146 research outputs found
Quasi-binarity of massive stars in young dense clusters - the case of the ONC
Observations indicate that in young stellar clusters the binary fraction for
massive stars is higher than for solar mass stars. For the Orion Nebula Cluster
(ONC) there is a binary frequency of ~ 50% for solar-mass stars compared to
70-100% for the massive O- and B-stars. We explore the reasons for this
discrepancy and come up with two possible answers: a) a primordially higher
binarity of massive stars could be inherent to the star formation process or b)
the primordial binary rate might be the same for solar-mass and massive stars,
but the higher capture cross section of the massive stars possibly leads to the
formation of additional massive binaries in the early cluster development. Here
we investigate the likelihood of the latter using the ONC as an example. N-body
simulations are performed to track the capture events in an ONC-like cluster.
We find that whereas low-mass stars rarely form bound systems through capture,
the dynamics of the massive stars - especially in the first 0.5 Myrs - is
dominated by a rapid succession of ``transient binary or multiple systems''. In
observations the transient nature of these systems would not be apparent, so
that they would be rated as binaries. At 1-2 Myrs, the supposed age of the ONC,
the ``transient'' massive systems become increasingly stable, lasting on
average several 10^6 yrs. Despite the ONC being so young, the observed binary
frequency for massive stars -- unlike that of solar-mass stars -- is not
identical to the primordial binary frequency but is increased by at least
10-15% through dynamical interaction processes. This value might be increased
to at least 20-25% by taking disc effects into account. The primordial binary
frequency could well be the same for massive and solar mass stars because the
observed difference can be explained by capture processes alone.Comment: 9 pages, 7 figures. accepted by A&
Stellar interactions in dense and sparse star clusters
Stellar encounters potentially affect the evolution of the protoplanetary
discs in the Orion Nebula Cluster (ONC). However, the role of encounters in
other cluster environments is less known. We investigate the effect of the
encounter-induced disc-mass loss in different cluster environments. Starting
from an ONC-like cluster we vary the cluster size and density to determine the
correlation of collision time scale and disc-mass loss. We use the NBODY6++
code to model the dynamics of these clusters and analyze the effect of
star-disc encounters. We find that the disc-mass loss depends strongly on the
cluster density but remains rather unaffected by the size of the stellar
population. The essential outcome of the simulations are: i) Even in clusters
four times sparser than the ONC the effect of encounters is still apparent. ii)
The density of the ONC itself marks a threshold: in less dense and less massive
clusters it is the massive stars that dominate the encounter-induced disc-mass
loss whereas in denser and more massive clusters the low-mass stars play the
major role for the disc mass removal. It seems that in the central regions of
young dense star clusters -- the common sites of star formation -- stellar
encounters do affect the evolution of the protoplanetary discs. With higher
cluster density low-mass stars become more heavily involved in this process.
This finding allows for the extrapolation towards extreme stellar systems: in
case of the Arches cluster one would expect stellar encounters to destroy the
discs of most of the low- and high-mass stars in several hundred thousand
years, whereas intermediate mass stars are able to retain to some extant their
discs even under these harsh environmental conditions.Comment: accepted by Astronomy and Astrophysic
Modes of clustered star formation
The realization that most stars form in clusters, raises the question of
whether star/planet formation are influenced by the cluster environment. The
stellar density in the most prevalent clusters is the key factor here. Whether
dominant modes of clustered star formation exist is a fundamental question.
Using near-neighbour searches in young clusters Bressert et al. (2010) claim
this not to be the case and conclude that star formation is continuous from
isolated to densely clustered. We investigate under which conditions
near-neighbour searches can distinguish between different modes of clustered
star formation. Near-neighbour searches are performed for model star clusters
investigating the influence of the combination of different cluster modes,
observational biases, and types of diagnostic and find that the cluster density
profile, the relative sample sizes, limitations in observations and the choice
of diagnostic method decides whether modelled modes of clustered star formation
are detected. For centrally concentrated density distributions spanning a wide
density range (King profiles) separate cluster modes are only detectable if the
mean density of the individual clusters differs by at least a factor of ~65.
Introducing a central cut-off can lead to underestimating the mean density by
more than a factor of ten. The environmental effect on star and planet
formation is underestimated for half of the population in dense systems. A
analysis of a sample of cluster environments involves effects of superposition
that suppress characteristic features and promotes erroneous conclusions. While
multiple peaks in the distribution of the local surface density imply the
existence of different modes, the reverse conclusion is not possible. Equally,
a smooth distribution is not a proof of continuous star formation, because such
a shape can easily hide modes of clustered star formation (abridged)Comment: 9 pages, 6 figures, accepted by A&
Gravitational Instabilities induced by Cluster Environment? - The encounter-induced angular momentum transfer in discs
AIM:The aim of this work is to understand to what extend gravitational
interactions between the stars in high-density young stellar clusters, like the
Orion Nebula Cluster (ONC), change the angular momentum in their protoplanetary
discs. METHOD:Two types of simulations were combined -- N-body simulations of
the dynamics of the stars in the ONC, and angular momentum loss results from
simulations of star-disc encounters. RESULTS:It is shown that in a star-disc
encounter the angular momentum loss is usually larger than the mass loss, so
that the disc remnant has a lower specific angular momentum. Assuming an age of
1-2 Myr for the ONC, the disc angular momentum in the higher density region of
the Trapezium is reduced by 15-20% on average. Encounters therefore play an
important part in the angular momentum transport in these central regions but
are not the dominant process. More importantly, even in the outer cluster
regions the angular momentum loss is on average 3-5%. Here it is shown that a
3-5% loss in angular momentum might be enough to trigger gravitational
instabilities even in low-mass discs - a possible prerequisite for the
formation of planetary systems.Comment: 9 pages, 8 figures, accepted by A&
Cluster-assisted accretion for massive stars
Gravitational interactions in very young high-density stellar clusters can to
some degree change the angular momentum in the circumstellar discs surrounding
initially the majority of stars. However, for most stars the cluster
environment alters the angular momentum only slightly. For example, in
simulations of the Orion Nebula cluster (ONC) encounters reduce the angular
momentum of the discs on average at most by 3-5% and in the higher density
region of the Trapezium %where encounters are more likely, the disc angular
momentum is on average lowered by 15-20% - still a minor loss process. However,
in this paper it is demonstrated that the situation is very different if one
considers high-mass stars (M* > 10 M(solar) only. Assuming an age of 2 Myr for
the ONC, their discs have on average a 50-90% lower angular momentum than
primordially. This enormous loss in angular momentum in the disc should result
in an equivalent increase in accretion, implying that the cluster environment
boosts accretion for high-mass stars, thus %in the cluster center, making them
even more massive.Comment: 10 pages including 2 figures, accepted for publication in ApJ
How Universal are the Young Cluster Sequences? - the Cases of LMC, SMC, M83 and the Antennae
Aims.Recently a new analysis of cluster observations in the Milky Way found
evidence that clustered star formation may work under tight constraints with
respect to cluster size and density, implying the presence of just two
sequences of young massive cluster. These two types of clusters each expand at
different rates with cluster age. Methods. Here we investigate whether similar
sequences exist in other nearby galaxies. Results:We find that while for the
extragalactic young stellar clusters the overall trend in the cluster-density
scaling is quite comparable to the relation obtained for Galactic clusters,
there are also possible difference. For the LMC and SMC clusters the densities
are below the Galactic data points and/or the core radii are smaller than those
of data points with comparable density. For M83 and the Antenna clusters the
core radii are possibly comparable to the Galactic clusters but it is not clear
whether they exhibit similar expansion speeds. These findings should serve as
an incentive to perform more systematic observations and analysis to answer the
question of a possible similarity between young galactic and extragalactic star
clusters sequences.Comment: 6 pages, 4 figures, A&A in pres
Towards the field binary population: Influence of orbital decay on close binaries
Surveys of the binary populations in the solar neighbourhood have shown that
the periods of G- and M-type stars are log-normally distributed. However,
observations of young binary populations suggest a log-uniform distribution.
Clearly some process(es) change the period distribution over time. Most stars
form in star clusters, in which two important dynamical processes occur: i)
gas-induced orbital decay of embedded binary systems and ii) destruction of
soft binaries in three-body interactions. The emphasis here is on orbital decay
which has been largely neglected so far. Using a combination of Monte-Carlo and
dynamical nbody modelling it is demonstrated here that the cluster dynamics
destroys the number of wide binaries, but leaves short-period binaries
basically undisturbed even for a initially log-uniform distribution. By
contrast orbital decay significantly reduces the number and changes the
properties of short-period binaries, but leaves wide binaries largely
uneffected. Until now it was unclear whether the short period distribution of
the field is unaltered since its formation. It is shown here, that orbital
decay is a prime candidate for such a task. In combination the dynamics of
these two processes, convert an initial log-uniform distribution to a
log-normal period distribution. The probability is 94% that the evolved and
observed period distribution were sampled from the same parent distribution.
This means binaries can be formed with periods that are sampled from the
log-uniform distribution. As the cluster evolves, short-period binaries are
merged to single stars by the gas-induced orbital decay while the dynamical
evolution in the cluster destroys wide binaries. The combination of these two
equally important processes reshapes a initial log-uniform period distribution
to the log-normal period distribution, that is observed in the field
(abridged).Comment: 9 pages, 9 figure
Ionization of clusters in intense laser pulses through collective electron dynamics
The motion of electrons and ions in medium-sized rare gas clusters (1000
atoms) exposed to intense laser pulses is studied microscopically by means of
classical molecular dynamics using a hierarchical tree code. Pulse parameters
for optimum ionization are found to be wavelength dependent. This resonant
behavior is traced back to a collective electron oscillation inside the charged
cluster. It is shown that this dynamics can be well described by a driven and
damped harmonic oscillator allowing for a clear discrimination against other
energy absorption mechanisms.Comment: 4 pages (4 figures
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