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Faure A.,CNRS Grenoble Institute for Particle Astrophysics and Cosmology Laboratory | Lique F.,CNRS Laboratory of Waves and Complex Media
Monthly Notices of the Royal Astronomical Society | Year: 2012

Nuclei with non-zero spin induce hyperfine splittings in the rotational spectrum of many commonly observed interstellar molecules. Radiative transfer modelling of such species requires in general a good knowledge of hyperfine selective collisional rate coefficients. We investigate in this work the impact of collisional rate coefficients on the molecular hyperfine excitation. The approximate sudden and statistical (proportional) methods are first compared to the almost exact recoupling approach. Rate coefficients are presented for a large number of CN and HCN transitions, with para-H 2(j = 0) as a collider. The sudden approximation and the recoupling approach, which both predict the propensity rule Δj = ΔF, are found to agree within a factor of 3 or better. Radiative transfer calculations are then performed using the large velocity gradient approximation. At low and moderate total optical depths (τ ≲ 10), where the relative hyperfine populations are close to the statistical weights, both the sudden and the statistical approximations are shown to provide accurate alternatives to the recoupling approach. At higher total opacities, however, the hyperfine propensity rule appears to matter and the sudden method is found to be significantly superior to the statistical approach. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


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.


Kunz M.W.,Princeton University | Lesur G.,CNRS Grenoble Institute for Particle Astrophysics and Cosmology Laboratory
Monthly Notices of the Royal Astronomical Society | Year: 2013

The magnetorotational instability (MRI) is the most promising mechanism by which angular momentum is efficiently transported outwards in astrophysical discs. However, its application to protoplanetary discs remains problematic. These discs are so poorly ionized that they may not support magnetorotational turbulence in regions referred to as 'dead zones'. It has recently been suggested that the Hall effect, a non-ideal magnetohydrodynamic (MHD) effect, could revive these dead zones by enhancing the magnetically active column density by an order of magnitude or more. We investigate this idea by performing local, three-dimensional, resistive Hall-MHD simulations of the MRI in situations where the Hall effect dominates over Ohmic dissipation. As expected from linear stability analysis, we find an exponentially growing instability in regimes otherwise linearly stable in resistive MHD. However, instead of vigorous and sustained magnetorotational turbulence, we find that the MRI saturates by producing large-scale, long-lived, axisymmetric structures in the magnetic and velocity fields. We refer to these structures as zonal fields and zonal flows, respectively. Their emergence causes a steep reduction in turbulent transport by at least two orders of magnitude from extrapolations based upon resistive MHD, a result that calls into question contemporary models of layered accretion. We construct a rigorous mean-field theory to explain this new behaviour and to predict when it should occur. Implications for protoplanetary disc structure and evolution, as well as for theories of planet formation, are briefly discussed. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.


Gallet F.,CNRS Grenoble Institute for Particle Astrophysics and Cosmology Laboratory | Bouvier J.,CNRS Grenoble Institute for Particle Astrophysics and Cosmology Laboratory
Astronomy and Astrophysics | Year: 2013

Context. Understanding the origin and evolution of stellar angular momentum is one of the major challenges of stellar physics. Aims. We present new models for the rotational evolution of solar-like stars between 1 Myr and 10 Gyr with the aim of reproducing the distributions of rotational periods observed for star forming regions and young open clusters within this age range. Methods. The models include a new wind braking law based on recent numerical simulations of magnetized stellar winds and specific dynamo and mass-loss prescriptions are adopted to tie angular momentum loss to angular velocity. The models additionally assume constant angular velocity during the disk accretion phase and allow for decoupling between the radiative core and the convective envelope as soon as the former develops. Results. We have developed rotational evolution models for slow, median, and fast rotators with initial periods of 10, 7, and 1.4 d, respectively. The models reproduce reasonably well the rotational behavior of solar-type stars between 1 Myr and 4.5 Gyr, including pre-main sequence (PMS) to zero-age main sequence (ZAMS) spin up, prompt ZAMS spin down, and the early-main sequence (MS) convergence of surface rotation rates. We find the model parameters accounting for the slow and median rotators are very similar to each other, with a disk lifetime of 5 Myr and a core-envelope coupling timescale of 28-30 Myr. In contrast, fast rotators have both shorter disk lifetimes (2.5 Myr) and core-envelope coupling timescales (12 Myr). We show that a large amount of angular momentum is hidden in the radiative core for as long as 1 Gyr in these models and we discuss the implications for internal differential rotation and lithium depletion. We emphasize that these results are highly dependent on the adopted braking law. We also report a tentative correlation between the initial rotational period and disk lifetime, which suggests that protostellar spin down by massive disks in the embedded phase is at the origin of the initial dispersion of rotation rates in young stars. Conclusions. We conclude that this class of semi-empirical models successfully grasp the main trends of the rotational behavior of solar-type stars as they evolve and make specific predictions that may serve as a guide for further development. © 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.


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.


Wiesenfeld L.,CNRS Grenoble Institute for Particle Astrophysics and Cosmology Laboratory | Faure A.,CNRS Grenoble Institute for Particle Astrophysics and Cosmology Laboratory
Monthly Notices of the Royal Astronomical Society | Year: 2013

We compute the rotational quenching rates of the first 81 rotational levels of ortho- and para- H2CO in collision with ortho- and para-H2, for a temperature range of 10-300 K. We make use of the quantum close-coupling and coupled-state scattering methods combined with the high accuracy potential energy surface of Troscompt et al. Rates are significantly different from the scaled rates of H2CO in collision with He; consequently, critical densities are notably lower. We compare a full close-coupling computation of pressure broadening cross-sections with experimental data and show that our results are compatible with the low-temperature measurements of Mengel & De Lucia, for a spin temperature of H2 around 50 K. © 2013 The Authors 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.


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.


Dubus G.,CNRS Grenoble Institute for Particle Astrophysics and Cosmology Laboratory
Astronomy and Astrophysics Review | Year: 2013

After initial claims and a long hiatus, it is now established that several binary stars emit high- (0.1-100 GeV) and very high-energy (>100 GeV) gamma rays. A new class has emerged called "gamma-ray binaries", since most of their radiated power is emitted beyond 1 MeV. Accreting X-ray binaries, novae and a colliding wind binary (η Car) have also been detected - "related systems" that confirm the ubiquity of particle acceleration in astrophysical sources. Do these systems have anything in common? What drives their high-energy emission? How do the processes involved compare to those in other sources of gamma rays: pulsars, active galactic nuclei, supernova remnants? I review the wealth of observational and theoretical work that have followed these detections, with an emphasis on gamma-ray binaries. I present the current evidence that gamma-ray binaries are driven by rotation-powered pulsars. Binaries are laboratories giving access to different vantage points or physical conditions on a regular timescale as the components revolve on their orbit. I explain the basic ingredients that models of gamma-ray binaries use, the challenges that they currently face, and how they can bring insights into the physics of pulsars. I discuss how gamma-ray emission from microquasars provides a window into the connection between accretion-ejection and acceleration, while η Car and novae raise new questions on the physics of these objects - or on the theory of diffusive shock acceleration. Indeed, explaining the gamma-ray emission from binaries strains our theories of high-energy astrophysical processes, by testing them on scales and in environments that were generally not foreseen, and this is how these detections are most valuable. © 2013 Springer-Verlag Berlin Heidelberg.

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