94 research outputs found
Two distinct sequences of blue straggler stars in the globular cluster M30
Stars in globular clusters are generally believed to have all formed at the
same time, early in the Galaxy's history. 'Blue stragglers' are stars massive
enough that they should have evolved into white dwarfs long ago. Two possible
mechanisms have been proposed for their formation: mass transfer between binary
companions and stellar mergers resulting from direct collisions between two
stars. Recently, the binary explanation was claimed to be dominant. Here we
report that there are two distinct parallel sequences of blue stragglers in
M30. This globular cluster is thought to have undergone 'core collapse', during
which both the collision rate and the mass transfer activity in binary systems
would have been enhanced. We suggest that the two observed sequences arise from
the cluster core collapse, with the bluer population arising from direct
stellar collisions and the redder one arising from the evolution of close
binaries that are probably still experiencing an active phase of mass transfer.Comment: Published on the 24th December 2009 issue of Natur
Dynamical age differences among coeval star clusters as revealed by blue stragglers
Globular star clusters that formed at the same cosmic time may have evolved
rather differently from a dynamical point of view (because that evolution
depends on the internal environment) through a variety of processes that tend
progressively to segregate stars more massive than the average towards the
cluster centre. Therefore clusters with the same chronological age may have
reached quite different stages of their dynamical history (that is, they may
have different dynamical ages). Blue straggler stars have masses greater than
those at the turn-off point on the main sequence and therefore must be the
result of either a collision or a mass-transfer event. Because they are among
the most massive and luminous objects in old clusters, they can be used as test
particles with which to probe dynamical evolution. Here we report that globular
clusters can be grouped into a few distinct families on the basis of the radial
distribution of blue stragglers. This grouping corresponds well to an effective
ranking of the dynamical stage reached by stellar systems, thereby permitting a
direct measure of the cluster dynamical age purely from observed properties.Comment: Published on the 20 December 2012 issue of Natur
Time as an Illusion
We review the idea, due to Einstein, Eddington, Hoyle and Ballard, that time
is a subjective label, whose primary purpose is to order events, perhaps in a
higher-dimensional universe. In this approach, all moments in time exist
simultaneously, but they are ordered to create the illusion of an unfolding
experience by some physical mechanism. This, in the language of relativity, may
be connected to a hypersurface in a world that extends beyond spacetime. Death
in such a scenario may be merely a phase change
Contributions of Albert Einstein to Earth Sciences: A review in Commemoration of the World Year of Physics
The World Year of Physics (2005) is an international celebration to
commemorate the one hundredth anniversary of Einstein's "Annus Mirabilis". The
United Nations has officially declared 2005 the International Year of Physics.
However, the impact of Einstein's ideas was not restricted to physics. Among
numerous other disciplines, Einstein also made significant and specific
contributions to Earth Sciences. His geosciences-related letters, comments, and
scientific articles, are dispersed, not easily accesible and are poorly known.
The present review attempts to integrate them, as a tribute to Einstein in
commemoration of this centenary. These contributions can be classified into
three basic areas: geodynamics, geological (planetary) catastrophism and
fluvial geomorphology.Comment: 17 pages, no figures, to be published in Naturwissenschafte
The Dynamical Structure and Evolution of Giant Molecular Clouds
Giant molecular clouds (GMCs) are the sites of star formation in the Galaxy. Many of their properties can be understood in terms of a model in which the GMCs and the star-forming clumps within them are in approximate pressure equilibrium, with turbulent motions treated as a separate pressure component
Estimation of population size when capture probability depends on individual state
We develop a multi-state model to estimate the size of a closed population from capture–recapture studies. We consider the case where capture–recapture data are not of a simple binary form, but where the state of an individual is also recorded upon every capture as a discrete variable. The proposed multi-state model can be regarded as a generalisation of the commonly applied set of closed population models to a multi-state form. The model allows for heterogeneity within the capture probabilities associated with each state while also permitting individuals to move between the different discrete states. A closed-form expression for the likelihood is presented in terms of a set of sufficient statistics. The link between existing models for capture heterogeneity is established, and simulation is used to show that the estimate of population size can be biased when movement between states is not accounted for. The proposed unconditional approach is also compared to a conditional approach to assess estimation bias. The model derived in this paper is motivated by a real ecological data set on great crested newts, Triturus cristatus. Supplementary materials accompanying this paper appear online
A prospective investigation of injury incidence and injury risk factors among army recruits in military police training
Preserved organic matter in a fossil Ocean Continent Transition in the Alps: the example of Totalp, SE Switzerland
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