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 ∼90 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 +2.6−2.3  per 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 ∼ 10−5 and at temperatures corresponding to blackbody radii 7–40 au, which scales to ∼40 au for realistic dust properties (similar to the current Kuiper belt). Two outlying populations are also evident; five stars have exceptionally bright emission ( f > 5 × 10−5), and one has unusually hot dust <4 au. 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 – likely originating in transient events – and bright emission systems – 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