759 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
The First Measurement of Spectral Lines in a Short-Period Star Bound to the Galaxy's Central Black Hole: A Paradox of Youth
We have obtained the first detection of spectral absorption lines in one of
the high-velocity stars in the vicinity of the Galaxy's central supermassive
black hole. Both Brgamma (2.1661 micron) and He I (2.1126 micron) are seen in
absorption in S0-2 with equivalent widths (2.8+-0.3 Ang & 1.7+-0.4 Ang) and an
inferred stellar rotational velocity (220+-40 km/s) that are consistent with
that of an O8-B0 dwarf, which suggests that it is a massive (~15 Msun), young
(<10 Myr) main sequence star. This presents a major challenge to star formation
theories, given the strong tidal forces that prevail over all distances reached
by S0-2 in its current orbit (130 - 1900 AU) and the difficulty in migrating
this star inward during its lifetime from further out where tidal forces should
no longer preclude star formation. The radial velocity measurements (-510+-40
km/s) and our reported proper motions for S0-2 strongly constrain its orbit,
providing a direct measure of the black hole mass of 4.1(+-0.6)x10^6(Ro/8kpc)^3
Msun. The Keplerian orbit parameters have uncertainities that are reduced by a
factor of 2-3 compared to previously reported values and include, for the first
time, an independent solution for the dynamical center; this location, while
consistent with the nominal infrared position of Sgr A*, is localized to a
factor of 5 more precisely (+-2 milli-arcsec). Furthermore, the ambiguity in
the inclination of the orbit is resolved with the addition of the radial
velocity measurement, indicating that the star is behind the black hole at the
time of closest approach and counter-revolving against the Galaxy. With further
radial velocity measurements in the next few years, the orbit of S0-2 will
provide the most robust estimate of the distance to the Galactic Center.Comment: 14 pages, Latex, Accepted for Publication in ApJ Letter
Spatially resolved mid-infrared observations of the triple system T Tauri
To enhance our knowledge of the characteristics and distribution of the
circumstellar dust associated with the individual components of the young
hierarchical triple system T Tau, observations in the N-band with MIDI at the
VLTI were performed. Our study is based on both the interferometric and the
spectrophotometric measurements and is supplemented by new visual and infrared
photometry. Also, the phases were investigated to determine the dominating
mid-infrared source in the close southern binary. The data were fit with the
help of a sophisticated physical disc model. This model utilises the radiative
transfer code MC3D that is based on the Monte-Carlo method. Extended
mid-infrared emission is found around all three components of the system.
Simultaneous fits to the photometric and interferometric data confirm the
picture of an almost face-on circumstellar disc around T Tau N. Towards this
star, the silicate band is seen in emission. This emission feature is used to
model the dust content of the circumstellar disc. Clear signs of dust
processing are found. Towards T Tau S, the silicate band is seen in absorption.
This absorption is strongly pronounced towards the infrared companion T Tau Sa
as can be seen from the first individual N-band spectra for the two southern
components. Our fits support the previous suggestion that an almost edge-on
disc is present around T Tau Sa. This disc is thus misaligned with respect to
the circumstellar disc around T Tau N. The interferometric data indicate that
the disc around T Tau Sa is oriented in the north-south direction, which
favours this source as launching site for the east-western jet. We further
determine from the interferometric data the relative positions of the
components of the southern binary.Comment: 24 pages, 19 figures, accepted for publication in A&
Asymptotic models for the generation of internal waves by a moving ship, and the dead-water phenomenon
This paper deals with the dead-water phenomenon, which occurs when a ship
sails in a stratified fluid, and experiences an important drag due to waves
below the surface. More generally, we study the generation of internal waves by
a disturbance moving at constant speed on top of two layers of fluids of
different densities. Starting from the full Euler equations, we present several
nonlinear asymptotic models, in the long wave regime. These models are
rigorously justified by consistency or convergence results. A careful
theoretical and numerical analysis is then provided, in order to predict the
behavior of the flow and in which situations the dead-water effect appears.Comment: To appear in Nonlinearit
Beating Your Fractional Beatty Game Opponent and: Whatâs the Question to Your Answer?
Influence of grape juice extraction methods on basic analytical parameters
Currently, for monitoring the ripening of grape berries, different devices are used to produce the juices to be analysed. Crushing the berries is a key step that determines the quantity of juice extracted and may impact it composition. The effect of different devices on analytical parameters of the musts produced were compared in this study. Samples from four grape varieties ('Cabernet-Sauvignon', 'EkigaĂŻna', 'Marselan' and 'Vermentino'), showing a variability of berry size and precocity, were crushed using six different devices (ASieves, Bag mixerÂź, Crusher, Manual, TPress and Blender). Whatever the pressing equipment, sugar concentrations of the must were not modified by the extraction method, unlike other parameters. pH and titratable acidity were slightly impacted by the crushing method without changing the ranking of the varieties. However, potassium concentrations were more impacted by the pressing method. Differences in mechanical forces applied to skins and seeds according to the pressing equipment used may release more or less potassium. This study clearly discarded a complete grinding of the samples for grape ripening monitoring: this method strongly modified the potassium content and, consequently, the pH and the titratable acidity of the musts
Geometrical and Physical Properties of Circumbinary Discs in Eccentric Stellar Binaries
In a previous work (Pichardo et al. 2005), we studied stable configurations
for circumstellar discs in eccentric binary systems. We searched for "invariant
loops": closed curves (analogous to stable periodic orbits in time-independent
potentials) that change shape with the binary orbital phase, as test particles
in them move under the influence of the binary potential. This approach allows
us to identify stable configurations when pressure forces are unimportant, and
dissipation acts only to prevent gas clouds from colliding with one another. We
now extend this work to study the main geometrical properties of circumbinary
discs. We have studied more than 100 cases with a range in eccentricity 0 .le.
e .le. 0.9, and mass ratio 0.1 .le. q .le. 0.9. Although gas dynamics may
impose further restrictions, our study sets lower stable bounds for the size of
the central hole in a simple and computationally cheap way, with a relation
that depends on the eccentricity and mass ratio of the central binary. We
extend our previous studies and focus on an important component of these
systems: circumbinary discs. The radii for stable orbits that can host gas in
circumbinary discs are sharply constrained as a function of the binary's
eccentricity. The circumbinary disc configurations are almost circular, with
eccentricity e_d < 0.15, but if the mass ratio is unequal the disk is offset
from the center of mass of the system. We compare our results with other
models, and with observations of specific systems like GG Tauri A, UY Aurigae,
HD 98800 B, and Fomalhaut, restricting the plausible parameters for the binary.Comment: 12 pages, 10 figures and 6 tables. MNRAS, accepte
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