30 research outputs found
Episodic accretion: the interplay of infall and disc instabilities
Using zoom-simulations carried out with the adaptive mesh-refinement code
RAMSES with a dynamic range of up to we
investigate the accretion profiles around six stars embedded in different
environments inside a (40 pc) giant molecular cloud, the role of mass
infall and disc instabilities on the accretion profile, and thus on the
luminosity of the forming protostar. Our results show that the environment in
which the protostar is embedded determines the overall accretion profile of the
protostar. Infall on to the circumstellar disc may trigger gravitational disc
instabilities in the disc at distances of around ~10 to ~50 au leading to rapid
transport of angular momentum and strong accretion bursts. These bursts
typically last for about ~10 to a ~100 yr, consistent with typical orbital
times at the location of the instability, and enhance the luminosity of the
protostar. Calculations with the stellar evolution code mesa show that the
accretion bursts induce significant changes in the protostellar proper- ties,
such as the stellar temperature and radius. We apply the obtained protostellar
properties to produce synthetic observables with RADMC3D and predict that
accretion bursts lead to ob- servable enhancements around 20 to 200 m in
the spectral energy distribution of Class 0 type young stellar objects.Comment: 17 pages, 14 figures, accepted by MNRA
The spatial scale of competition from recruits on an older cohort in Atlantic salmon
Competitive effects of younger cohorts on older ones are frequently assumed to be negligible in species where older, larger individuals dominate in pairwise behavioural interactions. Here, we provide field estimates of such competition by recruits on an older age class in Atlantic salmon (Salmo salar), a species where observational studies have documented strong body size advantages which should favour older individuals in direct interactions. By creating realistic levels of spatial variation in the density of underyearling (YOY) recruits over a 1-km stretch of a stream, and obtaining accurate measurements of individual growth rates of overyearlings (parr) from capture–mark–recapture data on a fine spatial scale, we demonstrate that high YOY density can substantially decrease parr growth. Models integrating multiple spatial scales indicated that parr were influenced by YOY density within 16 m. The preferred model suggested parr daily mass increase to be reduced by 39% when increasing YOY density from 0.0 to 1.0 m−2, which is well within the range of naturally occurring densities. Reduced juvenile growth rates will in general be expected to reduce juvenile survival (via increased length of exposure to freshwater mortality) and increase generation times (via increased age at seaward migrations). Thus, increased recruitment can significantly affect the performance of older cohorts, with important implications for population dynamics. Our results highlight that, even for the wide range of organisms that rely on defendable resources, the direction of competition among age classes cannot be assumed a priori or be inferred from behavioural observations alone
Risk Factors for Neck and Upper Extremity Disorders among Computers Users and the Effect of Interventions: An Overview of Systematic Reviews
Definition, aims, and implementation of GA2LEN/HAEi Angioedema Centers of Reference and Excellence
Identification of genetic variants associated with Huntington's disease progression: a genome-wide association study
Background Huntington's disease is caused by a CAG repeat expansion in the huntingtin gene, HTT. Age at onset has been used as a quantitative phenotype in genetic analysis looking for Huntington's disease modifiers, but is hard to define and not always available. Therefore, we aimed to generate a novel measure of disease progression and to identify genetic markers associated with this progression measure. Methods We generated a progression score on the basis of principal component analysis of prospectively acquired longitudinal changes in motor, cognitive, and imaging measures in the 218 indivduals in the TRACK-HD cohort of Huntington's disease gene mutation carriers (data collected 2008–11). We generated a parallel progression score using data from 1773 previously genotyped participants from the European Huntington's Disease Network REGISTRY study of Huntington's disease mutation carriers (data collected 2003–13). We did a genome-wide association analyses in terms of progression for 216 TRACK-HD participants and 1773 REGISTRY participants, then a meta-analysis of these results was undertaken. Findings Longitudinal motor, cognitive, and imaging scores were correlated with each other in TRACK-HD participants, justifying use of a single, cross-domain measure of disease progression in both studies. The TRACK-HD and REGISTRY progression measures were correlated with each other (r=0·674), and with age at onset (TRACK-HD, r=0·315; REGISTRY, r=0·234). The meta-analysis of progression in TRACK-HD and REGISTRY gave a genome-wide significant signal (p=1·12 × 10−10) on chromosome 5 spanning three genes: MSH3, DHFR, and MTRNR2L2. The genes in this locus were associated with progression in TRACK-HD (MSH3 p=2·94 × 10−8 DHFR p=8·37 × 10−7 MTRNR2L2 p=2·15 × 10−9) and to a lesser extent in REGISTRY (MSH3 p=9·36 × 10−4 DHFR p=8·45 × 10−4 MTRNR2L2 p=1·20 × 10−3). The lead single nucleotide polymorphism (SNP) in TRACK-HD (rs557874766) was genome-wide significant in the meta-analysis (p=1·58 × 10−8), and encodes an aminoacid change (Pro67Ala) in MSH3. In TRACK-HD, each copy of the minor allele at this SNP was associated with a 0·4 units per year (95% CI 0·16–0·66) reduction in the rate of change of the Unified Huntington's Disease Rating Scale (UHDRS) Total Motor Score, and a reduction of 0·12 units per year (95% CI 0·06–0·18) in the rate of change of UHDRS Total Functional Capacity score. These associations remained significant after adjusting for age of onset. Interpretation The multidomain progression measure in TRACK-HD was associated with a functional variant that was genome-wide significant in our meta-analysis. The association in only 216 participants implies that the progression measure is a sensitive reflection of disease burden, that the effect size at this locus is large, or both. Knockout of Msh3 reduces somatic expansion in Huntington's disease mouse models, suggesting this mechanism as an area for future therapeutic investigation
Explaining the luminosity spread in young clusters:proto and pre-main sequence stellar evolution in a molecular cloud environment
Hertzsprung-Russell diagrams of star forming regions show a large luminosity
spread. This is incompatible with well-defined isochrones based on classic
non-accreting protostellar evo- lution models. Protostars do not evolve in
isolation of their environment, but grow through accretion of gas. In addition,
while an age can be defined for a star forming region, the ages of individual
stars in the region will vary. We show how the combined effect of a
protostellar age spread, a consequence of sustained star formation in the
molecular cloud, and time-varying protostellar accretion for individual
protostars can explain the observed luminosity spread. We use a global MHD
simulation including a sub-scale sink particle model of a star forming region
to follow the accretion process of each star. The accretion profiles are used
to compute stellar evolution models for each star, incorporating a model of how
the accretion energy is distributed to the disk, radiated away at the accretion
shock, or incorporated into the outer layers of the protostar. Using a modelled
cluster age of 5 Myr we naturally reproduce the lumi- nosity spread and find
good agreement with observations of the Collinder 69 cluster, and the Orion
Nebular Cluster. It is shown how stars in binary and multiple systems can be
externally forced creating recurrent episodic accretion events. We find that in
a realistic global molecular cloud model massive stars build up mass over
relatively long time-scales. This leads to an important conceptual change
compared to the classic picture of non-accreting stellar evolution segmented in
to low-mass Hayashi tracks and high-mass Henyey tracks.Comment: 20 pages, 16 figures, Updated to match published article in MNRA