870 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
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
A layered edge-on circumstellar disk around HK Tau B
We present the first high angular resolution 1.4mm and 2.7mm continuum maps
of the T Tauri binary system HK Tau obtained with the Plateau de Bure
Interferometer. The contributions of both components are well disentangled at
1.4mm and the star previously known to host an edge-on circumstellar disk, HK
Tau B, is elongated along the disk's major axis. The optically bright primary
dominates the thermal emission from the system at both wavelengths, confirming
that it also has its own circumstellar disk. Its non-detection in scattered
light images indicates that the two disks in this binary system are not
parallel. Our data further indicate that the circumprimary disk is probably
significantly smaller than the circumsecondary disk.
We model the millimeter thermal emission from the circumstellar disk
surrounding HK Tau B. We show that the disk mass derived from scattered light
images cannot reproduce the 1.4mm emission using opacities of the same
population of submicron dust grains. However, grain growth alone cannot match
all the observed properties of this disk. We propose that this disk contains
three separate layers: two thin outer surfaces which contain dust grains that
are very similar to those of the ISM, and a disk interior which is relatively
massive and/or has experienced limited grain growth with the largest grains
significantly smaller than 1mm. Such a structure could naturally result from
dust settling in a protoplanetary disk.Comment: Accepted fopr publication in A&A, 8 pages, 1 embedded figur
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
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
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
The formation and evolution of binary systems. III. Low-mass binaries in the Praesepe cluster
With the aim of investigating the binary population of the 700 Myr old
Praesepe cluster, we have observed 149 G and K-type cluster members using
adaptive optics. We detected 26 binary systems with an angular separation
ranging from less than 0.08 to 3.3 arcsec (15-600 AU). After correcting for
detection biases, we derive a binary frequency (BF) in the logP (days) range
from 4.4 to 6.9 of 25.3 +/- 5.4%, which is similar to that of field G-type
dwarfs (23.8%, Duquennoy & Mayor 1991). This result, complemented by similar
ones obtained for the 2 Myr old star forming cluster IC 348 (Paper II) and the
120 Myr old Pleiades open cluster (Paper I), indicates that the fraction of
long-period binaries does not significantly evolve over the lifetime of
galactic open clusters. We compare the distribution of cluster binaries to the
binary populations of star forming regions, most notably Orion and Taurus, to
critically review current ideas regarding the binary formation process. We
conclude that it is still unclear whether the lower binary fraction observed in
young clusters compared to T associations is purely the result of the early
dynamical disruption of primordial binaries in dense clusters or whether it
reflects intrinsically different modes of star formation in clusters and
associations. We also note that if Taurus binaries result from the dynamical
decay of small-N protostellar aggregates, one would predict the existence of a
yet to be found dispersed population of mostly single substellar objects in the
Taurus cloud.Comment: 10 pages, 3 figure
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