907 research outputs found
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
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
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
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
Variable accretion as a mechanism for brightness variations in T Tau S
(Note: this is a shortened version of the original A&A-style structured
abstract). The physical nature of the strong photometric variability of T Tau
Sa, the more massive member of the Southern "infrared companion" to T Tau, has
long been debated. Intrinsic luminosity variations due to variable accretion
were originally proposed but later challenged in favor of apparent fluctuations
due to time-variable foreground extinction. In this paper we use the timescale
of the variability as a diagnostic for the underlying physical mechanism.
Because the IR emission emerging from Sa is dominantly thermal emission from
circumstellar dust at <=1500K, we can derive a minimum size of the region
responsible for the time-variable emission. In the context of the variable
foreground extinction scenario, this region must be (un-) covered within the
variability timescale, which implies a minimum velocity for the obscuring
foreground material. If this velocity supercedes the local Kepler velocity we
can reject foreground extinction as a valid variability mechanism. The variable
accretion scenario allows for shorter variability timescales since the
variations in luminosity occur on much smaller scales, essentially at the
surface of the star, and the disk surface can react almost instantly on the
changing irradiation with a higher or lower dust temperature and according
brightness. We have detected substantial variations at long wavelengths in T
Tau S: +26% within four days at 12.8 micron. We show that this short-term
variability cannot be due to variable extinction and instead must be due to
variable accretion. Using a radiative transfer model of the Sa disk we show
that variable accretion can in principle also account for the much larger
(several magnitude) variations observed on timescales of several years. For the
long-term variability, however, also variable foreground extinction is a viable
mechanism.Comment: 15 pages, 8 figures, Accepted for publication in Astronomy and
Astrophysic
- âŠ