902 research outputs found
Binaries in star clusters and the origin of the field stellar population
Many, possibly most, stars form in binary and higher-order multiple systems.
Therefore, the properties and frequency of binary systems provide strong clues
to the star-formation process, and constraints on star-formation models.
However, the majority of stars also form in star clusters in which the birth
binary properties and frequency can be altered rapidly by dynamical processing.
Thus, we almost never see the birth population, which makes it very difficult
to know if star formation (as traced by binaries, at least) is universal, or if
it depends on environment. In addition, the field population consists of a
mixture of systems from different clusters which have all been processed in
different ways.Comment: 16 pages, no figures. To appear as invited review article in a
special issue of the Phil. Trans. Royal Soc. A: Ch. 8 "Star clusters as
tracers of galactic star-formation histories" (ed. R. de Grijs). Fully peer
reviewed. LaTeX, requires rspublic.cls style fil
Multiple protostellar systems. II. A high resolution near-infrared imaging survey in nearby star-forming regions
(abridged) Our project endeavors to obtain a robust view of multiplicity
among embedded Class I and Flat Spectrum protostars in a wide array of nearby
molecular clouds to disentangle ``universal'' from cloud-dependent processes.
We have used near-infrared adaptive optics observations at the VLT through the
H, Ks and L' filters to search for tight companions to 45 Class I and Flat
Spectrum protostars located in 4 different molecular clouds (Taurus-Auriga,
Ophiuchus, Serpens and L1641 in Orion). We complemented these observations with
published high-resolution surveys of 13 additional objects in Taurus and
Ophiuchus. We found multiplicity rates of 32+/-6% and 47+/-8% over the 45-1400
AU and 14-1400 AU separation ranges, respectively. These rates are in excellent
agreement with those previously found among T Tauri stars in Taurus and
Ophiuchus, and represent an excess of a factor ~1.7 over the multiplicity rate
of solar-type field stars. We found no non-hierarchical triple systems, nor any
quadruple or higher-order systems. No significant cloud-to-cloud difference has
been found, except for the fact that all companions to low-mass Orion
protostars are found within 100 AU of their primaries whereas companions found
in other clouds span the whole range probed here. Based on this survey, we
conclude that core fragmentation always yields a high initial multiplicity
rate, even in giant molecular clouds such as the Orion cloud or in clustered
stellar populations as in Serpens, in contrast with predictions of numerical
simulations. The lower multiplicity rate observed in clustered Class II and
Class III populations can be accounted for by a universal set of properties for
young systems and subsequent ejections through close encounters with unrelated
cluster members.Comment: 15 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
Decoupled and unidirectional asymptotic models for the propagation of internal waves
We study the relevance of various scalar equations, such as inviscid
Burgers', Korteweg-de Vries (KdV), extended KdV, and higher order equations (of
Camassa-Holm type), as asymptotic models for the propagation of internal waves
in a two-fluid system. These scalar evolution equations may be justified with
two approaches. The first method consists in approximating the flow with two
decoupled, counterpropagating waves, each one satisfying such an equation. One
also recovers homologous equations when focusing on a given direction of
propagation, and seeking unidirectional approximate solutions. This second
justification is more restrictive as for the admissible initial data, but
yields greater accuracy. Additionally, we present several new coupled
asymptotic models: a Green-Naghdi type model, its simplified version in the
so-called Camassa-Holm regime, and a weakly decoupled model. All of the models
are rigorously justified in the sense of consistency
Resolving The Moth at Millimeter Wavelengths
HD 61005, also known as "The Moth," is one of only a handful of debris disks
that exhibit swept-back "wings" thought to be caused by interaction with the
ambient interstellar medium (ISM). We present 1.3 mm Submillimeter Array (SMA)
observations of the debris disk around HD 61005 at a spatial resolution of 1.9
arcsec that resolve the emission from large grains for the first time. The disk
exhibits a double-peaked morphology at millimeter wavelengths, consistent with
an optically thin ring viewed close to edge-on. To investigate the disk
structure and the properties of the dust grains we simultaneously model the
spatially resolved 1.3 mm visibilities and the unresolved spectral energy
distribution. The temperatures indicated by the SED are consistent with
expected temperatures for grains close to the blowout size located at radii
commensurate with the millimeter and scattered light data. We also perform a
visibility-domain analysis of the spatial distribution of millimeter-wavelength
flux, incorporating constraints on the disk geometry from scattered light
imaging, and find suggestive evidence of wavelength-dependent structure. The
millimeter-wavelength emission apparently originates predominantly from the
thin ring component rather than tracing the "wings" observed in scattered
light. The implied segregation of large dust grains in the ring is consistent
with an ISM-driven origin for the scattered light wings.Comment: 10 pages, 6 figure
Discovery of the Fomalhaut C debris disc
Fomalhaut is one of the most interesting and well studied nearby stars,
hosting at least one planet, a spectacular debris ring, and two distant
low-mass stellar companions (TW PsA and LP 876-10, a.k.a. Fomalhaut B & C). We
observed both companions with Herschel, and while no disc was detected around
the secondary, TW PsA, we have discovered the second debris disc in the
Fomalhaut system, around LP 876-10. This detection is only the second case of
two debris discs seen in a multiple system, both of which are relatively wide
(3000 AU for HD 223352/40 and 158 kAU [0.77 pc] for Fomalhaut/LP
876-10). The disc is cool (24K) and relatively bright, with a fractional
luminosity , and represents the rare
observation of a debris disc around an M dwarf. Further work should attempt to
find if the presence of two discs in the Fomalhaut system is coincidental,
perhaps simply due to the relatively young system age of 440 Myr, or if the
stellar components have dynamically interacted and the system is even more
complex than it currently appears.Comment: Published in MNRAS Letters. Merry Xma
The initial period function of late-type binary stars and its variation
The variation of the period distribution function of late-type binaries is
studied. It is shown that the Taurus--Auriga pre-main sequence population and
the main sequence G dwarf sample do not stem from the same parent period
distribution with better than 95 per cent confidence probability. The Lupus,
Upper Scorpius A and Taurus--Auriga populations are shown to be compatible with
being drawn from the same initial period function (IPF), which is inconsistent
with the main sequence data. Two possible IPF forms are used to find parent
distributions to various permutations of the available data which include Upper
Scorpius B (UScB), Chameleon and Orion Nebula Cluster pre-main sequence
samples. All the pre-main sequence samples studied here are consistent with the
hypothesis that there exists a universal IPF which is modified through
binary-star disruption if it forms in an embedded star cluster leading to a
general decline of the observed period function with increasing period. The
pre-main sequence data admit a log-normal IPF similar to that arrived at by
Duquennoy & Mayor (1991) for main sequence stars, provided the binary fraction
among pre-main sequence stars is significantly higher. But, for consistency
with proto-stellar data, the possibly universal IPF ought to be flat in log-P
or log-semi-major axis and must be similar to the K1 IPF form derived through
inverse dynamical population synthesis, which has been shown to lead to the
main sequence period function if most stars form in typical embedded clusters.Comment: 13 pages, 8 figures, LaTeX, accepted by A&A, minor change to
reference lis
Multiple protostellar systems. I. A deep near infrared survey of Taurus and Ophiuchus protostellar objects
(Abridged) We performed a deep infrared imaging survey of 63 embedded young
stellar objects (YSOs) located in the Taurus and Ophiuchus clouds to search for
companions. The sample includes Class I and flat infrared spectrum protostellar
objects. We find 17 companions physically bound to 15 YSOs with angular
separations in the range 0.8-10" (110-1400 AU) and derive a companion star
fraction of 23+/-9 % and 29+/-7 % for embedded YSOs in Taurus and Ophiuchus,
respectively. In spite of different properties of the clouds and especially of
the prestellar cores, the fraction of wide companions, 27+/-6 % for the
combined sample, is identical in the two star-forming regions. This suggests
that the frequency and properties of wide multiple protostellar systems are not
very sensitive to specific initial conditions. Comparing the companion star
fraction of the youngest YSOs still surrounded by extended envelopes to that of
more evolved YSOs, we find evidence for a possible evolution of the fraction of
wide multiple systems, which seems to decrease by a factor of about 2 on a
timescale of about 10^5 yr. Somewhat contrary to model predictions, we do not
find evidence for a sub-clustering of embedded sources at this stage on a scale
of a few 100 AU that could be related to the formation of small-N protostellar
clusters. Possible interpretations for this discrepancy are discussed.Comment: Accepted for publication in Astronomy & Astrophysics; 16 pages, 3
embedded figures, 1 JPEG figur
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
The EU and Asia within an evolving global order: what is Europe? Where is Asia?
The papers in this special edition are a very small selection from those presented at the EU-NESCA (Network of European Studies Centres in Asia) conference on "the EU and East Asia within an Evolving Global Order: Ideas, Actors and Processes" in November 2008 in Brussels. The conference was the culmination of three years of research activity involving workshops and conferences bringing together scholars from both regions primarily to discuss relations between Europe and Asia, perceptions of Europe in Asia, and the relationship between the European regional project and emerging regional forms in Asia. But although this was the last of the three major conferences organised by the consortium, it in many ways represented a starting point rather than the end; an opportunity to reflect on the conclusions of the first phase of collaboration and point towards new and continuing research agendas for the future
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