4,342 research outputs found
Evidence for a fundamental stellar upper mass limit from clustered star formation
The observed masses of the most massive stars do not surpass about 150Msun.
This may either be a fundamental upper mass limit which is defined by the
physics of massive stars and/or their formation, or it may simply reflect the
increasing sparsity of such very massive stars so that observing even
higher-mass stars becomes unlikely in the Galaxy and the Magellanic Clouds. It
is shown here that if the stellar initial mass function (IMF) is a power-law
with a Salpeter exponent (alpha=2.35) for massive stars then the richest very
young cluster R136 seen in the Large Magellanic Cloud (LMC) should contain
stars with masses larger than 750Msun. If, however, the IMF is formulated by
consistently incorporating a fundamental upper mass limit then the observed
upper mass limit is arrived at readily even if the IMF is invariant. An
explicit turn-down or cutoff of the IMF near 150Msun is not required; our
formulation of the problem contains this implicitly. We are therefore led to
conclude that a fundamental maximum stellar mass near 150Msun exists, unless
the true IMF has alpha>2.8.Comment: MNRAS, accepted, 6 page
A discontinuity in the low-mass IMF - the case of high multiplicity
The empirical binary properties of brown dwarfs (BDs) differ from those of
normal stars suggesting BDs form a separate population. Recent work by Thies &
Kroupa revealed a discontinuity of the initial mass function (IMF) in the
very-low-mass star regime under the assumption of a low multiplicity of BDs of
about 15 per cent. However, previous observations had suggested that the
multiplicity of BDs may be significantly higher, up to 45 per cent. This
contribution investigates the implication of a high BD multiplicity on the
appearance of the IMF for the Orion Nebula Cluster, Taurus-Auriga, IC 348 and
the Pleiades. We show that the discontinuity remains pronounced even if the
observed MF appears to be continuous, even for a BD binary fraction as high as
60%. We find no evidence for a variation of the BD IMF with star-forming
conditions. The BD IMF has a power-law index alpha = +0.3 and about two BDs
form per 10 low-mass stars assuming equal-mass pairing of BDs.Comment: 7 pages, 5 figures. Updated to match published versio
The mean surface density of companions in a stellar-dynamical context
Applying the mean surface density of companions, Sigma(r), to the dynamical
evolution of star clusters is an interesting approach to quantifying structural
changes in a cluster. It has the advantage that the entire density structure,
ranging from the closest binary separations, over the core-halo structure
through to the density distribution in moving groups that originate from
clusters, can be analysed coherently as one function of the stellar separations
r.
This contribution assesses the evolution of Sigma(r) for clusters with
different initial densities and binary populations. The changes in the binary,
cluster and halo branches as the clusters evolve are documented using direct
N-body calculations, and are correlated with the cluster core and half-mass
radius. The location of breaks in the slope of Sigma(r) and the possible
occurrence of a binary gap can be used to infer dynamical cluster properties.Comment: 12 pages including 7 figures, accepted for publication in A&
On the mass function of star clusters
Clusters that form in total 10^3 < N < 10^5 stars (type II clusters) lose
their gas within a dynamical time as a result of the photo-ionising flux from O
stars. Sparser (type I) clusters get rid of their residual gas on a timescale
longer or comparable to the nominal crossing time and thus evolve approximately
adiabatically. This is also true for massive embedded clusters (type III) for
which the velocity dispersion is larger than the sound speed of the ionised
gas. On expelling their residual gas, type I and III clusters are therefore
expected to lose a smaller fraction of their stellar component than type II
clusters. We outline the effect this has on the transformation of the mass
function of embedded clusters (ECMF), which is directly related to the mass
function of star-cluster-forming molecular cloud cores, to the ``initial'' MF
of bound gas-free star clusters (ICMF). The resulting ICMF has, for a
featureless power-law ECMF, a turnover near 10^{4.5} Msun and a peak near 10^3
Msun. The peak lies around the initial masses of the Hyades, Praesepe and
Pleiades clusters. We also find that the entire Galactic population II stellar
spheroid can be generated if star formation proceeded via embedded clusters
distributed like a power-law MF with exponent 0.9 < beta < 2.6.Comment: 10 pages, 4 figures, accepted by MNRAS, small adjustments for
consistency with published versio
Limits on the primordial stellar multiplicity
Most stars - especially young stars - are observed to be in multiple systems.
Dynamical evolution is unable to pair stars efficiently, which leads to the
conclusion that star-forming cores must usually fragment into \geq 2 stars.
However, the dynamical decay of systems with \geq 3 or 4 stars would result in
a large single-star population that is not seen in the young stellar
population. Additionally, ejections would produce a significant population of
hard binaries that are not observed. This leads to a strong constraint on star
formation theories that cores must typically produce only 2 or 3 stars. This
conclusion is in sharp disagreement with the results of currently available
numerical simulations that follow the fragmentation of molecular cores and
typically predict the formation of 5--10 seeds per core. In addition, open
cluster remnants may account for the majority of observed highly hierarchical
higher-order multiple systems in the field.Comment: A&A in press, 5 pages (no figures
Constraints on Stellar-Dynamical Models of the Orion Nebula Cluster
The results obtained by Kroupa, Petr & McCaughrean (1999) for specific models
of young compact binary-rich clusters are generalised using dynamical scaling
relations, to infer the candidate set of possible birth models leading to the
Orion Nebula Cluster (ONC), of which the Trapezium Cluster is the core. It is
found that candidate sets of solutions exist which allow the ONC to be in
virial equilibrium, expanding or contracting. The range of possible solutions
is quite narrow.
These results will serve as guidelines for future, CPU-intensive calculations
of the stellar-dynamical and astrophysical evolution of the entire ONC. These,
in turn, will be essential to quantify observables that will ultimately
discriminate between models, thus allowing us to understand if the ONC is in
the process of assembling a rich Galactic cluster, and, if this is the case,
how it occurs.Comment: 17 pages, 7 figures, New Astronomy, in pres
On the variation of the Initial Mass Function
(shortened) In this contribution an average or Galactic-field IMF is defined,
stressing that there is evidence for a change in the power-law index at only
two masses: near 0.5 Msun and 0.08 Msun. Using this supposed universal IMF, the
uncertainty inherent to any observational estimate of the IMF is investigated,
by studying the scatter introduced by Poisson noise and the dynamical evolution
of star clusters. It is found that this apparent scatter reproduces quite well
the observed scatter in power-law index determinations, thus defining the
fundamental limit within which any true variation becomes undetectable.
Determinations of the power-law indices alpha are subject to systematic errors
arising mostly from unresolved binaries. The systematic bias is quantified
here, with the result that the single-star IMFs for young star-clusters are
systematically steeper by d_alpha=0.5 between 0.1 and 1 Msun than the
Galactic-field IMF, which is populated by, on average, about 5 Gyr old stars.
The MFs in globular clusters appear to be, on average, systematically flatter
than the Galactic-field IMF, and the recent detection of ancient white-dwarf
candidates in the Galactic halo and absence of associated low-mass stars
suggests a radically different IMF for this ancient population. Star-formation
in higher-metallicity environments thus appears to produce relatively more
low-mass stars.Comment: MNRAS, in press; 34 pages, 14 figures (figs.1 and 14 in colour);
repl.vers: adjustments for consistency with published versio
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