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The second stars
Numerical simulations have shown that the first stars
to form in the Universe were very massive, with typical masses 10-100 times greater than the mass of the
Sun. However, we know from observations of old stars
in the Galactic halo that subsequent periods of star
formation produced much smaller stars, with typical
masses comparable to that of the Sun. It is theorized
that this change occurs as a result of the pollution of
the star-forming gas with heavy elements (“metals”)
and dust produced by the first supernovae. Two different scenarios have been proposed. In one, it is the additional cooling provided by atomic fine-structure lines
that allows the gas to fragment and to form low-mass
stars. In the other, this role is played by the dust.
Over the past few years, we have studied both of
these scenarios with the aid of high-resolution hydrodynamical (SPH) simulations. Despite our best efforts,
the importance of fine-structure cooling remains unclear: we find that in some circumstances it can promote fragmentation, but in others it is ineffective. To
properly understand its role, we must first develop a
much better understanding of the effects of the first
stars on their local environment. On the other hand,
the importance of dust cooling is clear: our results
show it to be highly effective, enabling the formation
of dense clusters of protostars even when the amount
of dust present in the gas is very small.
Mass determination for nearby open clusters
Based on a combined astrometric and photometric
membership analysis of 650 open clusters detected
in the all-sky catalogue ASCC-2.5, we have investigated the radial stellar density profile for 236 open
clusters within a few kiloparsecs around the Sun. For
each cluster, the mass was estimated from the tidal
radius determined from a fitting of three-parameter
King profiles to the observed integrated density distribution (Fig. 5). Different samples of members were
investigated. The distributions of the determined core
Fig. 4: A snapshot from a smoothed particle hydrodynamical simulation
of the effects of dust cooling in metal-poor gas, 420 years after the
formation of the first star. Each yellow dot is a protostar.
and tidal radii peak at about 1.5 pc and 7-10 pc, respectively. A typical relative error of the core radius lies
between 15% and 50%, whereas, for the majority of
clusters, the tidal radius was determined with a relative
accuracy better than 20%. Most of the clusters have
tidal masses between 50 and 1,000 solar masses, and
for about half of the clusters, the masses were ob-
die Massenfunktion derjenigen Haufen, die jünger als ein bestimmtes Alter sind). Dabei ist es wichtig, Korrekturen bzgl.
der Vollständigkeit der Stichprobe anzubringen. Wir finden,
dass die Massenfunktion (dN/dM) offener Sternhaufen mittlerer und hoher Massen bei deren Geburt einem Potenzgesetz mit einem Exponenten von ungefähr -1,7 folgt.
tained with a relative error better than 50%. The determined masses can be used to derive the distribution
of the cluster masses (the mass distribution of clusters
younger than a certain age, respectively). For that purpose it is essential to correct the results for completeness. We find that the initial mass function (dN/dM)