71 research outputs found
The physics and modes of star cluster formation: simulations
We review progress in numerical simulations of star cluster formation. These
simulations involve the bottom-up assembly of clusters through hierarchical
mergers, which produces a fractal stellar distribution at young (~0.5 Myr)
ages. The resulting clusters are predicted to be mildly aspherical and highly
mass-segregated, except in the immediate aftermath of mergers. The upper
initial mass function within individual clusters is generally somewhat flatter
than for the aggregate population. Recent work has begun to clarify the factors
that control the mean stellar mass in a star-forming cloud and also the
efficiency of star formation. The former is sensitive to the thermal properties
of the gas while the latter depends both on the magnetic field and the initial
degree of gravitational boundedness of the natal cloud. Unmagnetized clouds
that are initially bound undergo rapid collapse, which is difficult to reverse
by ionization feedback or stellar winds.Comment: 21 pages, 10 figures. To appear as invited review article in a
special issue of the Phil. Trans. Royal Soc. A: Ch. 3 "Star clusters as
tracers of galactic star-formation histories" (ed. R. de Grijs). Fully peer
reviewed. LaTeX, requires rspublic.cls style fil
A highly abnormal massive star mass function in the Orion Nebula cluster and the dynamical decay of trapezia systems
The ONC appears to be unusual on two grounds: The observed constellation of
the OB stars of the entire Orion Nebula cluster and its Trapezium at its centre
implies a time-scale problem given the age of the Trapezium, and an IMF problem
for the whole OB star population in the ONC. Given the estimated crossing time
of the Trapezium, it ought to have totally dynamically decayed by now.
Furthermore, by combining the lower limit of the ONC mass with a standard IMF
it emerges that the ONC should have formed at least about 40 stars heavier than
5 M_sun while only ten are observed. Using N-body experiments we (i) confirm
the expected instability of the trapezium and (ii) show that beginning with a
compact OB-star configuration of about 40 stars the number of observed OB stars
after 1 Myr within 1 pc radius and a compact trapezium configuration can both
be reproduced. These two empirical constraints thus support our estimate of 40
initial OB stars in the cluster. Interestingly, a more-evolved version of the
ONC resembles the Upper Scorpius OB association. The N-body experiments are
performed with the new C-code CATENA by integrating the equations of motion
using the chain-multiple-regularisation method. In addition, we present a new
numerical formulation of the initial mass function.Comment: Accepted by MNRAS, 11 pages, 5 figure
Constraints on the star formation histories of galaxies in the Local Cosmological Volume
IB is supported by an Alexander von Humboldt postdoctoral research fellowship. PK acknowledges support from the Grant Agency of the Czech Republic under grant number 20-21855S. This work benefited from the International Space Science Institute (ISSI/ISSI-BJ) in Bern and Beijing, thanks to the funding of the team âChemical abundances in the ISM: the litmus test of stellar IMF variations in galaxies across cosmic timeâ (Donatella Romano and Zhi-Yu Zhang).The majority of galaxies with current star formation rates (SFRs), SFRoâ„10â3Mâyrâ1â , in the Local Cosmological Volume, where observations should be reliable, have the property that their observed SFRo is larger than their average SFR. This is in tension with the evolution of galaxies described by delayed-Ï models, according to which the opposite would be expected. The tension is apparent in that local galaxies imply the star formation time-scale Ï â 6.7 Gyr, much longer than the 3.5â4.5 Gyr obtained using an empirically determined main sequence at several redshifts. Using models where the SFR is a power law in time of the form â(t â t1)η for t1 = 1.8 Gyr (with no stars forming prior to t1) implies that η = 0.18 ± 0.03. This suggested near-constancy of a galaxyâs SFR over time raises non-trivial problems for the evolution and formation time of galaxies, but is broadly consistent with the observed decreasing main sequence with increasing age of the Universe.Publisher PDFPeer reviewe
Top-heavy integrated galactic stellar initial mass functions (IGIMFs) in starbursts
Star formation rates (SFR) larger than 1000 Msun/ yr are observed in extreme
star bursts. This leads to the formation of star clusters with masses > 10^6
Msun in which crowding of the pre-stellar cores may lead to a change of the
stellar initial mass function (IMF). Indeed, the large mass-to-light ratios of
ultra-compact dwarf galaxies and recent results on globular clusters suggest
the IMF to become top-heavy with increasing star-forming density. We explore
the implications of top-heavy IMFs in these very massive and compact systems
for the integrated galactic initial mass function (IGIMF), which is the
galaxy-wide IMF, in dependence of the star-formation rate of galaxies. The
resulting IGIMFs can have slopes, alpha_3, for stars more massive than about 1
Msun between 1.5 and the Salpeter slope of 2.3 for an embedded cluster mass
function (ECMF) slope (beta) of 2.0, but only if the ECMF has no low-mass
clusters in galaxies with major starbursts. Alternatively, beta would have to
decrease with increasing SFR >10 Msun/ yr such that galaxies with major
starbursts have a top-heavy ECMF. The resulting IGIMFs are within the range of
observationally deduced IMF variations with redshift.Comment: Accepted for publication in MNRAS, reference adde
On the degree of stochastic asymmetry in the tidal tails of star clusters
Context: Tidal tails of star clusters are commonly understood to be populated
symmetrically. Recently, the analysis of Gaia data revealed large asymmetries
between the leading and trailing tidal tail arms of the four open star clusters
Hyades, Praesepe, Coma Berenices and NGC 752. Aims: As the evaporation of stars
from star clusters into the tidal tails is a stochastic process, the degree of
stochastic asymmetry is quantified in this work. Methods: For each star cluster
1000 configurations of test particles are integrated in the combined potential
of a Plummer sphere and the Galactic tidal field over the life time of the
particular star cluster. For each of the four star clusters the distribution
function of the stochastic asymmetry is determined and compared with the
observed asymmetry. Results: The probabilities for a stochastic origin of the
observed asymmetry of the four star clusters are: Praesepe ~1.7 sigma, Coma
Berenices ~2.4 sigma, Hyades ~6.7 sigma, NGC 752 ~1.6 sigma. Conclusions: In
the case of Praesepe, Coma Berenices and NGC 752 the observed asymmetry can be
interpreted as a stochastic evaporation event. However, for the formation of
the asymmetric tidal tails of the Hyades additional dynamical processes beyond
a pure statistical evaporation effect are required.Comment: accepted for publication by A&
Properties of hierarchically forming star clusters
We undertake a systematic analysis of the early (< 0.5 Myr) evolution of
clustering and the stellar initial mass function in turbulent fragmentation
simulations. These large scale simulations for the first time offer the
opportunity for a statistical analysis of IMF variations and correlations
between stellar properties and cluster richness. The typical evolutionary
scenario involves star formation in small-n clusters which then progressively
merge; the first stars to form are seeds of massive stars and achieve a
headstart in mass acquisition. These massive seeds end up in the cores of
clusters and a large fraction of new stars of lower mass is formed in the outer
parts of the clusters. The resulting clusters are therefore mass segregated at
an age of 0.5 Myr, although the signature of mass segregation is weakened
during mergers. We find that the resulting IMF has a smaller exponent
(alpha=1.8-2.2) than the Salpeter value (alpha=2.35). The IMFs in subclusters
are truncated at masses only somewhat larger than the most massive stars (which
depends on the richness of the cluster) and an universal upper mass limit of
150 Msun is ruled out. We also find that the simulations show signs of the
IGIMF effect proposed by Weidner & Kroupa, where the frequency of massive stars
is suppressed in the integrated IMF compared to the IMF in individual clusters.
We identify clusters through the use of a minimum spanning tree algorithm which
allows easy comparison between observational survey data and the predictions of
turbulent fragmentation models. In particular we present quantitative
predictions regarding properties such as cluster morphology, degree of mass
segregation, upper slope of the IMF and the relation between cluster richness
and maximum stellar mass. [abridged]Comment: 21 Pages, 25 Figure
Massive runaway stars in the Small Magellanic Cloud
Using archival Spitzer Space Telescope data, we identified for the first time
a dozen runaway OB stars in the Small Magellanic Cloud (SMC) through the
detection of their bow shocks. The geometry of detected bow shocks allows us to
infer the direction of motion of the associated stars and to determine their
possible parent clusters and associations. One of the identified runaway stars,
AzV 471, was already known as a high-velocity star on the basis of its high
peculiar radial velocity, which is offset by ~40 km/s from the local systemic
velocity. We discuss implications of our findings for the problem of the origin
of field OB stars. Several of the bow shock-producing stars are found in the
confines of associations, suggesting that these may be "alien" stars
contributing to the age spread observed for some young stellar systems. We also
report the discovery of a kidney-shaped nebula attached to the early WN-type
star SMC-WR3 (AzV 60a). We interpreted this nebula as an interstellar structure
created owing to the interaction between the stellar wind and the ambient
interstellar medium.Comment: Accepted by A&
Massive runaway stars in the Large Magellanic Cloud
The origin of massive field stars in the Large Magellanic Cloud (LMC) has
long been an enigma. The recent measurements of large offsets (~100 km/s)
between the heliocentric radial velocities of some very massive (O2-type) field
stars and the systemic LMC velocity provides a possible explanation of this
enigma and suggests that the field stars are runaway stars ejected from their
birth places at the very beginning of their parent cluster's dynamical
evolution. A straightforward way to prove this explanation is to measure the
proper motions of the field stars and to show that they are moving away from
one of the nearby star clusters or OB associations. This approach however is
complicated by the large distance to the LMC, which makes accurate proper
motion measurements difficult. We use an alternative approach for solving the
problem, based on the search for bow shocks produced by runaway stars. The
geometry of detected bow shocks would allow us to infer the direction of
stellar motion and thereby to determine their possible parent clusters. In this
paper we present the results of a search for bow shocks around six massive
field stars which were suggested in the literature as candidate runaway stars.
Using archival (Spitzer Space Telescope) data, we found a bow shock associated
with one of our program stars, the O2 V((f*)) star BI 237, which is the
first-ever detection of bow shocks in the LMC. Orientation of the bow shock
suggests that BI 237 was ejected from the OB association LH 82 (located at ~120
pc in projection from the star). A by-product of our search is the detection of
bow shocks generated by four OB stars in the field of the LMC and an arc-like
structure attached to the candidate luminous blue variable R81 (HD 269128). The
geometry of two of these bow shocks is consistent with the possibility that
their associated stars were ejected from the 30 Doradus star forming complex.Comment: 5 pages, 7 figures, accepted for publication in A&
Maximum stellar mass versus cluster membership number revisited
We have made a new compilation of observations of maximum stellar mass versus
cluster membership number from the literature, which we analyse for consistency
with the predictions of a simple random drawing hypothesis for stellar mass
selection in clusters. Previously, Weidner and Kroupa have suggested that the
maximum stellar mass is lower, in low mass clusters, than would be expected on
the basis of random drawing, and have pointed out that this could have
important implications for steepening the integrated initial mass function of
the Galaxy (the IGIMF) at high masses. Our compilation demonstrates how the
observed distribution in the plane of maximum stellar mass versus membership
number is affected by the method of target selection; in particular, rather low
n clusters with large maximum stellar masses are abundant in observational
datasets that specifically seek clusters in the environs of high mass stars.
Although we do not consider our compilation to be either complete or unbiased,
we discuss the method by which such data should be statistically analysed. Our
very provisional conclusion is that the data is not indicating any striking
deviation from the expectations of random drawing.Comment: 7 pages, 3 Figures; accepted by MNRAS; Reference added
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