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Maschberger T.,Joseph Fourier University | Maschberger T.,CNRS Grenoble Institute for Particle Astrophysics and Cosmology Laboratory
Monthly Notices of the Royal Astronomical Society | Year: 2013

We propose a functional form for the initial mass function (IMF), the L3 IMF, which is a natural heavy-tailed approximation to the log-normal distribution. It is composed of a low-mass power law and a high-mass power law which are smoothly joined together. Three parameters are needed to achieve this. The standard IMFs of Kroupa (2001, 2002) and Chabrier (2003a) (single stars or systems) are essentially indistinguishable from this form. Compared to other three-parameter functions of the IMF, the L3 IMF has the advantage that the cumulative distribution function and many other characteristic quantities have a closed form, the mass generating function, for example, can be written down explicitly. © 2012 The Author. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Zanni C.,National institute for astrophysics | Ferreira J.,CNRS Grenoble Institute for Particle Astrophysics and Cosmology Laboratory
Astronomy and Astrophysics | Year: 2013

Aims. This paper examines the outflows associated with the interaction of a stellar magnetosphere with an accretion disk. In particular, we investigate the magnetospheric ejections (MEs) due to the expansion and reconnection of the field lines connecting the star with the disk. Our aim is to study the dynamical properties of the outflows and evaluate their impact on the angular momentum evolution of young protostars. Methods. Our models are based on axisymmetric time-dependent magnetohydrodynamic simulations of the interaction of the dipolar magnetosphere of a rotating protostar with a viscous and resistive disk, using alpha prescriptions for the transport coefficients. Our simulations are designed to model the accretion process and the formation of accretion funnels, the periodic inflation/reconnection of the magnetosphere and the associated MEs, and the stellar wind. Results. Similar to a magnetic slingshot, MEs can be powered by the rotation of both the disk and the star so that they can efficiently remove angular momentum from both. Depending on the accretion rate, MEs can extract a relevant fraction of the accretion torque and, together with a weak but non-negligible stellar wind torque, can balance the spin-up due to accretion. When the disk truncation approaches the corotation radius, the system enters a propeller regime, where the torques exerted by the disk and the MEs can even balance the spin-up due to the stellar contraction. Conclusions. Magnetospheric ejections can play an important role in the stellar spin evolution. Their spin-down efficiency can be compared to other scenarios, such as the Ghosh and Lamb, X-wind, or stellar wind models. Nevertheless, for all scenarios, an efficient spin-down torque requires a rather strong dipolar component, which has seldom been observed in classical T Tauri stars. A better analysis of the torques acting on the protostar must consider non-axisymmetric and multipolar magnetic components consistent with observations. © ESO 2013.

Maschberger T.H.,CNRS Grenoble Institute for Particle Astrophysics and Cosmology Laboratory
Monthly Notices of the Royal Astronomical Society | Year: 2013

Stars form in regions of very inhomogeneous densities and may have chaotic orbital motions. This leads to a time variation of the accretion rate, which will spread the masses over some mass range.We investigate the mass distribution functions that arise from fluctuating accretion rates in non-linear accretion, m mα. The distribution functions evolve in time and develop a power-law tail attached to a lognormal body, like in numerical simulations of star formation. Small fluctuations may be modelled by a Gaussian and develop a power-law tail m-α at the high-mass side for α > 1 and at the low-mass side for α < 1. Large fluctuations require that their distribution is strictly positive, for example, lognormal. For positive fluctuations the mass distribution function develops the power-law tail always at the high-mass hand side, independent of α larger or smaller than unity. Furthermore, we discuss Bondi-Hoyle accretion in a supersonically turbulent medium, the range of parameters for which non-linear stochastic growth could shape the stellar initial mass function, as well as the effects of a distribution of initial masses and growth times. © 2013 The Author. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Duchene G.,University of California at Berkeley | Duchene G.,CNRS Grenoble Institute for Particle Astrophysics and Cosmology Laboratory | Kraus A.,Harvard - Smithsonian Center for Astrophysics
Annual Review of Astronomy and Astrophysics | Year: 2013

Stellar multiplicity is a ubiquitous outcome of the star-formation process. The frequency and main characteristics of multiple systems, and their dependence on primary mass and environment, are powerful tools to probe this process. Although early attempts were fraught with selection biases and limited completeness, instrumentation breakthroughs in the past two decades now enable robust statistical analyses. In this review, we summarize current empirical knowledge of stellar multiplicity for main sequence stars and brown dwarfs, as well as among populations of pre-main-sequence stars and embedded protostars. Among field objects, the multiplicity rate and breadth of the orbital period distribution are steep functions of the primary mass, whereas the mass ratio distribution is essentially flat for most populations other than the lowest mass objects. The time-variation of the frequency of visual companions follows two parallel, constant tracks corresponding to loose and dense stellar populations, although current observations do not yet distinguish whether initial multiplicity properties are universal or dependent on the physical conditions of the parent cloud. Nonetheless, these quantitative trends provide a rich comparison basis for numerical and analytical models of star formation. Copyright ©2013 by Annual Reviews. All rights reserved.

Steinacker J.,CNRS Grenoble Institute for Particle Astrophysics and Cosmology Laboratory | Steinacker J.,Max Planck Institute for Astronomy | Baes M.,Ghent University | Gordon K.D.,Ghent University | Gordon K.D.,US Space Telescope Science Institute
Annual Review of Astronomy and Astrophysics | Year: 2013

Cosmic dust is present in many astrophysical objects, and recent observations across the electromagnetic spectrum show that the dust distribution is often strongly three-dimensional (3D). Dust grains are effective in absorbing and scattering ultraviolet (UV)/optical radiation, and they re-emit the absorbed energy at infrared wavelengths. Understanding the intrinsic properties of these objects, including the dust itself, therefore requires 3D dust radiative transfer (RT) calculations. Unfortunately, the 3D dust RT problem is nonlocal and nonlinear, which makes it one of the hardest challenges in computational astrophysics. Nevertheless, significant progress has been made in the past decade, with an increasing number of codes capable of dealing with the complete 3D dust RT problem. We discuss the complexity of this problem, the two most successful solution techniques [ray-tracing (RayT) and Monte Carlo (MC)], and the state of the art in modeling observational data using 3D dust RT codes. We end with an outlook on the bright future of this field. Copyright ©2013 by Annual Reviews. All rights reserved.

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