimir force
measurements have tested zero point fluctuations up to energies of ~10 eV, well
above the dark energy scale of ~0.01 eV. Hence, the proposed cut-off in the
fluctuation spectrum is ruled out experimentally.Comment: 4 page
Quantum mechanics explained
The physical motivation for the mathematical formalism of quantum mechanics
is made clear and compelling by starting from an obvious fact - essentially,
the stability of matter - and inquiring into its preconditions: what does it
take to make this fact possible?Comment: 29 pages, 5 figures. v2: revised in response to referee comment
Chemical evolution of star clusters
I discuss the chemical evolution of star clusters, with emphasis on old
globular clusters, in relation to their formation histories. Globular clusters
clearly formed in a complex fashion, under markedly different conditions from
any younger clusters presently known. Those special conditions must be linked
to the early formation epoch of the Galaxy and must not have occurred since.
While a link to the formation of globular clusters in dwarf galaxies has been
suggested, present-day dwarf galaxies are not representative of the
gravitational potential wells within which the globular clusters formed.
Instead, a formation deep within the proto-Galaxy or within dark-matter
minihaloes might be favoured. Not all globular clusters may have formed and
evolved similarly. In particular, we may need to distinguish Galactic halo from
Galactic bulge clusters.Comment: 27 pages, 2 figures. To appear as invited review article in a special
issue of the Phil. Trans. Royal Soc. A: Ch. 6 "Star clusters as tracers of
galactic star-formation histories" (ed. R. de Grijs). Fully peer reviewed.
LaTeX, requires rspublic.cls style fil
Young accreted globular clusters in the outer halo of M31
We report on Gemini/GMOS observations of two newly discovered globular
clusters in the outskirts of M31. These objects, PAndAS-7 and PAndAS-8, lie at
a galactocentric radius of ~87 kpc and are projected, with separation ~19 kpc,
onto a field halo substructure known as the South-West Cloud. We measure radial
velocities for the two clusters which confirm that they are almost certainly
physically associated with this feature. Colour-magnitude diagrams reveal
strikingly short, exclusively red horizontal branches in both PA-7 and PA-8;
both also have photometric [Fe/H] = -1.35 +/- 0.15. At this metallicity, the
morphology of the horizontal branch is maximally sensitive to age, and we use
the distinctive configurations seen in PA-7 and PA-8 to demonstrate that both
objects are very likely to be at least 2 Gyr younger than the oldest Milky Way
globular clusters. Our observations provide strong evidence for young globular
clusters being accreted into the remote outer regions of M31 in a manner
entirely consistent with the established picture for the Milky Way, and add
credence to the idea that similar processes play a central role in determining
the composition of globular cluster systems in large spiral galaxies in
general.Comment: 14 pages, 8 figures, accepted for publication in MNRA
Group projector generalization of dirac-heisenberg model
The general form of the operators commuting with the ground representation
(appearing in many physical problems within single particle approximation) of
the group is found. With help of the modified group projector technique, this
result is applied to the system of identical particles with spin independent
interaction, to derive the Dirac-Heisenberg hamiltonian and its effective space
for arbitrary orbital occupation numbers and arbitrary spin. This gives
transparent insight into the physical contents of this hamiltonian, showing
that formal generalizations with spin greater than 1/2 involve nontrivial
additional physical assumptions.Comment: 10 page
Group theoretical construction of mutually unbiased bases in Hilbert spaces of prime dimensions
Mutually unbiased bases in Hilbert spaces of finite dimensions are closely
related to the quantal notion of complementarity. An alternative proof of
existence of a maximal collection of N+1 mutually unbiased bases in Hilbert
spaces of prime dimension N is given by exploiting the finite Heisenberg group
(also called the Pauli group) and the action of SL(2,Z_N) on finite phase space
Z_N x Z_N implemented by unitary operators in the Hilbert space. Crucial for
the proof is that, for prime N, Z_N is also a finite field.Comment: 13 pages; accepted in J. Phys. A: Math. Theo
The Outer Envelopes of Globular Clusters. II. NGC 1851, NGC 5824 and NGC 1261
We present a second set of results from a wide-field photometric survey of
the environs of Milky Way globular clusters. The clusters studied are NGC 1261,
NGC 1851 and NGC 5824: all have data from DECam on the Blanco 4m telescope. NGC
5824 also has data from the Magellan Clay telescope with MegaCam. We confirm
the existence of a large diffuse stellar envelope surrounding NGC 1851 of size
at least 240 pc in radius. The radial density profile of the envelope follows a
power-law decline with index and the projected shape is
slightly elliptical. For NGC 5824 there is no strong detection of a diffuse
stellar envelope, but we find the cluster is remarkably extended and is similar
in size (at least 230 pc in radius) to the envelope of NGC 1851. A stellar
envelope is also revealed around NGC 1261. However, it is notably smaller in
size with radius 105 pc. The radial density profile of the envelope is
also much steeper with . We discuss the possible nature
of the diffuse stellar envelopes, but are unable to draw definitive conclusions
based on the current data. NGC 1851, and potentially NGC 5824, could be
stripped dwarf galaxy nuclei, akin to the cases of Cen, M54 and M2. On
the other hand, the different characteristics of the NGC 1261 envelope suggest
that it may be the product of dynamical evolution of the cluster.Comment: 18 pages, 11 figures, accepted to MNRA
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