Constraints on the formation mechanism of the planetary mass companion of 2MASS 1207334-393254

In this paper we discuss the nature and the possible formation scenarios of the companion of the brown dwarf 2MASS 1207334-393254. We initially discuss the basic physical properties of this object and conclude that, although from its absolute mass ($5M_{\rm Jup}$), it is a planetary object, in terms of its mass ratio $q$ and of its separation $a$ with respect to the primary brown dwarf, it is consistent with the statistical properties of binaries with higher primary mass. We then explore the possible formation mechanism for this object. We show that the standard planet formation mechanism of core accretion is far too slow to form this object within 10 Myr, the observed age of the system. On the other hand, the alternative mechanism of gravitational instability (proposed both in the context of planet and of binary formation) may, in principle, work and form a system with the observed properties.Comment: 5 pages, MNRAS in pres

The dependence of the sub-stellar IMF on the initial conditions for star formation

Abridged: We have undertaken a series of hydrodynamical simulations of multiple star formation in small turbulent molecular clouds. Our goal is to determine the sensitivity of the properties of the resulting stars and brown dwarfs to variations in the initial conditions imposed. In this paper we report on the results obtained by applying two different initial turbulent velocity fields. The slope of the turbulent power-law spectrum alpha is set to -3 in half of the calculations and to -5 in the other half. We find that, whereas the stellar mass function seems to only be weakly dependent on the value of alpha, the sub-stellar mass function turns out to be more sensitive to the initial slope of the velocity field. We argue that, since the role of turbulence is to create substructure from which gravitational instabilities may grow, variations in other initial conditions that also determine the fragmentation process are likely to affect the shape of the sub-stellar mass function as well. The absence of many planetary mass `free-floaters' in our simulations, especially in the mass range 1-10 MJ, suggests that, if these objects are abundant, they are likely to form by similar mechanisms to those thought to operate in quiescent accretion discs, instead of via instabilities in gravitationally unstable discs. Finally, we find that multiple and single stars share comparable kinematical properties, both populations being able to attain velocities in the range 1-10 km/s. From these values we draw the conclusion that only low-mass star-forming regions such as Taurus-Auriga or Ophiuchus, where the escape speed is low, might have suffered some depletion of its single and binary stellar population.Comment: 13 pages, 6 figures, accepted by MNRA

Comparative statistics and origin of triple and quadruple stars

The statistics of catalogued quadruple stars consisting of two binaries (hierarchy 2+2) is studied in comparison with triple stars, with respective sample sizes of 81 and 724. Seven representative quadruple systems are discussed in greater detail. The properties of multiple stars do not correspond to the products of dynamical decay of small clusters, hence the N-body dynamics is not the dominant process of their formation. On the other hand, rotationally-driven (cascade) fragmentation possibly followed by migration of inner and/or outer orbits to shorter periods is a promising scenario to explain the origin of triple and quadruple stars. Our main results are: (i) Quadruple systems of Epsilon Lyr type with similar masses and inner periods are common. (ii) The distributions of the inner periods in triple and quadruple stars are similar and bimodal. The inner mass ratios do not correlate with the inner periods. (iii) The statistics of outer periods and mass ratios in triples and quadruples are different. The median outer mass ratio in triples is 0.39 independently of the outer period, which has a smooth distribution. In contrast, the outer periods of 25% quadruples concentrate in the narrow range from 10yr to 100yr, the outer mass ratios of these tight quadruples are above 0.6 and their two inner periods are similar to each other. (iv) The outer and inner mass ratios in triple and quadruple stars are not mutually correlated. (v) The inner and outer orbital angular momenta and periods in triple and quadruple systems with inner periods above 30d show some correlation, the ratio of outer-to-inner periods is mostly comprised between 5 and 10^4. In the systems with small period ratios the directions of the orbital spins are correlated, while in the systems with large ratios they are not.Comment: Accepted by MNRAS, 14 pages, 12 figures. Two electronic tables at http://www.ctio.noao.edu/ftp/pub/tokovinin/quadruples

Close binary systems among very low mass stars and brown dwarfs

Using Monte Carlo simulations and published radial velocity surveys we have constrained the frequency and separation (a) distribution of very low mass star (VLM) and brown dwarf (BD) binary systems. We find that simple Gaussian extensions of the observed wide binary distribution, with a peak at 4 au and 0.6< sigma_{log(a/au)}<1.0, correctly reproduce the observed number of close binary systems, implying a close (a<2.6 au) binary frequency of 17-30 per cent and overall frequency of 32-45 per cent. N-body models of the dynamical decay of unstable protostellar multiple systems are excluded with high confidence because they do not produce enough close binary VLMs/BDs. The large number of close binaries and high overall binary frequency are also completely inconsistent with published smoothed particle hydrodynamical modelling and argue against a dynamical origin for VLMs/BDs.Comment: Proceedings of the workshop on Ultra-Low Mass Star Formation, eds. E. Martin, A. Magazzu. To appear in Astron. Nach

The 0.03-10Mo mass function of young open clusters

We report the present day mass functions (PDMFs) of 3 young open clusters over a mass range from 30 Jupiter masses to 10~\msunn. The PDMFs of the 3 clusters are remarkably similar, suggesting little impact of specific conditions (stellar density, metallicity, early dynamical evolution) on the mass distribution. Functional forms are provided to allow quantitative comparison with MFs derived in other environments.Comment: 6 pages, to appear in "IMF@50", Corbelli, Palla, Zinnecker ed

The development of the European Union's strategy and its security policy : towards a multilateral security?

This paper is looking forward an answer to the question of the multilateralism in relation to the European Union (EU) security policy. Is it possible to say that multilateralism in this field of study has increased in the present times? Are the multilateral institutions of the contemporary international system evolving adequately or is there a setback in cooperation between international organisations (IGO) in terms of cooperation? All these questions are made in a context of globalisation. The interdependence between actors is every time bigger and the actions of one of them can provoke some effect in the other side of the globe disappearing the space-time dimension. New and more complex threats have emerged putting the European security into question

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

Resolution requirements for simulating gravitational fragmentation using SPH

Jeans showed analytically that, in an infinite uniform-density isothermal gas, plane-wave perturbations collapse to dense sheets if their wavelength, $\lambda$, satisfies $\lambda > \lambda_{_{\rm JEANS}} = (\pi a^2 / G \rho_{_0})^{1/2}$ (where $a$ is the isothermal sound speed and $\rho_{_0}$ is the unperturbed density); in contrast, perturbations with smaller $\lambda$ oscillate about the uniform density state. Here we show that Smoothed Particle Hydrodynamics reproduces these results well, even when the diameters of the SPH particles are twice the wavelength of the perturbation. Our simulations are performed in 3-D with initially settled (i.e. non-crystalline) distributions of particles. Therefore there exists the seed noise for artificial fragmentation, but it does not occur. We conclude that, although there may be -- as with any numerical scheme -- `skeletons in the SPH cupboard', a propensity to fragment artificially is evidently not one of them.Comment: 6 pages, 2 figures, accepted by A&