23 research outputs found
The Blue Stragglers of the Old Open Cluster NGC 188
The old (7 Gyr) open cluster NGC 188 has yielded a wealth of astrophysical
insight into its rich blue straggler population. Specifically, the NGC 188 blue
stragglers are characterized by: A binary frequency of 80% for orbital periods
less than days;Typical orbital periods around 1000 days;Typical
secondary star masses of 0.5 M; At least some white dwarf companion
stars; Modestly rapid rotation; A bimodal radial spatial distribution;
Dynamical masses greater than standard stellar evolution masses (based on
short-period binaries); Under-luminosity for dynamical masses (short-period
binaries). Extensive -body modeling of NGC 188 with empirical initial
conditions reproduces the properties of the cluster, and in particular the
main-sequence solar-type binary population. The current models also reproduce
well the binary orbital properties of the blue stragglers, but fall well short
of producing the observed number of blue stragglers. This deficit could be
resolved by reducing the frequency of common-envelope evolution during Roche
lobe overflow. Both the observations and the -body models strongly indicate
that the long-period blue-straggler binaries - which dominate the NGC 188 blue
straggler population - are formed by asymptotic-giant (primarily) and red-giant
mass transfer onto main sequence stars. The models suggest that the few
non-velocity-variable blue stragglers formed from mergers or collisions.
Several remarkable short-period double-lined binaries point to the importance
of subsequent dynamical exchange encounters, and provide at least one example
of a likely collisional origin for a blue straggler.Comment: Chapter 3, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
Imitating accelerated expansion of the Universe by matter inhomogeneities - corrections of some misunderstandings
A number of misunderstandings about modeling the apparent accelerated
expansion of the Universe, and about the `weak singularity' are clarified: 1.
Of the five definitions of the deceleration parameter given by Hirata and
Seljak (HS), only is a correct invariant measure of
acceleration/deceleration of expansion. The and are unrelated to
acceleration in an inhomogeneous model. 2. The averaging over directions
involved in the definition of does not correspond to what is done in
observational astronomy. 3. HS's equation (38) connecting to the flow
invariants gives self-contradictory results when applied at the centre of
symmetry of the Lema\^{\i}tre-Tolman (L-T) model. The intermediate equation
(31) that determines is correct, but approximate, so it cannot be used
for determining the sign of the deceleration parameter. Even so, at the centre
of symmetry of the L-T model, it puts no limitation on the sign of .
4. The `weak singularity' of Vanderveld {\it et al.} is a conical profile of
mass density at the centre - a perfectly acceptable configuration. 5. The
so-called `critical point' in the equations of the `inverse problem' for a
central observer in an L-T model is a manifestation of the apparent horizon - a
common property of the past light cones in zero-lambda L-T models, perfectly
manageable if the equations are correctly integrated.Comment: 15 pages. Completely rewritten to match the published version. We
added discussion of 2 key papers cited by VFW and identified more clearly the
assumptions, approximations and mistakes that led to certain misconceptions
Relative mobility determines the efficacy of MPAs in a two species mixed fishery with conflicting management objectives
Marine Protected Areas (MPAs) have been used to protect species in need of conservation and as a fisheries management tool. It has been suggested MPAs can benefit mobile stocks by protecting spawning grounds whilst also allowing yields to be maintained as mature fish move out of the protected areas. However, the robustness of this claim in mixed species fisheries has yet to be established. We use a simulation model to explore the efficacy of spatial closures and effort regulation when other forms of fishery control (e.g., Total Allowable Catches) are absent or non-enforced as ways of addressing management objectives that are difficult to reconcile due to the contrasting life-histories of a target and a bycatch, conservation species in a two-species fishery. The mobility of each stock in such a fishery affects the benefits conferred by an MPA. The differing management objectives of the two species can be partially met by effort regulations or closures when the species exhibit similar mobility. However, a more mobile conservation species prevents both sets of aims being met by either management tool. We use simulations to explore how spatial closures and effort regulation can be used to seek compromise between stakeholders when the mobility of one stock prevents conflicting management objectives to be fully met. Our results demonstrate that stock mobility is a key factor in considering whether an MPA can meet conflicting aims in a multispecies fishery compromised of stocks with differing life histories and mobilities
Regional variations in walking for different purposes - The South East Queensland quality of life study
Where people are located can influence behavioral choices and health outcomes through the effects of place on health. Walking is the most commonly reported form of nonoccupational and nonhousehold physical activity for adults. It is a behavior of particular interest to those in the transportation, urban planning, and public health fields. Researchers have examined patterns of walking from both an individual perspective (psychological and social factors) and from a broader community focus (location and built environment factors). The majority of studies have examined walking in the context of urban environments. Variations within regions (urban, periurban, and rural, for example) in walking have not been previously described. We use data from a regionally based quality of life survey to examine subregional variations in walking for particular purposes. Both the social and contextual variations that may underlie these differences are considered. This is useful in helping identify particular factors that may be further investigated in disaggregated analyses using GIS methods to identify specific differences in objective attributes between subregions that may influence peoples' choices to walk, such as walking infrastructure and the availability of destinations