60 research outputs found
The evolution of binary populations in cool, clumpy star clusters
Observations and theory suggest that star clusters can form in a subvirial
(cool) state and are highly substructured. Such initial conditions have been
proposed to explain the level of mass segregation in clusters through dynamics,
and have also been successful in explaining the origin of trapezium-like
systems. In this paper we investigate, using N-body simulations, whether such a
dynamical scenario is consistent with the observed binary properties in the
Orion Nebula Cluster (ONC). We find that several different primordial binary
populations are consistent with the overall fraction and separation
distribution of visual binaries in the ONC (in the range 67 - 670 au), and that
these binary systems are heavily processed. The substructured, cool-collapse
scenario requires a primordial binary fraction approaching 100 per cent. We
find that the most important factor in processing the primordial binaries is
the initial level of substructure; a highly substructured cluster processes up
to 20 per cent more systems than a less substructured cluster because of
localised pockets of high stellar density in the substructure. Binaries are
processed in the substructure before the cluster reaches its densest phase,
suggesting that even clusters remaining in virial equilibrium or undergoing
supervirial expansion would dynamically alter their primordial binary
population. Therefore even some expanding associations may not preserve their
primordial binary population.Comment: 12 pages, 7 figures; accepted for publication in MNRA
The dynamical evolution of very-low mass binaries in open clusters
Very low-mass binaries (VLMBs), with system masses <0.2 Msun appear to have
very different properties to stellar binaries. This has led to the suggestion
that VLMBs form a distinct and different population. As most stars are born in
clusters, dynamical evolution can significantly alter any initial binary
population, preferentially destroying wide binaries. In this paper we examine
the dynamical evolution of initially different VLMB distributions in clusters
to investigate how different the initial and final distributions can be.
We find that the majority of the observed VLMB systems, which have
separations <20 au, cannot be destroyed in even the densest clusters.
Therefore, the distribution of VLMBs with separations <20 au now must have been
the birth population (although we note that the observations of this population
may be very incomplete). Most VLMBs with separations >100 au can be destroyed
in high-density clusters, but are mainly unaffected in low-density clusters.
Therefore, the initial VLMB population must contain many more binaries with
these separations than now, or such systems must be made by capture during
cluster dissolution. M-dwarf binaries are processed in the same way as VLMBs
and so the difference in the current field populations either points to
fundamentally different birth populations, or significant observational
incompleteness in one or both samples.Comment: 11 pages, 10 figues, accepted for publication in MNRA
Spatial differences between stars and brown dwarfs: a dynamical origin?
We use -body simulations to compare the evolution of spatial distributions
of stars and brown dwarfs in young star-forming regions. We use three different
diagnostics; the ratio of stars to brown dwarfs as a function of distance from
the region's centre, , the local surface density of
stars compared to brown dwarfs, , and we compare the global
spatial distributions using the method. From a suite of
twenty initially statistically identical simulations, 6/20 attain
, indicating that dynamical interactions could be responsible for
observed differences in the spatial distributions of stars and brown dwarfs in
star-forming regions. However, many simulations also display apparently
contradictory results - for example, in some cases the brown dwarfs have much
lower local densities than stars (), but their global
spatial distributions are indistinguishable () and the
relative proportion of stars and brown dwarfs remains constant across the
region (). Our results suggest that extreme caution
should be exercised when interpreting any observed difference in the spatial
distribution of stars and brown dwarfs, and that a much larger observational
sample of regions/clusters (with complete mass functions) is necessary to
investigate whether or not brown dwarfs form through similar mechanisms to
stars.Comment: 7 pages, 5 figures, accepted for publication in MNRA
On the mass segregation of stars and brown dwarfs in Taurus
We use the new minimum spanning tree (MST) method to look for mass
segregation in the Taurus association. The method computes the ratio of MST
lengths of any chosen subset of objects, including the most massive stars and
brown dwarfs, to the MST lengths of random sets of stars and brown dwarfs in
the cluster. This mass segregation ratio (Lambda_MSR) enables a quantitative
measure of the spatial distribution of high-mass and low-mass stars, and brown
dwarfs to be made in Taurus.
We find that the most massive stars in Taurus are inversely mass segregated,
with Lambda_MSR = 0.70 +/- 0.10 (Lambda_MSR = 1 corresponds to no mass
segregation), which differs from the strong mass segregation signatures found
in more dense and massive clusters such as Orion. The brown dwarfs in Taurus
are not mass segregated, although we find evidence that some low-mass stars
are, with an Lambda_MSR = 1.25 +/- 0.15. Finally, we compare our results to
previous measures of the spatial distribution of stars and brown dwarfs in
Taurus, and briefly discuss their implications.Comment: 10 pages, 8 figures, accepted for publication in MNRA
Do binaries in clusters form in the same way as in the field?
We examine the dynamical destruction of binary systems in star clusters of
different densities. We find that at high densities (10^4 - 10^5 Msun pc^-3)
almost all binaries with separations > 10^3 AU are destroyed after a few
crossing times. At low densities (order(10^2) Msun pc^-3) many binaries with
separations > 10^3 AU are destroyed, and no binaries with separations > 10^4 AU
survive after a few crossing times. Therefore the binary separations in
clusters can be used as a tracer of the dynamical age and past density of a
cluster.
We argue that the central region of the Orion Nebula Cluster was around 100
times denser in the past with a half-mass radius of only 0.1 - 0.2 pc as (a) it
is expanding, (b) it has very few binaries with separations > 10^3 AU, and (c)
it is well-mixed and therefore dynamically old.
We also examine the origin of the field binary population. Binaries with
separations < 10^2 AU are not significantly modified in any cluster, therefore
at these separations the field reflects the sum of all star formation. Binaries
with separations in the range 10^2 - 10^4 AU are progressively more and more
heavily affected by dynamical disruption in increasingly dense clusters. If
most star formation is clustered, these binaries must be over-produced relative
to the field. Finally, no binary with a separation > 10^4 AU can survive in any
cluster and so must be produced by isolated star formation, but only if all
isolated star formation produces extremely wide binaries.Comment: 12 pages, 6 figures, accepted for publication in MNRA
Formation rates of star clusters in the hierarchical merging scenario
Stars form with a complex and highly structured distribution. For a smooth star cluster to form from these initial conditions, the star cluster must erase this substructure. We study how substructure is removed using N-body simulations that realistically handle two-body relaxation. In contrast to previous studies, we find that hierarchical cluster formation occurs chiefly as a result of scattering of stars out of clumps, and not through clump merging. Two-body relaxation, in particular within the body of a clump, can significantly increase the rate at which substructure is erased beyond that of clump merging alone. Hence the relaxation time of individual clumps is a key parameter controlling the rate at which smooth, spherical star clusters can form. The initial virial ratio of the clumps is an additional key parameter controlling the formation rate of a cluster. Reducing the initial virial ratio causes a star cluster to lose its substructure more rapidly
Binaries in the field: fossils of the star formation process?
Recent observations of binary stars in the Galactic field show that the binary fraction and the mean orbital separation both decrease as a function of decreasing primary mass. We present N-body simulations of the effects of dynamical evolution in star-forming regions on primordial binary stars to determine whether these observed trends can be explained by the dynamical processing of a common binary population. We find that dynamical processing of a binary population with an initial binary fraction of unity and an initial excess of intermediate/wide separation (100–104 au) binaries does not reproduce the observed properties in the field, even in initially dense (∼103 M⊙ pc−3) star-forming regions. If instead we adopt a field-like population as the initial conditions, most brown dwarf and M-dwarf binaries are dynamically hard and their overall fractions and separation distributions are unaffected by dynamical evolution. G-dwarf and A-star binaries in the field are dynamically intermediate in our simulated dense regions and dynamical processing does destroy some systems with separations >100 au. However, the formation of wide binaries through the dissolution of supervirial regions is a strong function of primary mass, and the wide G-dwarf and A-star binaries that are destroyed by dynamical evolution in subvirial regions are replenished by the formation of binaries in supervirial regions. We therefore suggest that the binary population in the field is indicative of the primordial binary population in star-forming regions, at least for systems with primary masses in the range 0.02–3.0 M⊙
Multi-messenger observations of a binary neutron star merger
On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40+8-8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Mo. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One- Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta
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