1,706 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
An analytical description of the disruption of star clusters in tidal fields with an application to Galactic open clusters
We present a simple analytical description of the disruption of star clusters
in a tidal field, which agrees excellently with detailed N-body simulations.
The analytic expression can be used to predict the mass and age histograms of
surviving clusters for any cluster initial mass function and any cluster
formation history. The method is applied to open clusters in the solar
neighbourhood, based on the new cluster sample of Kharchenko et al. From a
comparison between the observed and predicted age distributions in the age
range between 10 Myr to 3 Gyr we find the following results: (1) The disruption
time of a 10^4 M_sun cluster in the solar neighbourhood is about 1.3+/-0.5 Gyr.
This is a factor 5 shorter than derived from N-body simulations of clusters in
the tidal field of the galaxy. (2) The present starformation rate in bound
clusters within 600 pc from the Sun is 5.9+/-0.8 * 10^2 M_sun / Myr, which
corresponds to a surface star formation rate in bound clusters of 5.2+/-0.7
10^(-10) M_sun/yr/pc^2. (3) The age distribution of open clusters shows a bump
between 0.26 and 0.6 Gyr when the cluster formation rate was 2.5 times higher
than before and after. (4) The present star formation rate in bound clusters is
half as small as that derived from the study of embedded clusters. The
difference suggests that half of the clusters in the solar neighbourhood become
unbound within 10 Myr. (5) The most massive clusters within 600 pc had an
initial mass of 3*10^4 M_sun. This is in agreement with the statistically
expected value based on a cluster initial mass function with a slope of -2,
even if the physical upper mass limit is as high as 10^6 M_sun.Comment: 14 pages, 15 figures, to appear in Astronomy & Astrophysic
Systematic variation of the stellar Initial Mass Function with velocity dispersion in early-type galaxies
An essential component of galaxy formation theory is the stellar initial mass
function (IMF), that describes the parent distribution of stellar mass in star
forming regions. We present observational evidence in a sample of early-type
galaxies (ETGs) of a tight correlation between central velocity dispersion and
the strength of several absorption features sensitive to the presence of
low-mass stars. Our sample comprises ~40,000 ETGs from the SPIDER survey
(z<0.1). The data, extracted from the Sloan Digital Sky Survey, are combined,
rejecting both noisy data, and spectra with contamination from telluric lines,
resulting in a set of 18 stacked spectra at high signal-to-noise ratio (S/N>
400 per A). A combined analysis of IMF-sensitive line strengths and spectral
fitting is performed with the latest state-of the art population synthesis
models (an extended version of the MILES models). A significant trend is found
between IMF slope and velocity dispersion, towards an excess of low-mass stars
in the most massive galaxies. Although we emphasize that accurate values of the
IMF slope will require a detailed analysis of chemical composition (such as
[a/Fe] or even individual element abundance ratios), the observed trends
suggest that low-mass ETGs are better fit by a Kroupa-like IMF, whereas massive
galaxies require bottom-heavy IMFs, exceeding the Salpeter slope at velocity
dispersions above 200km/s.Comment: 5 pages, 4 figures, accepted for publication in MNRAS Letter
Hierarchical structures in the Large and Small Magellanic Clouds
We investigate the degree of spatial correlation among extended structures in
the LMC and SMC. To this purpose we work with sub-samples characterised by
different properties such as age and size, taken from the updated catalogue of
Bica et al. or gathered in the present work. The structures are classified as
star clusters or non-clusters (basically, nebular complexes and their stellar
associations). The radius distribution functions follow power-laws
() with slopes and maximum radius () that
depend on object class (and age). Non-clusters are characterised by
and R_{max}\la472 pc, while young clusters (age \la10
Myr) have and R_{max}\la15 pc, and old ones (age \ga600
Myr) have and R_{max}\la40 pc. Young clusters present a
high degree of spatial self-correlation and, especially, correlate with
star-forming structures, which does not occur with the old ones. This is
consistent with the old clusters having been heavily mixed up, since their ages
correspond to several LMC and SMC crossing times. On the other hand, with ages
corresponding to fractions of the respective crossing times, the young clusters
still trace most of their birthplace structural pattern. Also, small clusters
( pc), as well as small non-clusters ( pc), are spatially
self-correlated, while their large counterparts of both classes are not. The
above results are consistent with a hierarchical star-formation scenario for
the LMC and SMC.Comment: Accepted by MNRA
On the Disruption of Star Clusters in a Hierarchical Interstellar Medium
The distribution of the number of clusters as a function of mass M and age T
suggests that clusters get eroded or dispersed in a regular way over time, such
that the cluster number decreases inversely as an approximate power law with T
within each fixed interval of M. This power law is inconsistent with standard
dispersal mechanisms such as cluster evaporation and cloud collisions. In the
conventional interpretation, it requires the unlikely situation where diverse
mechanisms stitch together over time in a way that is independent of
environment or M. Here we consider another model in which the large scale
distribution of gas in each star-forming region plays an important role. We
note that star clusters form with positional and temporal correlations in giant
cloud complexes, and suggest that these complexes dominate the tidal force and
collisional influence on a cluster during its first several hundred million
years. Because the cloud complex density decreases regularly with position from
the cluster birth site, the harassment and collision rates between the cluster
and the cloud pieces decrease regularly with age as the cluster drifts. This
decrease is typically a power law of the form required to explain the mass-age
distribution. We reproduce this distribution for a variety of cases, including
rapid disruption, slow erosion, combinations of these two, cluster-cloud
collisions, cluster disruption by hierarchical disassembly, and partial cluster
disruption. We also consider apparent cluster mass loss by fading below the
surface brightness limit of a survey. In all cases, the observed log M - \log T
diagram can be reproduced under reasonable assumptions.Comment: ApJ vol. 712, March 20, 2010, 33 pages 15 figure
Simultaneous UBVRI observations of the cataclysmic variable AE Aquarii: temperature and mass of fireballs
We report simultaneous multicolour observations in 5 bands (UBVRI) of the
flickering variability of the cataclysmic variable AE Aqr. Our aim is to
estimate the parameters (colours, temperature, size) of the fireballs that
produce the optical flares.
The observed rise time of the optical flares is in the interval 220 - 440
sec. We estimate the dereddened colours of the fireballs: (U-B)_0 in the range
0.8-1.4, (B-V)_0 ~ 0.03-0.24, (V-I)_0 ~ 0.26-0.78. We find for the fireballs a
temperature in the range 10000 - 25000 K, mass (7-90).10^{19} g, size
(3-7).10^9 cm (using a distance of d=86 pc). These values refer to the peak of
the flares observed in UBVRI bands.
The data are available upon request from the authors.Comment: 8 pages, accepted in A
Old open clusters in the inner Galaxy: FSR1744, FSR89 and FSR31
We establish the nature and derive fundamental and structural parameters of
the recently catalogued objects FSR1744, FSR89 and FSR31. This work intends to
provide clues to constrain the Galactic tidal disruption efficiency, improve
statistics of the open cluster parameter space, and better define their
age-distribution function inside the Solar circle. Properties of the objects
are investigated by means of 2MASS colour-magnitude diagrams and stellar radial
density profiles built with field star decontaminated photometry.
Diagnostic-diagrams for structural parameters are used to help disentangle
dynamical from high-background effects affecting such centrally projected open
clusters. FSR1744, FSR89 and FSR31 are Gyr-class OCs located at Galactocentric
distances 4.0 - 5.6kpc. Compared to nearby OCs, they have small core and
limiting radii. With respect to the small number of OCs observed in the inner
Galaxy, the emerging scenario in the near-infrared favours disruption driven by
dynamical evolution rather than observational limitations associated with
absorption and/or high background levels. Internally, the main processes
associated with the dynamical evolution are, e.g. mass loss by stellar
evolution, mass segregation and evaporation. Externally they are, e.g. tidal
stress from the disk and bulge, and interactions with giant molecular clouds.
FSR1744, FSR89 and FSR31 have structural parameters consistent with their
Galactocentric distances, in the sense that tidally induced effects may have
accelerated the dynamical evolution.Comment: 12 pages and 13 figs; A&A, accepted, July 9, 200
Dynamical Evolution of Young Embedded Clusters: A Parameter Space Survey
This paper investigates the dynamical evolution of embedded stellar clusters
from the protocluster stage, through the embedded star-forming phase, and out
to ages of 10 Myr -- after the gas has been removed from the cluster. The
relevant dynamical properties of young stellar clusters are explored over a
wide range of possible star formation environments using N-body simulations.
Many realizations of equivalent initial conditions are used to produce robust
statistical descriptions of cluster evolution including the cluster bound
fraction, radial probability distributions, as well as the distributions of
close encounter distances and velocities. These cluster properties are
presented as a function of parameters describing the initial configuration of
the cluster, including the initial cluster membership N, initial stellar
velocities, cluster radii, star formation efficiency, embedding gas dispersal
time, and the degree of primordial mass segregation. The results of this
parameter space survey, which includes about 25,000 simulations, provide a
statistical description of cluster evolution as a function of the initial
conditions. We also present a compilation of the FUV radiation fields provided
by these same cluster environments. The output distributions from this study
can be combined with other calculations, such as disk photoevaporation models
and planetary scattering cross sections, to ascertain the effects of the
cluster environment on the processes involved in planet formation.Comment: 65 pages including 20 figures, accepted to ApJ Supplemen
Locomotor adaptability in persons with unilateral transtibial amputation
Background
Locomotor adaptation enables walkers to modify strategies when faced with challenging walking conditions. While a variety of neurological injuries can impair locomotor adaptability, the effect of a lower extremity amputation on adaptability is poorly understood. Objective
Determine if locomotor adaptability is impaired in persons with unilateral transtibial amputation (TTA). Methods
The locomotor adaptability of 10 persons with a TTA and 8 persons without an amputation was tested while walking on a split-belt treadmill with the parallel belts running at the same (tied) or different (split) speeds. In the split condition, participants walked for 15 minutes with the respective belts moving at 0.5 m/s and 1.5 m/s. Temporal spatial symmetry measures were used to evaluate reactive accommodations to the perturbation, and the adaptive/de-adaptive response. Results
Persons with TTA and the reference group of persons without amputation both demonstrated highly symmetric walking at baseline. During the split adaptation and tied post-adaptation walking both groups responded with the expected reactive accommodations. Likewise, adaptive and de-adaptive responses were observed. The magnitude and rate of change in the adaptive and de-adaptive responses were similar for persons with TTA and those without an amputation. Furthermore, adaptability was no different based on belt assignment for the prosthetic limb during split adaptation walking. Conclusions
Reactive changes and locomotor adaptation in response to a challenging and novel walking condition were similar in persons with TTA to those without an amputation. Results suggest persons with TTA have the capacity to modify locomotor strategies to meet the demands of most walking conditions despite challenges imposed by an amputation and use of a prosthetic limb
Close encounters in young stellar clusters: implications for planetary systems in the solar neighbourhood
The stars that populate the solar neighbourhood were formed in stellar
clusters. Through N-body simulations of these clusters, we measure the rate of
close encounters between stars. By monitoring the interaction histories of each
star, we investigate the singleton fraction in the solar neighbourhood. A
singleton is a star which formed as a single star, has never experienced any
close encounters with other stars or binaries, or undergone an exchange
encounter with a binary. We find that, of the stars which formed as single
stars, a significant fraction are not singletons once the clusters have
dispersed. If some of these stars had planetary systems, with properties
similar to those of the solar system, the planets orbits may have been
perturbed by the effects of close encounters with other stars or the effects of
a companion star within a binary. Such perturbations can lead to strong
planet-planet interactions which eject several planets, leaving the remaining
planets on eccentric orbits. Some of the single stars exchange into binaries.
Most of these binaries are broken up via subsequent interactions within the
cluster, but some remain intact beyond the lifetime of the cluster. The
properties of these binaries are similar to those of the observed binary
systems containing extra-solar planets. Thus, dynamical processes in young
stellar clusters will alter significantly any population of solar-system-like
planetary systems. In addition, beginning with a population of planetary
systems exactly resembling the solar system around single stars, dynamical
encounters in young stellar clusters may produce at least some of the
extra-solar planetary systems observed in the solar neighbourhood.Comment: 11 pages, 9 figures, 1 table. Accepted for publication in MNRA
- âŠ