50 research outputs found
Early evolution of the birth cluster of the solar system
The solar system was most likely born in a star cluster containing at least
1000 stars. It is highly probable that this cluster environment influenced
various properties of the solar system like its chemical composition, size and
the orbital parameters of some of its constituting bodies. In the Milky Way,
clusters with more than 2000 stars only form in two types - starburst clusters
and leaky clusters - each following a unique temporal development in the
mass-radius plane. The aim is here to determine the encounter probability in
the range relevant to solar system formation for starburst or leaky cluster
environments as a function of cluster age. N-body methods are used to
investigate the cluster dynamics and the effect of gravitational interactions
between cluster members on young solar-type stars surrounded by discs. Using
the now available knowledge of the cluster density at a given cluster age it is
demonstrated that in starburst clusters the central densities over the first
5Myr are so high (initially > 10^5 Msun pc^{-3}) that hardly any discs with
solar system building potential would survive this phase. This makes a
starburst clusters an unlikely environment for the formation of our solar
system. Instead it is highly probable that the solar system formed in a leaky
cluster (often classified as OB association). It is demonstrated that an
encounter determining the characteristic properties existing in our solar
systems most likely happened very early on (< 2Myr) in its formation history
and that after 5Myr the likelihood of a solar-type star experiencing such an
encounter in a leaky cluster is negligible even if it was still part of the
bound remnant. This explains why the solar system could develop and maintain
its high circularity later in its development.Comment: 11 pages, 7 figures, accepted by A&
Reaction of Massive Clusters to Gas Expulsion - The cluster density dependence
The expulsion of the unconverted gas at the end of the star formation process
potentially leads to the expansion of the just formed stellar cluster and
membership loss. The degree of expansion and mass loss depends largely on the
star formation efficiency and scales with the mass and size of the stellar
group as long as stellar interactions can be neglected. We investigate under
which circumstances stellar interactions between cluster members become so
important that the fraction of bound stars after gas expulsion is significantly
altered. The Nbody6 code is used to simulate the cluster dynamics after gas
expulsion for different SFEs. Concentrating on the most massive clusters
observed in the Milky Way, we test to what extend the results depend on the
model, i.e. stellar mass distribution, stellar density profile etc., and the
cluster parameters, such as cluster density and size.We find that stellar
interactions are responsible for up to 20% mass loss in the most compact
massive clusters in the Milky Way, making ejections the prime mass loss process
in such systems. Even in the loosely bound OB associations stellar interactions
are responsible for at least ~5% mass loss. The main reason why the importance
of encounters for massive clusters has been largely overlooked is the often
used approach of a single-mass representation instead of a realistic
distribution for the stellar masses. The density-dependence of the
encounter-induced mass loss is shallower than expected because of the
increasing importance of few-body interactions in dense clusters compared to
sparse clusters where 2-body encounters dominate.Comment: 9 pages, 6 figures, A&A accepte
Cluster dynamics largely shapes protoplanetary disc sizes
It is still on open question to what degree the cluster environment
influences the sizes of protoplanetary discs surrounding young stars.
Particularly so for the short-lived clusters typical for the solar
neighbourhood in which the stellar density and therefore the influence of the
cluster environment changes considerably over the first 10 Myr. In previous
studies often the effect of the gas on the cluster dynamics has been neglected,
this is remedied here. Using the code NBody6++ we study the stellar dynamics in
different developmental phases - embedded, expulsion, expansion - including the
gas and quantify the effect of fly-bys on the disc size. We concentrate on
massive clusters (),
which are representative for clusters like the Orion Nebula Cluster (ONC) or
NGC 6611. We find that not only the stellar density but also the duration of
the embedded phase matters. The densest clusters react fastest to the gas
expulsion and drop quickly in density, here 98% of relevant encounters happen
before gas expulsion. By contrast, discs in sparser clusters are initially less
affected but as they expand slower 13% of discs are truncated after gas
expulsion. For ONC-like clusters we find that usually discs larger than 500 AU
are affected by the environment, which corresponds to the observation that 200
AU-sized discs are common. For NGC 6611-like clusters disc sizes are cut-down
on average to roughly 100 AU. A testable hypothesis would be that the discs in
the centre of NGC 6611 should be on average ~20 AU and therefore considerably
smaller than in the ONC.Comment: Accepted for publication in Ap
How do disks and planetary systems in high-mass open clusters differ from those around field stars?
Only star clusters that are sufficiently compact and massive survive largely
unharmed beyond 10 Myr. However, their compactness means a high stellar density
which can lead to strong gravitational interactions between the stars. As young
stars are often initially surrounded by protoplanetary disks and later on
potentially by planetary systems, the question arises to what degree these
strong gravitational interactions influence planet formation and the properties
of planetary systems. Here, we perform simulations of the evolution of compact
high-mass clusters like Trumpler 14 and Westerlund 2 from the embedded to the
gas-free phase and study the influence of stellar interactions. We concentrate
on the development of the mean disk size in these environments. Our simulations
show that in high-mass open clusters of all disks/planetary systems
should be smaller than 50 AU just due to the strong stellar interactions in
these environments. Already in the initial phases, 3-4 close fly-bys lead to
typical disk sizes within the range of 18-27 AU. Afterwards, the disk sizes are
altered only to a small extent. Our findings agree with the recent observation
that the disk sizes in the once dense environment of the Upper Scorpio OB
association, NGC~2362, and h/Persei are at least three times smaller in
size than, for example, in Taurus. We conclude that the observed planetary
systems in high-mass open clusters should also be on average smaller than those
found around field stars; in particular, planets on wide orbits are expected to
be extremely rare in such environments.Comment: 20 pages, 9 figures, accepted for publication in Ap
Local-Density Driven Clustered Star Formation
A positive power-law trend between the local surface densities of molecular
gas, , and young stellar objects, , in molecular
clouds of the Solar Neighbourhood has been identified by Gutermuth et al. How
it relates to the properties of embedded clusters, in particular to the
recently established radius-density relation, has so far not been investigated.
In this paper, we model the development of the stellar component of molecular
clumps as a function of time and initial local volume density so as to provide
a coherent framework able to explain both the molecular-cloud and
embedded-cluster relations quoted above. To do so, we associate the observed
volume density gradient of molecular clumps to a density-dependent free-fall
time. The molecular clump star formation history is obtained by applying a
constant SFE per free-fall time, \eff.
For volume density profiles typical of observed molecular clumps (i.e.
power-law slope ), our model gives a star-gas surface-density
relation , in very good agreement with
the Gutermuth et al relation. Taking the case of a molecular clump of mass and radius experiencing star formation during
2 Myr, we derive what SFE per free-fall time matches best the normalizations of
the observed and predicted (, ) relations. We
find \eff \simeq 0.1. We show that the observed growth of embedded clusters,
embodied by their radius-density relation, corresponds to a surface density
threshold being applied to developing star-forming regions. The consequences of
our model in terms of cluster survivability after residual star-forming gas
expulsion are that due to the locally high SFE in the inner part of
star-forming regions, global SFE as low as 10% can lead to the formation of
bound gas-free star clusters.Comment: 16 pages, 15 figures, Astronomy & Astrophysics, in pres
Does the mass distribution in discs influence encounter-induced losses in young star clusters?
One mechanism for the external destruction of protoplanetary discs in young
dense clusters is tidal disruption during the flyby of another cluster member.
The degree of mass loss in such an encounter depends, among other parameters,
on the distribution of the material within the disc. Previous work showed that
this is especially so in encounters that truncate large parts of the outer
disc. The expectation is that the number of completely destroyed discs in a
cluster depends also on the mass distribution within the discs. Here we test
this hypothesis by determining the influence of encounters on the disc fraction
and average disc mass in clusters of various stellar densities for different
mass distributions in the discs. This is done by performing Nbody6 simulation
of a variety of cluster environments, where we track the encounter dynamics and
determine the mass loss due to these encounters for different disc-mass
distributions. We find that although the disc mass distribution has a
significant impact on the disc losses for specific star-disc encounters, the
overall disc frequency generally remains rather unaffected. The reason is that
in single encounters the dependence on the mass distribution is strongest if
both stars have very different masses. Such encounters are rather infrequent in
sparse clusters. In dense clusters such encounters are more common, however,
here the disc frequency is largely determined by encounters between low-mass
stars such that the overall disc frequency does not change significantly. For
tidal disruption the disc destruction in clusters is fairly independent of the
actual distribution of the material in the disc. The all determining factor
remains the cluster density.Comment: 7pages, 4 figures, accepted by A&
The expansion of massive young star clusters - observation meets theory
Most stars form as part of a star cluster. The most massive clusters in the
Milky Way exist in two groups - loose and compact clusters - with significantly
different sizes at the end of the star formation process. After their formation
both types of clusters expand up to a factor 10-20 within the first 20 Myr. Gas
expulsion at the end of the star formation process is usually regarded as only
possible process that can lead to such an expansion.We investigate the effect
of gas expulsion by a direct comparison between numerical models and observed
clusters concentrating on clusters with masses >10^3 M(sun). For these clusters
the initial conditions before gas expulsion, the characteristic cluster
development, its dependence on cluster mass, and the star formation efficiency
(SFE) are investigated. We perform N-body simulations of the cluster expansion
process after gas expulsion and compare the results with observations. We find
that the expansion processes of the observed loose and compact massive clusters
are driven by completely different physical processes. As expected the
expansion of loose massive clusters is largely driven by the gas loss due to
the low SFE of ~30%. One new revelation is that all the observed massive
clusters of this group seem to have a very similar size of 1-3 pc at the onset
of expansion. It is demonstrated that compact clusters have a much higher
effective SFE of 60-70% and are as a result much less affected by gas
expulsion. Their expansion is mainly driven by stellar ejections caused by
interactions between the cluster members. The reason why ejections are so
efficient in driving cluster expansion is that they occur dominantly from the
cluster centre and over an extended period of time. Thus during the first 10
Myr the internal dynamics of loose and compact clusters differ fundamentally.Comment: 10 pages, 9 figures, accepted by A&
From star-disc encounters to numerical solutions for a subset of the restricted three-body problem
Various astrophysical processes are known, where the fly-by of a massive
object affects matter initially supported against gravity by rotation. Examples
are perturbations of galaxies, protoplanetary discs or planetary systems. We
approximate such events as subset of the restricted three-body problem by
considering only perturbations of non-interacting low-mass objects initially on
circular Keplerian orbits. In this paper we present a new parametrisation of
the initial conditions of this problem. Under certain conditions the initial
positions of the low-mass objects can be specified largely independent of the
initial position of the perturber. Exploiting additionally the known scalings
of the problem reduces the parameter space of initial conditions for one
specific perturbation to two dimensions. To this two-dimensional initial
condition space we have related the final properties of the perturbed
trajectories of the low-mass objects from our numerical simulations. That way,
maps showing the effect of the perturbation on the low-mass objects have been
created, which provide a new view on the perturbation process. Comparing the
maps for different mass-ratios reveals that the perturbations by low- and
high-mass perturbers are dominated by different physical processes. The
equal-mass case is a complicated mixture of the other two cases. Since the
final properties of trajectories with similar initial conditions are usually
also similar, the results of the limited number of integrated trajectories can
be generalised to the full presented parameter space by interpolation. Since
our results are also unique within the accuracy strived for, they constitute
general numerical solutions for this subset of the restricted three-body
problem. As such, they can be used to predict the evolution of real physical
problems by simple transformations like scaling and without further
simulations. (...)Comment: 11 pages, 8 figures, + 2 pages appendix, published by A&A; This
version includes the Corrigendum (DOI: 10.1051/0004-6361/201526068e) and
changes from the editing proces