1,925 research outputs found
Top-heavy integrated galactic stellar initial mass functions (IGIMFs) in starbursts
Star formation rates (SFR) larger than 1000 Msun/ yr are observed in extreme
star bursts. This leads to the formation of star clusters with masses > 10^6
Msun in which crowding of the pre-stellar cores may lead to a change of the
stellar initial mass function (IMF). Indeed, the large mass-to-light ratios of
ultra-compact dwarf galaxies and recent results on globular clusters suggest
the IMF to become top-heavy with increasing star-forming density. We explore
the implications of top-heavy IMFs in these very massive and compact systems
for the integrated galactic initial mass function (IGIMF), which is the
galaxy-wide IMF, in dependence of the star-formation rate of galaxies. The
resulting IGIMFs can have slopes, alpha_3, for stars more massive than about 1
Msun between 1.5 and the Salpeter slope of 2.3 for an embedded cluster mass
function (ECMF) slope (beta) of 2.0, but only if the ECMF has no low-mass
clusters in galaxies with major starbursts. Alternatively, beta would have to
decrease with increasing SFR >10 Msun/ yr such that galaxies with major
starbursts have a top-heavy ECMF. The resulting IGIMFs are within the range of
observationally deduced IMF variations with redshift.Comment: Accepted for publication in MNRAS, reference adde
Properties of hierarchically forming star clusters
We undertake a systematic analysis of the early (< 0.5 Myr) evolution of
clustering and the stellar initial mass function in turbulent fragmentation
simulations. These large scale simulations for the first time offer the
opportunity for a statistical analysis of IMF variations and correlations
between stellar properties and cluster richness. The typical evolutionary
scenario involves star formation in small-n clusters which then progressively
merge; the first stars to form are seeds of massive stars and achieve a
headstart in mass acquisition. These massive seeds end up in the cores of
clusters and a large fraction of new stars of lower mass is formed in the outer
parts of the clusters. The resulting clusters are therefore mass segregated at
an age of 0.5 Myr, although the signature of mass segregation is weakened
during mergers. We find that the resulting IMF has a smaller exponent
(alpha=1.8-2.2) than the Salpeter value (alpha=2.35). The IMFs in subclusters
are truncated at masses only somewhat larger than the most massive stars (which
depends on the richness of the cluster) and an universal upper mass limit of
150 Msun is ruled out. We also find that the simulations show signs of the
IGIMF effect proposed by Weidner & Kroupa, where the frequency of massive stars
is suppressed in the integrated IMF compared to the IMF in individual clusters.
We identify clusters through the use of a minimum spanning tree algorithm which
allows easy comparison between observational survey data and the predictions of
turbulent fragmentation models. In particular we present quantitative
predictions regarding properties such as cluster morphology, degree of mass
segregation, upper slope of the IMF and the relation between cluster richness
and maximum stellar mass. [abridged]Comment: 21 Pages, 25 Figure
Maximum stellar mass versus cluster membership number revisited
We have made a new compilation of observations of maximum stellar mass versus
cluster membership number from the literature, which we analyse for consistency
with the predictions of a simple random drawing hypothesis for stellar mass
selection in clusters. Previously, Weidner and Kroupa have suggested that the
maximum stellar mass is lower, in low mass clusters, than would be expected on
the basis of random drawing, and have pointed out that this could have
important implications for steepening the integrated initial mass function of
the Galaxy (the IGIMF) at high masses. Our compilation demonstrates how the
observed distribution in the plane of maximum stellar mass versus membership
number is affected by the method of target selection; in particular, rather low
n clusters with large maximum stellar masses are abundant in observational
datasets that specifically seek clusters in the environs of high mass stars.
Although we do not consider our compilation to be either complete or unbiased,
we discuss the method by which such data should be statistically analysed. Our
very provisional conclusion is that the data is not indicating any striking
deviation from the expectations of random drawing.Comment: 7 pages, 3 Figures; accepted by MNRAS; Reference added
The ENCOMPASS framework:a practical guide for the evaluation of public health programmes in complex adaptive systems
BackgroundSystems thinking embraces the complexity of public health problems, including childhood overweight and obesity. It aids in understanding how factors are interrelated, and it can be targeted to produce favourable changes in a system. There is a growing call for systems approaches in public health research, yet limited practical guidance is available on how to evaluate public health programmes within complex adaptive systems. The aim of this paper is to present an evaluation framework that supports researchers in designing systems evaluations in a comprehensive and practical way.MethodsWe searched the literature for existing public health systems evaluation studies. Key characteristics on how to conduct a systems evaluation were extracted and compared across studies. Next, we overlaid the identified characteristics to the context of the Lifestyle Innovations Based on Youth Knowledge and Experience (LIKE) programme evaluation and analyzed which characteristics were essential to carry out the LIKE evaluation. This resulted in the Evaluation of Programmes in Complex Adaptive Systems (ENCOMPASS) framework.ResultsThe ENCOMPASS framework includes five iterative stages: (1) adopting a system dynamics perspective on the overall evaluation design; (2) defining the system boundaries; (3) understanding the pre-existing system to inform system changes; (4) monitoring dynamic programme output at different system levels; and (5) measuring programme outcome and impact in terms of system changes.ConclusionsThe value of ENCOMPASS lies in the integration of key characteristics from existing systems evaluation studies, as well as in its practical, applied focus. It can be employed in evaluating public health programmes in complex adaptive systems. Furthermore, ENCOMPASS provides guidance for the entire evaluation process, all the way from understanding the system to developing actions to change it and to measuring system changes. By the nature of systems thinking, the ENCOMPASS framework will likely evolve further over time, as the field expands with more completed studies
Integration of genetic and physical maps of the Primula vulgaris S locus and localization by chromosome in situ hybridization
•Heteromorphic flower development in Primula is controlled by the S locus. The S locus genes, which control anther position, pistil length and pollen size in pin and thrum flowers, have not yet been characterized. We have integrated S-linked genes, marker sequences and mutant phenotypes to create a map of the P. vulgaris S locus region that will facilitate the identification of key S locus genes. We have generated, sequenced and annotated BAC sequences spanning the S locus, and identified its chromosomal location. •We have employed a combination of classical genetics and three-point crosses with molecular genetic analysis of recombinants to generate the map. We have characterized this region by Illumina sequencing and bioinformatic analysis, together with chromosome in situ hybridization. •We present an integrated genetic and physical map across the P. vulgaris S locus flanked by phenotypic and DNA sequence markers. BAC contigs encompass a 1.5-Mb genomic region with 1 Mb of sequence containing 82 S-linked genes anchored to overlapping BACs. The S locus is located close to the centromere of the largest metacentric chromosome pair. •These data will facilitate the identification of the genes that orchestrate heterostyly in Primula and enable evolutionary analyses of the S locus
Learning to cycle: is velocity a control parameter for children’s cycle patterns on the balance bike?
The balance bike (BB) has been pointed out as being the most efficient learning bicycle due to its inherent stimulation of balance. However, the process of acquiring the control of balance on the BB has not been explored. This study aimed to: (i) categorize the cycle patterns of children on the BB, (ii) compare the cycle patterns in different stages of learning (before and after six sessions of a BB practice program), and (iii) verify whether velocity is a control parameter leading to transitions between different cycle patterns on a BB. The data were collected during the Learning to Cycle program from 12 children aged 6.06 ± 1.25 years. The velocity was measured using an inertial sensor. Seven different movement patterns were captured and categorized through video analysis. After practice, there was an increase in the mean number of different patterns and in the global mean and maximum velocity. These were interpreted as an improvement of the motor competence in the use of the BB. The results obtained support the hypothesis that velocity is a control parameter which leads to the emergence of diverse patterns of behavior. As the speed increased, the amount of foot contact with the ground became less frequent and the locomotor modes that imply that longer flight phases began to emerge.info:eu-repo/semantics/publishedVersio
A Laser System for the Spectroscopy of Highly-Charged Bismuth Ions
We present and characterize a laser system for the spectroscopy on
highly-charged ^209Bi^82+ ions at a wavelength of 243.87 nm. For absolute
frequency stabilization, the laser system is locked to a near-infra-red laser
stabilized to a rubidium transition line using a transfer cavity based locking
scheme. Tuning of the output frequency with high precision is achieved via a
tunable rf offset lock. A sample-and-hold technique gives an extended tuning
range of several THz in the UV. This scheme is universally applicable to the
stabilization of laser systems at wavelengths not directly accessible to atomic
or molecular resonances. We determine the frequency accuracy of the laser
system using Doppler-free absorption spectroscopy of Te_2 vapour at 488 nm.
Scaled to the target wavelength of 244 nm, we achieve a frequency uncertainty
of \sigma_{244nm} = 6.14 MHz (one standard deviation) over six days of
operation.Comment: Contribution to the special issue on "Trapped Ions" in "Applied
Physics B
Escaping stars from young low-N clusters
With the use of N-body calculations the amount and properties of escaping
stars from low-N (N = 100 and 1000) young embedded star clusters prior to gas
expulsion are studied over the first 5 Myr of their existence. Besides the
number of stars also different initial radii and binary populations are
examined as well as virialised and collapsing clusters. It is found that these
clusters can loose substantial amounts (up to 20%) of stars within 5 Myr with
considerable velocities up to more than 100 km/s. Even with their mean
velocities between 2 and 8 km/s these stars will still be travelling between 2
and 30 pc during the 5 Myr. Therefore can large amounts of distributed stars in
star-forming regions not necessarily be counted as evidence for the isolated
formation of stars.Comment: 10 pages, 10 figures, accepted for publication by MNRA
- …