3,296 research outputs found
The influence of star clusters on galactic disks: new insights on star-formation in galaxies
Stars form in embedded star clusters which play a key role in determining the
properties of a galaxy's stellar population. Physical mechanisms discussed in
this paper are runaway stars shot out from young clusters, binary-star
disruption in clusters, gas blow-out from clusters and the origin of thick
galactic disks. I emphasise that the SNIa rate per low-mass star depends on the
star-clusters formed in a galaxy and I discuss the IGIMF theory. Based on the
IGIMF theory, the re-calibrated Halpha-luminosity--SFR relation implies dwarf
irregular galaxies to have the same gas-depletion time-scale as major disk
galaxies, suggesting a major change in our understanding of dwarf-galaxy
evolution. The IGIMF-theory also naturally leads to the observed radial Halpha
cutoff in disk galaxies without a radial star-formation cutoff. It emerges that
the thorough understanding of the physics and distribution of star clusters may
be leading to a major paradigm shift in our understanding of galaxy evolution.Comment: 12 papges, to appear in The Galactic disk in a cosmological context,
IAUS254, eds J. Andersen, J. Bland-Hawthorn and B. Nordstro
On String Tunneling in Power Law Inflationary Universes
We consider the evolution of circular string loops in power law expanding
universes represented by a spatially flat Friedman-Robertson-Walker metric with
scale factor where is the cosmic time and . Our
main result is the existence of a "magic" power . In
spacetimes with a circular string expands either forever or to a
maximal radius and then contracts until it collapses into a point (black hole).
For , however, we find additional types of solutions. They include
configurations which contract from a positive initial radius to a minimal one
and then expand forever. Their existence we interpret as an indication for the
presence of a finite potential barrier. Equivalently the new solutions signal
string nucleation and tunneling, phenomena recently shown to occur in de Sitter
space.Comment: Latex, 9 pages + 4 figures (not included), NBI-HE-93-5
Optimal infinite scheduling for multi-priced timed automata
This paper is concerned with the derivation of infinite schedules for timed automata that are in some sense optimal. To cover a wide class of optimality criteria we start out by introducing an extension of the (priced) timed automata model that includes both costs and rewards as separate modelling features. A precise definition is then given of what constitutes optimal infinite behaviours for this class of models. We subsequently show that the derivation of optimal non-terminating schedules for such double-priced timed automata is computable. This is done by a reduction of the problem to the determination of optimal mean-cycles in finite graphs with weighted edges. This reduction is obtained by introducing the so-called corner-point abstraction, a powerful abstraction technique of which we show that it preserves optimal schedules
Cell Cycle-Dependent Differentiation Dynamics Balances Growth and Endocrine Differentiation in the Pancreas
Organogenesis relies on the spatiotemporal balancing of differentiation and proliferation driven by an expanding pool of progenitor cells. In the mouse pancreas, lineage tracing at the population level has shown that the expanding pancreas progenitors can initially give rise to all endocrine, ductal, and acinar cells but become bipotent by embryonic day 13.5, giving rise to endocrine cells and ductal cells. However, the dynamics of individual progenitors balancing self-renewal and lineage-specific differentiation has never been described. Using three-dimensional live imaging and in vivo clonal analysis, we reveal the contribution of individual cells to the global behaviour and demonstrate three modes of progenitor divisions: symmetric renewing, symmetric endocrinogenic, and asymmetric generating a progenitor and an endocrine progenitor. Quantitative analysis shows that the endocrine differentiation process is consistent with a simple model of cell cycle-dependent stochastic priming of progenitors to endocrine fate. The findings provide insights to define control parameters to optimize the generation of β-cells in vitro
BDM Dark Matter: CDM with a core profile and a free streaming scale
We present a new dark matter model BDM which is an hybrid between hot dark
matter HDM and cold dark matter CDM, in which the BDM particles behave as HDM
above the energy scale E_c and as CDM below this scale. Evolution of structure
formation is similar to that of CDM model but BDM predicts a nonvanishing free
streaming l_fs scale and a inner galaxy core radius r_core, both quantities
determined in terms of a single parameter E_c, which corresponds to the phase
transition energy scale of the subjacent elementary particle model. For
energies above E_c or for a scale factor a smaller then a_c, with a<a_c<a_{eq},
the particles are massless and rho redshifts as radiation. However, once the
energy becomes E\leq E_c or a>a_c then the BDM particles acquire a large mass
through a non perturbative mechanism, as baryons do, and rho redshifts as
matter with the particles having a vanishing velocity. Typical energies are
E_c=O(10-100) eV giving a l_fs \propto E_c^{-4/3}\lesssim Mpc and m_fs\propto
E_c^{-4}\lesssim 10^9 M\odot. A l_fs\neq 0, r_core\neq 0 help to resolve some
of the shortcomings of CDM such as overabundance substructure in CDM halos and
numerical fit to rotation curves in dwarf spheroidal and LSB galaxies. Finally,
our BDM model and the phase transition scale E_c can be derived from particle
physics.Comment: 7 pages, 8 figure
Thickening of galactic disks through clustered star formation
(Abridged) The building blocks of galaxies are star clusters. These form with
low-star formation efficiencies and, consequently, loose a large part of their
stars that expand outwards once the residual gas is expelled by the action of
the massive stars. Massive star clusters may thus add kinematically hot
components to galactic field populations. This kinematical imprint on the
stellar distribution function is estimated here by calculating the velocity
distribution function for ensembles of star-clusters distributed as power-law
or log-normal initial cluster mass functions (ICMFs). The resulting stellar
velocity distribution function is non-Gaussian and may be interpreted as being
composed of multiple kinematical sub-populations. The notion that the formation
of star-clusters may add hot kinematical components to a galaxy is applied to
the age--velocity-dispersion relation of the Milky Way disk to study the
implied history of clustered star formation, with an emphasis on the possible
origin of the thick disk.Comment: MNRAS, accepted, 27 pages, 9 figure
Interplay between telecommunications and face-to-face interactions - a study using mobile phone data
In this study we analyze one year of anonymized telecommunications data for
over one million customers from a large European cellphone operator, and we
investigate the relationship between people's calls and their physical
location. We discover that more than 90% of users who have called each other
have also shared the same space (cell tower), even if they live far apart.
Moreover, we find that close to 70% of users who call each other frequently (at
least once per month on average) have shared the same space at the same time -
an instance that we call co-location. Co-locations appear indicative of
coordination calls, which occur just before face-to-face meetings. Their number
is highly predictable based on the amount of calls between two users and the
distance between their home locations - suggesting a new way to quantify the
interplay between telecommunications and face-to-face interactions
Economic and other barriers to adopting recommendations to prevent childhood obesity: results of a focus group study with parents
Abstract Background Parents are integral to the implementation of obesity prevention and management recommendations for children. Exploration of barriers to and facilitators of parental decisions to adopt obesity prevention recommendations will inform future efforts to reduce childhood obesity. Methods We conducted 4 focus groups (2 English, 2 Spanish) among a total of 19 parents of overweight (BMI ≥ 85th percentile) children aged 5-17 years. The main discussion focused on 7 common obesity prevention recommendations: reducing television (TV) watching, removing TV from child's bedroom, increasing physically active games, participating in community or school-based athletics, walking to school, walking more in general, and eating less fast food. Parents were asked to discuss what factors would make each recommendation more difficult (barriers) or easier (facilitators) to follow. Participants were also asked about the relative importance of economic (time and dollar costs/savings) barriers and facilitators if these were not brought into the discussion unprompted. Results Parents identified many barriers but few facilitators to adopting obesity prevention recommendations for their children. Members of all groups identified economic barriers (time and dollar costs) among a variety of pertinent barriers, although the discussion of dollar costs often required prompting. Parents cited other barriers including child preference, difficulty with changing habits, lack of information, lack of transportation, difficulty with monitoring child behavior, need for assistance from family members, parity with other family members, and neighborhood walking safety. Facilitators identified included access to physical activity programs, availability of alternatives to fast food and TV which are acceptable to the child, enlisting outside support, dietary information, involving the child, setting limits, making behavior changes gradually, and parental change in shopping behaviors and own eating behaviors. Conclusions Parents identify numerous barriers to adopting obesity prevention recommendations, most notably child and family preferences and resistance to change, but also economic barriers. Intervention programs should consider the context of family priorities and how to overcome barriers and make use of relevant facilitators during program development.http://deepblue.lib.umich.edu/bitstream/2027.42/78270/1/1471-2431-9-81.xmlhttp://deepblue.lib.umich.edu/bitstream/2027.42/78270/2/1471-2431-9-81.pdfPeer Reviewe
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