1,155 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
Disc orientations in pre-main-sequence multiple systems. A study in southern star formation regions
Classical T Tauri stars are encircled by accretion discs most of the time
unresolved by conventional imaging observation. However, numerical simulations
show that unresolved aperture linear polarimetry can be used to extract
information about the geometry of the immediate circumstellar medium that
scatter the starlight. Monin, Menard & Duchene (1998) previously suggested that
polarimetry can be used to trace the relative orientation of discs in young
binary systems in order to shed light on the stellar and planet formation
process. In this paper, we report on new VLT/FORS1 optical linear polarisation
measurements of 23 southern binaries spanning a range of separation from 0.8''
to 10''. In each field, the polarisation of the central binary is extracted, as
well as the polarisation of nearby stars in order to estimate the local
interstellar polarisation. We find that, in general, the linear polarisation
vectors of individual components in binary systems tend to be parallel to each
other. The amplitude of their polarisations are also correlated. These findings
are in agreement with our previous work and extend the trend to smaller
separations. They are also similar to other studies, e.g., Donar et al. 1999;
Jensen et al. 2000, 2004; Wolf et al. 2001. However, we also find a few systems
showing large differences in polarisation level, possibly indicating different
inclinations to the line-of-sight for their discs.Comment: 13 pages, 11 figures, accepted in Astronomy and Astrophysics.
accepted in Astronomy and Astrophysics (A&A
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
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
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
Binary Stars in the Orion Nebula Cluster
We report on a high-spatial-resolution survey for binary stars in the
periphery of the Orion Nebula Cluster, at 5 - 15 arcmin (0.65 - 2 pc) from the
cluster center. We observed 228 stars with adaptive optics systems, in order to
find companions at separations of 0.13" - 1.12" (60 - 500 AU), and detected 13
new binaries. Combined with the results of Petr (1998), we have a sample of 275
objects, about half of which have masses from the literature and high
probabilities to be cluster members. We used an improved method to derive the
completeness limits of the observations, which takes into account the elongated
point spread function of stars at relatively large distances from the adaptive
optics guide star. The multiplicity of stars with masses >2 M_sun is found to
be significantly larger than that of low-mass stars. The companion star
frequency of low-mass stars is comparable to that of main-sequence M-dwarfs,
less than half that of solar-type main-sequence stars, and 3.5 to 5 times lower
than in the Taurus-Auriga and Scorpius-Centaurus star-forming regions. We find
the binary frequency of low-mass stars in the periphery of the cluster to be
the same or only slightly higher than for stars in the cluster core (<3 arcmin
from theta1C Ori). This is in contrast to the prediction of the theory that the
low binary frequency in the cluster is caused by the disruption of binaries due
to dynamical interactions. There are two ways out of this dilemma: Either the
initial binary frequency in the Orion Nebula Cluster was lower than in
Taurus-Auriga, or the Orion Nebula Cluster was originally much denser and
dynamically more active.Comment: 20 page
A fourth component in the young multiple system V 773 Tau
I report on a new component in the pre-main sequence multiple system V 773
Tauri. This second visual companion, V 773 Tau C, with a projected separation
of about 0."2 has been detected using speckle interferometry in the
near-infrared. Repeated observations from 1996 to 2002 show significant orbital
motion and thus confirm the character of the new companion as a gravitationally
bound star. Together with the two components of the spectroscopic binary V 773
Tau A and the previously known visual companion V 773 Tau B, the V 773 Tau
system appears as a young "mini-cluster" of four T Tauri stars within a sphere
of a radius less than 100 AU. V 773 Tau, A, B and C form a triple system that
is not hierarchic, but is apparently stable despite of this. The brightness of
V 773 Tau C has probably increased over the last years, which may explain its
non-detection in previous binary surveys.Comment: Accepted by Astronomy and Astrophysics, 5 pages, 2 figure
Partizan Subtraction Games
Partizan subtraction games are combinatorial games where two players, say
Left and Right, alternately remove a number n of tokens from a heap of tokens,
with (resp. ) when it is Left's (resp. Right's) turn.
The first player unable to move loses. These games were introduced by Fraenkel
and Kotzig in 1987, where they introduced the notion of dominance, i.e. an
asymptotic behavior of the outcome sequence where Left always wins if the heap
is sufficiently large. In the current paper, we investigate the other kinds of
behaviors for the outcome sequence. In addition to dominance, three other
disjoint behaviors are defined, namely weak dominance, fairness and ultimate
impartiality. We consider the problem of computing this behavior with respect
to and , which is connected to the well-known Frobenius coin
problem. General results are given, together with arithmetic and geometric
characterizations when the sets and have size at most 2
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