575 research outputs found
On the stratified dust distribution of the GG Tau circumbinary ring
Our objective is to study the vertical dust distribution in the circumbinary
ring of the binary system GG Tau and to search for evidence of stratification,
one of the first steps expected to occur during planet formation.
We present a simultaneous analysis of four scattered light images spanning a
range of wavelength from 800 nm to 3800 nm and compare them with (i) a
parametric prescription for the vertical dust stratification, and (ii) with the
results of SPH bi-fluid hydrodynamic calculations.
The parametric prescription and hydrodynamical calculations of stratification
both reproduce the observed brightness profiles well. These models also provide
a correct match for the observed star/ring integrated flux ratio. Another
solution with a well-mixed, but ``exotic'', dust size distribution also matches
the brightness profile ratios but fails to match the star/ring flux ratio.
These results give support to the presence of vertical stratification of the
dust in the ring of GG Tau and further predict the presence of a radial
stratification also.Comment: 9 pages, 11 figures. Accepted for publication in A&
On the alignment of T Tauri stars with the local magnetic field
Magnetic field is believed to play an important role in the collapse of a
molecular cloud. In particular, due to the properties of magnetic forces,
collapse should be easier along magnetic field lines, as supported by the
large-scale sheet-like structure of the Taurus giant molecular cloud for
instance. Here we investigate whether such a prefered orientation for collapse
is present at a much smaller scale, that of individual objects. We use recent
high-angular resolution images of T Tauri stars located in the Taurus
star-forming region to find the orientation of the symmetry axis of each
star+jet+disk system and compare it to that of the local magnetic field. We
find that i) the orientations of the symmetry axis of T Tauri stars are not
random with respect to the magnetic field, and ii) that young stars that are
associated to a jet or an outflow are oriented very differently from those
which do not have a detected outflow. We present some implications of this
puzzling new result.Comment: 8 pages, 2 figures, to appear in the conference proceedings of "Open
Issues in Local Star Formation and Early Stellar Evolution", held in Ouro
Preto (Brazil), April 5-10, 200
The Puzzling Mutual Orbit of the Binary Trojan Asteroid (624) Hektor
Asteroids with satellites are natural laboratories to constrain the formation
and evolution of our solar system. The binary Trojan asteroid (624) Hektor is
the only known Trojan asteroid to possess a small satellite. Based on W.M. Keck
adaptive optics observations, we found a unique and stable orbital solution,
which is uncommon in comparison to the orbits of other large multiple asteroid
systems studied so far. From lightcurve observations recorded since 1957, we
showed that because the large Req=125-km primary may be made of two joint
lobes, the moon could be ejecta of the low-velocity encounter, which formed the
system. The inferred density of Hektor's system is comparable to the L5 Trojan
doublet (617) Patroclus but due to their difference in physical properties and
in reflectance spectra, both captured Trojan asteroids could have a different
composition and origin.Comment: 13 pages, 3 figures, 2 table
Planets in binary systems: is the present configuration indicative of the formation process?
The present dynamical configuration of planets in binary star systems may not
reflect their formation process since the binary orbit may have changed in the
past after the planet formation process was completed. An observed binary
system may have been part of a former hierarchical triple that became unstable
after the planets completed their growth around the primary star.
Alternatively, in a dense stellar environment even a single stellar encounter
between the star pair and a singleton may singificantly alter the binary orbit.
In both cases the planets we observe at present would have formed when the
dynamical environment was different from the presently observed one.
We have numerically integrated the trajectories of the stars (binary plus
singleton) and of test planets to investigate the abovementioned mechanisms.
Our simulations show that the circumstellar environment during planetary
formation around the primary was gravitationally less perturbed when the binary
was part of a hierarchical triple because the binary was necessarely wider and,
possibly, less eccentric. This circumstance has consequences for the planetary
system in terms of orbital spacing, eccentricity, and mass of the individual
planets. Even in the case of a single stellar encounter the present appearance
of a planetary system in a binary may significantly differ from what it had
while planet formation was ongoing. However, while in the case of instability
of a triple the trend is always towards a tighter and more eccentric binary
system, when a single stellar encounter affects the system the orbit of the
binary can become wider and be circularized.Comment: 5 pages, 5 figures Accepted for publication on A&
Accurate stellar masses in the multiple system T Tau
The goal of this study is to obtain accurate estimates for the individual
masses of the components of the tight binary system T Tau S in order to settle
the ongoing debate on the nature of T Tau Sa, a so-called infrared companion.
We take advantage of the fact that T Tau S belongs to a triple system composed
of two hierarchical orbits to simultaneously analyze the motion of T Tau Sb in
the rest frames of T Tau Sa and T Tau N. With this method, it is possible to
pinpoint the location of the center of mass of T Tau S and, thereby, to
determine individual masses for T Tau Sa and T Tau Sb with no prior assumption
about the mass/flux ratio of the system. This improvement over previous studies
of the system results in much better constraints on orbital parameters. We find
individual masses of 2.73+/-0.31 Msun for T Tau Sa and of 0.61+/-0.17 Msun for
T Tau Sb (in agreement with its early-M spectral type), including the
uncertainty on the distance to the system. These are among the most precise
estimates of the mass of any Pre-Main Sequence star, a remarkable result since
this is the first system in which individual masses of T Tauri stars can be
determined from astrometry only. This model-independent analysis confirms that
T Tau Sa is an intermediate-mass star, presumably a very young Herbig Ae star,
that may possess an almost edge-on disk.Comment: 5 pages, 1 figure, accepted for publication in Astronomy &
Astrophysics Letter
High-spin structures of 88Kr and 89Rb: Evolution from collective to single-particle behaviors
The high-spin states of the two neutron-rich nuclei, 88Kr and 89R have been
studied from the 18O + 208Pb fusion-fission reaction. Their level schemes were
built from triple gamma-ray coincidence data and gamma-gamma angular
correlations were analyzed in order to assign spin and parity values to most of
the observed states. The two levels schemes evolve from collective structures
to single-particle excitations as a function of the excitation energy.
Comparison with results of shell-model calculations gives the specific proton
and neutron configurations which are involved to generate the angular momentum
along the yrast lines.Comment: 12 pages, 9 figures, Physical Review C (2013) in pres
Analysis of the Herschel DEBRIS Sun-like star sample
This paper presents a study of circumstellar debris around Sun-like stars using data from the Herschel DEBRIS Key Programme. DEBRIS is an unbiased survey comprising the nearest âŒ90 stars of each spectral type A-M. Analysis of the 275 F-K stars shows that excess emission from a debris disc was detected around 47 stars, giving a detection rate of 17.1
+2.6â2.3
âper cent, with lower rates for later spectral types. For each target a blackbody spectrum was fitted to the dust emission to determine its fractional luminosity and temperature. The derived underlying distribution of fractional luminosity versus blackbody radius in the population showed that most detected discs are concentrated at f ⌠10â5 and at temperatures corresponding to blackbody radii 7â40 au, which scales to âŒ40âau for realistic dust properties (similar to the current Kuiper belt). Two outlying populations are also evident; five stars have exceptionally bright emission (âf > 5 Ă 10â5), and one has unusually hot dust <4âau. The excess emission distributions at all wavelengths were fitted with a steady-state evolution model, showing that these are compatible with all stars being born with a narrow belt that then undergoes collisional grinding. However, the model cannot explain the hot dust systems â likely originating in transient events â and bright emission systems â arising potentially from atypically massive discs or recent stirring. The emission from the present-day Kuiper belt is predicted to be close to the median of the population, suggesting that half of stars have either depleted their Kuiper belts (similar to the Solar system) or had a lower planetesimal formation efficiency.This work was supported by the European Union through European Research Council grant number 279973 (MCW, GMK). GMK was also supported by the Royal Society as a Royal Society University Research Fellow
High-spin structures of 136Cs
Odd-odd 136Cs nuclei have been produced in the 18O + 208Pb and 12C + 238U
fusion-fission reactions and their gamma rays studied with the Euroball array.
The high-spin level scheme has been built up to ~ 4.7 MeV excitation energy and
spin I ~ 16 hbar from the triple gamma-ray coincidence data. The configurations
of the three structures observed above ~ 2 MeV excitation energy are first
discussed by analogy with the proton excitations identified in the semi-magic
137Cs nucleus, which involve the three high-j orbits lying above the Z=50 gap,
pi g_{7/2}, pi d_{5/2} and pi h_{11/2}. This is confirmed by the results of
shell-model calculations performed in this work.Comment: 6 pages, 4 figures, 3 table
No Fossil Disk in the T Tauri Multiple System V773 Tau
We present new multi-epoch near-infrared and optical high-angular images of
the V773 Tau pre-main sequence triple system, a weak-line T Tauri (WTTS) system
in which the presence of an evolved, ``fossil'' protoplanetary disk has been
inferred on the basis of a significant infrared excess. Our images reveal a
fourth object bound to the system, V773 Tau D. While it is much fainter than
all other components at 2 micron, it is the brightest source in the system at
4.7 micron. We also present medium-resolution K band adaptive optics
spectroscopy of this object, which is featureless with the exception of a weak
Br gamma emission line. Based on this spectrum and on the spectral energy
distribution of the system, we show that V773 Tau D is another member of the
small class of ``infrared companions'' (IRCs) to T Tauri stars. It is the least
luminous, and probably the least massive, component of the system, as opposed
to most other IRCs, which suggests that numerous low-luminosity IRCs such as
V773 Tau D may still remain to be discovered. Furthermore, it is the source of
the strong IR excess in the system. We therefore reject the interpretation of
this excess as the signature of a fossil (or ``passive'') disk and further
suggest that these systems may be much less frequent than previously thought.
We further show that V773 Tau C is a variable classical T Tauri star (CTTS)
and that its motion provides a well constrained orbital model. We show that
V773 Tau D can be dynamically stable within this quadruple system if its orbit
is highly inclined. Finally, V773 Tau is the first multiple system to display
such a variety of evolutionary states (WTTS, CTTS, IRC), which may be the
consequence of the strong star-star interactions in this compact quadruple
system.Comment: Accepted for publication in Astrophysical Journal, 29 pages, 2
tables, 5 figure
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